atomic energy &£^ l'enepgie atomique of canada …d.g. logan, a.b. mcdonald, j.c.d....
TRANSCRIPT
AECL-4913
ATOMIC ENERGY & £ ^ L'ENEPGIE ATOMIQUEOF CANADA LIMITED T £ 2 J r DU CANADA LIMITEE
A STUDY OF A SUPERCONDUCTING HEAVY ION CYCLOTRON
AS A POST ACCELERATOR FOR THE CRNL MP TANDEM
Edited by
J.S. FRASER and P.R. TUNNICLIFFE
Chalk River Nuclear Laboratories
Chalk River, Ontario
August 1975
A STUDY OF A SUPERCONDUCTING HEAVY ION CYCLOTROK A
POST ACCELERATOR FOR THE CRNL MP TANDK.M*
Edited by
J.S. Fraser and P.R. Tunnicliffe
Chalk River Nuclear Laboratories-.Ch P\ ] k R i w r . On t p\ r i o
August 19 7 5
AECL--
*Technical information taken from CR^L-1045, prepare ••-: i.Atomic Energy of Canada Limited in November 19?j.
Etude d'un cyclotron supraconducteur a ions lourds comme
post-accélérateur pour le Tandem MP de Chalk River*
Edité par
J.S. Fraser et P.R. Tunnicliffe
* Information technique provenant du rapportnon-publié CRNL-1045 préparé par l'EACL ennovembre 1973.
Résumé
Ce rapport donne le résultat d'une étude concernant lesaccélérateurs, effectuée dans le but de produire des faisceauxde tous les ions lourds, jusqu'il l'uranium, ayant une énergied'au moins 10 MeV par nucléon, à une résolution de ^ 0.01%.La conclusion est que le Tandem MP actuel, mis en liaisonavec un cyclotron supraconducteur compact, est une solutiontrès économique qui répond a la plupart des exigences.
Le cyclotron proposé est muni d'enroulements supracon-ducteurs S "âme d'air pour engendrer un champ magnétiqueinterne allant jusqu'à 5T. La variation de champ azimutalest suscitée par quatre paires de paies en fer saturé placésau-dessus et au-dessous du plan moyen et occupant moins dela moitié de 1'azimuth de chaque secteur. Des dés Rf rem-plissent l'autre moitié de chacun des quatre secteurs. Lesenroulements supraconducteurs en NbTi sont montés dans descryostats immergés dans de l'hélium liquide à 4.2 K.
Le faisceau de la source d'ions négatifs est groupé Sl'entrée du Tandem, stripé dans la borne fc haute tension etlorsqu'il est en un état de charge choisi il est stripé uneseconde fois au moyen d'une lame à l'orbjte d'injection inté-rieure du cyclotron. Avec un gain d'énergie de 0.8 MeV partour et par charge unitaire l'énergie de sortie réaliséevariera de 10 MeV/u pour l'uranium a 50 MeV/u pour des ionslégers complètement stripês. La gamme des radiofrëquencesnécessaire est 22-45 MHz. Le faisceau est extrait à un rayonfixe par deux impulsions successives dans des déflecteursélectrostatiques de ^ 4-x 10e- p-artïcules/s pour les ions légers"a ^ 1O10 particules/s pour l'uranium.
L'Energie Atomique du Canada, LimitéeLaboratoires Nucléaires de Chalk River
Chalk River, Ontario
Aoîlt 1975
AECL-4913
A STUDY OF A SUPERCONDUCTING HEAVY ION CYCLOTRON AS A
POST ACCELERATOR FOP. THE CRN!, MP TANDKM*
Kdited by
J.S. Fraser and P.R. Tunnicliffe
ABSTRACT
This report gives the result of a study of theaccelerators and research areas capable of producingbeams of all heavy ions up to uranium with energies 01at least 10 MeV per nucleon at an energy resolution of-.. 0.01%. It is concluded that the existing MP Tandeminjecting into a compact superconducting cyclotron isa very economical solution that meets most of therequirements.
The proposed cyclotron uses air cored super-conducting coils to generate an internal magneticfield of up to 5T. The azimuthal field variation (flut.Lois created by four pairs of saturated iron poles aboveand below the midplane and occupying less thai half theazimuth of each sector. Rf dees fill the ether halfof each of the four sectors. Tha NbTi superconductingcoils are mounted in cryostats immersed i: liquidhelium at 4.2 K.
The beam from the negative ion source ..s i.••• )\c'•»••• \at the input to the Tandorr-, sti ipped ir L;.c ";;1 g:--vo I I ,:•:(terminal, and a selected charge state stripped u secondtime with a foil at the inner injection orbit or thecyclotron. with an energy gain of 0.8 Mev per turn perunit charge, the output energy a.:hir\ 2c, w:]i \ary rroii;10 Mev/u for uranium to 50 M r,'/i: Jor fv.jly strippedlight ions. The range of radio "rcquerc , ea requ • r. d , :-:22-45 MKis . Tlie beam is £Xt: ac'«.'cd '• •- z\ f i.xoci :.V-!:;-;s ; vtwo successive impulses in electrostatic dc:lectors. 4 x 1 0 1 2 particles/s for ligr-t 'ors to 10'° p.ir'.ici-for uranium.
Chalk River Nuciear La;-:r . •.'j v i •_ sChalk River, Or.i.:;: ic
*Tcchnical information takor, fcor C'R'-T.by Atomic Energy of Canada Lii'.i'oc i:
(iv)
SUPERCONDUCTING CYCLOTRON
R F ACCELERATING ELECTRODES
IRON FLUTTER POLES
SUPERCONDUCTING COILS
TRIM MAIN
R F ACCELERATINGELECTRODE
ACKNOWLEDGEMENTS
Contributors to this study include H.R. Andrews,
G.C. Ball, C.B. Biyhnm, R.K. Elliott, A.J. Ferguson,
J. Fisher, J.S. Geiger, R.L. Graham, C.R. Hoffmann, J.A. Hulbert,
D.G. Logan, A.B. McDonald, J.C.D. Milton, H.M. Philippi,
H.R. Schneider and C.H. Westcott. individual credits are noted
in the by-lines of the various sections; those not credited
are the responsibility of the editors.
We must also acknowledge the patience of Mrs. Joyce Howes
in preparation of drafts and of the final manuscript.
( v i i )
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andur-
ot ron
injection
Rf system
M a m , Trim and shim coil
Flutter Poles
Extraction
Cryogenic s
Vacuum
Sy: :.e
if) i n s t r u m e n t a t i o n and con t roiy) Rad i a t i o n sViielding(b) Maqnotir s h i e l d i n g
oi-'i'. r r DYNAMICS - !(.A_S]C' wi-'f. U;N <'•'
'a) Klcmcnt.'iry !-'(.rmuJ ,;':•(h) l 'ocuss ing'c) Resonances
( i) Admittance at injectionfe) Cyclotron Hmittance
' f) Cyclotron Tolerances
^'^JA^^l I-^SI.ARCII EC:T'IPMKNT
a) scam Transpor t
;b) Research Kquipmont
PI Li.DVNc; ANT) SKkViyKK
( a ) S 11 e• ! ) A c c e l e r a t o r Room(c) !:;>:per imental Aro.ib(d) Heam Transpo r t Areasie) S e r v i c e Areas•:f} . " a x i l l a r y Systems anci Hu:ld
(v i i i)
CONTENTS (Cont'd)
Page
6 . DES IGK J\ND DE\TLOPMENT PROGRAM
i a) Summary 17 5(b) Accelerator physics 177(c) Magnet Development 179(d) Rf Modelling 182;e) Computer control Techniques 184(f) Shielding 190tg) Cryogenics 190(h) Mechanical and civil Design 191
REFERENCES 195
APPENDIX 200
(ix)
FIGURES
(1) General facility layout
(2) Operating limits
(3) Plan view of cyclotron
(4) Vertical section of cyclotron
(5) Phase plot for 'double'-drift, second harr.onic i
(6) Fraction of dc beam accepted as a function ot >::bucket width
(7) Equilibrium charge state distributions...
(8) Injection geometry
(9) Inner orbit radius/extraction radius vs. cha; IK >.
(10) Rf accelerating structure - plan view
(11) Rf accelerating structure - vertical section
(12) Rf accelerating structure - vertical section,
hot sector
(13) Rf accelerating structure - plan view of one <_• u;
(14) Operation in O- and TT-mode
(15) l/10th scale model of rf structure - assembled
(16) l/10th scale model of rf structure - disasjc"!!'
(17) Tuning range of rf model in O- & ;-modes
(18) Driver section of the TRIUMF rf power supply
(19) Cryogenic coil geometry
(20) Axial magnetic field in the midplane
(21) Magnetic field within the main coil
(22) Magnetic field within the trim coil
(23) Axial forces acting on the main and trim coils..(24) Typical critical current densities for
NIOMAX superconductor filaments(25) Effect of temperature on the critical current
of NIOMAX
(26) 1000 A superconductor cross section
(27) Components in a winding of a pancake coil
(28) Main coil power supply
(29) Dump resistor...
(x)
FIGURES ( C o n t ' d )
v iV y.a,;r.t%t..:a' ion curvi.- for i r o n 79
(31) Temperature variation of saturation magnetizationfor iron 80
(32) Induced poles due to magnetization... 82
(33) Fields at a boundary between a magnetized andunmagnetized region 83
(34) Current sheet equivalent of flutter pole 86
(3') Flutter-pole analogue model 87
(36) Azimuthal variation of axial field in the flutter-pole model 88
(37) Flutter field versus flutter-pole height 89
(38) Magnetic volume element 90
(39) Uniformly magnetized flutter-pole pair 91
(40) Magnetic force between 1/10 scale flutter poles 92
(41) Orbit separation vs. radius... 94
(42) Orbit separation vs. final specific energy 96
(43) Deflection angle vs. specific energy... 97
(44) Fringing field and extraction orbits... 98
(45) Vertical section of cryostat 101
(46) Plan view of cryostat 102
(47) Method of vacuum closure on beam trajectory 103
(48) Helium circuit 109
(49) Vacuum .components 114
(50) Beam and accelerator parameters 122
(51) Injection parameters 123
(52) Comparison between measured and calculated values ofattenuation length in concrete for neutrons 128
(53) Neutron dose rate vs. concrete shield thickness 129
(54) Concrete shielding required to reduce radiation field... 131
(55) Monte Carlo calculation for neutron sky shine... 132
(56) Magnetic shielding factor as a function of shield mass...137
(57) Magnetic rigidity versus ion specific energy... 141
(58) Ion cyclotron frequency versus specific energy 142
(xi)
FIGURES (Cont'd)
Page
(59) Magnetic field versus radius for uranium and carbonions 14 4
(60) Orbital resonance diagram 14 7
(61) Target room and beam transport layout 153
(62) Possible replacement system for the 90° analyzingmagnet 156
(63) Cyclotron service room layout 169
(64) Tandem-cyclotron computer system 185
Page
; Principal parameters 14
;;. Superconducting cyclotron parameters 17
HI. Negative ion currents from various types of
ion sources 22
TV. Tandem-cyclotron output beams 29
v. Some superconductor properties 58
vi. Minimum critical currents... 62
vir. Magnet characteristics 69
viil. Comparison of cyclotron magnet parameters-.. 77
IX. Saturation magnetization and other properties
of ferromagnetic materials 81
X. Cryostat heat influx... 106
XI. ORIC instrumentation and controls inventory 120
XII. Control computer and peripherals 120
XIIT. Neutron attenuation lengths*•• 130
XIV. Magnet field around a shielded dipole 136
XV. Shielding factors and masses for various
shields 139
XVI. Reference beam parameters 152
XVII. Bending angles for the dipole magnets 154
— 1 _
1. INTRODUCTION"
1(a) Summary
A major field of nuclear physics remaining to be explored
in detail is that of heavy ion reactions. while it is true that
linear accelerators and cyclotrons have been used for a number
of years to accelerate heavy ions, the results have fallen far-
short of the desirable objectives of high beam quality for
precision experiments and of sufficiently high energies for a wide
variety of projectiles to overcome the Coulomb barrier of any
target. In a recently published report^ , the Nuclear Physics
Branch of CRNL has examined a program of heavy ion research that
seems likely to lead to fundamentally new and exciting discoveries.
It concludes that a desirable accelerator to carry out all aspects
of the proposed program should produce beams of all ions
from hydrogen to uranium with energies variable from below the
Coulomb barrier for any target up to 300 MeV for hydrogen and
10 MeV/u for uranium. An intensity of 10 particles per
second and an energy resolution of 0.01% would be desirable.
This report discusses a cheap, compact, superconducting
cyclotron of novel design that will meet most of the above
requirements.
Three accelerator types and five accelerators were examined
to determine which would best satisfy the requirements. This
survey is described in Section 2 but may be summarized briefly
*Chalk River Nuclear Laboratories of Atomic Energy of Canada Limited
- 2 -
as roiiows. A r,»pid cycling synchrotron to give the neuessary
energv resolution would require a large aperture and is
consequently relatively expensive (see Table 1). Similarly a
conventional heavy ion linac requires a large expenditure(2)
to provide approximately 6 MW of rf power . This power and
consequently much of the cost can be reduced by going to a
superconducting linac. Although great strides forward in(3.4)the design of such a machine have recently been made
the necessary technology is not yet firmly established. The
technology for a conventional heavy ion cyclotron is highly
developed but the large amounts of iron involved again make it
expensive.
These studies indicate that a superconducting cyclotron
would be very much cheaper than all other accelerators except
possibly a superconducting heliac. Although in our estimate
the superconducting cyclotron is still cheaper, the difference
may not be as significant as the uncertain technology. The
idea of a superconducting cyclotron is new; nevertheless,
the necessary technology is well established. This gives us
considerable confidence in the cost estimates. we therefore
conclude that a superconducting cyclotron is a very attractive
solution.
Survey of Accelerators for Energetic Heavy ions
We have examined possible additions to the upgraded MP
- 3 -
Tandem Van de Graaf that would meet all, or most, of the
requirements . it was clear from the start that the intensity
requirement irould be difficult to meet for the heavier ions.
All types of accelerators using a Tandem as an injector would be-
most economically designed if the charge-to-mass ratio wore
increased by stripping before injection. For the heavier ions
this results in a fractionation of the beam current into 10
or more charge states, only one of which can be accepted for
acceleration. With this limitation in mind three types of
energy booster have been examined, viz., synchrotron, linear
accelerator and cyclotron.
1 (b) i Rapid-Cycling Synchrotron
A modern strong-focussing synchrotron should provide a
low cost per unit of final field-radius product but the very
high vacuum requirements and the inherently low duty cycle have
discouraged development of this type of accelerator for heavy ions
Advances in vacuum technology have now made the first problem
soluble but the low duty cycle can only be overcome by
successful development of multi-turn injection schemes, attainment.
of high peak currents from negative ion sources and design of a
slow beam extraction system for the synchrotron. Multiturn
injection appears possible, improved performance can be expected
for some negative ion sources, and slow extraction is
now an established technique in many existing synchrotrons.
- 4 -
in a study of the rapid-cycling synchrotron it became
apparent that the beam energy resolution requirement of 0.01%
would put severe demands on the rf system and on its control
during the acceleration cycle. The accelerating voltage would
have to be increased very slowly at the beginning of the cycle,
raised to a high value in the middle then slowly reduced to a
very low value at the end of the accelerating cycle. The energy
modulation and associated radial oscillations that result from
this rf program require a large aperture in the magnets. The
multi-turn injection scheme also requires a large aperture.
The large aperture requirement and the additions to
the design for slow beam extraction result in a cost similar to
that of a conventional separated-magnet isochronous cyclotron.
Beccuse of the difficulties associated with overcoming the low
duty cycle the synchrotron is not an attractive choice.
1(b)ii Linear Accelerator
A helical slow-wave structure has been developed in recent
years and proposals have been advanced to use it as an energy
booster for a Tandem accelerator at several laboratories. It
has been demonstrated that the structure can be adapted for low
particle velocities corresponding to the energies of ion beams
of all species that could be delivered by an MP Tandem.
The original concept of this accelerator consisted of a
large number of independently driven sections, each about 1.3m
Jong and providing an energy gain of 1.1 MeV per unit of charge.
— C! _
in this way the velocity profile along the machine could be
varied to suit any ion. The design developed at Frankfurt'1^'
operated at 108.48 MHz and each section consumed a maximum of
100 kw of rf power. For the 60 sections required to achieve
10 MeV/u, 6 MW of rf power would be needed.
Recent work at Karlsruhe and at the Argonne National(4)Laboratory has demonstrated that a superconducting vert-ion
of the helix can produce an accelerating gradient of more than
2 MV/m. The large saving in rf power cost is partially offset
by the refrigeration cost but a substantial cost reduction still
remains. A problem encountered in the superconducting helix
structure is the ease of deformation of the high Q structure.
This makes it prone to microphonics as well as detuning by
radiation pressure. Elaborate control devices are required for
fast control of the resonant frequency and these are currently
under development at several laboratories with satisfactory
solutions in sight. While the cost is probably smaller than
that of a synchrotron or conventional cyclotron, the necessary
technology for the superconducting machine is not yet firmly
established.
l(b)iii Heavy-ion cyclotron
The most highly developed accelerator for heavy ions is
the separated-magnet isochronous cyclotron. The large
cyclotron now under construction at the University of Indiana
is of this type, although it will not be capable of accelerating
- 6 -
the heaviest ions to 10 MeV/u. Detailed proposals have been
made for very large isochronous cyclotrons of the separated
magnet type at Oak Ridge , Michigan state University
and the Argonne National Laboratory ' . These concepts have the
advantages of highly developed magnet and accelerating system
technology as well as simplified injection and extraction
systems. The principal disadvantage is that of cost, estimates
being in the range of $4M to $6M for cyclotron hardware alone.
The subject of the current study is a superconducting heavy
ion cyclotron. The substitution of air-cored superconducting coils
for massive magnets and the overall reduction in size promises
to yield substantial cost reductions over the more conven-
tional design. A maximum average field of 5 Tesla (50 kG)
could be produced with state-of-the-art technology which
has been developed to f i l l the need for hydrogen bubble
chamber magnets in high energy physics experiments. A novel
feature of this concept is the use of saturated iron pole
tips to produce the azimuthal field flutter, an essential
requirement for focussing. in a previous air-cored design
the flutter was produced by a special set of coils.
Injection would be by stripping in the median plane with a
carbon foil placed normal to the inner equilibrium orbit. Four
sectors are proposed, each sector having a high-field or hil l
region and a low-field or valley region. An accelerating
electrode or "dee" is placed in each valley region. The dee
structure is coaxial with the magnetic field and tuning would
be by a movable short-circuit on coaxial support lines.
Extraction would be at a fixed maximum radius of 65 cm.
An orbit separation ranging from 3 mm to 15 mm (depending on the
charge-to-mass ratio of the ions) would be produced with 100 kv
_ 1 _
peak voltage on the four dees. Single turn extraction shouu!
be achievable with two electrostatic deflectors located in
hill half-sectors.
1(c) choice of a Superconducting cyclotron
This report recommends the superconducting cyclotron
concept as the basis for a detailed design study primarily
because it promises the minimum total cost. A superconductiny
cyclotron promises to fulfill all of the principal requirements
of a heavy ion booster accelerator with a minimum capital
cost and a minimum in development effort. The compactness
of the concept that results from the intense magnetic field
not only reduces civil costs but replaces the massive iron
magnets and associated power supplies with a much less costly
air-cored, superconducting coil system, cryostat and
refrigerator. For the superconducting cyclotron, the cost
benefit in cryogenic technique is much more clearly evident
than it is for a superconducting linear accelerator. That this
is so stems from the fact that in the cyclotron the large and
costly bulk of iron for the magnets is eliminated in favour of
a compact, air-cored coil system in a cryostat with only a
heat leak to be supplied by a refrigerator, whereas in the
linear accelerator, a costly rf power supply is eliminated in
favour of a less costly superconducting rf structure, cryostat
and refrigerator. The refrigerator in this case must cope with
the heat leak as well as the structure rf power dissipation.
- 8 -
a factor that is very much dependent on the state of the
art of rf superconducting structure fabrication.
The cryogenic techniques associated with the super-
conducting cyclotron have been well established over a number
of years in the development of intense magnetic fields for
hydrogen bubble chambers used in high energy particle physics.
The remainder of the concept does not pose any serious develop-
ment problems; tunable rf structures and cyclotron orbit
dynamics are well understood. By contrast, the superconducting
helix is only at this time being developed to the state where
it may provide a definite cost advantage over its room
temperature version. The cost advantage of the super-
conducting version of the helix over the normal one is
critically dependent on the accelerating gradient that
can be achieved reliably. The break even point has not
yet been clearly demonstrated.
The superconducting cyclotron, while it is a complex
machine, will, nevertheless, encompass far fewer adjustable
parameters than would the superconducting helix with its very
large number of independently driven and phased resonant
cavities.
At the present stage of development of the helix
concept it is difficult to forecast what success there will
be in achieving an energy resolution approaching 1 in 10 .
Cyclotrons, on the other hand, are by now sufficiently well
understood to know that the problem lies with achieving
correct and accurate magnetic field distributions and
- 9 -
stable rf accelerating voltage. These criteria have been
met in conventional versions of the isochronous cyclotron
and there are no presently evident reasons why they can-
not be met with some development effort in the superconducting
version.
The choice of a superconducting cyclotron is not
without its disadvantages. As a direct result of the neces-
sity to inject ions in the mid plane and to capture them
in stable orbits by changing their effective charge in a
foil stripper, it becomes difficult, but not impossible in
principle, to accelerate hydrogen and helium ions. For
hydrogen isotopes, only stripped ions are delivered by the
MP Tandem and for helium ions, the ionic charge changes by
a factor that is not compatible with the layout evolved
for all other ions from Li to U. If hydrogen molecular
ions could be injected they could be stripped to atomic
ions but again the geometrical requirements are not
compatible with the layout.
The internal foil stripper will be more difficult to
engineer and the general congestion of components in the
central region will be more severe than in a separated
sector iron-magnet cyclotron. However, the relative ease
of accommodation of a compact cyclotron in the existinq
- 10 -
building complex will minimize the disruption of the
ongoing nuclear physics program based on the MP Tandem
Accelerator. The uniqueness of a superconducting cyclotron
is an intangible but attractive feature in i ts favour.
- 11 -
2- .^P.NCKP'H Al, DUIMC,::
2i.i) :;unun.iry_ Doscr Ljrtj-on of _thq_ J- ac 11 i t y
Fig. 1 shows the layout of the propose; facility, ii .. -.
integrated with the existing Nuclear physics MP Tandem facility,
The accelerating system consists of
a) Negative Ion Source
b) Buncher System
c) MP Tandem Van de Graaff
and d) superconducting cyclotron
Negative ions of all atomic species are generated in a s 'liable
ion source (the new sputter source being the most likely c<inuubif;
accelerated by a single ^ 150 kv dc gap and velocity modulated
by two rf cavities. The ions are then accelerated to the Tandem
terminal, with up to 13 MV on it, stripped to multicharged
positive ions in the terminal and further accelerated back to
ground potential. The resulting distribution in charged species
has an appropriate component selected by an analyzing magnet ior
injection into the cyclotron. On arrival at :.he cyclotron the
velocity modulation imposed by the buncher system delivers
45% of the beam compressed in 3 of cyclotron rf phase. A
second stripping foil at the inner equilibrium orbit in the
cyclotron further increases the ion charge and the i-yclotr<n
captures and accelerates an appropriate charge species.
After extraction from the cyclotron a beam transport system
distributes the accelerated beam to any of six target areas.
The system is capable of bypassing the cyclotron and delivering
the output Tandem beam directly to these areas if desjred.
- 12 -
P.»«* IT EL.. t5S' 0"
0 •- • • ; • "•• ,<;,•
Fig. 1: General facility layout.
- 13 -
A computer-based digital control system will be ne.-v;,s r-
cope with the relatively complex adjustments oi~ a ].ir<:> •v-r .
of variables in optimizing and adjusting the whole tv/r.Lvm.
The principal feature of the proposed isochronous eye: •'.••!
is the use of air-cored superconducting coils to provide
a main magnetic field of 5 T resulting in a uniquely
compact machine-. The chosen design field is within, the r .\> • • '.•
of existing technology. The upper limit is as much dict.it.- . ;•
congestion due to the small size as by the superconductor
technology - in fact more detailed design study might sujaes*
of a somewhat lower field. Another important factor • \r-.i' ::.
the magnetic field is the orbit separation at extract.on.
The facility layout has been designed to take maximum
advantage of the existing structures. Except for the mst -W •.*, <. •
of the buncher and the exit beam transport, the present
Tandem remains undisturbed. The cyclotron because oi ,(s snull
size fits readily between the MP Tandem enclosure and the i K
machine room. The remaining space conveniently provides ,••:!;;
dation for services. The EN room provides a beam transport
gallery to target rooms grouped around it. Three new • r ;»
rooms (experimental areas) are proposed; these could ) <• ,. i'i< i
progressively if deemed prudent.
Table I and Fig. 2 indicate the main performance
parameters anticipated.
A plan view and a vertical section of the cyclotrr.;. ,tr-
shown in Figs. 3 and 4 and Table II lists its main d»•: i<m
parameters.
- 14 -
100 -
u
10
RF FRZCut^CY
L I M I T
INJECTION
LIMIT
MAGNETICFIELDL I M I T
INSUFFICIENTFOCUSING N
TANDEM OUTPUT
RF FREQUENCYLIMIT
NS
I I I10 30 60 100
MASS NUMBER (u)
238
J
Energy
Fig . 2; Operat ing l i m i t s
TABLE I
PRINCIPAL PARAMETERS
Energy Reso lu t ion , AE/E
Transverse Emittance
intensitv
3 to 10 MeV/u - uranium ionsto
50 MeV/u - lithium ions(see Fig. 2)
•~ 4 x 10"4
v mm-mrad
. 10 particles / s for most elements
ELECTROSTATIC
DEFLECTORS
A C C E L E R A T I N G
Pig. 3: Plan view of cyclotron
2.8m
SUPERCONDUCTINGCYCLOTRON
V 0 . 65m/ / / // / A
NbTi C O I L S MAIN
RESONATORNORMALSHIM C O I L S
Fig. 4; vertical section of cyclotron
5T0.130 . 6 5
J = 1 .003 -(4 spiral ridges)
ase 100 ]140 1
80 . 8 ]
22 - 4530 - 60
mm
1.050
z
<v
MVMHzkw
- 17 -
TABLE II
SUPERCONDUCTING CYCLOTRON PARAMETERS
Magnetic Field
Maximum average fieldOrbit radii Inner
Outer
Focussing frequenciesRadialAxial
RF Acceleration
Max. peak voltage resonators in phaseout of
No. accelerating gaps (four sectors)Max. accelerating voltage per turnFrequency rangeRF power
injection
Tandem beam to foil stripper on inner radius.
Extraction
Two 33 electrostatic extractors into fringe field(50 kv across 5 mm)
"Single Turn" extraction over most of the range.
Stability Requirements
Magnetic field 1 in 10RF frequency 1 in 10RF voltage 1 in 10Input beam phase bunch width 3
The main magnetic field is produced by a set of air-cored,
superconducting coils located in a cryostat and symmetrically
placed above and below the acceleration plane. The avorace
field at radius R in an isochronous cyclotron must follow t o
relation
<B--n -y(R)BR c
- 18 -
where (R) is ratio of the particle's total energy to its rest
energy and B is a constant. This radially varying field shape
can be approximated by two pairs of coils (main and trim coils)
but ripples in the field remain which must be corrected by the
shim coils. A set of air-cored normal coils is provided for
shimming.
