The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Study of Adiabatic Condition on Beam Compression with Barrier Bucket
Miho FUJIEDA, Yoshihisa !WASHITA, Akira NODA
Institute for Chemical Research, Kyoto University, Gokanosyo, Uji 611, Japan
Yoshiharu MORI, Ryosaku MURAMATSU: Chihiro OHMORI, Yasuo SATOj Masanobu YAMAMOTOt
KEK-Tanshi, 3-2-1, Midori-cho, Tanashi 188, Japan
Masahito YOSHII, KEK, 1-1, Oho, Tsukuba 305, Japan
cellation of a potential distortion was tried. The experimental results are described in this paper.

Abstract

A beam compression experiment by barrier buckets
was performed at the HIMAC using a high-field gradient cavity which involved magnetic alloy cores. Thecavity generated two barriers, the waveforms of whichwere improved to make a flat potential by optimizing
the drive signal. So that the smooth beam distribution
was achieved. In the beam compression process, the momentum spread was measured varying the sweep sp-eed
of barrier so as to study the adiabatic condition. It was
found that the speed 10 times slower than the drift timeof beam satisfied it.
1 IntroductionTune shift caused by space charge, which is in inverse
proportion to a bunching factor, limits the beam intensity in a proton synchrotron. In order to reduce the tune
shift and the resultant beam loss, it is necessary to increase the bunching factor, that is, to decrease the peakline density. A barrier bucket injection scheme[l, 2] enables to maximize the bunching factor, which becomesalmost 1, because a injected beam bunch is debunchingin a flat RF potential made by two barriers. It is also
possible to accumulate more bunches than the harmonic
number of the ring before the RF bucket becomes full.
It is a effective scheme to increase the beam intensity ofa proton synchrotron.
A barrier bucket experiment[3, 4] was performed at
the Brookhaven AGS during 1998 high intensity protonrun[5]. Two dedicated barrier cavities were used to carryout barrier gymnastics. One is a magnetic alloy(MA)loaded-cavity [6] developed by KEK, and the other is
one of the AGS accelerating cavity modified by BNL. In
order to conserve a longitudinal emittance, a barrier isrequired to be moved adiabatically. Therefore, the adiabatic condition for the debunching process was studied
in the experiment. A longitudinal emittance growth wasobserved during accumulation. The main causes wasa longitudinal mismatching between the bunch and theRF bucket. It was also found that the RF potential
distortion by an overshooting voltage of the MA cavity
and the beam loading of the ferrite cavity prevented thebeam from debunching smoothly.
In order to supplement the results of adiabatic condition, a beam compression experiment by barrier buckets
was performed using a high-field gradient MA(HGMA)
cavity[7, 8] at the HIMAC[9], where the adiabatic condition and longitudinal mismatching was studied and can• also: University of Tokyotalso: Japan Steel Works Co. Ltd.
+also: RCNP, Osaka University

2 Experimental setup

A beam compression experiment was performed at
the HIMAC(Heavy Ion Medical Accelerator in Chiba) in
the NIRS, where a HGMA cavity was installed for thebeam test. Two barriers of the peak voltage of 1.7kV
were generated by the cavity, which were triggered at
a revolution frequency of 261kHz. One barrier is fixed
and the other was moved by modulating the trigger frequency with a synthesizer. The adiabatic turning-off ofthe barrier was carried out with amplitude modulationof the drive signal by a PIN-diode attenuator.
3 Experimental results

3.1 Potential improvement
A flat potential is necessary to make a smooth beam
distribution in a barrier bucket injection scheme. In theexperiment at the AGS, a bump was formed in the potential well by an overshooting voltage of the MA cavity,
where some particles were trapped. The bottom of thebarrier potential made by the HGMA cavity was not flatas shown in Fig.l(d), because there was a small bump.
It has been improved, however, by mixing harmonics of
1 and 2 into the drive signal, because an MA cavity canbe driven by an arbitrary function due to its broad-band
impedance. The quite flat separatrix has been achievedby the correction, which is shown in Fig.l(b). A beamcompression have been performed using the waveforms.
Figure 2 shows the mountain range plots of the beam
monitor signal. In the case without correction, it isclearly seen that the bunch shape is distorted as sameas that of potential. On the other hand, the correction
makes the beam distribution smooth.

