sion. The parameters have been varied around apriori chosen mean values, and the capture effi­ciency CE computed. Some results are displayedin figs. 1 and 2. Fig. 1 shows the influence of

CEo/, 1: Kz 1 CE%K c 1

V =40kV aE· :!: 240keV

90 B= 0 90 B = '35 T.-1

1+-. ......

""2..... .....

so ......~ so

~---r

70 2: K=1 70V -S5kVB=·35T.-I

60 Measurement 60

ADIABATIC CAPTURE IN THE CERN PS

M. Boutheon and J. Gareyte

CERN, Geneva, Switzerland

(presented by J. Gareyte)

Abstract

In order to improve accelerated intensity, themain magnet cycling has been modified in the initialphase of acceleration to allow a quasi-adiabatic RFcapture. With a low injected current (20 rnA), acapture efficiency of 80% was recorded, very near tothe computed figure. With an operational injectedbeam of 160 rnA, difficulties appeared in the trans­verse planes, due to the reduced B. While thework is still in progress, an increase of 15% inthe accelerated intensity has already been obtained.

Introduction

Implementing adiabatic radio frequency capturein a synchrotron involves a compromise between cap­ture efficiency and dilution. As the CPS is goingto be an injector itself, the highest possible phasespace density must be achieved and the dilution atinjection avoided as far as possible. This led usto choose an initial CPS acceptance comparable tothe linac emittance. Capture parameters were thenoptimized to get the highest possible efficiency.

50

FIG.1200 250 300

50

:!:aE 60

keY FIG.2so 100 120 V

kV

Capture and Acceleration Optimization l )

Preliminary Considerations

The linac beam is usually such that, when de­bunched and circulating in the PS, 95% of its par­ticles lie within ~E = + 300 keV. With the newdebuncher, this could be-lowered to + 200 keV.Therefore we have centred our study ;round thesevalues. On the other hand, to make a real adiabaticcapture, one must have non-accelerating buckets,therefore a rate of rise of the magnetic field B=O.This is difficult to realize practically. In fact,we have chosen, for reasons of convenience, a B of0.35 Ts- l to make our experiments.

Capture

the energy spread, fig. 2 of the maximum voltageVmax '

Acceleration

Once captured in the buckets, the beam has tobe accelerated as fast as possible, to gain timeand to cross rapidly betatron stopbands. On theother hand, the bunching factor BF must not be de­creased too rapidly, in order to keep a favourablespace charge paramete~ and the radial amplitude ofthe synchrotron motion must not increase in ordernot to lose particles on the vacuum chamber. Allthese requirements are satisfied in an optimal man­ner if the bucket area is kept equal to the bucketarea after capture.

A computer program, written by U. Big1iani 2 ),

has been used to simulate the whole process underdifferent conditiohs and to find the optimum values.During capture, the RF voltage rises such that

where A is the bucket area, Ts the instantaneoussynchrotron period. K is a "coefficient of adia­bacity", which must not be much bigger than 1 3).The initial voltage Vmin is chosen to be 10% ofthe maximum voltage at the end of the capture, Vmax'Thus, the height of the bucket at injection is ap­proximately equal to half the linac energy disper-

dAA

K dtTs

(1)

A parabolic law of variation of B seemssuitable at the beginning, with parallel increaseof the RF voltage, such that the acceptance is keptconstant. When no~ina1 RF voltage is attained,near 200 MeV, the B is increased in such a waythat the ~cceptance remains constant, until thenominal B is reached. Such an optimized schemeis described in fig. 3, for an initial B of0.35 Ts-

1and an initial voltage of 85 kV.

Choice of Experimental Parameters

For a 1inac energy spread of ± 240 keV, thecorresponding PS acceptance (0.16 eVs) is obtained

- 95 -

t.2QV'V +0.003 cm- 2

t.R2 'V

which changes to +0.005 em- 2 , but the inaccuracyof the measurements has prevented ·confirmationof these values.