The azimuthal flutter that is required for radial and
vertical focussing is provided by saturated iron poles above
and below the plane. The field is thus logically divided into
sectors, in this case four (see Fig. 3). in each sector one pair
of flutter poles occupies somewhat less than one half of the
area, the other containing a segment of the accelerating structure.
The necessity of maintaining an orbit separation that
is adequate for clean beam extraction in a high magnetic field
cyclotron leads to the use of a multi-gap rf accelerating
structure. Eight gaps are provided by four "dees", one in each
sector of the proposed design. The rf accelerating structure is
mounted coaxially with the coils. Two opposite dees are connected
together and supported from the top coaxial feed line, the other
two dees being connected together and supported from below.
A foil stripper is located in one sector adjacent to a
pair of flutter poles. It is movable radially from 13 to 20 cm
to accommodate a range of ion injection requirements. ions from
the Tandem injector with charges between one and ten are stripped
in the foil to effective charges between three and 33, i.e. charge-
to-mass ratios, £, between 0.5 and 0.14 for lithium and uranium
respectively. Beam intensities are mainly defined by the
available intensities of negative ions and by the charge fraction-
ation in the two strippers. Expected intensities will range from
- 19 -
12 10
.4 x 10 particles/s for light ions to - 10 particles/s
for uranium.
Extraction will be initiated at a fixed radius of 65 cm in
two electrostatic deflectors located in adjacent sectors. These
provide sufficient orbit perturbation to move the orbit into the
fringing field region where the decreasing field causes the orbit
to spiral outwards.
The proposed cryogenic system is designed to minimize the
cooldown time required for maintenance or modification of the
cyclotron. The top trim coil is detachable in its own cryostat,
providing access to the accelerating structure without bringing
the entire magnet system up to room temperature. The accelerator
and the main cryostat use a common vacuum under normal operating
conditions; valves are provided to seal off the beam line between
the cryostat and accelerator during maintenance to the latter.
A pressure of ~~ 10 p a within the cyclotron is expected
to be adequate for good transmission of all ion species. The
common vacuum system provides a large cryogenic pumping surface
which will be supplemented by oil diffusion pumps to attain the
required pressure.
A large cylindrical iron magnetic shield is proposed to
enclose the cyclotron and reduce the external fields to less than
5 mT.
A positive ion source located in the terminal of the
Tandem could supply beams of ions not otherwise available from
negative ion sources. For this reason, developments elsewhere in
positive ion sources for linacs and cyclotrons are being carefully
followed and the present program of developing instrumentation
for the terminal of the Tandem is being actively pursued.
- 20 -
The possibility th^t the induced electric field (V x B force)
acting on an ion could strip off additional electrons, thus changing
the charge state is of no concern. For the H ions of TRIUMI•'*
this is a serious problem, and our magnetic fields are -. 10 times
greater than for TRIUMF. However, the factor •/-. is lower for
our worst case (50 MeV/u) than for TRIUMF by a factor of 3.5,
giving an electric field only , 2.85 times larger. This is much
more than compensated by the relative stability (binding energy)
of our ions, compared with the 0.755 eV for H ions; Ref. (5)
gives curves showing that the binding energy for an additional
electron being removed lies in the range of 100-200 eV for the
charge states likely to be used at extraction. For the charge
states at (before) injection, the binding energies are lower but
the B- factors are also much lower, so that no problems should
arise.
The possibility of multiplying the energy obtainable from
an isochronous cyclotron by beam recycling has been examined
by several authors ' . After acceleration and extraction,
a beam may be re-injected to a second stripper located between
the turns of the first-pass beam. On the second pass the beam
has a higher charge state and can be accelerated in the same rf
structure on a different harmonic than on the first pass. The
principal requirements of the scheme are a sufficiently high dee
voltage to overide the differing mass increase factors of the two
beams and an all-magnetic extraction system to ensure identical
paths for the two beams. The first requirement limits the final
specific energy to less than about 10 MeV/u. As the proposed
cyclotron already furnishes specific energies up to 50 MeV/u and
as the extraction system combines electrostatic and magnetic
deflection, a recycling scheme has not been pursued.
*The Tri Universities Meson Facility
- 21 -
2(b) ion Source (A.B. McDonald)
There are presently five main types of negative heavy ion
sources in use with tandem accelerators: the charge-exchange
duoplasmatron source, the direct extraction duoplasmntron source,
the Heinicke-Penning source, the sputter source and the diode
source. Of these, the first three are currently in use with the
CRNL-MP Tandem and a sputter source is presently on order.
Table III summarizes the intensities of negative ion beams
presently attainable from these sources. The information
contained in this table has been obtained from experience P.t
CRNL and from references (11) to (14). With the exception o: the
charge-exchange duoplasmatron, the emittance of all these sources
is known or predicted to be less than about 4~ mm mrad MeV
Negative ion sources are in constant development at
laboratories throughout the world and it can be expected that
increases in intensity and ion variety will be made, such as
which were achieved in the recent development of the sputter
source. Table HI indicates that a large variety of heavy ion
beams are already available in intensities of the order of
microamperes.
- 22 -
TABLE III
NEGATIVE ION CURRENTS FROM VARIOUS TYPES OF ION SOURCES*
ion
H~
H~
He"
Li"
Be"
B~
(BO)~
C~
(NH)~
(CN)"
O~
F~
Ne~
Na"
(MgH)"
(A1O)~
(PH2)"
P~
S~
300
80
5
5
0.3
1
20
10
2
30
100
100
none
0.5
0.3
0.7
2
4
50
Source
D
DXD
CXD
CXD
HP
S
D
S
DXD
HP
S
HP
HP
HP
S
HP
D
HP
ion
Cl~
Ar~
K~
Ca"
Fe~
Ti~
Ni~
Cu~
Se~
Br~
(Zro)~
(Mo02)~
Ag"
Te~
I~
Au~
Bi~
U~
tiA
50
none
0.2
1.5
1
1
12
20
25
40
3
0.5
25
20
50
8
0.2
5
SourceHP
HP
S
S
S
S
S
S
HP
S
S
S
S
HP
S
s
s
*D = Diode; DXD = Direct Extraction Duoplasmatron;
CXD = Charge Exchange Duoplasmatron; s - Sputter;
HP = Heinicke-Penning
- 23 -
2(c) Buncher (J.S. Fraser)
In an isochronous cyclotron there is no phase focussing.
If all ions are to gain the same amount of energy at each accel-
erating gap the accelerated beam must start and remain tightly
bunched in rf phase and the rf voltage must be stable. it follows
that the beam must be bunched prior to injection into the
cyclotron.* For a perfectly isochronized cyclotron,
O(15)
(16)AE/E = 4 x 10 corresponds to a phase spread A0 - 3
The proposed design is a double drift harmonic buncher
This device is capable of bunching 45% of a dc beam into a phase
width of 3 . With the buncher between the ion source and the
tandem accelerator, only small modulating voltages, having a
negligible effect on ion energy spread, are needed. Figs. 5 and
6 show results obtained in a preliminary calculation for mass 40
ions with fundamental and second harmonic amplitudes of 57 9
and 130 volts respectively giving 43% of the beam bunched into th*--
necessary 3 bucket at the cyclotron. The low modulation
amplitudes are made possible by the low ion energy (150 kev)
and long drift distances available in the tandem injection region.
The buncher gaps would be provided by a pair of tunable,
re-entrant coaxial cavities. Using sliding shorting-bars, such
cavities can be tuned conveniently over the required 2 to 1
frequency ratio.
*The rf waveform can be "flat-topped" by appropriate addition ofharmonics and the bunching requirements relaxed - a difficultprocedure and only essential if intensity limitations precludethe method proposed here.
- 24 -
-160 80 0 80QRPIUQL PHOSE,DEGREES
160
Fig. 5: phase plct for 'double'-drift, second-harmonic buncher
1. 0
0. 8
OLLJu
. 6
- 25 -
U
a0. 4
0. 2
0.020 4BUCKET WIDTH,DEC
6C
Fig. 6: Fraction of dc beam accepted as a function of rfbucket width
- 26 -
A thorough study is required to determine the choice
between 2nd and 3rd harmonic bunchers, whether more than two gaps
are required, the stability requirements of amplitude and phase
and the suitability of a given design for the whole range of
ions from Li to U.
2(d) MP Tandem (A.B. McDonald, H.R. Andrews)
The CRNL*MP Tandem Accelerator has been in operation since
1967 at voltages up to 10 MV and was recently upgraded by the
installation of a new set of accelerating tubes to permit terminal
voltages up to 13 MV. During the past year an active research
program with a variety of heavy ion beams has been carried out
at these higher voltages. Operation at voltages above 10 MV
has not been trouble-free, due mainly to a succession of failures
in charging belts. However, an alternative charging system,
using chains of pellets will be installed during 1974. Such a
system is in operation at several other laboratories and is
expected to solve the principal operating problem.
The terminal voltage stability of the Tandem accelerator
is presently excellent, mainly as a result of a new technique of
applying belt charge in the terminal, and a new electronic stabil-
izing system. Typical peak-to-peak ripple is less than 2 kv at
12 MV, which provides an energy resolution of better than
1.6 x 10~4.
Charge changing in the high voltage terminal can be
accomplished by thin foils or by gas. As may be seen in Fig. 7
foil stripping results in significantly higher charge states than
does gas. However, foil stripping suffers from several disad-
vantages relative to gas stripping. Firstly, for ions heavier
*Chalk River Nuclear Laboratories
100 1 I I I I I
1271 12 MeVr \
o
$>
|
O.I
\ \ \
Fig. 7:
8 10Charge State
12 14 16 18
Equilibrium charge state distributions for 12-MeViodine ions stripped in gases of hydrogen and oxygen,in a fluorocarbon vapor and in gold .nnd carbon foils
- 28 -
than ibout mass 32, the lifetimes of foils for injected beams
greater than about 2 uamps become shorter than 1 hour.
Secondly, the angular divergence due to multiple scattering
in the foil3 is much worse than for gas, since minimum foil
thicknesses are v 2 ug/cm whereas equilibrium charge distri-2
butions can be obtained for ^ 0.3 u.g/cm of gas. This angular
divergence increases the emittance of the heaviest beams
beyond the acceptance of the Tandem accelerator, reducing the
intensity considerably. Finally, the energy straggle from
stripping will be less with gas stripping. The overall energy
resolution for gas stripping can be expected to be about-4 -4
2 x 10 , compared to about 7 x 10 for foil stripping. For
these reasons, it has been decided to design the cyclotron to
operate with beams produced by gas stripping in the Tandem at
terminal voltages up to 13 MV. For such operation, the
transmission of the Tandem accelerator has been measured to be
greater than 40% for a large variety of heavy ions with masses
less than 35.
The intensities of beams from the Tandem accelerator can
therefore be estimated from the data presented in Table m combined
with estimates of charge state distributions produced at the
stripper. For the latter, the empirical formulae of Betz et al
have been found to match closely the charge distributions obtained
for beams in use at present. The results of calculations using
these formulae for the charge distributions after stripping in
the Tandem accelerator and in the cyclotron are presented in
Table iv. The cases presented are typical charge state combinations
which satisfy the requirements for acceleration in the cyclotron.
Transmission through the Tandem is assumed to be 40%, the buncher
- 29 -
c-Miciency is assumed to be 50%, and the beam current
into the Tandem is taken to be 5 uA.
TABLE IV
TANDEM-CYCLOTRON OUTPUT BEAMS
Beam currents from Tandem and cyclotron
Assumptions: Tandem transmission = 40%, Buncher efficiency - r)0,
Beam injected into Tandem = 5 -JLA. Tandem: gas stripping.
Cyclotron: foil
ION
7Li12c1 9F37ci63Cu1 0 3Rh153Eu
2O8pb2 3 8u2 3 8u
ChargeStatefrom
Tandem
1
2
2
6
7
7
6
9
10 (gas)
10 (foil)
ENERGY'T/A)(Mev)
3.7
3.3
2.1
2.5
1.7
1.0
0.6
0.6
0.6
0.6
c)INTENSITY
(Particle na)
7 20 a )
480 a )
400 a )
280
220
180
200
50
16
180b)
ChargeStatefrom
Cyclotron
3
6
7
15
21
24
25
30
33
33
ENERGY(T/A)(MeV)
47
41
39
18
21
18
18
10
10
10
(Part •<•]
640
24 0
IMP
3d
ih
39
10
4
3(}
ITYC)
c na)
))
a) Achieved by reducing stripper gas until charge state distri-bution is a maximum at this charge.
b) Transmission through the Tandem will be significantly less than40% for foil stripping, so that this intensity is an overestimate.
9c) 1 particle nA « 6 x 10 particles/s.
- 30 -
Poi all ions up to about mass 180, a terminal voltage of
1J MV permits operation with charge states at the peak of the
distributions, for both strippers. Above this mass, it becomes
necessary to use charge states in the Tandem and cyclotron which
are higher than the average value, so the intensities decrease.
For the lighter ions, it is necessary to use charge states
from the Tandem that are significantly lower than the average
equilibrium charge, in order to obtain the proper charge ratio for
acceleration in the cyclotron. It has been found in tests with
the Tandem accelerator that the yield of charge states lower
than the average equilibrium charge can be increased by reducing
the stripper gas pressure, so that nearly the same particle flux
can be obtained for any charge state below the equilibrium charge.
Few measurements exist for the emittance of the ion sources
used with the Tandem and for the emittance of beams after
acceleration. However, apparatus has been built for the
measurement of the output emittance of beams from the Tandem
accelerator using the techniques of Denhofer . Measurements
will also include emittances, foil lifetimes and charge state
distributions for Tandem beams bombarding carbon foils typical
of those to be used inside the cyclotron.
Estimates have been made of the emittance of the Tandem(19)
accelerator using the beam transport code OPTIC which
includes an estimate of multiple scattering effects based on the
theory of Moliere . These estimates suggest that for gas
stripping, the output emittance of the Tandem will be less
than - 50 rr mm mrad MeV .
A number of additions and modifications to the Tandem
accelerator are under consideration that should improve the beam
intensities attainable at high energies. A quadrupole lens is
being designed to improve the injection optics and provide better
- 31 -
focussing of the beam through the stripper canal. Another
modification under consideration is the addition of a lens am
charge state selector in the terminal. These elements would
greatly reduce the loading in the high energy beam tibes cv;seu
by unused charge states, thus increasing the average beam
currents that may be accelerated.
Another modification that has been discussed is the jdditi ••.
of a positive ion source in the terminal of the Tandem. Sources
of multiply charged positive ions are in use with many heavy
ion accelerators and development work is constantly in progress.
At present, typical sources capable of providing beams comparable
to those listed in Table IV are high power, pulsed sources which
would not be very suitable for operation in the terminal.
Future developments in this field should be carefully
followed, since the Tandem accelerator, with charge state selection
in the terminal should be capable of accelerating much larger
quantities of positive heavy ions than those listed in Table iv.
2(e) Cyclotron
2(e)i injection (J.S. Fraser)
Ions may be injected into a cyclotron in two ways, axially
or in the midplane. The former is ruled out for several reasons:
the coaxial mounting chosen for the rf structure would be in the
way of an axial injection channel, the magnitude of the magnet
rigidity of the ions from the Tandem would make electrostatic
inflection difficult and magnetic inflection would create jii
intolerable perturbation in the isochronous magnetic field.
Separated iron-magnet cyclotrons may use inflection magnets t
ic
tor
- 32 -
miciplane injection but these too are ruled out. The simpler
alternative is charge-change injection using a foil stripper at. , .. • ^ . t (21.22)
the inner equilibrium orbit.An idealized geometry for the midplane injection scheme is
shown in Fig. 8. The ion with charge Q. is injected at an
angle to the tangent of a hard edged magnetic field boundary.
The bending radius in the field is R. centered at the point P.
The stripping foil s is placed so that the injection orbit is
tangential to the inner equilibrium orbit of radius R1 at the
foil. Since the ion energy is unchanged in the stripping
process,
R i " Qo
where Q is the ion charge after stripping.
The triangle relation gives*
R.= COStf COS 9 -
"R.
The requirement that the radical be positive defines the
permissible integral charge ratios by the inequality
c o sQ i
Q
with an injection angle of 30 , for example, Q. Q / 2 .
The cyclotron equation for the specific energy T/A in MeV/nucleon
of an ion of effective charge Q and mass A can be written as
T/A = 48 (QBp/A)2
where Bp is the field radius product in Tesla-metres. The output
*The + sign is chosen because it corresponds to a more convenient
solution with R. £ R .
- 33 -
Pig. 8: Injection geometry. Angle of injection, 9? chargestate at injection, Q.; effective field boundary radius,R~; injection radius of curvature, R.; stripper foil, S;final charge state, Q ; inner equilibrium orbitradius, R,; extraction orbit radius, R .
- 34 -
to injection energy ratio, neglecting the relativistic m,iss
increase,
T / Ro
with a fixed extraction radius R , the inner equilibrium orbit
radius is determined by the ratio of charges before and after
stripping as illustrated in Fig. 9. Charge ratios for most of
the useful ion species lie in the range 0.25 to 0.4. An injection
angle of 30° will then require the stripper position to be
adjustable between 0.19 and 0.37 of the extraction radius (12.4
and 24.0 cm).
The stripping in the terminal of the Tandem can be accom-
plished either by a gas canal or by a carbon foil stripper.
The middle range of ions from Mg to Pb can be produced with Q.
values which will yield convenient Q./Q ratios, by operating the
gas stripper at its normal pressure. For the lighter ions,
low values of Q. can be enhanced in yield by reducing the
stripper gas pressure. For the heaviest ions, the maximum
intensity of 10 MeV/u ions will only be obtained if a foil
stripper is used in the terminal. As mentioned earlier, recent
measurements on the MP Tandem have shown that the lifetime of
foils in the terminal is markedly shorter with low energy
heavy ions than with light ions of the same energy. Ths effect
on the beam emittance of the gas and foil strippers will have to
be determined at an early stage in the cyclotron design.
The positioning of the stripper foil can be restricted to a
radial line provided that the injection angle 9 is allowed
to vary with the charge ratio Q./Q • Preliminary calculations
have shown that this can be accomplished by steering the beam
from a fixed point well outside the cryostat.
• 8 r
7k
. 6
. 5
. 4 I—
a:
. 2
. 1
e = 40
8 9 1 C
Fig. 9: Ratio of inner to final orbit radii, R^/R as a functionof charge ratio Q./Q for various injection angles.A hard-edge field is assumed.
;.-:e imection orbit is curved in the fringing fieJd of
•).v t-iul system; consequent ly the vicuuir line raus*. either be
o: suf: : • iently i arqe diameter to illow for the curvature or A
curved beam line must be provided.
in addition to a positioning mechanism, a remotely
operated roil changing mechanism is required. In the intense
magnetic field the forces on moving metallic structures will be
large. The foil changing mechanism could be constructed entirely
of non-metallic materials. A possible design comprises a magazine
placed outside of the cryostat and carrying, say, several hundred
foils mounted on plastic frames. The magazine couid be similar
to the familiar slide projector magazine. A single foil frame
would be carried in a guide by a nylon control string out of the
magazine to the proper position. Fine positioning of the foil at
the correct radius R, would be accomplished with the same mechanism.
After the foil stripper, the beam will be distributed over a
number of charge states varying from two for light ions to about
10 for the heaviest ions. Steps will be taken to remove the
unwanted charge states. The large energy gain per turn produces
a large inner orbit separation. This makes it possible to place
beam stops on either side of the desired orbit 180 azimuthally
from the foil stripper. At this point the displacement due to the
betatron oscillations of all but the proper charge state will
be at a maximum.
The detailed calculations of the injection orbit remain to
be carried out. The beam will be subject to several perturbing
forces as it crosses four accelerating gaps at varying angles and
through one hill region. Since the beam is bunched to about 3°
in phase the perturbing effects should be nearly the same for
all parts of the bunch.
- 37 -
.Me) I I Kf.jSY.stem (C.B. Bigham)
1. Accelerating Structure
The acceleration must, be accomplished in 100 ion revol-
utions to have sufficient orbit separation for efficient (sinqle
turn) extraction from this small cyclotron. This requires
1 MV accelerating voltage per turn and suggests a multigap
structure for obtaining this with reasonable rf voltages. small
cyclotrons have been operated with dee voltages in the 50-100 'A
range indicating 10 gaps.
A wide tuning range is required to cover the 3-50 MeV/t>
specific energy range - about two octaves in cyclotron frequency.
Quarter-wave resonators are readily tunable by adjusting the stub
length, in this cyclotron we have the advantage over an iron
cyclotron of no poles or yoke to impede axial access permitting
coaxial resonators to be used.
The required tuning range can be reduced by a factor of two
by using two resonators with each resonator driving alternate
"hot" sectors. A small coupling between the identically tuned
resonators gives rise to two normal modes of the system, separated
in frequency by an amount increasing with the magnitude of the
coupling. One mode, characterized by the fields in the two
resonators being in phase, is called the "0-mode"; the other,
with field 180° out of phase is called the "--mode". Operation
in the 0-mode covers the 3-12 MeV/u range and in the -mode
covers the 12-50 MeV/u range. The proposed structure is shown
in Figs. 10 to 13.
The four sectors provide 8 accelerating gaps between the
structure and ground as illustrated in Fig. 13. This gives
TOP RESONATOR
GROUNDEDSECTOR -
CONTAININGIRON POLES
BOTTOMRESONATOR
I . 55
Fig. 10: Rf accelerating structure - plan view; A sectorsconnected to bottom resonator; B sectors connected totop resonator; C sectors connected to ground andcontaining iron flutter poles.
- 39 -
2. 8m
•
IRON
IRONR,ft
n
TUNING SHORT
IRON
IRON
TUNING SHORT
0. 10
0. HO
Fig. 11: Rf accelerating structure - vertical section.
- 40
I I
c, p. „
0 2 Or. ,
V
\ S ' ^ A */•*,/ - y \ / \ NORMAL SHIM COILS / \ ^ / -
r 40mm
0. "44m
v\
Fig. 12: Rf accelerating structure - vertical section throughhot sector.
- 41 -
Fig. 13: Rf accelerating structure -plan view of one quadrant.
- 42 -
••.-i MV oer turn for • 100 kv structure voltage in the 0-mode. in
•.he -mode, the beam bunch crosses each gap when the rf phase is
-; " or I0S0 ;spe Fiq. 14) . The peak voltage must thei«fore be
raised to 140 kv in this mode. Under these conditions the energy
spread resulting from a phase spread in the beam bunch is the
same in both the O- and "-modes. The T-mode operation at
r. 140 kv would require twice the power.
A 1/10 scale model of the rf structure is shown in
Figs. 15 and 16. Fig. 15 shows the assembled structure. Fig.
16 shows the vane structure. The solid copper grounded sectors
would contain the iron flutter poles in the real device. The
tuning ranges for the 0- and ^-mode resonances are shown in
Fig. 17. These results indicate a 22-45 MHz frequency range
would be required in the full-scale device to cover 3-50 MeV/u.
the limits for the tv:o modes being indicated in the figure. The
stub short position would need to be variable over a i m range.
A 12 sector gap was chosen for the conceptual design giving
a transit time factor* of 0.97 for the 0-mode (0.99 for T-mode).
At the injection radius this gives a gap of 30 mm which is
quite conservative for 140 kv. The MSU cyclotron operates at
65 kv across a 7 mm gap but with severe sparking during
conditioning. The voltage across the axial gap between resonators
will be 280 kv for the Tr-mode. This gap can be large but because
sparks occur preferentially along the magnetic field lines it
may be the most likely region for sparks.
The capacitance of each pair of resonators is about 30 pf.
The estimated power per resonator is 10 kw at 100 kv or a total
-The effective energy gain in crossing the gap is reduced by thetransit time factor, sin(G/2)/(/2), where •* is the change in rfphase while the ion crosses the gap.
- 43 -
0 - MODE
0 OR TT - MODE A C C E L E R A T I O N
I T - M O D E
GAP 8 5 6 7 8
F i g . 1 4 : O p e r a t i o n i n 0 - and ;—mode, i n 0 - m o d e , s e c t o r s A. MrviB a r e i n p h a s e and g a p c r o s s i n g s o c c u r cit 0 _ / : I I E C .
i n Tr-mode, A and B a r e :.n a n t i p r i a s e w i ' h g-,p oror : - ; n ; :at 45° and 135°.
- 44 -
Fig. 15: l/10th scale model of rf structure - assembled.
I
c T f)«
Fig. 16: 1/lOth scale model of rf structure - disassembled,
- 4b -
600
3 MeV/A
12 MeV/A
100
01 L I I I 15 10
SHORT POSITION (cm)
Fig. 17: Tuning range of the l/10th scale rf model in 0- andTr-modes,
- 47 -
of 20 kw. This does not include losses in the adjustable short
or on the sectors. Adding 50% for these gives 30 kw total. '-no
amplifier driving one resonator is sufficient - the second
resonator being driven by the interelectrode coupling. The
choice of O- or -mode is made by setting the master oscillator
frequency. As mentioned above, operation in the --mode at 140 kv
to jive the full 0.8 MV acceleration per turn would require
abou t ou kw r f power.
2. RF Amplifier
The rf amplifier must provide up to 60 kw over the frequency
range 22-45 MHz. The required frequency stability can be obtained
if the amplifier is driven from a stable source but operation as
an oscillator with positive feedback from the resonator would not
have sufficient stability.
There are two amplifiers discussed in the recent 1iterature
which are relevant. The driver amplifier for the TRIl'MF rf(23)
system produces 120 kw at 23.1 MHz with adequate irequency9 4
stability, 2 in 10 , and output power stability of 1 i.n 10 .With output beam energy feedback from an analyzer, the beam energy
5stability is expected to approach 1 in 10 with this system.
This is, however, a fixed frequency system.
The 100 kw power amplifier developed for the NAL*.iccelerator
cavities covers almost the required frequency range, 30-53 MHz,
but probably does not have the required stability in output.
This amplifier is built into a very compact assembly of 4 modules
mounted directly on the accelerating cavity. It consists of a
6 tube (4CW800F) distributed amplifier driving a cascuie circuit
with a 4CW100.000E in the top half and 14 4CW800F tubes m ?W-
•National Accelerator Laboratory
- -18 -
lot torn :uilt". The accelerator rarity is driven directly
through blocking capacitors on the 4CW100.000E plate lead. It
covers the full frequency range without tuned stages.
We cannot however mount the power amplifier directly on the
cavity because of the high magnetic fields and a transmission
line will be required. This will need a matching circuit on the
power tube output and possibly a tuning circuit.
The TRIUMF driver amplifier is discussed here as an example
because it has all of the featiires required for cyclotron
operation except the broad frequency range. A block diagram of
the circuit is shown in Fig. 18. Transistor stages are used
to the 20W level, a 5CX1500A to 1600W and a 4cwl00,000E to 120 kw.
The output amplitude may be controlled by
(a) pulsed modulation for multipactor breakthrough
(b) linear control for wide dynamic range
and (c) screen grid modulation for maximum stability.
in (c) the driver stages are run near saturation which reduces4
their noise contribution. Amplitude stability of 1 in 10 has
ceen achieved in TRIUMF tests
The frequency may be controlled by the frequency
synthesizer or by the resonator via positive feedback. The latter
mode is used to track the frequency drift in the resonator during
warm up. The rf system must be readily tunable over the full
range and may not require the self-oscillator mode.
