ABALYZIBG MAGHET SYSTEM FOR THE

ELEC2B0STATIC ACCELERATOR

APPROVEDj

! T=~ j o r P r o f e s s o r

A ^(Ptyy^tk Minor P r o f e s s o r

at® S c h o o l of tarn

ABALYZIKG MAGNET SYSTEM FOR THE

ELECTROSTATIC ACCUSE* A TOR

THESIS

Presented to the Graduate Council ©f the

*orth Texas Stat® College in Partial

Fulfillment of the Requirements

For the Degree of

MASTER OF SCIENCE

by

fhonias 1. Young, B# S, in Physios

Dexter, Mew Mexico

August, 195k

TABLE OF COHTEITS

Pae© LIST OF ILLUSTRATIONS iy

Chapter I. HSRODOCTIOl 1

The Accelerator B©am Analyzing Devices

II, MODIFICATION OP THE ACCEL£EATOB. £

The Ion So\ire© The Accelerating Colum

III. THE MAGHET SISTER • 11

General Discussion fee Magnet The Power Supply fh* Ion Path Performance

APPBSDIX. # al|.

BIBLIOGRAPHY 2$

til

LIST OF ILLUSTRATIONS

Figure ?ag@

1, Electrostatic Analyser . . . . . . . . . . . . 3

2* Magnetic Analyzer. . . . . . . . . . . . . . . 3

3, Baai©~ft*eqtt®Bey Oscillator • • ?

ij.* Ionization Chamber 7

5# Accelerator Column 9

6* Ma$o@t C©iap©»@fit8 (D • • 2.3

7. Magnet Coaponenta (H) • • • • • • • • • • • • 15

8. Magnet Support Mechanism . . . . . . . . . . . 1?

f * Agsesfcled Magnet and Support . . . . . . . . . 19

10» Magnet Power Supply# . . . . . . . . . . . . . 20

!!• §rl£t Tube Assembly• • « • • • • • • • • • • * 21

12» Plot of Magnetic Induction • • • 23

It

CHAPTER 1

HIT HO DUG1101

fhe

I»ow energy positive-ion accelerators h a w recently

eoiae into rather common us© as sources of mono-energetic

neutron#*.- Using ions with energies ranging ft8)®® fifty kev

to a few hundred k«T it is possible to produce neutrons with

energies up to 13 *£ mev« Two coranon examples of this neutron

production process are the £$(dtn)H©3 reaction, which produces

8*5 iwrr neutrons,, and the H3(d,n)H@4 reaction, ifaich pro-

duces 13»5 aev neutrons, The existence of this process makes

possible the construction of & relatively inexpensive aouroe

of a large flux of mono-energetic neutrons-*, this neutron

flux may be controlled by regulating the intensity of the

beam reaching the target.

The components of tine usual linear accelerator w e m

positive-ion source, a high voltage supply, an accelerating

column, the necessary associated vacuum system, and a • beam

analyzing device, The first four coiaponents listed have

been designed and constructed,^ The ion soar©® and

*Fat M# Windham, "A Beuterium-Deuterluja Type Meutron

Source,* Unpublished Master^ thesis, Department of Physics, Horth feme State College. 1951 * ^ „ T. % , ,

Kenneth *« Hannah, & Positive Ion Source, unpublished

aeceleratIng column as originally constructed have been

modified t@ facilitate assembly and operation of the

accelerator* These modifications are discussed In

Chapter II,

- l&am

Bither electrostatic or electromagnetic deflection

principles may be used as the basis for the design of equip-

ment for separating beams of ©barged particles according to

mass, charge, and velocity. An electrostatic device operates

m an energy analyzer, whereas a magnetic device is a momen-

tum analyser* ' fielativistie effects are negligible in either

case since the maximum velocity of the particles of interest

'in' lets than 2 per cent of the velocity of light, r

An electrostatic analyzer (see Figure l) consists of

two concentric arcs, situated so as to form a portion of a

cylindrical condenser, with the beam path between them#

the foree exerted on a charged particle moving along th®

Ideal path (see Figure l) between th® surfaces la eV/x,

where e is the charge on the particle, f is the potential

difference between the arc®, and x is the radial separation

of the ares, the force required to keep a particle in this

Master*a thesis, Department of Physics, North Texas Stat®

°"^ ieorge^Tt Paulissen, "Operation and Control of a Kadt®-Frequency Ion Source,1' Unpublished M&ster *s thesis. Depart-ment of Physics. North Texas State College, 1953#

Vera A# McKay# "Design and testing of a Positive Ion Accelerator and Necessary Vacuum System,® Unpublished Master's thesis. Department of Physics, North ^exas State College, 1953•