The above changes have required a modificationof the pole-face windings' current at low energy soas to return to the optimum operating conditions of

-1cm

t.QV[;R :t -0.030and

~---::;oo"..:;;.----------v

2.0

1.5

c.s

ilT -).

Results

RF adiabatic trap­ping

_ Rise of RF vol­tage (outphasingof 2 groups ofcavities)

Beam bunching(inverse bunchlength).

With the reduced beam, RF trapping* figuresof 80% were achieved; as the overall trapping was50% one can see that significant transversal lossesoccur. In passing to a useful beam, further dif­ficulties arise.

100 ~s/cmPhoto 1

Radio-frequency Acceleration

The variations of accelerating voltage wereobtained using - the cavity AVC (slow variation atC = 350 MeV) - the dephasing of the cavities in two~roups (trapping at 50 MeV, see ~hoto 1). The;table phase is about 10 0 when B is 0.35 Ts- 1 and~oes to 32 0 for normal acceleration, beam control,orking around this value. The frequency programIpproximates to the theoretical curve by a series of;traight lines based on the B measurement.

OL---+----+--+---j----t:---+----::t::-----=~~_ ~ W W ~ ~. ~ ro 00

Experimental Work

Magnet Cycle

FIG.3 Parameters of optimized acceleration

1m,

with an accelerating voltage of 85 kV. Fig. 2shows that very little would b~ gained by increasingthe voltage further. For a B of 0.35 Ts- 1 , themore favourable value for the coefficient K isaround 1, which gives a capture duration of 250 ~s.

During this time, the beam, being insufficientlyaccelerated, will drift inwards by 1 cm. After80 ms of acceleration, the energy has reached 350MeV and the B can be increased to 1.9 Ts- 1 in60 ms, with parallel increase of RF voltage to140 kV.

Thanks to the flexibility of the CPS magnetsupply, the conditions described in the previousparagraph were achieved. 1500 volts is applied tothe magnet for 120 ms (starting 40 ms before injec­tion) which gives B= 0.35 Ts- 1 . Then the voltagerises linearly over 60 ms to reach 7600 volts(B = 1.9 Ts-1). The voltage program is obtainedby varying the firing angle of the rectifiers. The7600 volts is held until the required machine energyis reached in the operational cycle.

Transverse Beam Evolution at High Intensity

Beam Quantity Effect

It was necessary to increase the focusing tobalance the coherent space charge effects. With auniform distribution 4) , at 50 MeV we could calculate

Magnetic Field Changes at Low B

Studies have been made with a reduced beam of20 - 40 rnA and emittance about 5 TI mm mrad. Thisgave the following results:

a) the dipolar characteristics do not seem to bechanged, judging by the closed orbit;

b) no significant quadrupolar change was noted;

c) the sextupolar characteristics, however, werealtered; we have noted the following changes

andt.QV ~0.04 x N (in 1012 p+)

lIQR~ 0.01 x N (in 1012 p+)

for t.QR : +0. OlDt.R

-1em , forfor a non-bunched beam. With the bunched beam(BF'V 0.66 in our case), at 60 MeV, t.QV became

when B changes from 1.3 Ts-1 to 0.35 Ts- 1;

d) it seems that an octupolar field component:exists in the vertical plane, of the order of

* RF trapping is defined as follows: ratio ofprotons remaining 2 ms after injection to pro­tons present after the first five turns

- 96 -

0.05 x N (10 12 p+): measured values were in goodagreement. The corrections being DC, we have theworking point evolution shown in fig. 4.

Resonances - Transverse Losses

We can see from fig. 4 that many stopbands arepassed and losses were measured during this journey.Compensation (sometimes rather rudimentary: themachine itself making the filtering) of 2nd and 3rdorder stopbands is essential; e.g. acceleration isimpossible if one does not compensate the stopband2QV = 13. Photo 2 shows the losses occurring.

