beam energy measurements on the rutherford laboratory plaph ase w as found by settin g the attenuat...

BEAM ENERGY MEASUREMENTS ON THE RUTHERFORD LABORATORY P. L. A.

K. Batchelor, A. Carn e , J. M. Dickson, D. J . Warn er Rutherford High Energy Laboratory

1. Introduction

The results to be described a r e from measurements made us ing t he t ime-of-flight method described prev iously, 1 an d are a continuation of t he work reported at other acc elerator conferenc e s . 2, 3 As a. r e sult of r ecen t observations some of the work repor ted prev iously n e eds some qualificati on. In particular , t he 10 MeV and 30 MeV spect ra have p r ev iou s l y been affected by the residu al powe r in Tanks 2 and 3 which re s ults from the drive power feeding throu gh t he grounde d grid tr iode am plifiers into these cav it ies even t hough the E. H. T. is removed from the amplifiers. Also early comparison between theor y an d practice f rom t h e 10 MeV beam has been invalidated by the fact t hat the fie ld l aw in T ank 1 was distort ed from its o r igin al v alue , whi ch was used in t he com p ute r program. These effe cts are d i s cu s sed bel ow .

II . Effe ct of Residual Powe r on the Be a m

F igure 1 shows the layout of t h e r f sys tem of the P. L. A. , for r eference. The effect on t he 10 MeV be am of v a r yin g the amplitude of res idual power in Tanks 2 an d 3 r e spe ctively, with t hese t anks maint aine d at the cor rect r e sonant frequ ency, i s shown in Figs. 2a and 2b. The l ev e l of r es idual power was v a rie d by m eans of the vari able attenua tors Att 2, Att 3 in the powe r dividing network. T h e effe ct of v aryin g phase w as found by settin g t he attenuato r s at their n or m al operating v a lue s, and detun in g each tank in t urn . T he r esulting spe ctra ar e shown in F igs. 2c an d 2d for detunin g of T ank 2 and T ank 3 respectiv ely. It can b e seen t hat the res i dual power a ff ects the spectrum shape an d posi t i on quite considerably. For n or mal settings of att enuators and f r equ en cy t uners the residual voltages were 6.40/0 of op e rating v alu e in T ank 2, and S. S% in Tank 3 (see Fig. 2). An amplitude r e ducti on by a f a ctor O. 7 (to give 'half power point' amplit u de ) produced a dou blin g of t he beam energy spread at FWHH, and a small shift of mean energy ..-v50 k eY by each tank. A detun ing oft (f/ 2 Q ) = ±. 4 50 (to giv e the s ame redu ction in amplit ude, but now p l us a p h ase shift) produc e d a chan ge in beam ener~ spread at g'WHH by a f act or 3 (+ 45

0), 0.6 (- 450

) by Tank 2, and 3 (+ 45 ), 0.5 (- 45 ) by Tank, 3, and a s hift of mean energy of + 20 keY (+ 450

) by Tank 2, -and - 40 k eY (+4 50) , - 120 keY (- 45 0 ) by

Tank 3 .

Proceedings of the 1964 Linear Accelerator Conference, Madison, Wisconsin, USA

372

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373

Figures 3a and 3b show the effect of residual power in Tank 3 on the the 30 MeV beam. For normal settings of the attenuator and frequency tuner of Tank 3, the residual voltage was 10% of operating value. An amplitude reduction by O. 7 produced little or no effect on the 30 MeV beam energy spread, but a shift of 120 keV. Dehming by .~ 45 0 produced marginal changes in enere-v spread, and a shift of mean energy - 350 keV

o ~.-(+ 45 ) and + 50 keV (- 45 ).

It is now normal operatbg procedure to reduce t.he residual power to a minimum in nonoperatir::.g cavities, this being achieved by deturc.ing the tanks and insertion of maximum attenuation in the drive lines to the grounded grid triodes.