Poth of Particle

Fig. I.—Schematic Diagram of an Electrostatic Analyzer

Pieces

Path of Particle

Fig. 2 . — S c h e m a t i c Diagram of a Magnetic Analyzer

circular orbit is just th© centripetal fore©, *», /Jr#

Seating th© above ©agressions gives

J? = 1.1

fh© radius ©f the orbit is seen to be a linear function of

kinetic -«p©i»gy* $his analysar also provides a certain aaount

of spae© focusing in th© plane of the beara,̂

. A magnetic analyzer consists of an electromagnet with

parall®1 pole faces situated so as to create a

field 4$ right angles to the path of the particles

Figure " | f t h e for©# :©,x®rt©d on each chared particle la

B©yt inhere B la the Magnitude of the Magnetic induction '

erector* Equating thia expression to th© ©©nt»ip##^t fore©

»•= . 1«&

Xn this case the radius of the orbit is a linear function

4t 'Mmm&vm* In addition, tlx© magnetic analyzer pro Tides

the possibility ©f focusing the beam in the plane tangent to

th© curved part ©f th© pole pieces and parallel to the

direction of the magnetic induction as w«ll as iti the plan©

of the beaa,^

!Ph© accelerator under construction will produce sub-

stantial quantities of both and ions# It is 'desirable

ronic Y©l©oitl«* k --XlXIf^uly 15# 1929 h t8l|.

3l Plaid©,

t© keep B+ ions fro® reaching th© target sine© they heat th©

target without adding appreciably to th® neutron flux. Slae*

both the Bg and D"*~ ions ore accelerated to th# same energies,

It Is clear that an electrostatic analyzer could not possibly

fee used to separate these two components of the Ion beam#

fhus, the necessity of using a magnetic beam analyzer with

the linear accelerator in neutron production application® is

evident. In addition, the focusing flexibility of the mag-

netic analyzer Is advantageous»

CHAPTER IX

MODIFICATIQH OF THE ACCELERATOR

H a £b& tei

As originally designed,* the Ion source required

several hundred watts of electrical power and this, of

necessity, isolated at 2$0 kilovolts above ground. Further-

more, the nature of this radio-frequency ion source was

such as to require a continuous and complex timing pro-

cedure .during the operation of the accelerator»

A new radio-frequency exciter of very siraple design

has been constructed* The circuit for this oscillator is

shown in Figure 3». All the high voltage power necessary

for its operation Is supplied toy a small twenty-seven-volt

d.c. dynamo tor requiring a total power input of less than

fifty watts. The oscillator furnishes all the radio-frequency

power necessary to ionise the gas in a chamber modeled after

the Oak lidge ionization chamber described by Mosk#2 This

source is powered from storage batteriest which are easily

isolated at 250 kilovolts froa ground*

% Paulissen, clt.. p. 5* ? •

2C. D, Uowkt S. Beese, and W# M, Good, "Design and Operation of a Radio Frequency Ion Source for Particle Accelerators,w Mueleonlcs* IX {Septentoer, 19$D, 18-23.

Two 3-inch turns of l / 8 inch copper tube

,To ionization chamber

RF CHOKE

vJLQJL*—

ohms

+ 285 volts

(All tubes are 2C22's) (Filaments are connected in series.)

Fig. 3 . - -C i rcu i t Diagram of Radio-Frequency Oscillator

2 4 ground g lass

,25mm. I. D. Pyrex tube 4 0 / joint T *c» jCu f

8n

Fig. 4 .— Ionization Chamber

8

fh© oscillator operates at approximately eighty mega-

cycles, It does not require tuning in order to remain is

operation, sine# there is a single tuned circuit consisting

®f a three-inoh coil and the inter-electrode capacitances

of the tubes plus the distributed capacitances of the

circuit wiring# Necessary adjustment in operating frequency

amy he made by changing the dimensions of the coil# ;

The radio*frequency oscillator is capacitively coupled

to the ionization chamber by two copper straps around th®

chamber as shown in Figure 1{.#. Those straps are tapped

across the oscillator coil at points whioh were experi~

aientally determined as those giving the maximum tolerable

oscillator loading consistent with stability.