Beam injected (3 turns) : 165 mA,Le. 7 x 10 12 p+/p

Beam at 5th turn 5.25 x 10 12 p+/p (75%)RF trapping* ~ 65% leaving 3.5 x 10 12 p+/p (50%)at 60 MeV 2.5 x 10 12 p+/p (36%)

350 MeV (before 8=1. 9 Ts -1) 2.1 x 10 12 p+/p (30 %)high energy 2 x 10 12 p+/p(~30%)

These figures refer to a 50 MeV beam with anenergy spread of + 325 keV and with a 2.7 cm ptpradial and 0.7 cm-ptp vertical orbit.

Conclusion, Present and Future Improvements

Photo 3 shows that compensation of 2QV + QR=19allows an increase of approximately 20% in protonsaccelerated to high energy. Despite this, theoverall trapping remains low. One knows from

experience that it is advisable to have as many cor­rections as possible between injection and, say,800 MeV. The transverse losses are what might betermed "slow", the space charge effect causing theprotons to remain on the stopbands for a long time.As an example, !'>QV "single ~article" can be cal­culated as 0.075 x N (N in 10 2 protons), assumingthat PS admittance is uniformly fitled at 50 MeV.Unfortunately, factors of 2 or 3 '+ may be intro­duced to cope with real distributions. To fitphotos 2 and 3 with the measurements shown on fig.4,the exact distributions of the beam and their evolu­tion would be needed, as is discussed in 5)

Photo 2 Effect of stopband 2Qv = 13

Ip transformer: upper trace with compensationlower trace without

Injected protons ~ 6 x 10 12 (500 ~s/cm)

Working with a front-porch magnetic cycle andadiabatic trapping has brought an overall increaseof 15% in the intensity; this procedure also aidsthe measurements and studies in both 10ngitud~nal

and transverse planes. Future trials will include:

better approximation to theoretically optimizedcurve for Bprogramming of magnetic corrections with B(mainly PFW and quadrupoles)new (skew quadrupoles) and better non-linearcompensations will be used: the philosophy tobe adopted is 1 lens ++ 1 supply, control bycomputer, and circuit combination being soft­ware generated.

1.11.1

Normal operation

'.1'.r

Compensoted stop. bands

FIG.o4 Working point evolution

'.1

u

Results

The following results were obtained in opera-tion:

Photo 3 Effect of stopband 2QV + QR = 19

Ip transformer: upper trace with compensationlower trace without

Injected protons ~ 5 x 10 12 (20 ms/cm)

- 97 -

References

1) BOUTHEON, M., GAREYTE, J., SCHULTE, E., CERNinternal report MPS/CO 70-2

2) BIGLIANI, U., CERN into report SI/Int.EL/68-2

3) LILLIEQUIST, C. G., SYMON, K. R., MURA 491, July 1959

4) LEFEVRE, P., CERN into report MPS/DL Note 70-1

5) BACONNIER, Y., GAREYTE, J., LEFEVRE, P., Highintensity phenomena observed in the CPS, 1971Particle Accelerator Conference, Chicago

DISCUSSION

L. TENG : The N.A.L. booster synchrotron magnet isresonant excited. Hence Bis nearly zero. Adia­batic capture is achieved by turning the 16 RFcavities on one by one. The capture efficiency isestimated to be above 80 %, probably as high as 90 %.

Why don't you reduce the initial B to zero?I believe you will then have more than 90 %captureefficiency.

J. GAREYTE : This would have implied a lot of hard­ware changes. But as seen in Fig. 1 a gain of~ 10 %in efficiency would be possible.

K.H. REICH : Single-turn synchronous transfer intothe CPS will be used (at 800 MeV) as soon as thePSB becomes operational. At (50 MeV) injection in­to the booster,B ~ 0 and programmed adiabatic in­crease of RF voltage and synchronous phase angleare planned, which give, on paper, multiturn captureefficiencies > 95 %.

R.L. MARTIN : Did you get any idea of incoherentspace-charge defocusing forces by investigating theresonances as a function of intensity ?

J. GAREYTE : We made no systematic investigations.