3. cooling Requirements
The rf structure will require an estimated 30 kw of water
cooling for .100 kv operation, we are however proposing to go
to 60 kw for 140 kv light ion operation and the proposed
amplifier may be capable of . 100 kw. Flov rates, pressure drops
FREQUENCY
SYNTHESIZER
23 I MHz
VANUALON OFF
REMOTEON OFF
ARCSENSING
RESONATOR RF SAMPLE
DRIVER AMP
5CXI500 A
POWER AMP
4CM00.000 E
SCREEN]—MOD
SCREEN
SUPPLY
DETECTOR "I RESONATOR—• VOLTAGE
SAMPLE
Fig. 18:
_ DIGITAL CONTROL
Block diagram of the driver section of the TRIUMFrf power supply.
DRIVELINE
- 50 -
.iri.i temperature rises should be kept small so that vibration,
PA-ise and thermal expansion do not cause stabi l i ty di f f icult ies .
The- structure cooling channels could be designed for a maximumo
o- 3 l/s at 1388 kPa giving a temperature rise of 10 C at
lv.'O kw. The most troublesome parameter could then be reduced
considerably for the 30 or 60 kw levels. Hot, full-scale
experiments will be required to determine the optimum. The
temperature stability will also be very important.
The rf amplifier will require cooling up to -. 250 kw,
'.: 00 kw for the dummy load and . 150 kw for the amplifier.
The 4cwl00,000E requires 1.5 1/3 at 132 kPa. This pressure drop
is compatible with that suggested for the rf structure. The
dummy load might require -~ 0.8 I/a at a similar pressure drop.
2 ', e) I i i Main, Trim and Shim coil Systems (H. R. Schneider)
1. Geometry
An essentially uniform field with a maximum value of 5 Tesla
is required in the region between the injection radius at
0.13 m and the extraction radius at 0.65 m. To achieve this, a
set of four coils as illustrated in Fig. 19 and operated with a2
maximum overall current density of less than 2400 A/cm is used.
The main coils with 4.99 x 10 Ampere-turns (max) provide most
of the field, while a set of trim coils provide the requisite
field flattening. The axial magnetic field, B on the midplane,
computed as a function of radius using the MAGTWO computer(26)
program , is plotted in Fig. 20.
coil currents for Fig. 20 are adjusted for a nominally flat
field. By reducing the trim coil current a field rising with
radius, required for isochronism, can be produced. The amount
T I 0.70
0. 46
D I M E N S I O N S IN M E T R E S
Fig. 19: Schematic diagram of the cryogenic coil geometry.
CD
UJ
CLO
c_>r 2
-1
-2L-
VARIATION OF MAGNETIC FIELDABOUT 5T MEAN VALUE
. 2
1
. 8
i RA
I ME
.-——DIINTR
US
ES
.6 .8 1.0 I. 2 \). 4
RADIUS IN METRES
1.6 1.8 2. 0 2 2
Fig. 20s Axial magnetic field in the midplane. The insert showsthe deviation from the mean from the axis to 0.65 mradius.
- 53 -
of field increase with radius is small, being approximately 1,
tor 10 N,eV/u particles and 5% for 50 MeV/u . The four coil system
can provide the required field profile with a maximum deviation
from ideal of only a few millitesla. in the final design account
must also be taken of the effect of the flutter poles on the
radial profile of the azimuthally averaged field. This will
alter the coil currents required and may also require small
changes in the coil geometry. It will be necessary to provide
a set of auxiliary coils (shim coils) to give the exact field
desired. The number of coils necessary has not been determined
but if they were located on a plane approximately 2 5 cm frorr, the
midplane (see Fig. 4), the total ampere turns required is
estimated to be 10,000. This is small enough to allow use of
normal copper conductor for these coils.
2. Magnetic Forces
The interaction of the coil currents with the radial and
axial field components within the coil result in axial and radial
forces respectively, acting on the conductors. The forces are
not uniform since the field strength within the coil is not
uniform. The net force however consists of a radial component
outward and an axial component directed toward the midplane.
Supports between the coils must be sufficiently strong
to carry the axial force which is much larger than the coil wei'fh*
The radial force must be supported by the hoop stress in the con-
ductor and any reinforcing band wound into the coil and/or placed
around the outside.
The radial (B ) and axial (B ) fields at points within
the main coil and trim coil as calculated with MAC-TWO :or t 7
- rA -
r. iui m e : . 1 i are shown in rigs. 21 and 22. Maximum field
r-ren.j i. ::s within the main and trim coils are 6. 2T and 5.ST
re spec* i iTiely .
••or purposes of force calculations, the coils are divided
into eicrht pancake windings. in Fig. 23 the axial forces on
each nancake are shown for both the main coil and the trim coil.
The maximum axial force acting on the main coils and trim
coils is 34 x 106 N (7.63 x 10 lbs) and 6.14 x 10 N
i1.37 x 10 lbs) respectively. For windings nearest the midplane
the axial force is directed away from the midplane so the maximum
compressive forces within the coils are larger than the above
values. in the main coil the maximum axial compressive force
is 54 x 10 N while in the trim coil it is 0.66 x 10 N. The
corresponding stresses are modest, being only 16.3 x 10 N/m
(2 350 psil and 1.4 x 10 N/m (200 psi) in the main and triru '-oils
respect ively.
The large axial field within the coils results in large
radial rorces. Assuming that the radial force on a conductor at
•;. radius R is balanced completely by the hoop stress S(R) in the
coi.iuctor, then,
S(R) = B (R)JR
wi.ure J is the current density in the conductor. S(R) is largest7 2
•IL the inside winding and has a maximum value of 13.55 x 10 N/m
(10.6 x 10"" psi) in the main coil.
The overall current density was used in this calculation
so it represents the overall stress, i.e. the actual stress in
the conductor and its reinforcing could be greater since
ii lowance must be made for insulation and cooling channels.
01 STANCE_T0__
C O I L A X I S 0 . 9 2 m
• 24.
• 2*5.
9976
3444
.3.3.
• 2•3.
7807
8542
• 3.1 .
• 2
* 1.
8654
9164
• 3.
• 2
AS .03
5821 #
2.- 1
2.1
7222
12.66
1 176.08
.076 . 1 3
-1.05• 5 . 4 5
•x _J e•— Q . U3to 1} —
L_> -jr C3
1.293.83
- .1253.87
-1 .70
1.301.76
- .171.81
1.791.77
ST MIDPUNE FIELO
1.19- .38
0632
. 0 7I - 1 . 9 9
08-1 .93
- 9 6-1 .33
LB FIELD
KtY • J> COMPONENTS— "2 IN
Tt£LA
Fig. 21: Magnetic field within the main coil. The two numbersby each doc are the radial and axial components ofB in Tesla for a 5T midplane field.
I
KEYB,
i
FIELD^.COMPONENTS
INTESLA
^ B ,
D I S T A N C E TO C O I L A X I S 0 3 9 m
o
z
CO
a
h1.4
5
5.
5.
5.
r—co
o
a .a
-
0694
7632
4549
1552
1734
1 .354.70
# .904 95
.51* 5.10
.11# 5 . 1 5
- .355.10
5T
1.464 .46
• 994.62
.56• 4 . 7 5
134.83
- . 344.88
M I D P L A N E
1 474 24
o 1 014 31
.62* 4 . 4 0
• 22* 4 . 5 2
• 2 4
4 66
FIELD
<
1 314 00
993 93
684.01
.364.15
.034.44
0 1625m
i
Fig. 22: Magnetic field within the trim coil . Components asin Fig. 21.
1 2 . 5 7
f
5. 83I 2 . 7 a
1 1 3 . 93
9 . 05
0. 35
J3. 586.08J
F t o t a l = 3 4 . 0 1
R
FORCES IN VEGANEY.TCiJG(ONE N = 0 . 2 2 4 5 LB)
A X I A L M A G N E T I C F O R C E S A C T I N G O N T H E M A I N C O I L
0. 1*6
I0.091
o. o ? e• o . o i
0.177
— ^0 • 1 0 9
•0.065
0.021
\
Ftota I = 0.614FORCES IN MEGANEWTONS
A X I A L F O R C E S A C T I N G O N T H E T R I M C O I L
Fig. 23: Axial forces acting on the main and trim coils fora midplane field of 5T.
,-old worked 304 stainless steel has a yield strength* 2 S* i
of 231, '00 psi at 20K' ' so the hoop stresses while large, are
tractable using this material for support.
The foregoing calculation is pessimistic. If we allow
scne of the radial force on the inner winding to be carried to
the outer ones where the magnetic forces are lower, then the
overall hoop stress can be reduced. If, for example, the radial
forces are carried so that the hoop stress does not vary with
radius, then, for the main coil,the overall hoop stress will be
less than 5 x 107 N/m2 (7250 psi).
3. Superconductor Selection
(i) Conductor characteristics
Although a large number of superconducting alloys and com-
pounds have been discovered, only a few of the so-called type II
superconductors have found application in magnet construction. The
two most common, and commercially available, are niobium-tin and
niobium-titanium. Some properties of these are given in Table v.
TABLE V
SOME SUPERCONDUCTOR PROPERTIES
Superconductor: Nb^Sn Nb (48% Ti)
Critical Temperature 18.4 5 K 9.5 K(zero field)
Critical Field 22 T 12 T(at 4.2 K)
5 2 4 2^Critical Current 5 x 10 A/cm 8 x 10 A/cmDensity (4.2 K, 5T)
Mechanical Properties Brittle Ductile and hightensile strength.
*The critical current densities are very sensitive functionsof metallurgical treatment. The values given here aretypical values only
Clearly Nb^Sn has superior superconducting propert L<?S.
but because it is very brittle it is used in magnets (inly when
the need for high current density and/or high critical field
outweigh the fabrication difficulties.
NbTi alloy is the most commonly used conductor and some
i -i !. • V. • (29,30,31,32)
very large magnet coils have been wound with it.
The NbTi alloy is usually incorporated as a number of separate
wires in a copper matrix, which is then extruded and drawn to the
final conductor size. Intermediate heat treatments are often used
to get the highest possible critical current, conductors with as
many as 1000 superconducting filaments, each a few microns in
diameter, have been made in this way.
in the presence of a changing magnetic field the magnet-
izing currents may not remain confined to the superconducting
filaments but can cross over through the copper matrix to
adjacent filaments. The resulting trapped flux and subsequent
flux jumping can lead to magnet instabilities. To prevent tins
a twist is usually introduced in the conductor at n siige prior( 34 1
to final drawing to size. An analysis by Smith et al shows
that the twist pitch should be less than a characteristic length
given b ,,
I2 *. 10 8 X j dp/fi ' 1'. 1 )c c ^
where d(cm) is the superconductor thickness, or diameter,2
J (A/cm ) is the superconductor current density, , ( cm; is the
matrix resistivity. > is a geometric factor 1 and H (G/s)
is the rate of change of the magnetic field.
The critical current in a superconductor decreases with
increasing temperature or transverse field. This is shown :n
Figs. 24 and 25 where the critical current of a typical conductor,
(IMI Niomex) normalized to I at 4.2 K is plotted against
O
O
c_>Lu!Q-
C/3
C/)
LU
2000
1 500
1000
800
600 i
Pig. 24:
2 H * 6 8
A P P L I E D M A G N E T I C F I E L D . T E S L A
Typical critical current densities for 0.1 mm diam.and 0.25 mm diam. NIOMAX superconductor filaments(NIOMAX is a trade name for I.M.I. (U.K.) Ltd. NbTisuperconductor).
- 61 -
100
cvi
o
o 60
a:
CJ
o 20 -
6 7 8
TEMPERATURE K
Fig. 25: Effect of temperature above 4.2K on the c r i t i c a lcurrent of NIOMAX at 0, 2, 4, 6, and 8 Tcsla.
- 62 -
temperature for several values of transverse field strength, and
in Table VI where the guaranteed minimum currents for three
commercially available conductors are given for magnetic
fields up to 10 T.
TABLE VI
GUARANTEED MINIMUM CRITICAL CURRENTS (AMPERES) FOR THREECOMMERCIALLY AVAILABLE CONDUCTORS WITH A
SINGLE 0.25 mm DIAMETER NbTi CORE
BTesla
2345678910
Supercon(1)
14411287715847352414
IMI(2)
105877262514130—
VMC(3)
91786655463832—
(1) Supercon Types S, M, and G wiresSupercon inc.9 Erie DriveNatick, Mass. 01760
(2) Niomax Simperial Metal Industries (Kynoch) Ltd.New Metals DivisionP.O. Box 216,Birmingham, U.K.
(3) Super SW-U20,Vacuum Metallurgical CO. Ltd.Hattori Building,1, 1-Chome, Kyobashi, chou-KuTokyo, Japan
' i i) st.ib M i snt ion of Superconducting c-i.;?-.
[n the early l'J60's, when the first magnet coils were
wound with the newly available hard superconductors, porfor^-nct
well below expectations was often obtained. In part icr.l.ir it
was often found that for currents considerably smaller than tie
short sample critical current of the conductor, non- supercondu.111 ng
regions would suddenly develop within the coil. These norm • I
regions propagated rapidly through the coil and if the current
was not reduced rapidly, permanent coil damage resulted from
overheating or arcing.
The problem, as it is now understood, was caused by flux
movement whic"'1 r. ~y '.. e place through type II superconductors.
Such flux motion {or flux jump) generates an electric field and
dissipates energy in the superconductor. Since the heat difrnsior.
in the superconductor is much slower than the flux motion, in
increase in superconductor temperature with a concoritt :nt
decrease in its critical current results. This is an unstable
situation and if unchecked can result in total col laps.-..- o* •!•••
superconducting state.
A number of methods for stabilizing a coil against such -m
occurrence have been devised, but here we shall consider only
one - full cryostatic stabilization. This represents the most
conservative approach to coil stabilization and is used on all
big coils where reliable operation is a necessity.
The superconductor in this case has sufficient high-conductivity
normal metal (usually copper) in parallel with it so that i f
a portion of the coil should go normal due to a thermal transient,
the current will be shunted by the normal metal without a
significant temperature rise. After decay of the transient
the superconductor will return to its superconducting s<..it e.
I'.r thit. to be so, heat transfer from the conductor to the
liquid helium must be good. This means uhe surface area of the
copper in contact with the liquid helium must be large enough to
allow efficient heat transfer through nucleate boiling. For the
worst case with all of the current in the copper the power
densities at the cooled surface must be kept well below the point
where the transition from nucleate to film boiling occurs.
Also, by dividing the superconductor into fine filaments with a
good metallurgical bond to the copper matrix, low thermal
gradients in the superconductor and good heat transfer to the
copper are assured.
An exact calculation of the heat transfer from the super-
conductor through the copper to the liquid helium is complex.
Fortunately such calculations are not necessary (at least at
the preliminary design stage of a coil) since some limiting
case calculations are usually sufficient to check the adequacy
of stabilization.
(iii) conductor Specifications
The selection of the maximum conductor current represents
a compromise between the desire to keep the current leads small,
to minimize heat leak into the cryostat, while at the same time
keeping the number of turns in the coils and hence inductance,
small enough to give reasonable charging and discharging times.
The selection of 1000 Amperes was made on the basis of
these considerations but does not necessarily represent the
optimum value since calculations have not been done in enough
detail to determine this.
A 1000 Ampere conductor patterned after that used for
the NAL bubble chamber magnet' is shown in Fig. 26.
6r> -
, 3
t ° , k e P e0 ? • ° ° L'1'
i '• - - ^
CASTELLATIONS
Cu— COOLING
STRIPS
MULTIFILAMENTCOMPOSITESUPERCONDUCTOR
\ Cu\BACKING
STRIP
(Dimensions in millimetres)
Fig. 26: 1OOO A superconductor cross section.
The conductor consists of four components viz. a backing
strip, two side cooling strips and a central multifLlament super-
conductor, all of which are soft soldered together to form th»-
conductor used in winding the coils.
The cooling strips are copper of rectangular cross section,
2.15 mm x 3 mm with 0.5 mm deep transverse grooves, or castel lat i or;
spaced 6 mm apart, on one side. When wound into a coil the
cooling strip grooves form channels to allow liquid helium to
contact one surface of the superconductor.
The superconductor is a copper-NbTi composite with a cross
section of 1.65 mm x 4 mm and contains thirty-three 0.25 mm di^iwt
NbTi filaments with a twist pitch of less than 5 cm. i;,ich
filament therefore carries a maximum current of 30 . '.i A v-'i i<-- • '' >'•
Table VII is well below the critical current for th i r, f lini<-)'
- 66 -
r.^L- it. 7 T. liquation 2.1 gives the value (7000/H) : for the
•:.;r iC cr i st :c- length of this conductor. Except for fast'dis-
•:arje o: 'he magnet, H will always be less than 1 m T/s
so 20. 'J cm. Thus the condition that the twist pitch be lessc
: :.an - is met and no effects due to eddy currents should occur.
As noted above, this conductor is similar to that used in
the NAL bubble chamber magnet, and the technique for fabrication
• ,s beer, developed by Supercon. There are two advantages to
::;.i.-: ing the conductor in this way. First, large aspect ratios
:u.'CL5sary tor winding pancake coils are easily realized even with
'visted iilament, and second t' e cooling channels are an integral
part of the conductor. The latter represents a cost saving over
the -ti tern.it ive of fabricating a special cooling strip which is
v/cx;rid into the coil along with the conductor.
4. coil Fabrication
construction of the coils as a series of pancake windings
.:: >rds the most practical way of supporting the large forces
v. t;.m then. The four parts of a winding are illustrated in
A .062 err. (.025"' stainless steel reinforcing band is
vrwov.r. -I with the conductor. Following the practice at NAL this
. : ; ;; si Krht.lv wider than the copper conductor so that when
:janc I'-'.OS are stacked into a coil, the axial load is not
:i:nr.. ' \ o-j to the copper. This also reduces the width tolerance
• :.<•> c'.jiH-r ,
:•••/ •;s::"!>j .; double layer of mylar insulation continuous
..'•::"• 'or shorts durinq winding is oossibie.
- 67 -
C O O L I N GS T R I P S
C O N D U C T G R
1
11
r
o<? c s
t o
* / /
o c °
! I
n0.62 S T A ^ I L E S S S T
10
0. 13 MYLAR
(Dimensions in millimetres)
Fig. 27: Components in a winding of a pancake roil
- 68 -
A pancake for a main coil has 130 turns and there are 38
pancakes in a coil. To form a coil the pancakes are stacked
(alternate ones counter wound) with 0.2 cm thick x 2 cm wide
micarta spacers between them, giving 50% coverage of the coil
surface. The stack is securely clamped to a base which might
be part of the bridge between the coils. If NAL practice is
followed the pancake coils are connected in series by soft soldering
and rivetting. The stainless steel band is left free on the
inside - to prevent stress build-up any clamping would produce,
and at the outside the final turn is edge welded to the
penultimate one, thus obviating radial clamping.
A summary of the magnet characteristics is given in
Table VII. It is assumed that the same conductor would be
used for both the main and trim coils.
TABT.F, VTT
MAONF.T CHARACTERISTICS
Main Co_i i Tr i m <'•
Winding Inside Diameter 1.84 m "."•'
Winding Outside Diameter 2.76 m '..!••
Coil Height .46 m . 1<
Coil Spacing .32 5 m .7 "
Number of Pancake windings 38 '. '-
Turns/Pancake 130
Length of conductor in a Pancake Winding 939 m 1•
Total No. of Turns/Coil 4940 * '
Weight of Conductor/Coil 8200 kg 4? -
Total weight (1 coil) 10,000 kg .: '7 2
Average Coil Hoop stress 5 x 10 N/m i. 3 :< ' '(7250 psi) ..47 7. •
Axial Force 34 x 10 N ( . i ', x •(7.48 x 1C6 1L) <1 . i^ x
oOverall Current Density (max) 2 360 A/cm." J4';J<•• \
Midplane Field (max) vr
Field at conductor (max) 6. 2T •.'••
Coil Inductance 140 H '.
Total Stored Energy 7;; •••,
H a v i n g d e t e r m i n e d the m a i n c h a r a c t e r i s t i c s sever.-t! :: • • '.
calculations can now be made to check the coil stabilJ'V.
The maximum power is dissipated in the coil when \:•<..•
is carried entirely in the copper. Using a value r>\ . "
ohm-cm for the resistivity of copper at 4.2 K in-i r : ' •
of 7 T, we find the normal conductor resistance is 1'».7 ;•: i
ohm/cm and the maximum power dissipation at 1000 A is '.'•)! r:,.. •
- 70 -
The conductor is cooled on the edges and one face. Because
of the interpancake spacers only 7 5% of the edge area is exposed.2
Using a heat transfer rate of 0.4 W/cm * edge cooling
can account for 159 mw/cm.2
The exposed area for the cooled face is 0.7 cm /cm. Using2
a value of 0.15 W/cm for the heat transfer rate through
the channels* gives a heat exchange capacity of 10 5 mw/cm so the
total heat exchange capacity for the conductor is 264 mw/cm,
which is comfortably above the maximum dissipation, so full
"external" stabilization should be achieved.
One must also check the "internal" stabilization i.e. the
thermal behaviour of the NbTi in the copper. This is in general
complex since the thermal gradient and current density in the
NbTi are interdependent and moreover the NbTi thermal conductivity
is a sensitive function of temperature. Since, however, small
filaments are used, the gradient will be small. To simplify the
calculation then, assume the thermal gradient, current density
and thermal conductivity are functions of the mean filament
temperature only. Then the temperature rise AT in a circular
superconducting filament is
— (2.2)
where I , I are the currents in the copper and
superconductor respectively.
p, is the resistivity of the copper
A is the cross-sectional area of the copper
k is the thermal conductivity of the NbTi
= 1.2 x 10~J W/cm/K at 4.2 K ( 3 5 )
1fThis value was used by Purcell for the NAL magnet design.
- 71 -
To e n s u r e t h a t w i t h a t h e r m a l t r a n s i e n t . !:o e r r
t r a n s f e r smoothly from the NbTi t o the coppe r and :...:<•
i t i s n e c e s s a r y t h a t ,
AT '• T - TKTc N
where T is the critical temperature of the NbTi -THJ
temperature.
The numerator of equation (2.2) is a maximum whun l:
so for the 1000 A conductor with 33 filaments the m.-x.::.:i;i v .• l i
for /T is 0.1 K.
Now the critical temperature of the NbTi at !"•: \r- t •
(see Fig. 24) and the copper temperature is 4 . :> v s • "::•
condition (2.3) is easily met.
5. Power supplies and coil Protection
(i) power Supplies
Regulated power supplies are required for the throe --oil
systems; a) Main coils, b) Trim coils and c) Shir, coils. T'n-
power supply requirements for (a) and (b) are somewhat uncon-
ventional, i.e. peak power demand only occurs during chargiiv'
the coils. At other times the supply is required to carry H.<
full current while delivering only a small amount or power t .
make up rectifier and line losses.
A main coil power supply designed for continuous operi1.!'
at 10 volts, 1000 Amperes, would be a conservative one tor t :•
application. The main coil inductance is 140 H, so wirh >
10 volt supply the charging rate would be 71 mA/s, giviri'i a
total charging time from zero to maximum field o.f J.'-' ho> r • .
A higher voltage would speed up the charging, but in s idit , :.
- 72 -
: the larger power supply required, account also has to be
i .ikon of the increased power dissipated in the dump resistor
during charging.
Fig. 28 is a simplified schematic of the main coil power
supply. Two primary windings are used. One of these is a high
power winding with a simple controller such as a motor driven
autotransformer which is used during magnet charging. Once the
magnet current is established the high power winding is de-energized,
and the second primary winding, fed from a low power precision
regulator, is used to maintain the magnetic field at the desired
level with a stability of ^ 1 part in 10 . The error signal is
derived from an NMR probe located at some convenient position
near the magnet.
To reduce the magnet current, switch S is closed and S
opened. The discharge resistor value is chosen to give a discharge
rate equal to the charging rate. For faster discharging, S. is
opened and the stored energy dissipated in the dump resistor
described below.
A similar but smaller power supply could be used for the
superconducting trim coil. The shim coil supplies are conventional.
It is estimated that ten coil pairs will be necessary for final
field trimming. Each pair will require its own power supply,
with a typical rating of 300 A at 7.5 volts and regulation better4
than 1 part in 10 .
(ii) current Leads
Current leads for the superconducting coils must carry 1000 A
from outside the cryostat to the superconductor in the liquid
helium bath. Such leads are commercially available. One model*
'•"American Magnecics Inc., Model L-1000
CHARGING CONTROLLER
AC LINE
lOOOA(MAX).
AC LINE
CURRENTSENSOR
-AAA/—FAST DISCHARGE
RESISTOR
,\s.
LOW POWER HIGH PRECISION REGULATOR
SUPERCONDUCTINGCOIL
DUMP
SWITCH
F i g . 2 8 : S c h e m a t i c .Uagrarn o f m a i n c o i l pcv.-ir s u p p l y .
- 74 -
r~. :o.i . t 1 .'00 A consists basically of a thin wall stainless
si. ol -.-;be, enclosing a 3/4" diameter copper bus which is
ii\>.r.Lc-.i by Nb Sn superconductor. The bus is cooled by helium
vapor irom the bath and the specified heat loss equivalent for
. 2i>' long pair of leads is 2.8 l/h of liquid helium.
following NAL design philosophy it is also required that
the leads be able to discharge the magnet without burning out
even if cooling for them is interrupted, i.e. they must carry
1000 A for approximately ten minutes without overheating.
(ni) Dump Resistor
The dump resistor is connected permanently across the
magnet coil leads. Its purpose is to discharge the coils in a
controlled way in the event of a fault in the magnet or
cryogenic system.
Following the specification for the NAL magnet, the maximum
voltage during discharge will be limited to 200 volts. This
means the dump resistor must be no greater than 0.2 ohms, and
the discharge time constant for the main coils would be
6 50 i, - a reasonable value comparable to that of other large
superconducting magnets. The resistor can be made from stainless
steel sheet 50 cm x 100 cm x 0.5 cm thick, slit as shown in
F ig . 29 .
If the resistor is immersed in a tank 130 cm long x 80 cm
high x 30 cm wide, containing 312 kg (68 gal) of water, then a
temperature rise of less than 50 c would result from dumping the
maximum magnet stored energy into the resistor. The peak power
density at the surface of the resistor would be 40 w/cm .
- 75 -
R = 0. 2n
500
20
7&
! V
i !
1000
(Dimensions in millimetres)
Fig. 29: Dump resistor fabricated from stainless s teel .
- 76 -
{iv) coi1 Protection
At 0 Tesla the energy in the magnetic field is approximately
;n MJ. such a large stored energy makes careful design of
protective systems imperative, to avoid dissipation of this
energy in unwanted and uncontrolled ways.
Proper superconducting operation of the magnet can be
monitored by measuring the voltage across each pancake winding.
Appearance of a potential difference at any of the monitor
points indicates that part of the corresponding winding has gone
normal and may be used as a fault signal to initiate either a
partial or complete dump of the magnetic energy.
in the event of failure of the liquid helium cooling of
the superconductor - say due to a loss of vacuum in the cryostat,
the magnet would quench itself, and essentially all of the
energy would be dissipated in the windings. Taking into account
the variation of the specific heat of copper with temperature ,
the calculated temperature rise in the windings is only 7 5 K.