III. Measurements on Tank 1

Due to the discovery of the effect of residual power on t.he spectra it was decided that some of the early work on the 10 Me'F beam should be repeated to try to improve the agreement between theory and practice which, at this energy, had not been good. Figure 4 shows some typical spectra for different injection energies and rf field] evels, the tilt tuners being set at their assumed "nat field" setting (tilt 50 .. :)0). Comp'Jted spectra and phase acceptar..ce did not agree Nell with meastaed Ii al ;)es. Also the transparency of the tank was dow[\. by a factor 5 on its origin;,.) value. It was therefore decided to remeasure the axial field in Ta~:k 1 to see if this could be the cause of the disagreemenL The field law measured for "tilt 50-50" is plotted in Fig. 5 and is compared with the field law measured when the cavity field was SE:t up seY-en years ago (i. e., the field law used for computer calculations). It can be seen that gross distortion of the field h5:.s occurrf:d, probably due to a charcge of dime:-~sions caused by temperature cycling aDd periociic dLsmartling over the past seven years.

The actual measured field law has been used m the computer pro~ gram to give comparison with some of the measured data of Fig. 4, ar~d

the computed spectra ace giver! in Fig. 6. The experimental spectra of Fig. 4 do not show the same fine structure as the comp-:.lted spectra, due ." th' t"·b· .. L • f" 1 1 --l h d· . . . to smoo· lng ou . y J1ner In r· eve~ arJu p ase, ant lL~JecttOn er.ergy. But, with this proviso, comparison of correspondLl:.g speci:ra (A, 13, C, etc. ) now shows quite good agreement.

Fig. (i. e.

The field law has been restored to the proper value shown in 5. The transpareEcy of Tank 1 ha.s retuYr.ed to 11.S or'iginal vabJe 1/50). Results for the new fleld .setting are not yet available.


374

-1

~ ~

~ 7-w u:: 9·1

o -' w u:

"

TANK 1 ACCURATE R.E METER READING-675 (460 kw)

TILT so -SO ir ' ~~~t :~I~d = om

" ~ Hr---------' ~ _____ z

~ ~

a:

~ -' -' ~ 41 i~48r-----/ '------

2.,;H ~65r--------.J "-___ _ u..:.<5·8 u...,' Oz Oz w« w-8.11-------' ........ -----~1-6-3 norma optro ing le~1 0-'

~~ <~ ~~6~ i~~~O~PE~R_~_IN~G~lE_V_El~~_~~~ 9·5 10·0 10.\ Mrv. FIG 2 b 9.5 _JO.O 10·5 Mev

FIG 2(0) 10 Mev BEAM ENERGY VARIATION 10 Mov BEAM ENERGY VARIATION WITH WITH RESIDUAL RE LEVEL IN RESIDUAL R.F LEVEL IN TANK 3.

-' ~ w -' o -' \!I

TANK 2.

~ 3~,1-------/ '-----Z

1441------/

-,', «-~~5-Ir_:===J o )lif-z

-' w ~ 5-01-------- ~~----'

i) J.~~t lKo§!nlJtj!uniS]f:J' ~~&§ffii}!!!lliiC,~ 4·81----- -./ '-------

~i6.3 ~ @;~ ~i ~i47 ~~ 01-------' ~-------

9·5 10.0 10.5 Mrv. 9·5 10·0 10. Mrv. FIG Z~) 10 Mrv BEAM ENERGY AS A FIG 2\1) 10Mrv BEAM ENERGY AS A

FUNCTION OF TUNER SETTING FUNCTION OF TUNER SETTING FOR TANK 2. FOR TANK 3 .

TANK I ACCURATE R.F. LEVEL 675 TILT 50-50 TANK 2 ACCURATE R.F. LEVEL 765 TILT 60-40

-1

1 ~ "- 9

I.!J u.

liloo!---' ~9'3r-------

lonk on tune f = fo T I

l? z 5'8

~ ffi 731-----" Q.

o 6-0f--_./

-J. ~ 0 u. o W l? ~ Z

~ Q. 29.5 3O{) 3O'S - 31·0 Mev.

~ 1/

FIG 3\0) 30 Mev ENERGY v TUNER SETTING IN TANK 3

~ 7'8

-1 « & l}O

~ l) ~ 10{)1-----'

g ~ 29·5 FIG 3(b)

30·0 3O'S 31·0 Mev. 30 Mel/ ENERGY v RESIDUAL POWER

IN TANK 3


375

INJECTION ENERGY 502 Kev. INJECTION ENERGY 518 K<v.