the Accelerating Column

A recent change in th© location of th© accelerator

necessitated a horizontal position for the column rather

than the vertical arrangement as specified by McKay«3

horizontal arrangement required a sealing compound with a

much higher tensile strength than the Apieson W with which

the coltusn was originally sealed* Several unsuccessful

attempts were aaade to seal a column with Araldite 100, which

requires a higfr curing temperature (110®C), but in each case

uneven cooling of the sealed column caused the porcelains to

^McKay, o£. clt«. pp. 8-15*

SCALE

Ion-Source-End Plate Side View of Accelerator Column Al Plates

JJ l Stf 16 Disks

Groove

J j_«. O-Ring 2 Target-End Plate H T -

Porcelain lnsulator«

Steel Disk (see cut)

(actual size)

Groove for No. 69 Parker O-R ln f l

Removable Al Electrode

Hole for Al Electrode

Pumping Holes

Fig. 5. - - T h e Accelerator Column

1©

mmsk* 4 modified «o1.uhhi 1mm Figure f>) was eonstructed

at the Bmleonis Hesea^ch Canter of Th« University #£ ̂ ©x&a

with tJa* h«lp and advice of ®». I** Hudspeth t»4 *f». ̂ ©opl^s#

CHAPTER III

A relatively simple magnet design was chosen so that

the various part# could be machined on available equipinent*

The first detail to be considered in the magnet design was

the relationship between the radius of curvature of the

beam path and the masher of ampere-turns required in the

©oils. It was obviously desirable to use the least radius

consistent with a relatively low power dissipation in the

windings • A few siwple computations indicated that a radius

of ten inches would meet all the requirements for the present

magnet mast be capable of bending the most magnet-

ically rigid ions encountered into, the ten-Inch orbit when

they have an energy of 2$Q k®v*. A rearrangement of Bquation

siv̂ e#

1= mv/re 2*1

it® Mi® magnitude of the repaired magnetic induction* SHirther-

asr«# Si# relation of energy to velocity is

v=\&W® • . ' 4' ' 4-

Ration 2*2 gives a velocity of 3«1*6 X 10 ^/s#d for

ions accelerated to 2$0 Stev* fhen Equation ;g*t stoowi that

a magnetic field strength of 56<?0 gauss is required to bend

*%%

12

;l;hese p a r t i c l e s into * c i r c u l a r orbit of t en - inch r a d i u s ,

A® approjd matiosa t o the r e q u i r e d Mgnetoiaotiir© fore©

(mst) mm••obMJamd by using the equation for 'in\i4MiEU.si4 2.

.da#a»t£* c i r c u i t $

mmf-- »i= +

*here W i s the mmbmt of turaa in to® winding, i i s t he

iwiivmM# i n each. t u r n , @ I s tim l eng th of the f l u x p & t h i a

the s t e e l , d is.- t he a i r - g a p wMttt , /*0 I s t h e Stegpertte perme-

a b i l i t y of f&m spac®, said^t i s the pe rmeab i l i t y o f laild

at©©! Cl88/i0 )•* A s e t 4$ magnet dimensions, — #*<6$ meters

and 4 = 0,0127 me te r s , chosen by & s e r i e s of fecial, i a l e t t*

l e t i o n a g i v e s , f r o a Bqimtion 2 # 3 , irsaf = 7320 ampere-turns • f»%

The Magnet !

Flux Pativ

The magnet proper was cons t r u e t e d of eoa«@r©i a l -"grade

mild s t e e l , which has a carbon c o n t e n t ©f .not more than 0^30

per cent# A carbon conten t lauch g r e a t e r than t h i s s e r i o u s l y

a f f e c t s t he pe rmeab i l i ty of s t e e l •-* f h e s t e e l p a r t s of the

Mm gnat a re shown i n Figure 6* Ha l f - i nch s t e e l b o l t s were

N a t h a n i e l H. f'r&nk, I n t r a d i c t i o n S M Opt ics , 1950, p . 271. ' _

2ArthuP W# Smith. E. D. Campbell, and W. I>. f i n k , BJhe E f f e c t of Changes in Total Carbon and in the Condition of Carbides on the Magnetic, P r o p e r t i e s of S t e e l , f f tya l f ta l Heview, XXHIiMarfdj, 192J+), 377-85•

3 l b i d .

13

Scale; -J- "s I " 4

FRONT VIEW TOP VIEW

4 Pole ^ % Pieces

s;d,r(B)

4"

4

diameter

Core

Fig. 6. —Magnet Components (I)

Ik

nsed to fasten th© mtLtims (B) to th® central cor© {$),,

around '«hloih the ©©11 fonas f i t , and t© fasten th© pol»

pieces (A} to the s ides, All wmtm&s l a direct contact

with ot&er surfaces were machined to an accuracy of two

thousandths of an Inch. Other surfaces were sawed to shape

on a band saw and groiand to a smooth f in i sh .