During such a quench,a maximum potential difference of about
40 volts would appear between adjacent pancake windings. This
should not cause any problems.
7. comparison with other magnets
A comparison of various cyciot .-on magnet parameters with
those of other large superconducting ruagnets is made in Table VIII,
As can be seen, no large departures from what has been achieved
are proposed.
Higher central fields and much larger stored energies have
been achieved. In fact most of the magnets listed, have larger
stored energies than the proposed cyclotron magnet. The CERN
magnet is an order of magnitude larger.
TABLE VIII
COMPARISON OF CYCLOTRON MAGNET PARAMETERS WITH THOSE OF SOMESUPERCONDUCTING MAGNETS THAT HAVE BEEN BUILT
Central Field (T)
Stored Energy (MJ)
Current DensityConductor (A/cm )Overall (A/cm2)
Conductor current(max)(A)
coil DiameterIns ide (m)Outside (m)
.Magnet Height •;m)
y,;ig ne t We igh t ( kg}
Reference
NASALev; is
Researchcentre^
8.8
8.5
6312-89855160-6790
427
. 51
.908
.64
(74)
...__ANL_
1.8
80
1700775
2200
4.785.28
3.04
45,400
(29)
BNL
3.0
72
60002500
6000
2.402.75
1.90
18,600
(32)
SACLAY__
4.0
10
80005100
1500
1 .00] .27
.80
2800
(30)
CKRN___
3.5
800
30 301030
8000
4 .726.35
4.5
120,000 7
(38)(39)
NAJL- _3.5
400
37001885
5000
3. 554.20
2.45
3,000
(J3)
PROPOSEDCYCLOTRON
5
70
40802360
1000
1 .842.76
1.25
17,2 50
I
iv
- 78 -
riutter Poles (H.R- Schneider)
I. purpose and Requirements
As described in Chapter 3 an azimuthally varying field is
required to provide axial focussing of the beam. The field
variation, characterized by the flutter factor F, (F = <B >/<B-> -1),
together with a spiral curve of both edges of the flutter pole
can provide adequate axial focussing. If F ^ 0.018, a pole
spiral angle with a maximum value of 36 at the extraction radius
assures v .1, i.e. adequate focussing for all ions includingz
50 MeV/u light ions.
2. Magnetization of Ferromagnetic Materials
When ferromagnetic materials such as iron, nickel, or
certain alloys are placed in a magnetic field H, the dipole
moments of the magnetic domains in these materials are aligned
with H, resulting in a net magnetic dipole moment for the material.
The induced dipole moment per unit volume is called the magnet-
ization M. in Fig. 30 the variation of M with H for ircn is shown.
M increases with H until it reaches its saturation value,
,; M = 2.16T. Beyond this M changes very little. For example,
over the range 1.5 T to 15 T the measured increase in M for iron
is less than 0. 5% .
The temperature variation of saturation magnetization in
iron is shown in Fig. 31. As the temperature increases, thermal
agitation decreases the spin alignment until at the curie
temperature (1042K for iron) the ferromagnetic behaviour
disappears completely. For the present purpose the point to
O
Mo
CD
MAGNETIZATION TESLA
o
- 6Z. -
F .6<
(/>UJ> 2
03O
1 1 1 1 i I I-100 0 100 200 300 400 500 600 700
TEMPERATURE °c
Fig. 31: Temperature variation of saturation magnetization foriron. (From Ref. 43)
- 81 -
note about this curve is that M is not a sensitive u,i:..--s
temperature at room temperature.
Values of M along with some other properties o: ,, ^
of terromagnetic materials are given in Table ix.TABLE IX (From Reference
SATURATION MAGNETIZATION AND OTHER PROPERTT Kf!OK A SELECTION OF FERROMAGNETIC MATERIALS
. ^
Material
lron ( 1 )
! cobalt(2)
1 Permendur
! 2V-Permendur
j Dysprosium
Max. SaturationMagnetization
0 K
Tesla
2.20
1.82
....
• • • •
3.77
293 K
Tesla
2.16
1.76
2.45
2.38
curie 1Temperature
K
104 3
1400
930
980
8r
•)cv.
gm-
7.
6.
H.
8.
8.
cir/
9
Q
2
i
'I) Low carbon steel
(2) 50% Fe + 50% Co
(3) 49% Fe + 4 9 % Co + 2% V
Ti.e iron-cobalt alloy Permendur has a saturation r : r.r •-
ization about 10% greater than that of iron. Tts h:;:. <•<>-.•••
however rules out its use for the entire flutter polo, but •
may be used as part of the pole face if special field s)•.•!;>!!;>:
- 82 -
•>. Demagnetizing Field and Saturation Magnetization
The maqnetizing field H in Fig. 30 is the field within the
ic material and is in general different from the external
field.
Qualitatively this can be seen by considering an iron bar
in a magnetic field H 4 .. The effect of the magnetization can
be represented by induced poles as illustrated in Fig. 32.
H e x t
F I G U R E 32: induced poles due to magnetization of an iron bar.
Superposition of the dipole field due to magnetization and
the external magnetizing field H then determines the actual
field in the presence of the iron.
Now inside the bar there is also a field due to the induced
poles, with the field lines directed from + to - and hence opposed
to M and the magnetizing field He x t
Th 1 s dem wme t: z i r..:
must be added v e c t o r i a l l y t o H to o b t a i n *he ac t 'Ml r.: i r';ext
field which in turn then determines M.
H is in general not uniform but it does have a 1 uniting
maximum value which is related to the saturation maqnot iz^tio:
To determine this maximum, consider a boundary between .
magnetized and unmagnetized region as illustrated T. 1 .;.
MAGNETIZED
H,
UNMAGNETlZtD
M -- 0
PIG. 33; Fields at a boundary between a magnetized .in>Junmagnetized region.
V is a pillbox volume enclosing an area S .it the t,uum2.«rv .
Let the field within V, to the ]eft and right of the boundary
be H and H respectively.
By Gauss' theorem,
ffn.nds =r I f(E +H )ds =- f f f {dim) dV . .-(27)
Now H may be defined by
and since divB - 0
we find that.
divH - -divM.
- 84 -
:..-Hi 2.: into 2.4 and using Gauss' theorem again gives,
v ;s considered as an elemental volume enclosing a boundary
s'.ri-.iCti S then 2.6 becomes,
(H + H ) AS = (M) .iSo o
or H = M-H 2.7
D o
I he- si.gn change on the right hand side occurs since there is a
contribution only from the magnetized region where M and n
.ii o oppositely directed.
The field and magnetization directions assumed in Fig. 33
mean that H is greater than zero, and in view of 2.7 this meanso
I: >' and has a direction opposed to M.D
In general then, if M is the saturation magnetization of
., material, the demagnetizing field in it satisfies the inequality,H •' M 2.8
D s
Figure 30 shows that for an internal magnetizing field
greater than 10 A/m, (.: H - .13T) iron is saturated, w.'th
y. - 2.16?.G S
It follows then from 2.8, that in external fields larger
than 2.29T iron will be completely saturated.4. Flutter Pole Field
The minimum operating magnetic field for the cyclotron is
about 3T, complete saturation and hence uniform magnetization of
the flutter poles may be assured.*
*The assumption that the poles are uniformly magnetized is notstrictly correct at the pole edges, since the demagnetizing fieldthere has a radial component which alters the direction of M. Theeffect is believed to be small but calculations to check this arenot complete.
.. 85 -
The magnetic field due to these uniformly nagne* l/'y] :»> oy\
is, in principle, calculable. However, because of t.Le;: ocinv.ili.-v
shape, analogue model measurements are useful.
It is shown in Appendix I that there is a magnetic cqi.iv.1-
ence between a surface current and a uniformly magnetized volume.
In particular consider a typical flutter pole illustrated in
F ig . 34 .
In this case if there is uniform current I flowing m the-
wall of the pole, then the surface current density is r/h whei >•
h is the height of the pole, and this is the magnetic equiv.^.-r;1
of a uniformly magnetized pole with M = I/h.
A four sector flutter pole analogue shown in r-'ia. 3 5 w-is
constructed to determine the flutter pole field. The model is
2/7 scale with 76 turns of #22 copper wire wound as a single
layer around the periphery of each pole.
Typical results of a polar field plot are shown in Fig. 3b
where the midplane field variation in one 90 sector is plotted
as a function of angle for several radii. The coil current 4or
these measurements was 3 amperes, corresponding to a surface
current density and hence magnetization of 3993 A/m ( M 5.018 nrr >
To scale these fields to saturated iron then, one must multiply
them by (2.16/5.018 x 10 ) = 430, giving a maximum flutter f i*.-ld
of 1.4 T.
The magnitude of the midplane flutter field depends on • h«-
pole height. This dependence, obtained from the mode] ind :><-nl<:'.i
to full size, is shown in Fig. 37.
With the adopted position of trin\ coils, n 20 cm high ]x>U'
is about all that can be accommodated. The change m ! iold w,th
pole height at 20 cm is not large and, in fact, if i' is necess.ir •.
to ease congestion in the vicinity of the trim coils this en•.. 1 1
- 86 -
Fig. 34: current sheet equivalent of a flutter pole,
- 87 -
Fig. 35: Flutter-pole analogue model
- 88 -
3 0 1—
i
Q-
2. 0
-1.0
1 . 0 -
10 20 30 40 50 60 70
AZIMUTHAL ANGLE. DEGREES
80 90
Fig. 36: Azimuthal variation of axial field in the flutter-polemodel at various radii in cm.
- 89 -
1. 5
i . o -
a:
. 5
—
—
4
1
1
/
/
1
s
1
1
1 ! i
1
ii
1 1 I
0 4 8 12 16 20FLUTTER POLE H E I G H T , CM
Fig. 37: Flutter field versus flutter-pole height
- 90 -
:.<.• done Ly reducing the pole height, with a small sacrifice in
iv.axir.iuKi tlutter field.
5. .Magnetic Forces on the Flutter Poles
The main force acting on the flutter poles is one of attraction
between pole pairs. Other forces arise because of the interaction
of the magnetic dipole moment and the field gradient. Since the
latter is small however, such forces are also ^mall and will not
be considered here.
To calculate the attractive forse, we use the usual,
F =4 T 2
r
(2.9)
where F is the force between two magnetic poles of strength m
and m , and separated by a distance r.
if an elemental volume such
as illustrated in Fig. 38 is
uniformly magnetized, then the
dipole moment D isc m
D - -f-MdA =m (2.10)
Fig. 38:
Magnetic volume element.where dm = MdA is the magnetic
pole strength.
Now consider two uniformly magnetized flutter poles as
illustrated in Fig. 39.
- 91 -
Fig. 39: Uniformly magnetized flutter-pole pair.
Using equations 2.9 and 2.10 the force between the two
poles can be written as,
,y Two similar integrals represent: rwthe contribution of the inducedpoles on the top and bot'.oir, :.resof the flutter poles.A 2 *
/A1 R
The integrals in the square brackets depend on the tint tor
pole geometry only. Therefore for a given geometry, wo can
wr ite,2
F = k(uQM)
The constant k is easily determined experimentally. In FJJ. •'<
the measured magnetic force between two 1/10 sc^le iron Hi;?'or
poles in a uniform magnetic field is plotted ac.
of the induced magnetization (>i M) .1 o
' :. i • S ' ! ' • i 1 ''
EOO
.as
FORC
E.MA
GNET
IC
. HO
. 35
. 30
. 25
. 20
. 15
. 10
. 05
n
i i
F = M A I 0 M
k = 18. 85
-
i i i
kgm/(TESLA)2
y
/
1 1 I 1
1 1
/
1
T 1 •" f ~ T ' .
y
—
—
—
—
i i i i10 12 14
O J 0 M ) 2 . TESLA2
toI
18 20 22 * 1 0 3
Fig. 40; Measured magnetic force between two 1/10 scale ironflutter poles in a uniform magnetic field.
- 93 -
Since the magnelizing field and hence magnetization are
small (far from saturation), uniform magnetization of the poles
is assumed.
From this plot, the value of k for the model is found to be
18.9 kgm/(Tesla) . For full-size poles with a saturation
magnetization (;i M) of 2.16 Tesla the attractive iorco would bo,
F = (18.9) x 100 x (2.16) 2 kgm
= 8818 kg.
The total load tending to pull all the poles together would be
four times greater or about 3 5.3 tonnes.
2(e)v Extraction (C.B. Bighan)
For single turn extraction there must be complete separation
between the beam in succeeding turns at the extraction radius.
Fig. 41 shows the orbit separation vs. radius for heavy ions
(£ = Q A> -• 0.14) with 3 and 10 MeV/u output energy and light
ions (C - 0.5) with 50 MeV/u.
Figure 42 shows the orbit separation at the extraction radius
vs. output energy for light {' •: 0.5) and heavy '' L- . 14) iors.
These curves are all for 0.8 MV acceleration per turn corresponding
to peak structure voltages of 100 kv in O-mode up to 12 MeV/u
and 140 kv in --mode for 12-50 MeV/u. The orbit separation can
be effectively increased by a factor of at least four ; sing a
resonant extraction technique. This would allow either larger
beam size, lower rf power or a stronger magnetic field should
other considerations require it.
Separation of orbits depends on beam size as well is ST^ICIP.;.
h typical heavy-ion beam is expected to have an eniu.im-c ••:
- 94 -
50
20 f~
Q.UJ00
o
10 -
L
2 *~
0. 14
I
OUTPUTENERGY
V / u
o 0 . 2 0 . 3 0. 4
RADIUS. M
0. 5 0. 6 0. 7
Fig. 41: Orbit separation vs. radius for final specificenergies T/u = 3, 10, and 50 and for £ = Q /u = 0.14,0.14 and 0.5 respectively.
about 12 mm-mrad before the second striuper. Scatter m a in the
stripper foil increases this to 24 mm mrad fsee Sec. 3d'!') .
If an energy resolution of 1 in 10 is achieved, • he radial
increase in beam width during acceleration is negligible so
that the expected emittance at the extractor is 24/.T6 - 6 iran-mrad
If this can be brought to another radial waist at the extractor
septum, it should be possible to separate orbits with spacing
down to 2.5 mm corresponding to 50 MeV u light ions - the
worst case shown in Fig. 42.
The proposed extraction system uses two electrostatic
deflectors in adjacent grounded sectors of the cyclotron. They
are in the grounded sectors so that the rf will not ho.v. the
septum.
Considering the extractor as a deflector the anale of
deflection is
g *£ = — — ~ R sir. radians.
2 T/u o
where E is the deflecting iiela in volts/m
Z is the charge-to-mass number ratio
T/u is the specific energy In MeV/u
R is the extraction radius; in n>o
and • is the azimuthaJ extent of the extractor.
For E = 100 kv/cm (50 kv across 5 mm say) R •= o.f,r. m and
• = 33 we obtain the deflections shown in Fig. Ai for charge to
mass ratios of 0.14, 0.3 and 0.5. Heavy ions f" - 0.14' with an
energy of 10 MeV/u are deflected by 0.025 radians and 1iaht ions
(C ~ 0.5) with 50 MeV/u by 0.0376 radians. The traiec-n ies
after the extractor depend strongly on the radial dep«-i> i'-ncu ••:
the magnetic field outside the last ort it.
The radial variation in magnetic field outside the extract i;n
orbit at 65 cm is shown in Fig. 44. The estimated orbit v>^sif
- 96 -
0 >-
X
•cl
a.
CDQiO
0I I
20 30 40
OUTPUT ENERGY (MeV/u )
50
Fig. 42: Orbit separation vs. final specific energy T/u for= Qo/
U =0.14 and 0.5,
0 . 0 1
- 97 -
ONE 3 3 ° D E F L E C T O R 1 0 0 K V / C M
0. 05 -
toz-a.
Q
0.
oi—CJUJ
u_
0. 03
0. 02
= 0 . 5
Fig. 43:
_L
10 2 0 3 0 4 0
E N E R G Y . MeV u
J
50
Deflection angle vs . specific energy following oneelectrostat ic deflector spanning an arc of 21 witha f ie ld of 100 kV/cm. for C - 0.14. 0.3 and <? . c,.
- 98 -
//
A
180 c
FLUTTER POLE EDGE
t360°
90e»
180°
4-100KV/CM DEFLECTORS
2-200KV/CM DEFLECTORS
65
Fig. 44:
70 . 75 . 80
RADIUS. M
85 90
Fringing field and extraction orbits with four100 kv/cm deflectors or two 200 kv/ctn deflectors
- 99 -
at. 'to intervals for either four 100 kV/cm or two 20') '-•.•./err,
tie t lectors at the azimuthal angles, indicated from *;he beq:nninq
of the deflector, are also shown. outside the iron flutter pules
the orbit is almost closed so that a shielded beam tube would be
required to extract the beam. it may also be necessary to use a
longer deflector extending through the rf electrode instead of
using a higher voltage.
A better approach would be to "sharpen up" the fringing
field of the coils. The coil geometry of Fig. 19 was arranged to
give nufficient clearance for separate cryostat and rf vacuum
tanks. if the vacuum systems are integrated, the coil radius
reduced, the coil separation reduced and the coil aspect ratio
increased (radial dimension reduced), a sharper edge in the coil
field is obtained. The iron pole thickness could also be varied
radially to help maintain isochronism into the fringing field of
the coils and allow a sharper drop off beyond. These cnanges
reduce coil size and cost but reduce the space available for
extractors etc. A compromise will be required at the detailed-
design stage.
The beam passing through the fringing field of the coils
experiences axial and radial focussing if the radial gradient is
not too large i.e. n = -rdB/(Bdr) • 1. But if the beam is to be
extracted without a magnetic shield, n must be greater than unity
in the early part of the orbit. However, after a substantial
radial angle has been established, the local gradient could be
reduced to n .v $ by an arrangement of iron shims; in this manner
both axial and radial focussing would be achieved.
In summary, the orbit separation is adequate for single *irn
extraction with electrostatic deflectors but the present coils
- 100 -
A-oiilfi require a magnetic channel to extract the beam from the
coil iringe field. Modifications to the coil geometry making
the fringing field fall off more rapidly will allow extraction
with two 100 kv/cm deflectors and without a magnetic channel.
The required modifications do not seem to be prohibited by other
considerations. The detailed orbit calculations to establish
the focussing requirements have yet to be done.
2(e)vi Cryogenics (J.A. Hulbert)
1. cryostat
(i) Mechanical
In operating a cryogenic system, time is consumed in cycling
between room temperature and the operating temperature, when
maintenance or modification is to be carried out. In the super-
conducting cyclotron direct access to the accelerating structure
and stripper is blocked by the magnet assembly and it is there-
fore important that, if possible, the top trim coil be detach-
able without bringing the magnet system up to room temperature.
Figs. 45 and 46 show a layout which assumes that such an
assembly is possible. This arrangement is achievable if accelerator
and cryostat use a common vacuum under normal operating condi-
tions and if the positions of entrance and exit beams are confined
to fixed apertures for all particles at some location just inside
the main coils. It is then a straightforward matter to seal off
the beam line between cryostat and accelerator and let down the
accelerator vacuum without disturbing the cryostat vacuum. in
the design the cryostat support structures should be as simple as
possible to permit access and to allow dismantling of sections
with a minimum of pipework disconnection. One mechanism for doing
this is illustrated in Fig. 47.
- 101 -
I m M^r /r
v
Fig. 45: Vertical section of cryostat (1/20 so;le).
A Upper main coil KB Lower main coil LC Access pipe (one of 12 for gas, liquid, M
leads, monitoring) - main dryostat ND Access pipe - top trim coil cryostat oF. Probe and monitoring access to cyclotron
- outer radius pF Probe and monitoring access to cyclotron Q
- inner radius RG Vacuum tank SH 80K radiation shield TI Helium vessel - main cryostat UJ Main coil suspension V
Main c o i l spacing br I •-igeworkTrim c o i l bridgeworkA u x i l i a r y shim c o i lTypica l beam a p e r t u r eDemountable j o i n t to r e l e a s e -vce l e i » t i ° '
chamberRac ia l l ack tor top t r i m c o i lPariial b r a c i n g for top t r i m o i lRadial b r a c i n g for main c o i l . i s scni lyTop trim coilLower trim coilTop trim coil cryostat bridcjpworkMachine support frame
- 102 -
Fig. 46: Plan view of cryostat.
ABCDE
Injection beam lineExtraction beam lineBeam line closure - injectionBeam line closure - extractionMain cryostat - helium vessel
suspension pointTop trim coil cryostat - helium
vessel suspension pointTop trim coil cryootat - bridge work
H Current leads (main)I Current leads (top trim)J Refrigeration connections (main)K Refrigeration connections (top trim)L Monitor and probe accessM Main cryostat vacuum pumps (two)N Top trim coil cryostat vacuum pump0 Stripper foil changer lock
- 103 -
uO
nu
o
B3au-3
O
OA :0)
- 104 -
The cryostat envelope will be subjected to large stresses,
arising from magnetic forces, which are not commonly encountered in
low temperature engineering. The standard forces are the
pressure loading due to the evacuated interior and the weight of
helium vessels, radiation shields and magnets, hanging from the
interior of the envelope. It will be a matter for the final
design group to decide whether the cryostat envelope should support
the approximately 40 ton weight of the magnet assembly unaided
or whether the envelope should be suspended from an external
framework (or the iron shield) or whether interior legs should
carry the weight to the machine foundation. The latter approach
would seriously affect the estimate of cooling requirements
where suspension of the magnets has been assumed. The standard
forces can be adequately met by the use of a thick-walled
aluminum alloy welded shell with a thickened and reinforced
machined cover plate carrying the interior load.
The non-conventional "stresses" arise from eddy currents
induced in the multiple walls of the cryostat when the coil
currents are changed and from the forces between the parts of the
magnet system where these forces have, unavoidably, to be
transferred through the cryostat walls.
To minimize eddy currents the radiation shielding in the
cryostat will need to be sectored and joined with insulating
connectors. The material for all non-sectored cryostat walls will
be chosen to have the highest possible electrical resistance
consistent with mechanical requirements and economics.
The magnetic forces which will require special structural
treatment are those acting on the iron flutter poles, both axially
and radially, and the axial interaction between the separated trim
coil and the grouped superconducting coils. Means to prevent
- 105 -
relative radial displacement and oscillation oi the co-la ->r:
poles is also required. in Fig. 4 5 radial check-:ars :cr t •.
coils and an external support frame are shown diagrammuLicai J.y.
Bracing, as necessary, for the poles will have to be fitted
inside the rf "pill box".
The magnet coils are each bolted down to a stainless steel
strongback and welded into individual helium vessels. "he vessel.•:
are then bolted down to aluminum alloy (or stainless steel)
bridgework and interconnected with two-inch diameter stainless
steel pipes at four, or six azimuthal locations to provide he!in-
flow channels and access for filling and emptying syphons, clrc* -
rical feeder cables for the coils and monitoring leads. The
coil vessels hang by stainless steel tubes from the top plate of
the cryostat and are braced radially with check bars which may
be stainless steel or titanium alloy. The check bars will be
loaded by differential thermal contraction during cooling and
their anchorages will need to be designed so as tc use this pre-
loading to advantage. Provision will need to be made, at the
interface between the top trim coil cryostat and the main
cryostat, for controlled mechanical loading of the radial ci.ecV
bars so that the ends of the bars may be freed for the removal
of the trim coil cryostat.
Radiation shielding is provided by aluminum segmented sv.r t .ic<>s
cooled to 80 K by an auxiliary circuit from the refrigerator.
Part of the shield cooling may be provided by natural boil-o'*
from the cryostat but detailed information on the available-
cooling from this source will have to await a proper issessncn*
of the detailed cooling requirements following T fl 1 ' nrch.if: :<•.-..
d e s i g n i n c l u d i n g b r a c i n g a n d s u p p o r t l o s s e s . 1" •' n? t 1 !••• '
i n f l u x t h r o u g h t h e r a d i a l c h e c k b a r s , e a c h l-.it wil l '•!'<• t • '••<
t h e r m a l l y c l a m p e d a t i t s m i d p o i n t to t h e s 1 i^ 1 i • > ' > n • ••••*..
- 106 -
Since the accelerator and cryostat have a common vacuum
•..he use of multi-layer "super-insulation" is not thought to be
advisable because of possible outgassing problems, insulation
clearances should be adequate to allow for flange protrusions,
overlaps, bolt heads, cooling pipes and eccentricities due to
manufacturing tolerances and cooldown strains in large diameter
shells.
Reserve capacity for liquid helium above the magnet coils,
totalling 900 litres permits operation of the cyclotron for a
period of 24 hours during refrigerator servicing or breakdown.
Temporary shield cooling would be provided by a "liquid nitrogen
powered" gas cooler.
(ii) Operational heat load
In Table X , various sources of heat influx to the liquid
helium stages of the cryostats are recognized and preliminary
estimates given of the magnitude of each.
TABLE X
CRYOSTAT HEAT INFLUX TO 4.2 K IN WATTS
Superficial 3
Current leads 10
Magnet suspension 3
Radial bracing 10
Total 26
These estimates assume that the superficial loss can be
kept down to about 0.3% per day, considering the cryostat as a
simple helium storage vessel, and that the practical limit for a
- 307 -
good electrical lead design between room teir.perature and ^ ..: ••.
aives a flux of not less than 1 mw per ampove capacity .sn,: uses
extrapolation of the data given in ref. (2^) tor the contribution
due to support structure and lateral bracing. The tot.,-1 is roimi-:
of i to 30 watts. The heat load on the radiation shielc wov. 1 rl .<••
expected to be around ten times this figure or 300 watts.
fiii) cooldown heat load
The weight of the coils plus helium vessels and support
structure is expected to be . 33,000kg. Tho equ:valent V O ' I T P S
of liquid nitrogen and liquid helium for cooldown rror 300 - *-
78 K and from 78 K to 4.5 K respectively with perfect enthnlp\
transfer are 9500 litres and 5000 litres. Allowing an efficiency
of 2/3, cooldown would require 14 500 litres of liquid nitroqen
and 9500 litres of liquid helium (including 1500 litres to 'ill
the cryostat). However, the figure for nitrogen assumes direr'
contact with the copper. If helium flow is used to fransfcr the
"cold", to avoid the need for separate liquid nitroaer. cool; no
pipes, then the nitrogen requirement becomes 55000 litres. Tne.se
quantities provide further incentive for arranging access to t he-
room temperature parts of the machine without heating up the cils
Provision of reserve liquid capacity, high pressure g.is
storage and compressor capability for occasional use o: such 1 IT <
quantities of liquid helium is not considered oconom' -a]1/
justifiable in the present system and the scheme outlined in ' ' <•
following section uses direct refrigerator circulation for the
cooldown process with the acknowledged inconvenience o:
extended cooldown time.
- 108 -
:'-oino lspei-ts ot the design which cannot be dealt with in
•iet.ii 1 until i proper structural design is complete concern the
possibility of separating the accelerator chamber and cryostat
vacuums, access for probe drivers and probe leads and the position
and mechanism of the stripper-foil changer.
The difficulty in the way of separating the two vacuum
spaces is in knowing in advance of proper trajectory calculations
whether it will be possible to thread a thermally isolated vacuum-
tight beam tube, curved suitably and dimensioned adequately to
accommodate all ion species along both injection and extraction
paths. Th? main uncertainty here is in the detailed requirements
of the magnetic extraction. The need for special ferromagnetic
or superconducting elements might interfere with the use of a
beam pipe. The scheme proposed above for internal shut off
valves within the cryostat places reliance on the correct oper-
ation of these valves to avoid the need to warm up the magnets.