720 720

700 700

C

680 680 11

'"' ~ « '-' <I'>

,..66 66 A

'" "'" 9·5 10-0 10·5 H<v. 9·5 0·0 10·5 Htv. '" >-

'" 545 K<v. 72 INJECTION ENERGY 555 K<v. '" « ~72

~

'"' ~ 700 ~

10·5 Mev. 9·5 10-0 10·5 H<v. 114

FIG 4. 10 Hev. ENERGY AS A FUNCTION OF R.F. LEVEL IN TANK I AND VARIOUS INJECTION ENERGIES.

'" ~

;;: .., "' ~ 0 .., ~ 10

~ .. ~

'" " UNCORRlCTED fiELD LAW CORRESPONDING TO SO-SOTILT

AS MEASURED ON IS .... ,lPRll 196-i USIHG PERTUR.BATION METHOD :E:

't IC CORRECTED FIELD LAW

~ 5

PoE·SET lOTHE lA ..... HEASUftEO 6Y PER1URIIATl01'4 METHOD

SEVEN YEARS AGO

10 20 30 .0 GAP HUHIER

FIG 5 . AXIAL fiELD LAWS fOR TANK I (BEfORE AND AfTER CORRECTION)


376

51

50

.. c; u <II .. E 0 <II 26 c 0

~

..... 25

I r <l

!J) w w a: ~ o

I NJECTION ENERGY 502 Kev.

A

9·5 10·0 0·5 Me~

INJECTION ENERGY 545 Kev.

'4 24I--"C_~./~ \'------

...J W > ~ 21 L....!1B'---___ ~

u.: c:i

17.5 A

9·5 10·0 105 Mev.

FIG 6. BEAM ENERGY IN

Tunc Z 0 ACCURATE " f )( A(CUilAn AF' "ACCUllAn AI o A(CUR.n It'

T UK I ACCU un If

-'0 -20

LIYEL 7" LIYH 7n LEYH 720 LEYEL 690

LIYH US

TILT 60-40

+20

INJECTION ENERGY 518 Kev.

28

24 C

22 8

20 A

9·5 10~ (l·5 Mev.

INJECTION ENERGY 555 Kn

c

8

A

9·5 D~ K)·5 Mev.

TANK 1 [THEORETICAL]

+'0 +60

PH.t.\( IN Of.(iIlH!.

FIG 7 30 M.V ENERGY V INPUT PHASE

+'0


377

IV. Setting of Tanks 2 and 3

Before correcting the field law for Tank 1 measurements on the 30 MeV and 50 MeV beams were made to determine good operating levels of Tanks 2 and 3 for minimum energy spread in the beam. For the 30 MeV beam it is possible to run Tank 2 at an rf field level which gives an integral number of phase oscillation half wavelengths along the tank, and hence a flat energy vs. input phase region for the linear phase oscillation range of 0 s ~ 15

0• Additionally it is required that the beam energy

spectrum should be narrow. Figure 7 shows the variation of output energy with input phase: in particular, the rf level 735 is nearest to the required condition. The effect of input phase on output energy for the chosen rf level in Tank 2 (735) is plotted in Fig. 8, showing a roughly constant energy over the phase Tange, with little change in energy spread.

In Tank 3 it is not desirable to operate at a level corresponding to an integral number of phase oscillation half wavelengths, since the 'A~ /2 level is too near threshold for acceleration, a:::-"d the 2 'A (J /2 level is such that sparking would occur in ~he ta..'1.k. Thus Tank 2 is operated at a different level (765) to give nearly integral number of phase oscillation half wavelengths between the input of Tank 2 and the output of Tank 3. The resulting 30 MeV spectra as a function of input phase to Tank 2 are shown in Fig. 9; the 50 MeV spectra versus input phase to Tank 3 is shown in Fig. 10. Finally Fig. 11 shows the energy spectra as a function of rf level in Tank 3.

V. Tank Toleraro.ces

From the above res"Jlts it is possible to rnake2stimaies of the stability of the output beam in terms of mean energy and energy spread as :\:nctions of luachine pa!"'ameters. Also ese has been made of the Beam Energy Monitor described by Haro.na and Hodge[~ 4 to complete some of the resr:lts not determined by time-of"-flight methods. This monitor, which uti} izes a combination of degrading wedges for range cetermiclation, and a split ion chamber as a detector system, is very useful for determining mean energy, and can measure shifts of the order of 5 keY. A seT of measurements with this monitor takes only a few minutes compared with several hours by time-of-flight methods. In the future the energy monitor will be used as a survey instrument to determine machine conditions worthy of further jnvestigation by the longer method.