Should future applications ©f Mia accelerator require

sharper focusing of th© beam, 0.faaa©,» equations^ could be

ms#d as a guide for reshaping th# pole pieces.

the winding was divided evenly among three evils ' of

approximately 1500 terns each, for a to ta l of IJ5©0 turns*

Therefore, a current of approximately 1,65 araperes Is r e -

quired to produce the rmximm of 5^90 gauss seeded to de- •

f l e e t 2$6 Jcev lone into fee ten-Inch orbi t •

fee eoi l forms were constructed of aliasdnum disks

xaomted on steel cylinders which were made to f i t closely

around the central s teel cor© to prevent movement of th#

col ls (see Figure ?1* the finished forms were lined with

heavy f i sh paper before being wound with l6~gauge General

Electric "Haa^y ^oraix" vir«« fho ends of each length of

wire were connected -to a "fextolite* binding post attached

to the outer edges of the alustlnam disks* A potential

\ # r a * » ® . *11. , W-30U.

1$

I " , " SCALE: 4" * 1

- L " — 0

4 ® Outer Discs Inner Discs

Top View Side Views

Coil Form Disc 6

Top View Outer Cylinders Middle Cylinder

Side Views

Fig. 7 . - - M a g n e t Components (II)

16

mStmmmrn of approximately fifty volts- is required across

the terminals of the winding, which consists of the three

ooils connected in sirlt#*.

Support

In of to to facilitate- alignment of the la&gnet a support

was designed uhieh allows im-mmnt in the horizontal plan©*

botii pirtUftX and at right angles to th* length of tto*

©®2ws.#- iM addition t® providing for rotation of tfo* magnet

about a vertical axis* Hi® support also permits United

movement in the vortical direction,

A H parts of the support (see Figure 8) near the magnet

are made of brass, because of its magnetic properties, &

disk (A) $m$ inches in diaBseter is attached to the lower aid®

;<sf th® laagaet with four machine screws* To this is soldered

a brass pip® 01 of $•?$* laches outside diameter. ' A slx-ineh

ring of eighth~inch stock is. soldered at th« bottom of th*#

pips t& that it extends one~elghth«ineh beyond the pip®#

fhree labors <D#B,F) of brass plat® form a ©import for tfa#

ring* ftie ring rests on th© solid plate (F), fitting into

•Hi® plates (S)t which also rest on C?) t ^h® top plat© (^)

flta around th© brass pipe W and is bolted to th® lower

plates* In order to facilitate -angular movement of th® mag-

net & rod (I) 1® attached to the lower part of the brass

pip©, A clamping device <B) is provided with which the end

of the rod is held, thereby holding the magnet in the correct

17

£ •

L 5

(A)

I " . i" SCALE: •§• " 1

(B)

y

(C)

F » ) ' 6 "

[ < L y

[ * -6" -

(E)

15

Top View

•

T 8 "

L , 10

(G)

+; HH.V- H V f _ B I S9_ L_

16'

End View

I '* t SCALES •£- « f

+. i ) f

_ _ _ — — —- — —— - i

b 6" >. Top View

n i l

(H) End View

l t d )

Fig. 8.--Magnet Support and Adjustment Mechanism

angular position. fee lower brass plat# rests on a half-

Inch steel plat® (S) upon which it slides to provide adjust-

ment at Fight angles to the accelerator axis* fee steel

plate is mounted on angle-irons so that it can he nursed in

a direction parallel to the accelerator axis by means of &

threaded rod# Small changes in the vertical position of the

steel plate ©an he made by adjusting four supporting screws,

•font SmsSki

fee magnet power supply consists of a conventional

full-wave bridge rectifier using "fengar* diodes (see

Figure 10)• fee filament power is obtained from a trans-

former with a single primary and four isolated secondary

windings# fee output voltage is controlled by a variable

transformer• Flu# adjustment is provided by a two-ohm

power resistor connected in series with the output* Power

for both filaments and a.c. input is controlled by a single

master switch. Smoothing of the d,e. output is accomplished

by a small L-section filter and a large condenser in parallel

with the magnet load*

fee Ion Path

fee beam ©merges from the vacuum pump manifold through

a small hole in the wall opposite that connected to the

accelerating eoluaai» It then enters the drift tube, which

carries it between the pole faces and on t® the target*

fee drift tube (see Figure 115 is constructed of 1*37$ lneh

19

L_»

Fig. 9, —Assembled Magnet and Support

20

OUTPUT

D. C. OUTPUT SWITCH

3 Amp.

2 [ L i 2 0 0 0 v

Voriabe two-ohm resistor

v a r i a b l e J S l y M S u U L ; MASTER SWITCH TRANSFORMER

a. c.