Total separation of the vacuum systems appears to be a more
reliable arrangement.
The preferred entry for probes and stripper mechanism is
through the top plate of the accelerator chamber, either close
to the rf line or in line with the separation between the two
cryostats. In either of these positions the complete system can
be tested before final assemoly of the machine. Otherwise access
could be obtained radially, just off the midplane, to avoid
interference with the beam, when system tests could be made only
after final machine assembly.
2. Refrigeration
The components of the helium refrigeration circuit, omitting
details of the refrigerator itself are snown in Fig. 48.
- 1 ()') -
0
0)E
CO
• C1ZD
- no -
• -i 1 >wn :r.>m room temperature to 100 K is achieved by
t . •:., ,• .:;>; ;ow pressure helium gas through a bulk liquid nitrogen
v- ; -*vi in : passing the -old gas through the cryostat. As the
t envoerat.:re of the cryostat approaches 100 K the gas will return
to the gas holder at a very low temperature and will nave to be
heated, either directly or by use of a heat exchanger, before
recirci.lation. It may be possible to obtain a turbine pump which
can be operated at low temperature in which case the cold gas
would not require reheating and nitrogen consumption would be
reduce'! by about one half.
The "100 cfm"rating for the gas circulating turbine allows
for the :irst stage of cooling to be completed in about two days
(48 hours).
Cooling down from the region of 100 K is carried out using
the refrigerator. A single CTI Model 1400 would cool the 33000 kg
mass of the low temperature assembly from 100 K to 4.2 K in a
time ranging from 120 to 200 hours, depending on how efficiently
the refrigerator heat exchangers can be made to operate at high
temperatures. A liquid helium storage vessel of 2000 litre
capacity is provided whose essential function is to store the
cryostat liquid charge in the event of a shut down. However,
under the circumstances that a cooldown cycle starts with
this vessel full, its contents may be used to accelerate the
final stages of the cooldown from, say, 40 K down, saving about
10 hours.
The medium pressure gas storage system is made up from five
standard commercial natural gas containers of 12,000 litres
capacity each. These permit the system operating volume of 2000
litres of liquid helium to be stored as room temperature gas at
1700 kPa iiuring long term system shut down. They also provide
- Ill -
buffer vessels for the periodic cleanup of ..-on*-air. in ,'::.* •• -• • •
helium charge by circulation through an adsorption tv ;• • •
'gas purifier'). The refrigerator compressors rruv.ic u-
motive power for gas circulation in the storage ,mJ pur:: . r. .
system. Storage of the complete gas charge can be ,ic!':"'.v-: ?
about 2O hours with the normal pair of compressors s: pj.,! * i
with the CTI 1400 refrigerator.
Details of the connections between the cr yes1..it:-; .•_•-.-.
refrigerator are omitted from Fig. 48 because these -'.<••• • :. ••:
details of the refrigerator heat-exchanger circi:*r\ t ^ 0 0
extent.
The cooling specification of the CTI Model 140'j rofr: -<:r or
liquefier should provide ample cooling for the ste.Tiy-statt
operation of the cyclotron magnet system and gives 1 c>'>IdowT;
time as indicated above. A specially modified version would '; o
necessary to give the shield cooling of 300 w at "0-idn K ;n : to
cope with the multiple cold gas and liquid interconnecting
s iphon.
3. Safety
Conventional low temperature systems are equipped with
safety valves for the release of pressure build-up occurring :ror
the accidental heating of condensed gases. All enclosed space;•:.
both in vacuum and pressure components require this protection.
In addition, the structure of the magnet system of the cycjo*: .n
will require continuous monitoring with strain gauges <*
strategic points both to check on performance vit\ : n :. s : ::.
strains during cooldown and to give warnm: o: .»!!• i : ' • '
stress which could lead to catastrophic faili.r<_ .
- 112 -
, • ; •. . . u-,.,.r,i ' J . A . i l u l b e r t )
. . .-.L .V. : remtni
:c deterrr. m e trie necessary quality of vacuum along the beaff.
... iL\i throug':: t/ne cyclotron, the data on charge change cross-(45)
sections given by Schmeltzer and Bohne is employed.
The fraction of incident ions transmitted without charge
change throi.gr. a gas--filled channel is given by
L
f - exp -A I • (q, )p(z)dzo
where I is the channel length in cm
is the total charge-change cross section per atomt
q is the ion chargeB Iity/c)
, .. . . both functions of z in general
is tne ion veloci f l j / o 1
, is the channel pressure
A ^ 3,3 x 10 torr cm for monatomic molecules
is a function of the gas filling the channel and is given by
Schmeltzer and 3ohne for nitrogen. Measurements by Nikolaev and
coworkers show that the cross section is directly related
to the number of electrons available for exchange in the target
gas atom. At higher energies the electron numbers in deeper
shells determine the relative cross sections. Hence at high
e n e r g i e s ( i . e . c - ' 0 . 0 5 ) he l ium has the most advantageous c r o s s
section of any gas except hydrogen.
To estimate a value for the pressure corresponding to a
transmission of 90%, we approximate
f =- exp(-AJ pL)
where A- hL V 0.105
- 1 1 3 -
T h e a p p r o x i m a t e i o n p a t h i n th<-> c y : 2 ' ' r T. . ; .-'"• :: ;• ;
c o n d i t i o n b e c o m e s
- 2 2 - 2, • 1 . 5 x 10 cm t o r r .
Assuming a pressure of 10 torr then • should be less •_':..in-15 -2 t
10 cm . Schineltzer and Bohne aive calculated cr >ss sec : on?
for a range of charge states of S, As, I, " :n N.,. m i!J
10 for ion kinetic energy greater -.h-in 0.4 Me\ vor
nucleon.
Thus it is to be expected that a pressure or i>> '..:>*-•
w i t h i n the c y c l o t r o n should b e adequate for all :on ST>. •'•:••
be accelerated.
2. Pumping
We assume that the presence of strong magnetic fields near
the cyclotron will prevent the use of rotating machinery and
possibly ion pumps unless placed well outside the iron shiel i.
Under this circumstance the application of oil difiv;:-,->.>n pup-:>;na
seems to be indicated particularly when the pressure r: surf tc s
at 4.2 K will provide efficient cryopumpina for all <• .ses r-X'.-opt.
'47 xhelium and hydrogen. For example Dawson et ^i ' -.'ivf t)•<- pmnyir.
speed for condensable gases on to a 12 K surface as rang in-i betwee
-1 -25 and 7 -C.sec per cm of cold surface with a capture coe: : >>!.
of over 97% for gas in equilibrium with a 77 K surroun-.i J nu
surface.
Modern oil diffusion pumps will maintain ^s ).iq}. a press ir«.
differential when pumping helium as when pump rn: lu'rixren
The system illustrated in Fig. 49 d e p e n d on ' • .<>> • r • : r. :
common vacuum pumping for the acceleration t-r :ml r .:;: ' '•>.•
cryostat. If it is found possible and iesir
- 114 -
E! O
~o
m• 4 -
CQ)C
aoe3o(0
design to separate the two vacuum spaces an .;;id: t: • >r:..' : :
pump and a cryopump in a separate envelope v v. 1^ be :c-.v. lie 1
to pump the acceleration chamber.
2(1) rnstrumentat ion and Control V.S. O O I T C T , .'•:.' . "r :'• ;:r,
and ,! .C.D. Milt on)
During the past few years the adver;* o; the :cv-c.:st meii r
sized computer has resulted in a rapid evolution ;:. -ys'.on.- • •. r
instrumenting and controlling large acceJ or ;torr;. .'[M-:/.-:... •-,.,..:••
systems have been chosen by the designers o* the ?•-:''v-
'50) (51^
SIN and Indiana strong focus: p.., cy.-. : r .n.., .. •. • .
are currently under construction. Also - x . r ' m : (per . ::;'.
cyclotron facilities of earlier designs, for ex r.i-'o ;•! -! ,'
and the Eindhoven AVF cyclotron'" are currently ni.yment in.;
their conventional instrumentation with computer-: :r>. : yyy.->-r.i-.
The ORIC facility is to be completely switched over -ron •'.
present conventional system to a compute; b.isej c.^'io] syv'. or.:.
A m o n g t h e a d v a n t a g e s w h i c h t h e C-WLC p e o p l e : o r e s ' ' - !?•- ';•«.-
following - "converting ORIC to computerize-] control • v::]; : ;•.•<•
hundreds of hours in set up and maintenance fine. r:
addition, experiments requiring frequeni energy change1- v: • I '.
become practical, and the operating staff will be able t :rrvi(52)
more assistance to users" . in view o: 'hose advances H I
design thinking and operational experience it seems IT-M,I.S < >,,.
the design study proposed in this report will ;ead 'r "ornijuvu
b a s e d i n s t r u m e n t a t i o n a n d c o n t r o l s y s t e r f'.r !-he supt-i - -• •:-> 1 '••<*'
cyclotron.
T h i s v i e w p o i n t is reinforceci !.y thf r--' .' :v< V ' rr.lex
p r o l . l e m o f s e t t i n g u p tlie a c c e l e r a t ; n u s y : * e H ••> -r-c 1 •:• at. • :
d e s i r e d a t o m i c s p e c i e s t o a s p e c i f i e d e n e r - ' y . A t ' ': - n : * •:.
- 116 -
:-ecf ion (2(f)iv) a possible method of setting up the accelerator
• .<-• sketched and from this description those interested can
iorive some appreciation of the complexity of the problem. in
what follows immediately we discuss the problems in more general
terms.
2 if) i Function gatetories
A computer-based instrumentation and control system can, in
principle, perform the following groups of functions: monitoring
and logging data, assisting the operator in manual operation, and
closed loop control operation, we shall consider each group of
functions in turn.
1. Data Monitoring
This involves the periodic measurement of analogue signals
such as voltages, currents, temperatures and pressure and the
sensing of such things as switch positions, rate flow sensors
and valve positions, with the computer system it is easy to
compare each of the hundred or more analogue signals being
monitored with reference values appropriate to the desired
operating conditions and draw attention to the abnormal conditions,
The maintenance of operational records of machine parameters in a
readily searchable form, for example on magnetic tape, is easily
done with such a system. The functions in this group are purely
passive in nature. The computer system simply assembles and
evaluates data for presentation to the machine operator or
iccelerator physicist.
- 117 -
2. Operator Assistance
The number of parameters to be sot to obtain a given i,c,ir
from the cyclotron is likely to be more than a hundred. In iiuli ,
operations the values of many of these settings will be derived
from particle orbit calculations. As experience is gained it is
probable that a library of settings will be accumulated base? on
operating experience. It may well prove necessary to adjnst
the parameters in a time controlled fashion. The computer is
ideally suited to the execution of these tasks and in so doinq
provides an important operational aid to the machine operator.
3. closed-loop control Operations
in this mode of operation and in response to a beam energy
request, for example, the computer would not only make an initial
setting of all parameters in accordance with stored reference
values but it would then follow an optimization procedure to
achieve the best beam quality at this new energy. The optimization
procedure is a variational problem which may involve many variable:.-
and is likely to be plagued by the very difficult problem ot
distinguishing local optimums from the true optimum for the
system. Closed-loop control of accelerators is in an embryonic
state of development. Significant progress in this direction is
likely to require very elaborate beam monitoring facilit.es. it
is important to note that the optimization process is a v\>r iat: on t,
procedure and would require large amounts of central processor
t ime.
- 118 -
2 v f)i i The LAMPF Computer System
A good example of the current 'state-of-the-art' is the(54)computer control system for the LAMPF linear accelerator
It is now operational anc' is satisfactorily performing functions
1. and 2. as described above. Design concepts in this system
(H.S. Butler - private communication) which permit 'reliable'
accelerator operation despite the presence of software and
hardware (mainly control console hardware) problems are the
following.
1. Accelerator component protection functions are
independent from the computer control system.
2. The interfaces are such that parameter values are
hardware retained until altered on instruction from the computer.
Thus parameter values are maintained in 'crash' situations and
the accelerator status is unaffected although there is of course
a temporary loss in "communication". Program reloading from a
disc keeps this lost time to a minimum.
Economies were realized in accelerator component interfacing
costs by standardizing on a set of nine standard interface modules.
Successful implementation of this standardized system required
continuing close consultation between the instrumentation and
accelerator design engineers to ensure that specifications for
all accelerator components were compatible with the system.
While the LAMPF hardware .servicing subroutines are written in
machine language they are Fortran callable and the operating
programs are written in Fortran. This use of a high level
programming language allows accelerator physicists to do their
own programming with immediate benefits from reduced programming
cost, increased utilization of the systems potential, and more
favourable user acceptance of the system.
- 119 -
Finally, the control console design is < vory :.r.v..-• r : .,ii '
aspect of any computer control system. it should ::c r.<jv • • • .
in the LASL system this control console " ' provides * he
means of communication between the control room operators .,r.<l
the accelerator - no meters, no switches, no lights. one .>r-.;on
the LASI. designers took to simplify operation was to make
provision tor t lie p>:sh button selection of arcol er a tor s c " r : r: •
prog r onus .
2 ( f) i i i Proposed System for the Superconduc t ing _cycl_ot_ron
h computer-based system is proposed to facilitate operation
and development of the superconducting cyclotron. From the
experience at other accelerators it seems clear that development
of the computer system should commence in the earliest stages of
the cyclotron design so that maximum advantage can be taken of
its potentialities as has been done in the LAMPF facility.
The initial aim should be for a systeir. that monitors and loqs
performance data and aids the operator in setting up and
optimizing performance. It is suggested that, wherever possible
during this development phase, features should be included * hat
will make it possible to implement whole or partial closed-loop
control of the cylotron at some later date.
As a rough estimate of the magnitude of the hardware for
the computer-based system one can cite the features of the system
now being installed at the ORIC cyclotron. Table XT , taken fron.
ref. 52, gives an approximate inventory of the instrumentation
and controls prior to installing their computer-based system.
- 120 -
TABLE XI
ORIC INSTRUMENTATION AND CONTROLS INVENTORY
High resolution analog reference (1 in 10 ) 34
Medium resolution analog references (1 in 10 ) 80
Contact closures 240
High resolution analog data (1 in 10 ) 46
Low resolution analog data (1 in 10 ) 110
Contact sensors 425
The components of their control computer system are summarized
in Table XII.
TABLE XII
CONTROL COMPUTER AND PERIPHERALS
A general purpose computer, 16 bits/word, 24K memory.
An operator keyboard/printer.
A high speed paper tape reader.
A high speed paper punch.
A magnetic tape drive.
A disc storage device.
A card reader.
A high speed serial printer.
A CRT graphic and alphanumeric display with semi-conductor memory and keyboard.
An 80-channel, multiplexed input, analog-to-digitalconverter/system.
This would be typical of a minimal system.
- 121 -
It should be noted that the Mr Tandem accelerator tc .
as the injector for the superconducting cyclotron . r- i^rvi :'.
interfaced to the PDP-10 computer for data monitoring purposer.
The superconducting cyclotron control system must therefore
communicate with the PDP-10. The final system capabilit-.ee and
computer configuration would result from studies already beci.n
based on experience with control of the MP tandem .iccp'^ra' r
•is discussed in Section 6(d).
2 ( f) iv Se_tting up the Superconduct ing cyclo_^ron
To give some idea of the complexity of obtaining the desired
beam from the tandem plus cyclotron we consider here a possible
procedure for setting up. There are four basic parameters which
in combination determine the output beam energy, T (MeV), of
the cyclotron. The basic relations between parameters listed on
Figs. 50 and 51 can be summarized as follows:
Average magnetic field B = 0.222 (T A 0 ™ f ir. Teslai i:}
Initial orbit radius R, = 0.65(T1/To) < in netres) </•
Injection energy T^ = (Q. + l)v+ (in MeV) 3;
Radio frequency f f = 60.8 (Q B A ) (in M H Z ) '4)
where Q. is the charge state of the ions emerging from the tandem
which have been selected for injection into the cyclotron, v\ is
the tandem terminal voltage in megavolts, <j is the ion char re-
state after stripping in the cyclotron, R1 is the radius of the
first equilibrium orbit in metres, T is the desired output beam
energy in MeV and u is the atomic mass of the ion being accelerated
(An output radius of 0.65 metres has been assumed. The -\cloM .;.
is designed to accommodate charge state ratios <j /<_> ;n the
- 122 -
SUPERCONDUCTING CYCLOTRON
B , F r f , V r f , * r f
FOIL /STRIPPER-^
HEAVY IONS 10 MeV/u
F IONS 50Mev/u
r
/
\ I
GAS OR FOIL STRIPPER
MP TANDEM
13.5 MV
01BUNCHER
NEGATIVE ION SOURCE
rf
Fig. 50: Beam and accelerator parameters.
- 123 -
M I D - P L A N E I N J E C T I O N G E O M E T R Y
6
S T E E R I N GM A G N E T
Fig. 51: Injection parameters.
- 124 -
r :o> • :, <j o • 4. In general, the set of parameterso i
tv'oss -.rv to produce the desired output energy will not be <J
,r,,j;;e one. selection from within the subset of energy acceptable
er combinations will be made on the basis of desired beam
r.-rrent .
To obtain a desired beam of energy T will require the follow-
.n - procedure. First, a set of parameters V , Q , Q , B and f
:;K,st. DC selected which is consistent with the limitations of the
'-.-ciotron and tandem design (V '- 13 MV, 3T '» B 5T, 22 s f c
•-.'-, V:':iz, and 2.5 Q ,/Q. < 4) and which lead to the desired output
flux and energy. The adjustments proceed as follows:
i) choose Q and set B according to eqn. (1); this
involves adjustment of the main coil current, the
trim coil current, and the 10 shim coil currents.
:i) choose T. and set stripper foil at R according to
eqn. (2) .
m ) choose Q. and set V as required by eqn. (3) .
iv) set up beam transport system in the tandem-cyclotron
area including adjustment of B to vary 6 to finds s
the correct injection orbit leading to the stripper
foil.
v) Set f according to eqn. (4).
vi) set buncher voltages V . and V o.Bl B2
vii) Set buncher phase 0 relative to 0 according toB rf
cyclotron operating mode (0 or v).
viii) optimize beam on stripper foil in cyclotron by adjusting
tandem parameters, beam transport and B .
lx) Trim the adjustments of B or f to obtain beam at
extractor.
x) Adjust v so that beam is properly centered on
extraction beam channel.
- 125 -
x i ) A d j u s t e x t r a c t o r v o l t r : g t - V,, t o o p t i m i z e ; . i : t p i . t .
x i 1) S e t u p b e a m t r a n s p o r t s y s t e m i n t h e c y r l o l r .<;.- ;..; i •.'•"••
area.
x i i i ) Optimize a l l s e t t i n g s .
Smooth variat ion of T can be accomplished by tuning :••., i .o r:
and V together with the other energy dependent elements. The
range of this tuning will be limited by charge state yields and
focussing near the upper right of Fig. 2. For T values past aparticular limit one must make a new choice of Q and r anr?c o i
then adjust the ether variables accordingly.
For heavy ions, covering the T ,/u range 3-10 MeV/u corres-
ponds to a B range 2.7-5 T for a single Q . In general, higher
beam currents are available at lower Q values so probably theo
cyclotron would be operated at higher B and lower Q for the
lower energies.
For light ions, 50 MeV/u can be obtained for Q /u 0.5 at
B --. 3T. The range is limited by f and focussing (see chapter 31*
and not by B. It is unlikely however that E v-11 be operated
much below 3T as higher currents will be obtained for lower Qo
values.
The expected method for varying the energy over a wide range
then involves continuous tuning over 10-20% intervals with
changes in charge states to cover the full range, setting up
the initial conditions after each charge change would be very
difficult without full computer control.
2 b -
.:\ ... u:.dt ion -'• r. 1 ei^aj^na ^'..C. {-.all)
r: : Neutron Pi eduction by T Vi J: jind! Heavy_ JLo.i3
•;':\o r,i iiatxon shielding requirements will be governed by
• •• i.r tons? flux oi -as neutrons generated when a high energy
,-o ;"i . f .-harqed particles strikes solid material such as a beam
a-.op or the walls of the accelerator and/or beam transport system.
The neutrons produced when high energy protons (E "•20 MeV)
;r£ shopped in a thick target are of two types: (a) evaporation
r.e.;irons with a mean energy of < 10 Mev and an isotropic angular
distribution and i.b) cascade neutrons with energies up to the
energy of the incident proton and angular distributions which
are very strongly forward peaked. Monte-Carlo calculations of
tr.e spectra of neutrons produced in (p,n) reactions on various
target nuclides for proton energies 25 ^ E^ £ 400 MeV, have been
Dnab(55)
p
carried out by Bertini ' . These results are in reasonably
good agreement with a number of experimental measurements
The neutron spectra produced by other ions are less well
understood. Spectra, similar to those produced by protons, have
; een observed for 10 and 20 MeV/u alpha particles stoppingI 571
m tantalum ' . These results suggest that a significant number
of cascade neutrons will probably be produced by other light
,A ' 10) ions at 50 MeV/u.
For heavier ions at energies of 5-10 MeV/u the (Hl.xn)
reaction is known to dominate the total reaction cross section
resulting in a large flux c* evaporation neutrons. At energies
above 10 MeV/u, break up of the compound system into several
reaction products could result in the production of higher
energy neutrons.
- 127 -
2 'a)ii Neutron Attenuation
The attenuation of fast neutrons in sh if -2 -] i ng r- ;'.ori.ils ;-
achieved by inelastic nuclear collisions. j.xpor imentaZ and
calculated attenuation lengths ('• ) for neutron energies of
1 • K 1000 MeV in a variety of materials''3 ' '"' aren
given in Fig. 52 and Table XT 11. The results reflect the fact
that the inelastic cross sectionsfor all elements are constant
above L 100 MeV, increase rapidly with decreasing er.orgvr*
until E 2 5 MeV and remain reasonably constant down to Kr - 4 Ve\
Since the neutron attenuation lengths .r.crease rapidly
between 20 and 200 MeV the flux of high energy cascade r.eu'rons
produced when protons are stopped in solid materials determines
the thickness of shielding required to reduce the radiation level
below the biological tolerance (for E 10 MeV a flux of-2-1 n 7
10 n cm s is equivalent to 2.5 mrem/h ) . Detailed cal-
culations of the shielding required for proton energies - 400 MeV
have been carried out by Braid et al . using the Monte Carlo
calculations of Bertini as parameterized by Aismiller to
calculate the neutron spectra, and the P spherica.l harmonic
method to calculate the neutron transport through the
shielding material.
The results obtained for neutrons produced in the angular
region 0-30 by protons stopping in a copper target and a
concrete shielding wall of density 2.3 g/cm are shown in
Fig. 53. in general, the thickness of the shielding wall
required for the angular ranges 30-60°, 60-90° and 90°-180 are
85%, 65% and 48% of those required for 0-30 respectively.
A comparison of the calculations of Braid et al, for
25 MeV protons stopping in Cu with experimental results of
Wadman et al , for 80 MeV al/>ha particles stopping in Ta
200
(NJ
e 160
£ I 20ce
I 80
r 40
pO L R L experimental doto
A 0 R N L experimental doto
a Calculated-poor geometry00
I
10 100
Neutron energy
1000 10000
( MeV)
Fig. 52: Comparison between measured and calculated values ofattenuation length (X ) in concrete for neutrons(see ref. 59) . w*
- 129 -
IO
- ? L
10
UJ
GOO
cz
r
10 . 5 i
10
7 L
2 . 5 M R E M / H R
4 0 8 0 1 2 0 1 6 0 2 0 0
C O N C R E T E S H I E L D T H I C K N E S S ( i n . )
Fig. 53: Neutron dose rate per u.A of protons vs. concrete shieldthickness.
- 130 -
shews that the shielding required for one particle n.A of 20 MeV/u
alpha particles is roughly equivalent to that required for protons
with the same E/u.TABLE XIII
NEUTRON ATTENUATION LENGTHS X (in) FOR VARIOUS MATERIALS
AS A FUNCTION OF NEUTRON ENERGY
E —(MeV)n
Klement cr mater
Al
Fe
Cu
Pb
Heavy concrete
,-• = 3.5 g
ial
/cm
14
4.
4.
2.
2.
3.
3.
a)
1
5
5
2
3
5
90 b )
15.1
10.7
4.2
4.7
140C)
12.1
11.9
5.3
4.9
5.4
9.8
270
18.
5.
5.
b)
0
7
8
Std. concrete
t =2.3 g/cm2 4.0 9.5 12.9 18.0
-•L) calculated from measured inelastic cross reactions for14 Mev neutrons.
b; ref. 6
c) ref. 7
2(g)iii shielding Requirements
The shielding requirements have been calculated on the
basis of 1 }j.A of 50 MeV protons using the results of Braid
e^ al . The wall thicknesses shown in the site plan (Fig. 1)
are derived from the curves of Fig. 54 which show that wall
•n
O R D I N A R Y C O N C R E T E S H I E L D T H I C K N E S S ( f t . )CT) CD
I I
0 Oa
• o
: • " ? » - • •
•-3
n;v(DWW
o
0
—H
2 »Z
m
o
mm toO
O
I CD
3
0
")
O
O
"J5
om
o~r\
- HI Tm
•
COo
-t; ra
0
~>!—'
0<
df t
o3
M
'-'i-t
n
_ i- » — io !
|i
Or-O
-i
- T£T -
- 132 -
SOURCE-DETECTORSEPARATION DISTANCE
60 90 120 150
SOURCE ANGLE. K (deg)
180
Fig. 55: Monte Carlo calculation for neutron sky shine as afunction of angle and distance from a monodirectionalpoint source of 14 MeV neutrons (see ref. 63).
- 133 -
thicknesses around these areas are from 4 to 7 feet cf ordir. -•
concrete or its equivalent depending on the situation o -'
the walls with respect to the beam direction and whether the
walls are internal or external. Heavily shielded beam stops
are provided for all beam lines. The shielding is designed so
that set-up or maintenance work can be carried on in any room
not receiving the accelerator beam. In rare situation? it might.
be necessary to restrict access to certain areas either inside or
outside the buildings. If iron or heavy concrete (ilmcnite^ ire
used instead of ordinary concrete the wall thicknesses car. ho
reduced by factors of about 0.5 and 0.8 respectively. simla-ly
a factor of 10 reduction of the beam intensity to 0.1 .A reduces
the shielding requirement by one foot of concrete.
The problem of neutron sky-shine determines the necessary
roof thickness. The amount of sky shine is estimated from the
Monte Carlo calculations of Wells . Figure 55 shows represent
ative neutron dose rates for air scattering as a function oi
distance and angle for a monodirectional point source of 14 VeV
neutrons. At 35 feet, corresponding to a typical separation
between target locations in adjacent rooms, the dose rate is
th12
-12approximately 10 rem/h per neutron/second, with roof thick-
nesses of 1.5 feet of ordinary concrete and a source oi 10
neutrons/s of which 1/3 pass through the roof, the dose rate in
the adjacent room is about 0.7 mrem/h which is well below the
permissible value. This calculation represents worst case
conditions which would rarely if ever obtain and thus roof
thicknesses of 1.5 feet should be ample.
- 134 -
. h) Maano t 1 c t5ju_eId 1 mj {A..I. Ferguson)
There will be a significant external magnetic field extending
over a fairly large area around t.ie superconducting cyclotron (see
Table XIV) . The field can be expected to cause a number of
problems and for the purposes of this study we have assumed some
form of magnetic shield will be necessary.