The following tables summarize the effect of injection energies, rf field lev-els and intertank phases on the output energies of the P. L. A., for the settings specified in Section IV.


378

TANK

1 A

CCUR

ATE

R.F

.675

, TI

LT

50

-50

TA

NK

1 AC

CURA

TE

R.F

67

5,

TILT

50

-5

0.

I TA

NK 2

ACC

URAT

E R.

F. 7

6~,

TILT

60

-40

TA

NK

2 AC

CURA

TE

R.F

. 73

5,

TILT

6

0-4

0.

70

66·5

-

60

59·5

50

525

~ 40

45·5

301 O

PERA

TING

PH

ASE

(f) ~'I

j~

tJ)

20

UJ

~31.5

UJ a: ~ 1

0 0

z 24·5

A--

z -0

UJ ~17.

5 U

J tJ

) I

~-IO

a.

a.

10

·5 -2

0

+3·5

0 -3

0

29

30

31

Mev

. -4

0

FIG

8

30 M

ev.

EN

ER

GY

v

INP

UT

P

HA

SE

29

30

31

M

ev.

FIG

9.

30

Mev

. E

NE

RG

Y

v IN

PU

T

PH

AS

E


379

Tolerances on Various Machine Parameters Affecting the Output Energies of the P. L. Ao

(a) Injection Energy

Change of Injection Change in Mean Energy Output Energy Energy Measured

% kV MeV keV by

10 Not measured yet 30 Not measured yet

+1 +5 50 -12 (-0. 024%) B.EoMo -1 -5 50 +12 (+0.024)

(b) RF Levels

-----------~-----------

'I ""hange in RF Acc. Output Energy II 0/0

Change in Mean Measured

ank 1

ank 2

.1

rrank 3 L-

RF MeV

+1 +5.5 10 -1 -5.5 10

+1 +5.5 30 --1 30

Energy by keV

--4------~~-~--------+_.---------

-30 (-0.3%) +30 (+0.3(1:)

-+--

1'.0. F.

-')5 (-0 08 m ) -I 'T' F L .;0 -'- 0 O •. :.

-5.5 +25 (+0.08%) I I----+---"--+----------f-------------------------50 50 -+----f------t------------ ,;N01-~t measured ________ _

+1 -1

+1

+8 -8

30 30

50

..L.80 (+O. 270/0) Bo Eo M. -80 (-0.270/0)

Not yet measured ---------------f---.-----

+9 50 0 B.EoM. _--'-_-_9_--'-____ 5_0 ______ -I---_5_QJ: 0 ~f0 _____ , _______ _ -1 ..

(c) Interi:ank Phases

Change in Output Energy Intertank MeV

Phase. Degsees

oks 1/2 10 30 1° ~jO

Tan ------

l.ks 2/3 10 - 50 --

Chan ge in Mean nergy E keV

24 (0. 08%) 18 (0.

---~-

~6%) ___ _

45 (0. 09%)

Measured by

B.EoM. T. o. F.

~-------'----


380

60

SO

40

U) 30

lJ..

I lJ..

I a:

<.!)

TANK

1

ACCU

RATE

R.

F.

675,

TI

LT

SO -

SO

. TA

NK

2 AC

CURA

TE

R.F.

76

5;

TILT

6

0-4

0.

TANK

3

ACCU

RATE

R.

F.

888;

TI

LT

70

-30

.

1r;.~

./, A

\\ ./

,,\

lJ..I

02

0

OPER

ATIN

G PH

ASE

~ ~

z lJ..I

10

U) « I Il.

o

-10

-20

L 40

FIG

10

\

-J;~

~ ~

"""-

45

SO

5S

Mev

. SO

M

(v.

EN

ER

GY

v

INP

UT

P

HA

SE

TANK

1 A

CCUR

ATE

R.F.

675;

TI

LT 5

0-

SO.

TANK

2 A

CCUR

ATE

Rf

765;

TI

LT 6

0-4

0.

TANK

3

TILT

70

-3

0.