3.2 Adiabatic condition on beam compression process
In order to study the adiabatic condition, a beam
bunch was spread changing the sweep speed of barrierin the experiment at the AGS, the momentum spread ofwhich was measured. It was found that the sweep time
as slow as 10 times of the drift time of beam was adiabatic there. Moreover, the simulation was performed
by a multi-particle tracking code, the result of which
was consistent with the experimental ones. In the debunching process, however, the potential distortion by
the beam loading of the ferrite cavity induced the growthof the momentum spreads. The momentum spreads were-251-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

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Fig. 1 Gap voltages and RF buckets: (a), (c), gapvoltages; (b), (d), RF buckets. (a), (b), With
waveform correction; (c), (d), without correction. One revolution period is 3826ns.
not decreased as same as spread phase gap.
An adiabatic condition measurement was performed
during the beam compression process by barriers without any potential distortion at the HIMAC. The injectedbeam was coasting in the ring, where a barrier voltage
was increased adiabatically. Then the barrier was divided into two barriers. While one of them was fixedat the phase, which was assumed to be 0°, the otherwas moved from 0° to 180°, where the one revolution
period corresponded to 360°. As the barrier was moved,
the width of beam was shortened as shown in Fig. 2.
As soon as the barrier was moved by 180°, two barriers
were turned off suddenly. After cmpletely debunching,
the momentum spreads were measured by the Schottky
signal while varying sweep speed of the barrier. The
initial momentum spread was measured when the RF
voltage was off, which was ±0.1% and corresponded tothe drift time of 4.3ms. If the barrier is moved adiabatically, the beam emittance is conserved, the momentum
spread of which should become double the initial one.
The experimental results are shown in Fig. 3.
The experimental results almost agree with double
the initial momentum spread, which show that the adiabatic condition is satisfied. Moreover, the simulation results are compatible with the experimental ones. Whenthe sweep times are from 5ms to 50ms, the momentum
spreads are slightly larger than the other values. Therefore, the results seem to be in agreement with the conclusion about adiabaticity which was derived from theexperiment at the AGS.

3.3 Longitudinal mismatcJhing
The longitudinal mismatching mainly caused thelongitudinal emittance growth in the experiment at theFig. 2 Mountain range plots of beam monitor during
the beam compression. (a): with correction,(b): without correction. The period between
traces is 5001-Ls. In both plots, the sweep times
are 100ms/180°. One can see that the beam
distribution is not flat around 10 ,. . ., 80th tracein the case without correction(b).
AGS. In the experiment at the HIMAC, the longitudinalmismatching was caused on purpose to study the emittance growth by the mismatch, where the momentum
spread of beam was measured. In the case aiming tocause the mismatching, a barrier was moved from 0° to180° adiabatically. Then the barrier was jumped backto 0° in 0.2ms and stayed at 0° in 50ms, then turned
off suddenly. In the matching case, after the barrier wasmoved from 0° to 180°, it was moved back to 0° adiabatically. Then it was turned off suddenly. Those patternsof the barriers are shown in Fig. 4. The measured momentum spreads are listed in Table 1. In the ordinary
compression, the momentum spread was measured after
the barrier was moved from 0° to 180° adiabatically.
Table 1
The momentum spreads.
initialordinary compression
mismatching
matchingmomentum spread(±%)
0.10
0.17
0.180.13

In the mismatching case, the momentum spreadshould become double the initial momentum spread,
while its initial value should be kept in the matchingcase. The experimental results almost agree with the
expected values. A longitudinal mismatching induced
the longitudinal emittance growth.

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The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999
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sweep time(ms)-~-Fig. 3 Momentum spreads for the sweep time. The
solid circles and lines are the experimental andsimulation results, respectively. The initialmomentum spread is ±0.1%.
4 Conclusion
A beam compression experiment with barriers wasperformed at the HIMAC. A high-field gradient MA cavity was used here, which generated two barriers. The
distorted potential by the barriers was improved successfully with mixture of a few harmonics, so that thesmooth distribution of beam was achieved. The momentum spreads were measured varying the sweep speed. Itturned out that it was necessary for the sweep time to beslower than 10 times of drift time of the beam, which satisfy the adiabatic condition. A longitudinal mismatching
between the RF bucket and the beam bunch induced thelongitudinal emittance growth.
Acknowledgement
We would like to be grateful to Dr. M. Kanazawa,Dr. K. Noda and all the machine crew in the HIMAC.
References[1] J. Griffin et al., IEEE Trans. Nucl. Sci. 30, pp. 35023504, 1983.
[2] M. Blaskiewicz and J. M. Brennan, Proc. of the 5thEPAC, pp.2373-2376, 1996.
[3] M. Fujieda et al., Proc. of 1999 PAC, NY, USA, 1999;
M. Fujieda et al., "Barrier Bucket Experiment at theAGS", submitted.
[4] M. Blaskiewicz et al., Proc. of 1999 PAC, NY, USA,1999.
[5] L. Ahrens et al., Proc. of 1999 PAC, NY, USA, 1999.
[6] M. Fujieda et al., Proc. of the 1st APAC, pp.408-410,1998.
[7] C. Ohmori et al., Proc. of 1999 PAC, NY, USA, 1999.
[8] Y. Mori et al., Proc. of the 6th EPAC, pp. 299-301,
1998.
[9] M. Kanazawa et al., Proc. of 1999 PAC, NY, USA,1999.

Fig. 4 Mountain range plots of gap voltage. (a): mismatching case, (b): matching case. The period
between traces is lms.

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