Estimates of the effectiveness of an iron magnetic shield
have been made with the following simplifying assumptions:
1. The shield is a spherical shell of specified
thickness and known permeability.
2. The cyclotron magnetic field can be approximated by
a dipole field beyond a radius of 1 metre.
with these assumptions the field everywhere can be calculated
exactly. A set of linear equations is obtained from the boundary
conditions on the normal and tangential components of the
magnetic induction, B, at each spherical boundary. The solution
of these equations determines the coefficients of the field
expressions for each spherical region. A computer program has
been written to allow an arbitrary number of spherical sub-
divisions within the shield so that variation of the permeability
.., can be accommodated.
In the results given here the dependence of \i and B has been
approximated by an assumed B-H relation. The relation is
B = H + 19.0 (1 - exp (-28.491H))
where B is expressed in kiloGauss and H in kiloOersted. This
relation fits a typical B-H curve at the points
H(kOe) B(kG)
0.0 18
1.0 20
- 135 -
The actual B at a point in the iron is obtained by a self-
consistency condition. The calculation st-uls with m issiinr ,
the corresponding B is found and the u that corresponds to this
is found. An adjustment to u. is made and the calculation is
repeated until successive n's in the cycle are equal. The cal-
culation is made on the equator, °- - 90 , where B is largest.
Since u is assumed to be constant in a shell, the self-consistency
condition will not be obtained for other •- values where a < )
is smaller. This constitutes one of the approximations «\ade in
the calculation.
in Table XIV, |B| in kG is listed for radii 1 £ R S 4 metres
and azimuth angles 0 s c « 90 for no shield. The 1st column
is R and the 7 next columns are IB(~)| at 15 intervals in
The field here and in all subsequent results is normalized so
that B(90°) - 36 kG at 1 metre.
in Table XV are shown shielding factors for a variety of
shields. The shielding factor S is defined by
S = B(90°)/B (90°), where B (90°) is the field witho o
no shield, at a point outside the shield. S is independent of
R and outside the shield. The mass of the shield in metric
tonnes is also given. In computing the mass it is assumed that
the shield can be thinner at the top and bottom since the flux
is smaller here. Thus, for computing the IMSS, the inner surface
is assumed spherical and the outer ellipsoidal with the thick-
ness at the top 1/4 of that at the equator.
In Fig. 56 the shielding factor, S, is shown as a function
of the mass for several internal radii which are indicated by
numbers labelling the curves. For thin shields, i.e. those
for which S ^ 1, the shielding effectiveness is almost independent
of the radius. For cases with S « 1 the smallest radius provides
the most economical result.
- 136 -
TABLE XIV
MAGNET FIELD AROUND A SHIELDED DIPOLE
IN Al.ONC. VARI'll'S RADII FOR 1 40 m WITH NO SHIELD
1.
4 .
6.7
a .t
10.11.12.13.14.15.16 .17 .18.19.20.2122,232425262728293031323334353637383940
R
0O
•J
, • • 1
Q
0P
c[:.
0
00,00.0,0.0.0.0.0.0. 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0
.0
.0
.0
n .'AiAXIS
72.
2.1.0 .0.
0 .
;i.
0.
0.0.0.0.0.0.0,0.0,0.0
0
0
00000
0
00000000
000
c00000000666712 50576033332099140609S307 JO0541.0417.0328.0262.0213.0176.0147.0123.0105.0090.0073.0068.0059.0052.0046.0041.0037.0033.0030.0027.0024.0022.0020.0018.0017.0015.0014.0013.00x2.0011
70.8.^ .1.0.0.
0.
0.0.
0.0.0.0,0.0,0,0.00000000000000000
o000
16807710598809645613324920461370096307020527,0406,0319.0256,0208.0171.0143.0120.0102.0088.0076.0066.0058.005].004 5.0040.0036.0032.0029.0026.0024.0021.0020.0018.0016.0015.0014.0013.0012.0011
6482
1000000000000000c00
000000000000000000
iO
.8999
.1125
.4037
.0141
.5192
.300 5
.1892
.1268
.0890
.0649
.0488
.0376
.0295
.0237
.0192
.0158
.0132
.0111
.0095
.0081
.0070
.0061
.0053
.0047
.0042
.0037
.0033
.0030
.0027
.0024
.0022
.0020
.0018
.0017
.0015
.0014
.0013
.0012
.0011
.0010
56.7.2.0 .
0.
0.0.0.0.
0.
0.
0.0.0,0.0,0,0,0,0,00000000000000000000
4 5
92101151108288944 55426351660111207810 5690428,0329,0259.0207.0169.0139,0116.0098.0083.0071.0061.0053.0047.0041.0036.0032.0029.0026.0023.0021.0019.0017.0016.0014.0013.0012.0011.0010.0010.0009
47.5.1.0.
0.
0.0.0.0.0.0.0.0.
c.0.0,0.0.0,0,000000000
60
62359529763874413810
2205
1388
0930
0653
0476
0 3 58
,0276
.0217
.0174
,0141
.0116
,0097
,0082
.0069
.0060
.0051
.004 5
.0039
.0034
.0030
.0027
.0024
.0022
.00200.00180000000000
.0016
.0015
.0013
.0012
.0011
.0010
.0009
.0009
.0008
.0007
39.4.1.0.0.0.0.0.0.0.0.0.0.0.0.0,0,
0.0,0,0,00800000000
00000000
75
45189315461261643156182611500771054103950 296,0228,0180,0144,0117,0096.0080.0068.0058.0049.0043.0037.0032.0029.0025.0022.0020.0018.0016.0015.0013.0012.0011.0010.0009.0008.0008.0007.0007.0006
HORI7.ONTALPLAIIE
36.4.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0,0,0.0.0000000000000
90
000050003333562 5288016671050070 304940.3600270020801640131,0107,0088007 3,0062,0052,004 5.0039.00 34,0030
.0026
.0023
.0020
.0018
.0016
.0015
.0013
.0012
.0011
.0010
.0009
.0008
.0008
.0007
.0007
.000b
.0006
- 137 -
I 0
exo
C5
zI—I
o
XCO
. 01100
MASS IN METRIC TONNES
1000
Fig. 56: Magnetic shielding factor as a function of shield mas:for several internal radii (in metres).
- 138 -
:.e internal field due to the induced magnetism of the
i ion w.i :es :"rom ai>oi.t 2.5 kG for R - 2.5 m to 0 . <Jb KG for
K •; m.
i ;r. the basis of these calculations, an iron shield with
,,r. inner radius of 2.5 m and an outer radius of 3 m was chosen
reduce tne magnetic field immediately outside the shield to
.iuoui -> mT in the horizontal plane. The model used in these
orelm-: nary calculations is not convenient to construct, and so
cost estimates are based on a closed cylinder with the above
dimensions. The shielding of the field in the horizontal plane
for chis geometry should be similar to the results presented in
Table :•?•/ for a spherical shield.
Further calculations should be carried out lor the cylindrical
geometry to determine both external and internal effects of such
a shield. Another possibility which should be considered is
the elimination of the shield completely. The results of Table XV
indicate that the field beyond a distance of 6 metres is less
than about 20 mT which could be handled by local shielding on
equipment and beam lines. The effects of such fields on equip-
ment, the effects of the larger fields near the cyclotron on
personnel, and the effects of local magnetic objects on the
internal field of the cyclotron should be carefully considered
to determine the necessity for the magnetic shield.
- 139 -
TABLE XV
SHIELDING FACTORS AND MASSES FOR VARIOUS SHTKLDF
Innerr a d
m.
2 . 52. <>2. '.,2 . 52 . 52 . 52 . 5
3 . 03 . 03 . 03 . 03 . 03 . 0
4 . 04 . 04 . 04 . 04 . 0
5 . 05 . 05 . 0
Outerr a d
m.
2 . 62.12 . 82 . 93 . 03 . 13 . 2
3 . 13 . 23 . 33 . 43 . 53 . 6
4.054 . 14.154 . 24 . 3
5 . 15 . 25 . 3
ShieldingFactor
0000
on
0
0000Q
0
00000
0
n0
. 8 1
. 6 1
. 3 8
. 2 1
.C47
.032
.025
. 7 0
. 4 0
.101
.048
.0 34
.028
. 7 3
. 4 5
. 2 0
.110
.0 59
. 2 5
.096
.061
Massmet]r i cTonnes
4797
1 -/JJO 5?6 33 24387
6 81302 1 32 9 13 7 14 3 5
591201 8 224 537 3
1873 « 0V77
_ r
. J
. 0, '
( •
(•
• ?
. 0
. 0
. 0
.0
. 0
. 0
.7
. 0
. 0
. 0
.'.)
. 0
. 0
. 0
NSl I >i' !y\T' IONS 'U.K. Schnc iiior)
:cr the purposes of generating a conceptual design for the
.:vc.'Urcn, simple analytic relations describing the motion of
cr. rqeri particles in a magnetic field are adequate. Thus, the
..••r;.-t radius r of an ion in a magnetic field B is given by
r - 3-^- ;- IT/U) + 2(T/u)Eo]' 3.1
•»•••:.ere 7 u is the ion kinetic energy per nucleon in MeV
i. is the nucleon rest energy in MeVo J
and, * is the ion charge to mass ratio relative to the
charge to mass ratio of the proton (e/m )
;, plot of Br against T/u for several ions is shown in Fig. 57.
From this we see that a uranium 33+ ion (£ = .14) with an energy
of 10 MeV per nucleon has an orbit radius of .65 m in a 5 Tesla
iielJ. A proton on the other hand would have an energy of
4'JO XeV at the same radius.
The ion cyclotron frequency (f ) J.s given by,
f A e ' B * -fc = 2- nT •- • v 3'^
P
wiiere is the relativistic factor equal to the ratio of the
icial energy to rest energy of the ion.
The variation of the cyclotron frequency with final ion
energy in a cyclotron with a fixed extraction radius can be
ortaineu from equations 3.1 and 3.2. This is shown in Fig. 58
for the case of 0.65 m extraction radius.
- 141 -
<-t 1
zoUJ
1CJ
z
UJ
o.
>-' JexUJ—^
• * ^
UJ
zot—<
oE!fC
V)<y
c
•71
cr;
LTi
r |*J
V)3C
u
•'u0
Oc0
&
O
atr.
o oo to
W-V1S31
I I ! 1 1
20
16
>-
oUJ
S 12
o
ro
-.12 6 8 1 0 2 0
S P E C I F I C E N E R G Y M e V ' u
MO 6 0 8 0
Fig. 58: Ton cyclotron frequency versus specific eneigy.
As an ion is acceleratc-d both. its ind ., • <- • »•'•;!* »•
;ncr>;:se. I f, theroforp, the i or.s ore o arr ve :' :: - ••
accelerating aaps at the same relative ri ^ha^e the maan
! : i-](i TUJst also increase with radius so that '• ' , an•.• r.o
remain constant. The required radial vari.it-on of )• io»
lyo-.-lironous acceleration can be obtainc.-i !ro"" <?•_•'. ,;* : cis
ti'i'J •'..'. w!ii; (i .»111.-r some manipulation ;.fcr;f-.
r v
w h e r e is the for the ion ?.t the c;W: ;•>»-"•;-'1
R is the final orbit radius.
The radial variation of B is quite small for ion energies o:
interest here, as can be seen in Fig. c>9 where the required
field for 10 MeVAi '.'ranium 33+ and "0 MeVA; c"~ ro :-l:^' :
D - .63 m.
J (b» Focussing
Focussing in c i r c u l a r machines is usua l ly deser beri in
terms of axia l and r a d i a l be t a t ron frequenc ie:; . "'hese art' *
beam o s c i l l a t i o n frequencies associa ted wi^h the ax> :i -.n•:
r a d i a l focussing (or defocussina) forces ac t ing on Lhe ;<-ir.
in a magnetic f i e l d with, no ar.imuthal v a r i a t - o n and .; i
g r a d i e n t , k = (dB/B)/(dR/R) , the be ta t ron frequencies r ..rs
normalized to the cyc lo t ron frequency are approximately i : v
2
v = -k7.
- 144 -
c
in'o.
oi
L D 1
CD
L 12 .3 .4 ,5
RADIUS IN .METRES
. 6 . 7
Fig. 59: Magnetic field versus radius for uranium andcarbon ions.
- 14 5 -
For the isochronous field discussed above, where k car. be2
shown to be equal to ." -1, v is imaginary i.e. the field .s
axially defocussing.
This defocussing force can be overcome by providing an
azimuthal variation in the field (Thomas focussing). Moreover
if the poles used to produce the azi:au*"hal variation have a spiral
shape, advantages of edge focussing can also be realized.
Kqunt ion 3. '2. for this case then becomes,
v 2 - -k + -~— F(l f 2 tan2?) + 3.CZ N -1
where F is the field flutter factor defined by,
F <B2V<B>2 - 1 3.7
N is the number of sectors
and
Z is the average angle between the orbit and the normals to
the pole edges.
A hard edge approximation for the magnetic field is often
used. in this case, if the azimuthal variation is AB and h
is the fractional azimuthal width of the pole in a sector, then2
(ir)F = (ir) h(1-h) 3-8These formulae can be applied to the flutter poles
discussed in section 2(e)ii and 2(e)iv. For 10 MeV/u uranium
at the final orbit, B = 5T, y = 1.01, AB - 1.4T, h - 33/90,
N - 4 and % = 20 . From 3.3 and 3.5 then we get v = .06.
This is generally considered to be too low for adequate axiul
focussing. The usual design range for v is 0.1 to 0.3. with tn
above pole shape then, focussing would be adequate for
uranium only up to 8 MeV/u.
- 14b -
2
i'r.e oroblem is however not serious since the tan " term in
. >. represents a powerful focussing Torce. By increasing the
•vor.wc spiral angle to 36 adequate focussing, not only tor
.ran.urn but also for the more demanding case of 50 MeV/u light
ions can be achieved.
3;c) Resonances
The oscillatory motion of the ion beam about its equilibrium
orbit, admits the possibility of resonances in the orbital motion.
Resonances occur when the relation av + bv = n is satisfied,r z
a, b, and n being integers including zero. A v , v plot of
this relation for the important resonances in a four sector
machine appears in Fig. 60. The shaded rectangle encloses
the operating range of v and v for the superconducting
cyclotron. In this essentially low energy cyclotron, i.e. , s 1.05,
there is no great problem presented by resonances for most
operating conditions since neither v nor v change very much
during acceleration.The half integral imperfection resonance, v = | and the
non-linear coupling resonance (Walkinshaw resonance) v = 2vr z
must be crossed only when accelerating light ions (C = .5) to
final energies below about 15 MeV/u. with care in the design
of the magnetic field neither of these resonances should givedifficulty*64^.
3(d) Admittance at injection (C.H. Westcott)
The emittance of the beam from the Tandem after passing the injec-
tion stripper is not yet exactly known. Fortunately the admittance
AXIAL FOCUSSING FREQUENCY
0 o
01 rtrt ft)
h w(j o3 3
it) £Dfl> 3
OO fl>Mi
art h>-3* D>(0 01C 3
fl enO 31
a ac reo art-
0)O
O 3i^ aO H-r+ Ot-t [UO rt3 fO• tn
- 148 -
appears relatively large and the feasibility
•. ; he sri-.omc is not in serious doubt.
The beam emittance depends on its diameter at the stripper
•.•nd the r.m.s. scattering angle introduced in stripping as well
as its emittance before stripping. The latter is only about
12 mir-mrad, so that the increase due to the stripping is apprec-
iable. The spot-size attainable at the injection foil could be
-.3 small as 2 mm diameter if the last lens could be located
s:.y 3 m from the foil. However, it is not clear this would yield
an acceptable foil life - to be reasonably safe we assume a 5 or
c ran spot diameter (say 5 mm horizontally and 6 mm vertically.
Then, with a scattering angle in the stripper of say 3 mrad (full
angle for 30% intensity ) we would expect* an emittance contri-
bution from the stripper (in the z-plane) of about 6~ mm-mrad,
giving about 24 mm-mrad for the stripped beam, the r-plane value
being somewhat smaller.
The cyclotron admittance can be calculated approximately
based on . =0.1. The r-plane conditions are much lessz
stringent (on this approximation), since v « 1. in the final
design, extraction considerations will probably be such as to
limit the infection r-emittance to not much more than the z-plane
value, this being the objective in design optimization.
The -function of the Twiss matrix, which determines the
beam envelope, is a function of azimuth in the machine, but its
•average" (strictly its harmonic mean, the reciprocal of < 1/f.
round the orbit) is given by v = R/<P>* , where R is thez
average radius of the orbit at the energy under consideration.
*The 3 mrad 80% value is taken as about ± 2 mrad for say 95%
transmission.
- 149 -
At injection R 16 cm, so that ' " 1.6 m.
since the flutter near injection is small, to assume that
does not exceed 2.8 m (which woulr^ mean : /: . - 3 anywheremax m m ' 2
on the orbit. If we then provide for a maximum vertical beam-
width of i 1.8 cm, the admittance A, given by z =~\/3z , becomes360 mm-mrad.
This admittance appears completely adequate, and if an
optimization is carried out the vertical aperture may perhaps
be reducible by a factor of 2 or 3 from the value assumed in
this study.
3(e) cyclotron Emittance (c.H. westcott)
As a result of the acceleration process the 24 mm-mrad beam
emittance at injection is reduced to 24//T6 - 6 mm-mrad at the
extraction radius of the cyclotron, subject to any effective
emittance increases produced by errors in the electric or
magnetic fields or space charge effects. Beam emittance and
energy resolution both depend on these errors, with the energy
resolution being more sensitive to them. If we assume that the
desired energy resolution is achieved, the effect of these
errors on beam emittance should be negligible. The radial
dimension of the beam at the extractor septum must be less than
the orbit separation (see Fig. 41) which is as small as 2.5 mm
unless resonant extraction used. If we assume that a radial
aperture at the extraction septum of 2 mm is available, then the
required emittance for the approaching beam can be estimated if
we know p at the septum position, using &r ~ ~\l •*._ • However,r max v
at the present stage of the design we only know which ior
- 150 -
1, using <l- = R/v , has the value 0.65 metres. If at the
^u;»a » the required beam emit Lance becomes 5 mm-mrad,r r
•or r 2 mm. This is to be compared with the prediction of[TUXC mm-mrad in the previous paragraph; while detailed design is
still necessary, to ensure that ^ £ <p > at the septum position,
the numbers quoted do not appear to set any particularly difficult
criterion to be satisfied in connection with this design.
The alternative of using "resonant" extraction techniques,
which may be desirable especially for some of the lighter ion
cases, should allow the orbit separation at ejection to increase
by a factor of about four. However, some beam emittance increase
necessarily accompanies this technique, with attention to the
design constants this should be less than the factor of sixteen
which is available from the postulated increase by four of the
orbit separation.
3(f) cyclotron Tolerances
The beam dynamics are similar to those for a number of
operating 50 MeV cyclotrons but the desired beam quality is'66)
comparable with the best that has yet been achievedv , i.e. an-4
energy resolution of 4 x 10 and a beam emittance of rr mm-mrad.
The tolerances will then be determined by this desired beam
quality and will be calculated in detail during the design phase.
For now, qualitative arguments are used to show the requirement
for SCHIC are no more difficult to achieve than for an iron
cyclotron. Parameters from the MSU cyclotron will be used for
comparison.
The number of accelerated orbits are 90-125 instead of
210 for MSU so that tolerances are higher for perturbations that
- 151 -
drive resonances. The only strong resonance involved is
near the extraction orbit.
Setting up an accurate isochronous field is more difficult-
at MSI1 than for the superconducting cyclotron. At MSU, eight
trim coils are used for profile changes of several percent.
In our case, the profile can be made correct to -- 0.1% by suitably
choosing the superconducting trim coil current and the normal
coils nujte only - 0.1% corrections. Use of a program such as
"Fielder", developed at MSU , should allow the "setting up"
of fields to an accuracy at least as good as at MSU. This moans
that the phase error of the bunch can be more nearly zero through-
out the acceleration, giving better "onergy focussing" and{67 \
"energy stability" . Also no central "magnetic hill" is
required for focussing and accurate isochronism can be maintained
all the way from the stripper foil.(67)
Calculations at MSU show that the fractional energyo -4
spread for a 2 bunch length is near the ideal 1.4 x 10 witho
rms phase deviation during acceleration of ^ 3 - corresponding4
to local deviations from isochronism of 1 in 10 . With the
proposed 3 bunch length, a fractional energy spread of 4 x 10
can be achieved with comparable magnetic field errors.
Magnetic field level (or frequency) changes of ± 10 ppm-4
at MSU result in an energy variation of «* 0.3 x 10 . Frequency
stability can easily be much better so the allowed magnetic fiol •:
level stability could be ± 30 ppm, i.e. contributing - 1 x 10
to the energy spread.
The accelerating voltage should have a stability of
-v- 1 in 10 - this level of stability has been achieved in a much
larger system at TRIUMF.
- 152 -
4. NUCLEAR RESEARCH EQUIPMENT
4,a) Beam Transport (C.R.J. Hoffmann)
The layout of the beam transport system is determined by
the relative locations of the target rooms, the cyclotron and
the MP Tandem in conjunction with the following considerations:
1. disruption of Tandem operation and experiments should
be a minimum during construction of the new facility.
2. the new beam line system must be able to transport both
the Tandem and the cyclotron output beam to the new target rooms.
3. a given beam line should not pass through one new target
room to get to another.
The layout shown in Fig. 61, is used for this study. The
reference beam parameters are given in Table XVI.
TABLE XVI
REFERENCE BEAM PARAMETERS
Horizontal half width (cm)
Horizontal half angle (mrad)
Vertical half width (cm)
Vertical half angle (mrad)
Momen turn (GeV/c)
One half the momentum spread (1%)
*for U+ at 8.5 MeV/u.
The reference Tandem beam of maximum magnetic rigidity - 3Tm+8
(u ,0.5 MeV/u) is assumed to form a double waist at slit S-3 on
the +33 beam line of the existing beam transport system. The
reference cyclotron output beam has a double waist at A and for
Tandem atslit S-3
0.13
3.0
0.076
5.0
7.3
0.02
Cyclotronat A
0.14
0.88
0.044
2.3
30*
0.02
T A R G E T S C O M 5TARGET ROOM
TARGET ROOM6
IMAGE SLITTANDEM
OBJECTSLIT
Fig. 61: Target room and beam transport .layout. "B" indicatesa dipole magnet; the remaining units .ire qwa<Jrupol «_»s.
- 154 -
convenience is assigned the same momentum per unit charge as
the Tandem reference output beam (which is a aood approximation
for !' with specific energy 8.5 MeV/u. if" ions of 10 MeV/u
can be accommodated by increasing the field strengths in the
various beam line elements by roughly 8%). Double waists are
formed at the targets for both beams. Tailoring will be required
to meet specific experimental requirements.
Each quadrupole has an aperture of 5 cm and an effective
length of 28.1 cm. The effective drift distance between the
quadrupoles forming the doublets and triplet is 12 cm. The pole
tip magnetic fields required are all less than 0.43T for the
reference beams, except for the central element of the triplet
which needs 0.51T.
The bending angles for the dipole magnets are given in
Table XVII. The magnets are the window frame type and
operate at 1.5T (bending radius 2.0 m) for the reference beams.
The vertical aperture for each is 5 cm. The beams enter and
exit normally i.e. no pole face rotation is used.
TABLE XVII
BENDING ANGLES FOR THE DIPOLE MAGNETS
Magnet Angle (degrees)
Bl 38
B2 19
B3 47
B4 76
B5 26
B6 33
B7 25
:ing 90 analyzing magnet at t
not capable of bending the beams of high mass-energy product
The existing 90 analyzing magnet at the Tandem output is
- 1 5 b -
( 4 rj0) w h i c h a r c e n v i s a g e d f o r t h e c y c l o t r o n i n a t . It c a n
>.•iiii'Jie a m a x i m u m m r t s s - e n e r a y p r o d u c t o*~ 1 7 5 . /"(">"<=r""''.cr.t 1 ••. •'•
analyzing magnet, must be replaced. it appears that the sw:-_c".,r.-
magnet SMl of the existing transport system can bend the mere
rigid beams through 33 into the new transport system.
A plausible replacement system for the 90 analyzing magnet
is shown in Fig. 62. The system consists of a symmetrical
arrangement of two 4 5 bending magnets (window frame type
operating at 1.5T for the Tandem reference beam) separated by
a symmetrical triplet. A doublet is used after SMl in produci.na
the double waist assumed at slit S-3. The existing triplet
immediately before SMl is retained and operated as a doublet.
The replacement system should allow acceptable transport of
beams having a mass-energy product ^ 450 to target rooms 1, 2
and 3 of the existing facility.
All of the beam lines in Figs. 61 and 62 except those
shown dotted, have been analyzed using the TRANSPORT program- '.
The properties and costs for the dotted portions have been
inferred from those of the rest of the system. The bendingo o
angles shown dotted in target rooms 4, 5 and 6 are 15 , =25
and = 30 respectively.
Calculated emittance values for the Tandem beam using the
computer code OPTIC (A.B. McDonald, memorandum, July 197 3)
show that the acceptance of the transport system ( 4 ' mm mrad)
is adequate. In particular, for U+ , 13 MV terminal voltage2
and a 0.3 ug/cm nitrogen gas stripper the emittance is 1.85- mm
mrad. The estimated maximum emittance of the cyclotron output
beam is 2ir mm-mrad. Although a value of ,••• mm-mrad was used ior
the preliminary design of the segments from location A to i•'.<
13IMAGE ISLIT
IOBJECTSLIT
Fig. 62: A possible replacement system for the 90 analyzingmagnet.
- 157 -
and A to the QD spectrometer (which are traversed by the cyclo-
tron beam only), these segments should be readily adjusted t or *.:.•
larger emittance, and the concept given here is sufficiently
explicit for the present purpose.
information from detailed orbit dynamics calculations
is needed to complete the beam transport system into and out of
the cyclotron. Subsequent adjustments to the rest of the systerr
may be required.
4 (b) Research Equipment (H.R. Andrews)
The research equipment and experimental techniques developed
for the MP Tandem will also prove applicable to the proposed
high energy heavy ion facility. The major pieces of apparatus to
be used with the new accelerator include the QD particle
spectrometer, an on-line isotope separator (ISOL), the
Lotus gamma ray angular correlation goniometer, the gas target,
a large scattering chamber, and the on-line 7-gap orange beta ray
spectrometer. There will also be target positions for angular
correlation . :. -idiations and other, as yet unspecified, uses.
?ro;-4
The QD magnetic spectrometer is a high resolution broad
range instrument with an energy resolution AE/E of 2 x 10
a solid angle of 15 msr, an energy range E /E . of 1.5, and
a maximum analyzable energy of 156.3 q /M MeV. The spectrometero o
can be operated in an angular range from 0 to 160 to the beam
direction. This instrument is particularly useful for heavy ion
reactions because its design permits kinematic compensation of
dE/d^ over the large available solid angle. The focal plane will
be fitted with a multiwire proportional counter system which cin
- 158 -
detect. low rates in one part of the plane in the presence of
I.;rqe nnckground rates elsewhere.
An important aspect of heavy ion reactions is the
identification of the detected particle. Since all particles2
with the same value of ME/q are focussed at the same place in the
focal plane of the QD , further information is necessary.
An energy measurement by the focal plane detector will determine
the ratio q"/M. The remaining information can be obtained from
the time of flight through the spectrometer. This can be measured
by delayed coincidence with a scattered particle in the target
chamber or by using the pulse structure of the accelerator beam.