927~

,-

90

0

880

uj86

0 >

~

o ...J lJ..I8

40

u::

u.:

ci 82

0

800 FIG

II

.

OPER

ATIN

G LE

VEL

45

50

55

Mev

. 50

Mev

EN

ERGY

AS

A

FUN

CTIO

N OF

R.F

. LE

VEL

IN

TANK

3


381

Conclusions

The change of energy and energy spread in a beam drifted through a cavity containing some residual rf power has been shown to be such as to cause concern if a precise output spectrum is required.

Agreement between computati.on and ex."Periment at 10 MeV has been improved, after finding the actual field law in Tar..k L This field law has now been set to its correct value, and it is iEtended to check the field laws ir: Tanks 2 and 3 in the near future.

The settings and tolerances giv"en for the P. L. A. represent the state of the art at the present time. Detailed work IS continuing, by the use of the Beam Energy Monitor and time-()f'~flight apparatus, to determbe more cJ.osely the optimum operatir:g conditions.

Acknowledgements

The authors wish to acknowledge the help of Messrs. A. P. Banford, D. H. Carpenter, E. M. Mott, and S. Webb during these measurements, and members of the P. L. A. OperatiEg Staff.

TENG: Do you understand all these?

CARNE: Well, certainly the agreement between axial motion computations and the experiment is qu.ite good. This is particularly true in the case of tanks two and three, where we have used a large signal calculation, that is, a 32 step -by-step integratioE per cell. ThE; agre'cment in tank one is now quite good and we think we can improve the situation. We think in terms of phase oscillation wavelengths, and the pict'J.re ties up fairly well.

TENG: The shifts you are talking about ar'2 very large, 50 kV In 50 MeV.

CARNE: Sure that! s alarmi::lg, but these figures in fact agree with eql~aHons given by Lloyd Smith some time ago in LS-3. These give quite tight tolerances.

WHEELER: When you change the level in a cavity by say 1%, did you change the phase between the cavities to compensate for the change in the position of the bucket?

CARNE: No.


382

LAPOSTOLLE: When you spoke about the inference of residual fie-Ld in tanks two ard three on measurements done ai, 10 MeV, do you thInk that even the field induced by the beam could a:J:ect such meas'.lrements ?

CARNE: There could be some effec~, but as JohI"'. DicksOE said ~,ar1ier J

we have a peak current of the oy"der of 200 PA" so that ~ields iLduu::d by the beam are of the order of a factor 1000 riowr' on the res:'.dual fields in the cavities, where there are ,,-'- fE"cw ki}owa:Js of rf pOlii;-er.

TAYLOR: I will just men~.ion thaivve have had exac-t:ly ~:hE ;"amF C'

Unless we drop the field to zero J we fInd that thE~ m,c~aSUl'·,:.;meDts aloe turned about all over the place ar,d we have a high cn:'Te:"1.t too,

REF'ERENC:ES

L An Accurate Determinatjon of the P. L. A .. Beam Energy by a Time of Flight by C. J. Batty and D. J. WarV\er. 1'1lRL/H/ g.

2. Accurate Energy MeasuremeT,.ls on a ;:::0 Me\/ P. L. A. r!t'port S:JD'

ml'HtJd tCl 196":! Ac('eler,q+o~" roY{"'r~r'-'e ;n r)· .. b-n···· n S co R !"1'~o •• ...,l,.o ........ , oJ ___ . ~..;;J.,'" ....... ~. ~ ~.'!'_ .., .• "'; _ '_'.'.i. .~ U 1.'..I;::!l .. ,~,).. 0 }.)" • 1\ ...... 1-1. .:::;:, _

P. L. A. Acc/Phys. 18}.

3, Beam Observations in th.:; P. L. A. by K. 'F3atchE-lor> A. CarnE:, J. Dickson and D. J. Warner. Repo!'i: on 1962 Cor1'.er=:nctc on Linear Accelerators for High Enecgies at:-:3~'oo.khaven NatLor:.al Laborato:cy. BNL 6511.

4. A Beam Energy MonU,orbyR. C. HardJ.ac:.xld T. A. Hodgei:'. P.L.A. Progress Report 1963 NIHL/H/ ;,0.


383

beam energy measurements on the rutherford laboratory plaph ase w as found by settin g the attenuat...

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