Fig. 10. - - M a g n e t Power Supply

21

SCALE: » I "

Vacuufn Manifold End

Target End

SCALE: r * r

(Screw Holes)

Groove - ^ deep, for

No. 2 6 Parker O-Ring Target Vacuum Manifold

Connection Connection

Fig. I I . — D r i f t Tube Assembly

copper tubing* Machined brass discs were soldered to th©

©nd of this tub© to form O-Eing joints, with, th© pump mani-

fold on on© ©nd and th© target assembly on the other,

^ho tub© was first filled with sand and, after heating,

was bent to a thirty-degree angle around a wooden frame • A

Jig which conformed to th© desired shape of the finished

tab© was used to hold the tube while it was being flattened.

Annealing was necessary twice during the shaping process

because of work-hardening of th© copper«

*

gjiTftfflMflffll •

Measurements were carried out to check th© magnitude and

uniformity of the magnetic induction between the pole faees •

A plot la shorn in Figure 11 of the induction as a function

of the current In th© winding* It is obvious fpoat this plot

that the aagnot is capable of producing th© necessary flu*

density*

Th® first ©heck for uniformity of th© nagn©tle induction

was mad© by determining the flux density at different posi-

tions in th© air gap. The jerk coil of the flux-meter was

then stored along th® center of th© gap to detect amy variation

of th© induction along the path of th© low beam. leither of

these checks showed any variation of strength with position,

whieh indicates that the magnet should have satisfactory

focusing properties#

Operating procedures ar@ given in the appendix*

23

7 .0

6.0

I* <6 3 O o» o

0 o> u .©

o CL

JC

c 4> 41

5 41 m

a "O c

5.0

4.0

3.0

zx>

i

i

i

>

\ <

i . O ' 0 * 5 U 3 t . 5 2 . 0 2 * 5

1.0

0.<

Current in t h t Magnet Winding, amperes

Fig. 1 2 . — A plot of magnetic induction between the poles against current in the winding.

APPEBDIX

OPERATION OF THE MAOMlf

flftagtfng m & MlBfMfufe

The following steps constitute the correct procedure

for placing the magnet Is operation:

1, S#t the variable transformer to "0* wit®,

2* Turn the d,c« output switch to w0»w#

3, Turn til© master switch t® "ON®.

4, Adjust the variable transformer slowly until appro*-imately the correct d.e, voltage is obtained*

5, Make fine voltage adjustments with the resistor•

®tm %$sm

Correct shutdown procedure is as follows:

1* Turn the variable transformer slowly to MQ" volts,

2« Turn the d«c • output switch to "OFF®,

3. Turn the master switch to

2%

BIBLIOGRAPKT

Brailsford. P.. Magnetic Materials. second edition revised. London, M«t&u»naind Sc., I9SI.

Camae, Horton, "Double Poetising with Sedge Shaped Magnetic Fields," Review of Scientific Instruments. XXII {March 1*51), 297-30J. '

Prank* Bathaniel H., Introduction to Electricity and Optics, lew York, McGraw-Hill Book Co., Inc., 195>0«

Hannah- Kenneth W#, "A Positive Ion Source," Unpublished Master's thesis, Department of Physies, Horth Texas State College, 191*8.

Hughes, A* L,f and Rojansky, "On the Analysis of !l®c« tronic Velocities by Electrostatic Means," gfcarslsal Review. XXXIV {July, 1929),

McKay, Vern A,, "Design and Testing of a Positive Ion Accelerator and lecessary Y&ewm System,R Unpublished Masterrs thesis, Department of Physics, lorth Texas State College, 1953» PP* S-l5»

Moak, C. C.f lees®# H., and Good, "Design and Operation of a Radio-Frequency Ion Source for Particle Accelerators," lueleoaies, IX CS®Pteab#r, 1951)» 18-23.

Paulissen, George T#, "Operation and Control of a ladio-Frequency Ion Source," Unpublished Master*s thesis, Department of Physics, lorth Texas State College, 1953 •

Smith, Arthur W», Csapbell, B. D*, and Flok, W. L«, ®Th® Effect of Changes in Total Carbon and in the Condition of Carbides on the Magnetic Properties of Steel," Physioal Review. XXIII (March, 192U, 377-85.

Stoner, Edmund C., Magnetism. London, Methuen and Co», Ltd., W .

Windham, Pat M,, % Deuterium-Deuterium Type Neutron Source, Unpublished Master's thesis. Department of Physios, Horth Texas State College, 1951.

M