Because of the variation of path length with various trajectories,
the time resolution is proportional to the solid angle. Thus
the best resolution requires a smaller solid angle or a two
counter system to determine the direction of arrival at the focal
plane. For energetic heavy ions a solid state counter telescope
can unambiguously determine Z, and M can ba deduced from the
flight time data.
On-line computer control and data handling will be important
in the operation of the QD spectrometer. The interfacing between
the spectrometer and the linked PBP-1, PDP-10, CDC-6600 computing
system will be ready when the QD becomes operational in 1974.
Charged particle reactions will also be studied in a
scattering chamber with solid state counter telescopes used
for particle identification and energy measurements. A 19 inch
chamber, presently in use with the Tandem, has top and bottom
independently rotatable detector positions, an eight position
target turret and a vacuum lock for transferring targets from
the preparation area to the chamber. The present chamber could
- 159 -
be used for most experiments with the new accelerator although
a larger chamber with computer control of detector positions
would be advantageous in many cases. At 10 Mev/u the counter
telescope can determine the nuclear charge for all ions. com-
bining the AE, E data with time of flight will allow a mass
determination up to about 60 amu. Discrimination between higher
masses requires longer flight paths so that the QD becomes
the instrument of choice.
The other major facility which will be installed in 1974
for use with the MP Tandem and later with a heavy ion facility
is an on-line isotope separator (ISOL). Such an instrument is
used for studying the decay of nuclei far from the region of
beta stability. Although some degree of selectivity in production
is possible with the proper reaction, the rarer species will
.always be swamped by unwanted activities. The ISOL system
allows the mass separation of the activities at speeds which
permit the study of the fastest beta decays. The recoiling
radioactive nuclei stop in He gas following the reaction and a
gas flow transports the activity to the separator ion source.
A limited amount of chemical separation is possible during this
transport through the use of various traps. The proposed separatoi
will occupy one of the new target areas with heavy shielding
between the target-ion source area and the counting area. The
separator will also be used off-line in isotope separation and
target preparation. Finally, beams of separated short-lived
isotopes can be used in more elaborate optical pumping or
atomic beam experiments for the determination of spins, nuclotr
magnetic dipole and electric quadrupole moments.
- 160 -
Gamma ray measurements are presently made with a selection
or high resolution Ge(Li) detectors and various Nal(T-t)
scintillation counters. Angular correlation measurements are
made on two relatively simple correlation tables as well as with
the existing 'Lotus1 goniometer. The latter is a precision
instrument which positions up to six Nal (T-t) or Ge(Li) detectors
on a spherical surface around the target. The target chamber
can contain several particle detectors for particle-gamma
coincidences. This type of gamma ray measurement will continue
to be important in heavy ion experiments. The usefulness of
t_he recoil distance Doppler shift and the Doppler shift
attenuation methods will be enhanced with the larger recoil
velocities available in energetic heavy ion collisions.
The seven-gap orange spectrometer is a large solid angle,
medium resolution instrument ideal for in-beam study of
conversion electrons. This device is currently operated with a
momentum resolution of 0.5% and a transmission of 0.4 steradians.
ir.t- resolution can be relaxed to 2% with an increased transmission— PI
: 1.4 steradlans. l.'siny a pulsed beam, lifetimes ^ 10 w second
,ii..t delayed activities can be measured.
The last major facility to be mentioned is the differentially
;s :r;pe1 wmuowless gas target. This permits the study of inverse
:•.• xtions on gaseous targets which avoid straggling in entrance
•.:. i exit windows and generally enjoy a greatly reduced background.
\-..i'r. higher energy heavy ions the use of inverse reactions c .i
:.•• .jrc.it iy extended with both gas and solid targets. The latter
••.'.:.. .'» 1 o-..; reco:. i distance lifetime measurements in situations
v:.i..-<.- ' :• :.rii-st.T,'iv used reaction produces too small a recoil
- 161 -
The nuclear research area has been planned wi tk :Vi.r
guiding principles:
1. That all target rooms be accessible for set up
purposes when not in use for a running experiment.
2. That the MP Tandem beam as well as the cyclotron
beam be available at all target locations.
3. That the QD magnet, now installed in Target i • •< >x\\ ,
remain there.
4. That disruption of normal Tandem operation di/rin.:
the building and commissioning of the cyclotron bo
minimized.
Figure 1 shows the proposed layout for the new target rooms
and experimental equipment. Target room 1 will be largely used
lor beam transport with the possibility of one or two largo!
positions in the south end for Tandem experiments. si.nil,-.r
considerations apply to Targ2t room 2. Target room 3 is presently
occupied by the QD spectrometer and the orange beta ray s]iw!r'!-
meter. The latter, now located in the west corner, will )..•<•
moved because of the large bending angle required to reach .•
with the cyclotron beam.
Target room 4 will be devoted to the proposed on-line isotope
separator and its associated equipment. As shown, heavy :sh ie.1 d in<?
will be required between the ion source region and the count in:
area. Three secondary beazn transport lines for the sep.ir.i'ci
isotopes are planned as shown. Two bean lines are provided '<>
the separator - o n e for a d i r e c t b o m b a r d m e n t ion source sn<i !.n>-
for a g a s t r a n s p o r t system w h i c h will c a r r y a c t i v i t i e s •>: t >-. <-r '<
the s e p a r a t o r ion source or di r e c t l y to the shielded c-^w. ::; • ire,
as in the present gas transport system.
- 162 -
It is proposed to use Target room 5 for relatively high
background irradiations and particle counting experiments. The
orange spectrometer would also be located here. Two scattering
chambers are shown schematically on two of the beam lines. Target
room 6 will be reserved for low background experiments usually
involving gamma ray measurements. The regions shown correspond
to the approximate areas presently occupied by the gas target
system, the Lotus goniometer, and two simple angular correlation
tables. Target rooms 5 and 6 are planned to provide ample space
for equipment displaced from the old target areas as well as
plenty of room for setting up new apparatus as required in the
future.
- 163 -
5. BUILDING AND SERVICES
(K.K. Elliott, D.G. Logan, H.M. Philippi, j. Fisher)
5(a) Site
Figure 1 shows the superconducting cyclotron, beam transport
and new target rooms beside the Building 137 Tandem facility.
Two conditions as follows determined this site for the cyclotron
and new experimental area:
1. The existing Tandem Van de Graaff accelerator shall : <•
the heavy ion injector.
2. The Tandem shall provide particle beams to the
existing target rooms during construction to minimize
the interruption of the nuclear physics experimental
programs.
These conditions preclude the relocation of the Tandem to
a more favourable site for the enlarged facility. The three
Target Rooms 4, 5 and 6 are sited to avoid the underground st.ro. IP
to the west, where foundations for the heavy shielding walls tnl
beam dumps would tend to be unstable and expensive. This layou*
limits space available for the service room and makes vehicle
access difficult.
The proposed site is freely accessible to water lines and
electrical power feeders, to satisfy the increased demand for
cooling water and power.
5(b) Accelerator Room
The accelerator room is located between the old and new
machine rooms, beside Target Room (TR) 1 to make both Tan'lc-r •.;.•:
cyclotron beams available to TR-4, 5 and-6. The superconduc' ::;••
cyclotron, within its magnetic shielding, is located in a shiei •)<
- 164 -
room approximately 33 by 33 feet in area and 33 feet high to allow
the beam chamber and radiofrequency feed and tuning structure to
be removed intact from the top of the magnetic shield. The walls
and ceiling are constructed from four feet and l| feet of
ordinary concrete respectively.
Adequate space is provided between the cyclotron, magnetic
shield and radiation shielding walls for installation of beam
pipes and services and for later maintenance. A service tunnel
under the magnetic shield and 15 feet of headroom between the
magnetic shield and overhead crane provides adequate access below
and above the cyclotron. Access for equipment, service lines and
personnel between cyclotron and service room is available at four
service room levels.
5(c) Experimental Areas
Three new target areas as follows are shown in Fig. 1.
TR-4 for the isotope separatorArea 30 ft x 60 ft
TR-5 for high background experimentsArea 40 ft x 40 ft
TR-6 for low background gamma ray countingArea 60 ft x 40 ft
Heavier shielding will be added to TR-3 so that its QD
particle spectrometer, for study of heavy ion reactions, may be
used with cyclotron beams.
Access doors between the experimental rooms and other areas
are dictated by the following considerations.
1. There shall be outside floor level access to each of
TR-4, -5 and -6.
2. There shall be inside floor level access through TR-3
and the Tandem machine room so that the rooms not in
- 16 5 -
use can be reached easily regardless oi r.uii.it i .>n
fields in other areas or rooms.
3. There shall be doors from the ocam transport roora ..'.'.
TR-5 and doors between TR 2-4, 4-5, and 5-0.
4. Many of these doors will require a maze structure :ui
radiation shielding.
Radiation shielding walls and ceilings \.re determine"! i
the following considerations.
1. Thicknesses are based on one ^amp of 50 MeV protons.
This is believed to be a worst case calculation lor
50 MeV/u heavier ions such as lithium. Experiments
are planned to test this hypothesis on the Tandem it
the highest available energies.
2. The shielding is planned so that set-up work will be
possible in any rooms not then receiving the accoler.it or
beam.
3. Beam stops will be necessary in the wall for each
beam line. Approximate dimensions may be scaled 1 rorri
Fig. 1.
4. The possibility of an outside restricted area should be
considered which could be invoked for the highest back-
ground experiments. Most experiments are not expected
to cause radiation problems.
5. The highest fields will be generated in the ISOL region
of TR-4 and in TR-5. TR-6 will be reserved for lower
background experiments.
6. In the case of high energy light particle be,tms, 1 he
neutron radiation is peaked forward so that the shiel-l. r
requirements at 60 - 90 are about 30% lower »th..iri
at 0° - 30°.
- 166 -
7. Roofs to be ii-foot regular concrete to reduce
neutron shy shine.
8. The wall thicknesses can be scaled on Fig. 1.
5(d) Be am Transport Areas
The low and high energy beam transport systems will occupy
portions of existing TR-1 and TR-2 and the old machine room. The
beam tunnel in the old machine room is approximately 85 ft x
22 ft wide and 20 ft high.
Ordinary concrete shielding between the beam tunnel and
TR-4 and TR-5 will be added to satisfy the access and radiation
shielding requirements described in 5(c) above.
5(e) Service Areas
The Tandem accelerator sulphur hexafluoride insulating gas
system is located in the old machine room and in a separate
building on the proposed site for TR-4 and TR-5. This proposal
assumes it will have been relocated to a new building addition,
45 ft x 25 ft in area, on the north east side of the new machine
room conveniently beside the Tandem to make piping connections.
The new monitoring and control computer and remote
instrumentation for the superconducting cyclotron will be
housed in a 50 ft x 20 ft addition on the north end of the present
Tandem control room. A new personnel passage will connect the
control room, SF& handling and storage area and the ion source
end of the Tandem machine room.
Most of the cyclotron auxiliary systems and services will be
located in a 'four floor service room beside the cyclotron room as
shown in Figs. 1 and 63. The inside dimensions of the top,
- 167 -
mezzanine and main floors are 32 ft x 34 ft whil • the basoiv"
dimensions are 32 ft x 28 ft to avoid undermining the ox.st •r .
foundations under the old machine room wall. The walls are
constructed from ordinary concrete shielding of sufficient
thickness to allow personnel in the service room while adjacent
areas are being used with a cyclotron beam.
Equipment and personnel can enter the service room v a a
10 ft x 20 ft high opening to the unloading area. Largo finer
hatches just inside this opening allow equipment to be placed
on any floor. A monorail hoist and 10 ft x 10 ft opening in ;he
shielding wall on the top floor allows large equipment to bo nov
through the service room to the cyclotron room. Five ft x 8 '"'.
high openings to the cyclotron room at the three lower service'
room levels are provided for entry of services and personnel.
Stairs are provided between all four levels o^ the service r" r>.
The beam line ion pumps, ion pump power supplies, magnet
power supplies and other beam line services are located in the
beam tunnel close to the equipment they service. Similar service
for the beam transport and research equipment m the Target
Rooms are located inside the respective target rooms conveniently
close to the serviced equipment. sufficient remote instruments
and controls are located in the control room to provide
sufficient monitoring and control of these services when
radiation fields are present in the area they occupy.
5(f) Auxiliary Systems and Building Services
The cyclotron requires many auxiliary systems including
special power supplies, vacuum pumping, low temperature
refrigeration and centralized controls and instrumentation.
- 168 -
These auxiliary systems are described in Chapter 2. The cyclo-
tron and the building require other services including steam, raw,
tempered and deionized water, electrical power and lighting,
air handling equipment and radiation alarms. Section 7(c),
Equipment and Civil Costs, includes a work Breakdown Structure
which lists the main systems, auxiliary systems and important
components for the new Tandem-Cyclotron facility.
The more important cyclotron auxiliary systems and building
services are described below. Most of these systems are located
in the four floor service room immediately north west of the
cyclotron.
5(f)i Vacuum System
The cyclotron vacuum requirements and proposed system are
described in section 2(e)vii. A common superconducting coil
cryostat and beam chamber vacuum is proposed. Six diffusion
pumps with liquid JSL traps and valves, two mechanical roughing
pumps, an off gases gas holder and local control and instrument
panel compose the vacuum system. The diffusion pumps, traps and
valves are located in the cyclotron room close to the components
they evacuate. The roughing pumps, off gases gas holder and
controls are located in the service room as shown in Fig. 63.
5(f)i i Radiofrequency Power Supplies
The cyclotron rf power supply to the dees and the dc power
supplies for the extractor deflection plates are described in
Sections 2(e)ii and 2(e)v respectively. The main rf system
includes main amplifier, driver and low power amplifier with 6c
power supplies for each. This equipment will be located in the
Service Room iMezzanine as shown in Fig. 63 to be close to the
T™~
j
TOP M. :J J? Pi A N I L i V
c:::;
Li U__1
- 1 69 -
3
d LLEZ]
1 .:.- <j« Pv>tt\ I
I
U
r—i ! »1 LJ
1—li
i
'ig. 03: Cyclotron service roorr layout
- 170 -
cyclotron and the supply of electric power. The me.-. ::!,;.(.• • in,
will also be dry and free from excessive no i so oi ";i i '.;• : i; wi. c
would interfere witli operation or maintenance o' t no s<-ti.'-:' . /«
electronics find high frequency circuits.
rj ( f) i L i Sj^er^onduct_in2 Magnet Power^Sugpl ies
The proposed power supply system is csesorLoed in : ••••'. .1 •
iii. It contains separate dc power supplies tor the 'wo in-i.-:
coils connected in series and the two superconducting; trim <-;l;
also connected in series. Dump resistors are provider :or e.:*•:.
coil system. Ten small dc power supplies arc provide*: :r>: •.•it-
ten pairs of normal shim coils to tailor the raui.;I m i-nct..-
field profile. These power supplies and dump resistor: art-
located on the Service Roon mezzanine floor as shown ;:. i-'ur. > .
Here they will be close to the cyclotron ..nd .1 supply •• • t. ; ,r.v
electric power. The mezzanine should also be dry, .r«.i..--, ii, 1
relatively free of vibration which coi;ld .ifqr.de p. •; : o: ;. ;::>-. • .
5(f)iv Refrigeration System
The low temperature refrigeration syste;;. a. s -JUSI-I .;,, 1 .:.
Section 2(e)vi. it includes the liquid nitrogen ana h..• 1 -...:;;
storage tanks, low temperature helium refrigerator v/.tii c••!:,-
pressors, helium gas holder, low pressure heliuni a.ib .-.--, .fr,.] .,• ,
cooler and heater, five 250 psi helium gas s.orage t,in.'..•; .;.';
helium purification system. This equipment is loca-e-1 . r. - ••
Service Room as shown in Fig. 63. The two compressors ire ^ • .
in the basement to provide solid foundations for the rcc-ipr . • it
equipment and to isolate the noise ana any \'i;>rai ..r-r. to ;i•.;• .:
The 1000 cubic ft helium gas holder an'i tiie 40nf> i;'r.- 1 ;... :
- 171 -
nitrogen storage tank are 18 and 12 feet high respect .vi' \
project above the mezzanine floor. The liquid helium '..ank . :•
located close to the cyclotron room wall to minimize the lcr>;v
of insulated piping between -*yclotron and tank. The i ive ta;i'-.:-
for pressurized helium gas storage are located on the :our*'
floor and roof.
1 > ( f) v centralized contro 1 and Iiistrument a110n
This system is described in Section 2 f H . The noni'.o'-ir.!
and control computer will be installed in the control roor. • I'- '
described in the preceding Section 5(e). The field sensors,
remote operators and wiring will be installed throughout the
Tandem, cyclotron, beam transport system and experimental aroa;-
as required.
5(f)vi Access Roads and Tunnels
Three new access roads will provide vehicle access 'rorr ->•
existing road to the three new target rooms, TR-4, T> and 4>.
These roads become tunnels as the roads approach the target. ro<>n>s
through the earth shielding. A new branch from the sane rn-id
will provide vehicle access to the unloading area beside th>-
Tandem machine room. Service Room and TR-6.
S(f)vi i Mechanical Services
Crane beams, spanning each new area, will be provide: w'i.
three-and five-ton electric hoists and wL«_h manual beam and
trolley travel.
Make-up air will be distributed by ducts from the sup;>i \
fan on the top floor of the Service Room. Heating and c ' ' n.-
in each area will be by fan coils. Each area wi]I )>v '•>:'*. ' ' ••<- • :
- 172 -
roof-mounted fans. A centrifugal chiller of about 200-ton capacity
will be located in the Service Room basement, chilled water and
steam or hot water will be piped from the service room to the
fan coil units, control room cooling and heating will be done
by a packaged air conditioner. Approximately 280 kw of heat will
be removed from cyclotron, beam transport, experimental equipment
and instrumentation by the ail conditioning and ventilation
system.
Process air will be supplied from Bldg. 137. Steam will be
supplied from the 4-inch main on the west side of Bldg. 114.
Condensate will be discharged to the drain. All areas will be
provided with fire protection sprinklers and will be connected
to the Plant Supervisory system.
5(f)viii Water Systems
Approximately 514 IGPM of raw water will be required to
satisfy the cooling water requirements for the cyclotron, beam
transport and auxiliary systems. Building service requirements will
increase this additional flow to approximately 1000 IGPM. This
flow may be supplied from the 10-inch diameter fire water line
in front of Bldg. 114. All cooling water will drain to the
front of Bldg. 114 and then south to the 6-foot culvert.
The cooling water flow to the cyclotron, beam transport and
auxiliary systems is pressure and temperature controlled,
constant supply pressure will ensure steady flows independent of
mains pressure which depends upon plant wide demands on the
process water system. Controlled water temperature, above the
dew point, will stabilize equipment temperatures under normal
operation and will avoid condensation and consequent insulation
- 173 -
lailure of magnet coils. The pressure and temperature contr * ;
water system includes two circulating pumps, a filled mixina
tank and a control valve across the pumps to regulate supply
pressure, A three-way valve on the mixing tank outlet controls
the percentage of flow recirculated and dumped to drain to
control supply water temperature. When the mains supply is
cold, most of the cooling water will be recirculated, while most
of the cooling water will be dumped to drain during late summer-
when the river water temperature is high.
A 70 IGPM closed recirculation deionized cooling water
system will be installed in the Service Room basement to cool
the radiofrequency power supplies and accelerating structure.
It will be cooled by chilled water.
The drinking water system will be supplied from the existing
building 114 system. The sanitary system may be expanded
slightly. Sewage lines may be connected to the existing 6-inch
line near Bldg. 115.
5(f)ix Electrical Services
The electrical demand will increase by 2000 kVA maximum
through the addition of the cyclotron and other new equipment.
Existing feeders and transformers are adequate to supply the low
energy beam transport, experimental equipment and miscellaneou
building services. A new 2400-volt 3 phase supply will be
required for the rectifier to the radiofrequency power system md
the new 200-ton water chiller. A new 1500 kVA, ' 30 volt, 3 phase
unit substation, for cyclotron auxiliary systems and high oner-ry
beam transport, will also be fed by the new 2400 volt, 1 ph..s<-
supply.
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The 2400-volt switchgear, 1500 kVA transformer and 600 volt
feeder breakers will be purchased in an outdoor metal-clad
enclosure, approximately 24 ft long, 54 in. deep and 90 in. high.
It will be installed outside TR-3 close to the overhead 2400 volt
feeders as shown on Fig. 1. Two 2400 volt feeders and six 600
volt feeders will be installed between the outdoor substation and
the power distribution centres and dependable AC power supplies
in the Service Room as shown in Fig. 63. The electrical system
includes circuit breaker panels, motor controllers and secondary
wiring to power consuming equipment.
New building additions will be supplied with fluorescent
lighting, fire alarms, high radiation alarms, telephones and
connection to the guard tour supervisory system.
- 17 5 -
6 . DESIGN AND DEVELOPMENT PROGRAM
6(a) Summary
A des ign and development program w i l l be needed t o p rov ide
detailed information for preparation of manufacturing drawings
and procurement specifications.
The concept does not invoke the development of new technology
or pushing known technology beyond established ximits; i t does
however propose a novel combination of technologies in the
cyclotron component of the system. Consequently a substantial
amount of theoretical, experimental and engineering conceptual
design work will be needed to establish a satisfactory and most
economical design. The design work will necessarily be an iterative
process involving interaction between engineering and accelerator
physics disciplines.
More conventional parts of the proposal will no doubt also
benefit by more detailed examination. However, most effort will
need to be concentrated on the superconducting cyclotron and its
associated systems. Apart from some verification of i ts perfor-
mance, l i t t l e work,beyond what is anticipated anyway, is
foreseen for the MP Tandem itself.
Isochronous cyclotron technology is well established. There
are 70 AVF cyclotrons listed in reference (49), 45 of which are
in the relevant 10-50 MeV energy range with respect to orbit
dynamics. There are only two with comparable Bp ratings, TRI'W
and SIN, both meson factories accelerating protons to 500 and
600 MeV. Limiting our specific energy to 50 MeV/u simplifies the
orbit dynamics so that the computer codes available for studies
of detailed beam behaviour should be able to cope easily. On
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ihe other hand, only one of the small cyclotrons (MSD) seems
to have achieved the beam quality we desire. The most troublesome
factors affecting beam quality are associated with the rf system
so that some major improvements may be required there. However,
at IRIUMF, close to the desired stability has been achieved in
a much larger rf system.
The major change from existing machines is the use of
superconducting coils. Relevant experience in coil design is in
the large bubble chamber magnets built at ANL, CERN and NAL. The
most recent at NAL, was completed in a period of two
years ending in 1972 at a cost of $2M. Our cyclotron magnet is
almost an order of magnitude smaller in terms of stored energy
but the requirements on field quality are much higher. The
mounts and cryostats are therefore more difficult to design.
However, adopting the NAL magnet as a pattern should reduce the
development to one of scaling.
The use of saturated iron pole pieces to provide the
flutter field is new. This has been modeled by current sheets
and theae experiments show that sufficient flutter can be
achieved. The current sheet model together with possible
supplementary calculations for edge effects is expected to provide
sufficient information for accurate definition of the pole shape.
Some field trimming will be required during final set up of the
magnet, consequently, early construction of the main cyclotron
coil system is indicated so that detailed field mapping can
be used to determine the final design of the flutter poles.
Because of the high field, leading to a relatively small
device, the cyclotron will be congested and V3 anticipate several
rounds of detailed mechanical conceptual design and accelerator
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physics studies before an optimum configuration is found.
may become appropriate to lower the field somewhat but it is
unlikely an optimum will be found at higher fields because of
limitations in the superconducting technology. Discussion ot
specific items in more detail follows.
6(b) Accelerator Physics - cyclotron (C.H. Westcott)
Work involving detailed orbit-dynamics c-alculations as has
been found for similar projects is likely to be extensive and
include -
i. Iterative procedure; computations alternating with mag-
netic field measurements for actual fields produced
from superconducting coils with iron blocks to produce
the flutter, or simulations thereof,
ii. Study of changes of (i) when mean field is reduced for
lighter ions,
iii. Study of injection for different stripping charge
ratios in actual fields,
iv. Optimization of (i), (ii), (iii) above for flexibility
in ion types.
v. Extraction; this is expected to be a major problem but
a tractable one with suitable computational techniques.
vi. Exploration of other changes, e.g. larger sizes, lower
fields and possible optimization in respect of the::.<••.
vi i . Misalignment effects, magnetic hysteresis,
v i i i . Bunching requirements, energy spread resulting.
ix. Possible rf third harmonic for improving &E.
x. Misalignment of rf electrodes, phasing errors.
xi. Emittance effects generally, checking vertical aperture
- 17 8 -
xii. Extracted beams; fringe field effects, use of
quadrupoles for beam transport; various ions,
xiii. Tracking ions of undesired charge state.
xiv. Other calculations to devise and support diagnostic
methods and for planning of the commissioning stages,
xv. Although the beam intensities to be used are small,
space charge calculations should be made - they will
probably verify that the effects concerned can be
neglected.
Particular attention will be required to factors affecting
the beam energy spread after acceleration.
Computer programs are available from other laboratories
where similar work has been done. The first two groups and number 6
have already been adapted and run (subject to some verification)
on the Chalk River CDC-6600 computer; the others require such
adaptation or are already partly so adapted. From TRIUMF, the
programs of prime interest are:
1. REPLOT + POLICY + CYCLOP
The first two treat magnetic field values (measured or
estimated) so as to be suitable for use by CYCLOP or GOBLIN -
this includes analysis into harmonics and may also adjust
values to give isochronism. CYCLOP has more sophisticated
isochronization routines and locates closed orbits.
2. GOBLIN is a Runge-Kutta particle tracking routine, and
can track backwards or forwards; acceleration is localized
(whenever a plane is crossed). For a 2-or 4- dee system
modification will be necessary to use this feature.
3. TRIWHEEL (see PINWHEEL below). Differs from GOBLIN in
that the electrostatic accelerating field is stated in three
dimensions.
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From i.uii, Berkeley, Calif, are -
4. £INWHEEL like TRIWHEEL but with a loss ex.ic'-, two-
dimensional accelerating field description. This is a
CDC 6600 "RUN" code.
•3. G0C-3D is similar to GOBLIN, also written for a '•G'"\
like GOBLIN is based on the original report by ^o: don in.:
Welton . The magnetic field input can be 2-dir-iens i on.:]
with median plane symmetry, or 3-dimensional, and for .sm :
displacements a first order transfer matrix method is
available as well as Runge-Kutta tracing.
Also from MSU, Michigan -
6. SYMEON an improved equilibrium orbit program requiring
smoothed magnetic field data as input.
7. INDIGO an accelerated orbit tracincr code.
6(c) Magnet Development (H.R. Schneider)
6(c)i Field Shape Studies
The various orbit dynamic programs used to determine be<;m
behaviour from injection to extraction require an accurate map
of the cyclotron magnetic field. For the superconducting cyr'i.-
tron, this can be generated in a semi-empirical way. The main
coil and trim coils are concentric, and their fields easily
calculated using a computer program such as MAGTWO ' . The f]>•'
pole field which is superimposed on the coil field can be oijt <..n<-
from measurements on a current sheet model.
Since the current sheet model is an analogue of uniiorr-Iy
magnetized poles, and this is not so at the edges, it is
possible that correction for edge effects may b- necessary. < n
- 180 -
way to estimate the correction is through the use of the
computer program TRIM , which is widely used for calculation
of fields in iron magnets. This is basically a two-dimensional
program so it cannot calculate the field due to complex flutter
poles shapes. It can however be used to calculate the field
between poles of simpler shape, e.g. either rectangular or
circular, and this can be compared with the equivalent current
sheet calculation to determine corrections due to non-uniform
magnetization at the edges. similar corrections can then be
applied to the current sheet model measurements.
It is expected that the field so determined will be adequate
for beam dynamics calculations during the design stage. Of
course, as is usual in cyclotron construction, final field
plotting in the full size magnet will also be required to
determine the final field shimming.
As part of the design and development program, then, an
accurate current sheet model will be required along with an
automatic field mapping system. Proper design of the model would
allow the pole shape to be changed easily and hence permit
investigation of the focussing properties of a variety of pole
shapes.
Computer programs to combine the field measurements with the
coil calculations must be written so that fields of various coil
configurations and pole shapes can be determined easily, for use
in the appropriate beam dynamics codes.
6(c)ii Test Magnet
The construction of a magnet model is usually necessary in
designing an iron cyclotron magnet, since field calculations in
the iron cannot be made with sufficient precision. The super-
- 181 -
conducting case is somewhat different, since the coil fields
can be calculated accurately and a current-sheet model provides •-
adequate analogue of the flutter poles. Nevertheless construction
of a small superconducting test magnet does have merit. in
particular if it is wound with the conductor chosen for the full
size magnet, it would allow checking of the heat transfer
calculations and determine whether full stabilization can be
achieved at all operating points. Checks of residual magnet-
ization in the coil could also be made as a function of rate
of change of the magnetic field, in addition forces on the coils
due to the magnetic shield (if one is used) can be measured,
as can the effect of the shield on the midplane field.
For a 1.1. of these measurements a model about 1/4 full scale
(-v 60 cm o.d.) would probably be adequate.
6(c)iii Superconductor
The demand for superconductor is too small for industry to
have evolved standard sizes and shapes. Consequently each large
magnet that is built has the conductor custom fabricated. This
allows the greatest freedom in design of a particular coil but
it also means that some development and testing is inevitably
required to optimize fabrication methods and check any chosen
conductor design. Such work is best done through a development
contract with a superconductor manufacturer.
An independent checking of the H-I characteristic for
possible conductor designs should also be made. This could be
done under contract with a laboratory that has adequate high field
magnets, or facilities could be set up at CRNL. For the latter
option, the major apparatus required is a cryostat and a high
- 182 -
field solenoid with a field in excess of 6T over length of at
least 30 cm.
6(c)iv Magnetic Field stability
The achievement of a magnetic field stability of 1 in 10
depends on three factors? current stability, mechanical stability
and cryostatic stability. Current stability of 1 in 10 is a
realistic objective for a dc superconducting magnet. Following(29)
the practice for the ANL bubble chamber magnet control ', the
short-term current errors can be Kept within 4 in 10 and the
long-term errors within 15 parts in 10 . The very long time
constant ( 10 s) eases the control problem. The design proposed
here should have great mechanical and cryostatic stability, in
view of the fact that the stability requirement exceeds that of4
bubble chamber users (1 in 10 ) it would be desirable to
carry out stability measurements on an existing large magnet
such as the one at NAL.
6(d) Rf Modelling (c.B. Bigham)
Since the acceleration in a cyclotron comes solely from the
rf system, the energy spread in the beam must come from some
variation in the relationship of the beam to the rf.
There are three effects.
(a) time fluctuations in the amplitude of the dee voltage,
i.e. "rf ripple".
(b) time variation in the gap crossings of the particles
i.e. phase width.
and (c) time fluctuations in rf frequency or magnetic field which
produce a phase shift of the beam relative to the rf.
- 183 -
The "phase width" will be determined by the bunch width
the injected beam but (a) and (c) depend on engineering of the
rf and magnet. Stabilization of the magnetic field level to
1 in 10 is not difficult for an iron magnet and is expected
to be no more difficult for a superconducting magnet. The rf
frequency can be controlled to 1 in 10 using a master
oscillator - power amplifier system . This leaves stabilis-
ation of the rf voltage as the most difficult development problem.4
At TRIUMF, amplifier stability of 1 in 10 has been achieved and
feedback from the beam energy analyzer is expected to give
1 in 10 at a single frequency, in our case there is the
added requirement of obtaining the desired stability over a
broad frequency range.
Small cold test models (1/10 scale) can be used to determine
tuning ranges. A larger ( or i scale) model would be required
for reasonable field strength measurements. However, since the
main development will be in obtaining high stability, the full-
scale high power system should be built early in the program.
The amplifier can probably be purchased. The problem will be
to develop the complete system including the accelerating
structure, amplifier, cooling systems and control circuits to the
required level of stability over the required tuning range.
Full-scale models of the harmonic buncher cavities will be
made. Tuning and phasing controls will have to be developed
to provide tracking of the two cavities with the cyclotron
oscillator at all frequencies.
- 184 -
6(e) computer Control Techniques (R.L. Graham, J.S. Geiger,J.C.D. Milton)
The computer control system for the superconducting
cyclotron is envisaged as evolving naturally out of the system
now being developed for the MP Tandem accelerator. in this
section we discuss the present system and its design and
construction period of the cyclotron; some of the additional
computing facilities described in Section 2(f) would be
integrated into the existing system and we would begin to gain
experience in their use. Upon its installation the cyclotron
would therefore find a control system many aspects of which had
been tested and were already in use.
6(e)i Existing computer Facilities
The Tandem accelerator monitor and control system is
based on the very powerful computing facilities already situated
at the Tandem accelerator. The extent of these facilities is
illustrated in Fig. 64. Briefly they consist of an 80K (36 bits)
PDP 10 and a 24 K (18 bits) PDP 1. The PDP 10 is a remote
terminal of the computing centre's CDC 6600 and can submit jobs
to and receive results from that computing system. The PDP 10
system is backed up by a 1/2 million word fixed head disc and
a 10 million word disc pack. For the purposes of accelerator
control, the features to note are the 8 digital and 8 analogue
output buffers on the PDP 1, and the 128 point programmable process
scanner and special device controller on the PDP 10. This latter
is able to control, and receive information from, a very large
number of low speed devices. The limit would be set by the flow
of information on the I/O bus.
- 185 -
I FORKICKSOBTEP
SCALE«SSTORAGE SCOPE
0 0 ' SPECT.I ADC
l« IN JZ OUT
Fig. 64: Tandem-cyclotron computer system.
- 186 -
Both the PDP 1 and PDP 10 have powerful priority interrupt
systems. They both thus have time sharing capabilities. With
the PDP 1 this is essentially single user time sharing. The PDP 10
however runs under one of the most sophisticated and reliable
multi-user time sharing monitors in existence. In addition to the
usual capabilities of such monitors, it has real time capabilities.
It also supports high level languages such as FORTRAN, ALGOL and
BLISS as well as MACRO, a flexible assembly language. BLISS is a
high level language particularly well suited to easy writing of
efficient service routines and other monitor interface routines.
Furthermore, several simulation programs, such as MIMIC for example,
exist for simulating mini-computer systems on the PDP 10. These
allow not only creating, editing and assembling of programs but
extensive debugging as well. Programs for the mini-computer can
thus be developed and largely debugged before the mini-computer
is delivered, or while it is on-line with the accelerator.
6(e)ii Computer Control of Beam-Transport system
The beam transport system was designed with computer cor.wrol
in mind; the power supplies for the analyzing magnet, switching
magnets and quadrupole lenses are referenced to digitally
controlled voltage supplies, computer control of these components
was completed in 1968 and first used in yield curve measurements
inl969 ( 7 2 ).
6 (e)iii computer control of the Tandem Accelerator
1. Present Status
The MP Tandem Accelerator, as purchased from High Voltage
Engineering Inc., was not designed for computer control. To
facilitate such control, the ion-source power supplies are being
- 187 -
replaced with highly stabilized, remotely programmable units.
Digital conunun ic at ion for both monitoring and control of
ion-source functions goes via a pulsed light-pipe system to the
ion-source assembly which operates at potentials up to 300 kv.
The data-monitoring facility now allows the PDP-10, through the
Special Device controller, to log individually, or sequentially
at 10 kHz, 32 parameters from the extraction deck with 12-bit
resolution. The design presupposes an additional 32 twelve-bit
parameters from the main deck as well but these are not yet
installed.
The ion-source control system is not multiplexed; each
parameter has a dedicated 4 channel light pipe assembly. Six
channels were installed to the extraction deck during 197 2 and
many additional channels to both decks could be added in the
future when needed. At the present time these control functions
are set manually.
Provision now exists for monitoring up to 128 parameters at
ground potential using a multiplexed ADC. At the present time
about twenty functions are being monitored. These include, low-
and high-energy vacuum, column currents, up charge, corona current
and tank pressure. The transfer of monitored data to the PDP-10
computer memory is done by a 128 point process scanner which
places the information directly into memory through the DMlO.
software is now available for the following functions;
(i) maintaining and updating a disc file of monitored
parameter values.
(ii) the time-history of one or more parameters can be
plotted on a display CRT. The operator can obtain
the digital value and measurement time of any point
in this plot through use of the light pen.
- 188 -
(iii) via teletype input the operator may set upper and
lower limits on any of the monitored parameters. If,
at any time, this parameter strays outside the
permissible range, its name, value and corresponding
limit values are displayed on a storage scope on the
operator' s console.
(iv) the monitored data is, of course, also available
on the usual output devices (teletype, plotter,
magnetic tape, DEC tape and computer links) .
2. Development Program
As noted above, the routine expansion of the data logging
facility to the main deck of the ion-source is planned. Attention
is now centred on the computer-operator interface and
communication to the high voltage terminal. Effort on the former
will likely revolve around a new keyboard-display tenninal with a
hard copy option.
we are just beginning to attack the problems associated
with communicating between the computer and the high voltage
terminal of the Tandem accelerator. Experience over the past two
years with the terminal "wobbler" has shown that solid state
electronics in the terminal is highly vulnerable to electrical
discharges (breakdowns). The first communication link to the
computer is now under test; it consists of an analogue light source
in the terminal, a single fibre-optic light pipe and a sensor
at ground potential. It will be used to monitor the voltage and
frequency of the alternator in the terminal, when perfected,
six such channels will be implemented (3 up and 3 down) and more
are envisaged.
- 189 -
With the completion of the systems just described the
PDP-10 computer will be able to monitor essentially all functions
of the Tandem accelerator apart from beam quality. The next
phase of development must be to gather experience in how to use
this monitored information most effectively in improving machine
performance. Several professional man years of thought and
program development are needed for this goal to be achieved,
i.e. the goals outlined in Sec. 2(f) (data monitoring). Sorce
additional software development and control hardware will be
needed to realize the level of operation assistance envisaged in
Sec. 2(f). In particular, computer-control will be needed for
such items as corona-point position, high- and low-energy steering
and machine quadrupoles.
Within two to three years we will be able to monitor all,
and set most, of the accelerator's functions by computer. it
may then be feasible to embark on the third category of
Sec. 2(f), closed-loop control. Progress in this area will only
follow a considerable improvement in our understanding of the
inter-relationships of the many adjustable machine parameters.
It should be noted nonetheless that if we had adequate programming
effort, we would be in a very strong position to enter this
field because of the computational power of the PDP-10 backed up
by the CDC 6600. Such power would be essential for the extensive
variational calculations required.
The development of a computer-based logging and control
system as described here would provide facilities on the scale
referred to in Sec. 2(f). The extension of the control system to
include the cyclotron and additional beam transport will
approximately double the required number of analogue and digital
- 190 -
channels. By that time the development of computer control
Hardware and software for the Tandem accelerator will be
sufficiently advanced to minimize the effort in extending the
system to include the cyclotron,
6 \ f) shielding
Measurements of neutron fluxes produced by the highest
energy light and heavy ion beams available from the upgraded
MP Tandem should be made prior to final design of the shielding.
6(g) cryogenics (J.A. Hulbert)
There are now sufficient tonnage-scale liquid helium cooled
magnets in operation that all problems associated with such
installations should have we11-documented solutions. However the
superconducting cyclotron has a number of non-trivial mechanical
differences from the NAL and ANL bubble chamber magnets. Not
the least of these are the necessarily tighter tolerances on
field distribution and stability, the more complex magneto-
mechanical coupling and general spatial complexity due to the
small size of the machine.
It is intended that full use shall be made of all available
expertise outside CRNL to advise on the detailed cryogenic design.
If possible that design will be contracted out to a company with
appropriate experience. Nevertheless the possibility does arise
that during design development, some tests may need to be
carried out at CRNL, particularly if, for example, some design
recommendation of the contractor needs independent experimental
con f irmat ion.
191 -
Typical areas in which experimental tests may be required
include:
Current leads.
Tank suspension.
Low temperature stressing and performance of structures.
Cooldown - thermal efficiency of coolant circulation.
Magnet winding strip short sample tests .
Most tests could probably be performed in a cryostat 1/2 metre
diameter and 2 metres high using liquid helium transported from
Montreal in 100 l i t re lots.
6(h) Mechanical and Civil Design (R.K. Elliott)
6(h)i cyclotron Magnet support and cryostat
The superconducting cyclotron concept includes two large
superconducting main coils and two smaller superconducting trim
coils as shown in Fig. 45. The 5 Tesla magnetic field at the
mid-plane generates large outward radial forces on each coil
and large axial forces tending to bring the two sets of coils
together. The coils must be supported to restrict conductor
movement to a very small amount to avoid sudden changes from
the superconducting to the normal conducting state.
The coils must be maintained at or below liquid helium
temperature (4.2 K) to keep them in the superconducting state.
Therefore, the coils must be contained in a cryostat to minimize
the heat input from nearby conventional equipment and the air
in the accelerator room. Heat gains to coils must be minimized
to keep the refrigeration system at a reasonable size and cost.
The large coil forces, small allowable conductor deflection,
low operating temperature and small allowable heat gain, pose
a unique engineering problem for the design of the coil support
system and low temperature cryostat. Although the requirements
suggest that a l l four coils be supported within a single
- 192 -
cryostat, this cryostat must be split to allow beam vacuum
chamber, rf accelerating structure, flutter poles and other
equipment to oe installed or changed at a later date.
Much engineering analysis and design of alternative layouts
will be required to propose a feasible, optimized design and
fabrication route and to generate reliable cost estimates and
schedules.
6(h)ii cyclotron vacuum Chamber
Layouts showing the vacuum chamber, inlet beam pipe, radio-
frequency acceleidLing structure, rf power transmission line,
electrostatic deflectors and outlet beam pipe will be necessary
to determine how to meld the physics, electrical and mechanical
requirements. They will establish mechanical feasibility and
help to assess relative merits of alternative machines with varia-
tions in important accelerator parameters and optimize the
accelerator design. The variable parameters should include
magnetic fields up to 5 Tesla, number of sectors and accelerating
gaps per revolution and alternative beam extraction systems.
Sufficient detailed design of the optimum layout should be
prepared to estimate reliably the fabrication and installation
costs.
6(h)iii Magnet Shield
The air-cored superconducting magnets, which produce a
central magnetic field of 5T, will also produce high stray fields
outside the cyclotron. For example, the 14 foot superconducting
magnets for the 15 foot NAL bubble chamber produces a 3T central
field and an estimated 5mT stray field on the central plane 12
metres from the centre of the bubble chamber .
It is considered that the radial fields just outside the
magnetic shield should be less than 4mT to avoid troublesome
- 193 -
effects on nearby research equipment and experiments. Section
2(h) Magnetic Shielding, gives calculations of the magnetic
field at various radii and angles from the magnetic axis, with
and without a spherical iron shield, since a spherical shield
is impractical to build and contain the cylindrical accelerator
geometry, the capital cost estimates in section 7(c) assume a
vertical cylindrical shield 2.5 metres inner radius, 3.6 metres
inner height and 0.5 metres thick. The shielding effectiveness
should be roughly equal to that for a sphere of 5.0 metres I.D.
and 6.0 metres O.D. For cyclotron output energies below 10 Mev/u,
the magnetic shield will also provide some degree of radiation
shielding.
Since the stray field beyond 6 metres radius is less than
20 mT, local magnetic shielding may be adequate for equipment
and beam lines. Therefore, shielding requirements should be
evaluated carefully to determine if the magnetic shield may be
eliminated entirely at the expense of increasing the cyclotron
room radiation shielding walls from 4 to 7 feet. If the magnetic
shield is required,a mathematical or experimental model of the
cylindrical geometry should be developed to predict shielding
factors and to optimize the shield design.
This magnetic shield is a large, heavy engineering structure
which dwarfs the cyclotron proper. Its cost will be a significant
fraction of machine cost. Many penetrations will be required
for rf power, magnet power, liquid helium cooling, water cooling,
vacuum lines and beam lines, and these penetrations must not
degrade the magnetic shielding properties significantly. Therefore,
the magnetic and radiation shielding and penetration requirements
should be studied carefully to specify a feasible an<? economical
magnetic shield.
- 194 -
The magnetic shield could be constructed from one thick
shield or two or more concentric shields and could be built
from rolled steel sheets or cast iron blocks- Other alternatives
may be possible to minimize the size or cost of the shield.
Alternative designs must be prepared and evaluated to optimize
the design of magnetic shield. Sufficient detail drawings of
the selected design must be prepared to generate reliable time
and cost estimates for fabrication and installation.
6(h)iv Engineering Support for Development
During the development phase, engineering problems would be
solved as they arise, some of the special engineering problems
were discussed in the paragraphs above. Prototype equipment would
be designed for testing. Preliminary specifications and
drawings of final machine concepts would be prepared.
6(h)v Buildings and Services
Service requirements for the accelerator and beam line
equipment must be studied more deeply than was possible for this
proposal. The services, including power supplies, cooling systems
and instrumentation, will occupy considerable space and will
represent a significant share of the total facility cost.
Further study of the beam transport and experimental room
requirements can be expected to suggest revised facility layouts
to increase the value and versatility of the accelerator complex
or to reduce civil costs.
The building layout will be modified to reflect improvements
in the accelerator, beam transport system and experimental facili-
ties and to provide adequate space and an economical layout for
all auxiliary equipment and services.
- 195 -
7. REFERENCES
1. "A Program for Nuclear Physics in the Seventies and Beyond",ed. J.C. Hardy, A.B. MacDonald, and J.C. D. Milton, AtomicEnergy of Canada Limited report, AECL-4 596 October 197 2.
2. H. Klein, H. Herminghaus, P. junior and j. Klabunde, Nucl.inst. and Meth. ST7, 41 (1971) .
3. First Operation of a Superconducting Proton Accelerator,A- Brandelik et al, Proc. of the 1972 Proton Linear AcceleratorConf. Los Alamos, LA-5115, p. 93, (1972).
4. "Progress in the Development of Superconducting Resonatorsfor Heavy ion Acceleration", R. Benaroya et al, ibid, p. 168.
5. National Heavy-ion Laboratory (NHL), A Proposal, Oak RidgeNational Laboratory, private communication, (1972).
6. "Proposal for a versatile Trans-Uranic Research Facility",Dept. of Physics, Michigan State University, private communi-cation, May 1969. See also H. Blosser, M.M. Gordon andD.A. Johnson, Proc. Fifth mt'l. Cycl. Conf., Oxford, England,Butterworths, 1969, p. 50.
7. Midwest Tandem cyclotron, A Proposal for a Regional AcceleratorFacility, Argonne National Laboratory report ANL-7 582, June 1969
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9. E.D. Hudson, M.L. Mallory. R.S. Lord, A. Zucker, Proc. 1973rarticle Ace. conf., IEEE NS-20 (3), 173 (1973).
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11. E. Heinicke and H. Baumann, Nucl. inst. & Meth. 74, 229, (1969).
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13. High voltage Engineering Corp. - private communication.
14. R.H.V.M. Dawton, IEEE Trans. Nucl. Sci. NS-19 (2), 231 (1972).
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15. See Ref. 5, Fig. 6.19.
16. Blasehe et al.. Proc. Int. conf. NUcl. Reactions by Heavyions. Heidelberg. 1969; c.R. Emigh, 1968 Proton LinearAce. Conf. BNL-50120 (C-54) , p. 338.
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R. Poirier, O.K. Fredriksson. J.F. Weldon and W.A. Grundman,Cyclotrons-1972, 451 (1972).
24. 100 kw RF power Amplifier, Q.A. Kerns and H.W. Miller,1971 Particle Accelerator Conference, NS-18, p. 246,(1971) .
25. Some Aspects of the control and Stabilization of the RFAccelerating Voltage in the TRIUMF cyclotron, K.L. Erdman,K.H. Brachaus & R.H.M. Gummer, cyclotrons-1972, p. 444 (1972).
26. MAGTWO, Computation of Magnetic Field and Vector potentialDue to a Set of coaxial. Rectangular Winding Section Air-CoredMagnet coils, J.A. Hulbert, Atomic Energy of Canada Limitedunpublished internal report CRNL-605.
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29. "The Superconducting Magnet System for the 12 Foot BubbleChamber", J. Purcell, Argonne National Laboratory reportANL/HEP-6813 (1968).
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30. BIM, A Large 10 MJ Magnet at Saclay, A. Berruger, R. Blondct,G. Bronca, J. Holtz, J. Krikorian, j. Neal, Advances inCryogenic Engineering, 1969.
31. "Preliminary Test Results of BEBC Superconducting Magnet",F. Wittgenstein, Proc. of the Fourth int. Conf. on MagnetTech. 1972, 295.
32. The Superconducting Magnet for the BNL Seven Foot BubbleChamber, D.P. Brown, R.W. Burgess, G.T. Mulholland. Proc.of the 1968 Brookhaven Summer Study on SuperconductingDevices and Accelerators, p. 794.
33. Superconducting Magnet for the 15 Foot NAL Bubble Chamber,J. Purcell, H. Desportes & D. Jones, Argonne National Laboratoryreport, ANL/HEP-7215, February, 1973.
34. intrinsically Stable Conductors, P.F. Smith, M.N. Wilson,C.R. Walters, J.D. Lewis, proc. of the 1968 BrookhavenSummer Study on Superconducting Devices anfi Accelerators,p. 913.
35. Materials and conductor Configurations in SuperconductingMagnets, H. Brechna, ibid, p. 478.
36. Solenoid Magnet Design, D.B. Montgomery, Wiley interscience 1969
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38. Development Program for the Magnet of the European 3.7 metreBubble Chamber, F. Wittganstein, Proc. of the 1968 BrookhavenSummer Study on Superconducting Devices and Accelerators,p. 828.
39. CERN Courier, 1£ (2) Feb. 1970.
40. S. Foner, A.J. Freeman, N.A- Blum, R.B. Frankel. E.J. McNiff,H.C. Praddaude, Phys. Rev. 181, 863 (1969) .
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43. Magnetism in solids, D.H. Martin, Iliffe (London) BooksLtd.. 15 (1967).
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49. Cyclotrons 1972, AIP Conference Proceedings No. 9 (Americaninstitute of Physics, N.Y., N.Y. 1972) p. 476.
50. Ibid. p. 510.
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52. Ibid. p. 500.
53. Ibid. p. 515.
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55. H. Bertini, phys. Rev. 131, 1801 (1963) and references citedtherein.
56. R.G. Alsniller, M. Leimdorfer and J. Barish, Oak RidgeNational Laboratory Report ORNL-4046 (1967) and referencescited therein.
57. W.W. Wadmann III, Nucl. Sci. Eng. 2s.. 220 (1969) and HealthPhys. 11, 659 (1965).
58. w.G. Simon and S.T. Ahrens, Phys. Rev. C2, 1292 (1970) andreferences cited therin.
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60. S.J. Lindenbaum, Ann. Rev. Nucl. Science 1_1, 213 ;1961) andreferences cited therein.
61. The AECL Study for an Intense Neutron Generator, AtomicEnergy of Canada Limited report,AEC\-2600 (1966).
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63. M.B. Wells, reproduced in Reactor Handbook, 2nd ed., vol.IIIB, Ch. 15 (edited by E.P. Blizard), Inter Sciencepublishers, New York (1962).
64. Orbit Properties of the Isochronous Cyclotron Ring with RadialSectors. M.M. Gordon, Annals of physics J30, 571 (1968) .
65. Accelerateurs circulaires de Particules, H. Bruck, PressesUniversitaires de France, 1966.
66. Kosby, Blosser and Johnson, cyclotrons 1972 p. 430.
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68. J.E. Allinger, G.T. Danby, J.W. Jacxson, L.W. Smith,Economic Beam Transport for Utilization of the post-ConversionAGS, BNL E.P. and S. Div. internal Report 70-5, Dec. 1970.
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- 200 -
APPENDIX
uniform Magnetizations: Equivalent current Shell (H.R. Schneider)
The equivalence of current distributions and magnetized
media is dealt with in many standard texts on electromagnetic
theory ' ' . we shall consider he re a special case only,
appropriate to the flutter poles.
Consider an arbitrary volume with parallel plane ends and
containing a magnetically polarizable material, as illustrated
in Fig. 1. If the material is uniformly magnetized - that is
the magnetization M has the same value and direction (assumed
normal to the plane ends) everywhere within V then, the magnetic
Fig. 1: Uniformly magnetized volume and its equivalentcurrent shell.
- 201 -
iield due to the magnetization is identical to that of a current
flowing in the wall of the cylinder, perpendicular to the
direction of M, provided that the surface current density is
equal to M. This is illustrated in Fig. 1. This can be seen
as follows.
We note first that the vector potential of a circular
current loop such as illustrated in Fig. 2 is given by (4)
m x rA = it = -=-
O
(1)
Fig. 2.
where m is the magnetic dipole moment of the loop, given by,
m = Ta I n.
If now M is defined as the magnetic dipole moment per unit
volume at a point within a volume V, then the dipole moment of an
elemental volume dV is MdV and the vector potential at the
point p illustrated in figure 1 is.
llo fff M x r (2)V r
The minus sign appears because r is measured from p.
Equation 2 can be rewritten as.
= ~£fffusing the vector relations
Curl (va) = (grad cp) x a +
andcurl a dV =
V
curl a
a dS
(3)
(4)
(5)
- 20 2 -
equation 3 becomes,
where the volume integration is over the volume containing the
magnetized material and the surface integration over the surface
bounding it.
Since we are considering uniform magnetization, the first
integral of (6) is zero, so the vector potential of a uniformly
magnetized region bounded by S is given by,
11
1 x n
s r
For an arbitrary current distribution and in the absence of
magnetic material A satisfies the Poisson equation,
V2A = -'i J (8)
which in integral form is,
A = 77 fff J/r dv (9)
The integration is over a volume containing the current
distribution.
If now the current is confined to a thin layer T in the
wall bounding the volume V in Fig. 1, then the vector potential
due to this current is,
comparing (7) and (10) we see that the vector potentials are
identical if the surface current density, Js = JT, and the
magnetization are related by,
Js = M x n
- 203 -
REFERENCES
1. J .A. S t r a t t o n , Electromagnet ic Theory, p . 242, McGraw-Hill (1941)
2. W.R. Smythe, S t a t i c and Dynamic E l e c t r i c i t y , p . 355,McGraw-Hill(1968) .
3 . W.K.H. Panofsky and M. P h i l l i p s , C l a s s i c a l E l e c t r i c i t y andMagnetism, p . 124, Addison-Wesley (1955) .
4 . Smythe, op . c i t . p. 290.
5. Stratton, op. c i t . p. 604.
6. Stratton, ibid. p. 230.
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