JAERI-Conf 95-021

2p1-5
RF Beam Chopping in a Surface-Plasma Type Negative Ion Source
K. Shinto• A. Takagi, M. Kinsho, K. lkegami,
Z. Igarashi, S. Machida, M Yoshii, Y. Morit
National Laboratory for High Energy Physics (KEK)
1-1 Oho, Tsukuba, Ibaraki 305, JAPANAbstract
A direct fast chopped beam extracted froma surface-plasma H- ion source is proposed anda preliminary test has been examined. The converter bias voltage is modulated by rf pulses andthe extracted H- beam is observed. The direct fast chopped H- beam extracted from theion source has a good response to the modulation of the converter bias voltage as expected.
The chopped H- beam extracted by this method
has been injected into the 500 MeV booster synchrotron at KEK-PS.
1

arc pulser

filament

fast chopped

pulserIntroduction

DCRecently, the increase of the beam intensity
is more desired at 12 GeV proton synchrotron inKEK(KEK-PS). One of the difficulties to increase
the beam intensity is the beam loss at the beam injection from the linac to the booster synchrotron.
In order to eliminate the beam loss at the beam injection, the fast chopped beam synchronized withthe rf frequency of the booster synchrotron is required.
The fast beam choppers such as electrostatic
deflection devices have been developed and successfully achieved. However the space charge neutralization is distracted, and then the beam loss
and the emittance growth become severe in thisdevice. Therefore, it would be ideal that the fast
beam chopping can be achieved the H- formation in the ion source. Some new methods are attempted to make the fast chopped beam in theion source. For example, there are two methods to make the fast chopped beam by applying
the pulsed high voltage at the collar electrode in
the PIG type H- ion source[1] and at the plasma
electrode in the volume-production- type H- 1011
source[2].
At KEK-PS, a surface-plasma type H- ionsource(BLAKE)[3][4][5] is used for the H- beam
formation. In this ion source, the negative ions areproduced by the interaction between the positive•The Graduate University for Advanced Studies, 1-1
Oho, Tsukuba, Ibaraki 305, JAPAN1Institute for Nuclear Study, University of Tokyo, 3-2-1
Midori-cho, Tanashi, Tokyo 188, JAPANfilament P.S.

T"'•""'

P.S

Fig.1:The schematic drawing of the surface-plasma H- ion source.
ions in the plasma and the metal surface called
converter that is shielded from plasma. The converter bias voltage is modulated by the synchronized pulses with the rf frequency of the booster
synchrotron. The direct fast chopped H- beam
is produced by changing the H- production efficiency.
In this paper, a new method of the fast beam
chopping in the surface-plasma H- ion source at
KEK and preliminary results from the direct fast
H- beam chopping are presented.
2Experimental setup

Schematic drawing of a surface-plasma type
H- ion source used in this experiment is shown inFig. 1. Permanent magnets surrounding a plasmachamber are used to confine the plasma by cusp
magnetic field. The hydrogen plasma is producedby the electron emission from a couple of LaB 6filaments. To enhance the H- beam, the work
function of the converter must be lowered. Cs vapor is introduced into the plasma chamber and-230-JAERI-Conf 95-021

r------ ---i1

R1

C1I5Q0.1mF

1osc
Converter
electrode(Plasmasheath)

Baisl

DC

0.01t~~......;I-:.::U.....L..._..................,..._........,.........&&&L._._...............J10°

Fig.2: The circuit diagram of the rf modulated
power supply for the converter.

101

103
102
frequency f (kHz)104

Fig.3: The characteristi c of the ratio of the output
voltage for input voltage as a function of the rf
frequency.

the thin layer of Cs atom is formed onto the converter surface. H- ion is produced by the interaction between the plasma and the converter surface.
The direct fast chopped H- beam is produced bychanging the efficiency of H- formation following
the frequently changed voltage of the converterbias. The extracted H- beam is accelerated up to750 ke V by Cock croft-Walton type pre-accelera tor
and up to 40 MeV by the linac. The frequency ofthe direct chopped H- beam is synchronized withthat of the rf cavity in the 500 MeV booster synchrotron. Therefore by injecting a chopped Hbeam into each rf bucket, the beam loss caused by
the leak of the rf bucket is decreased in principle.
33.1

where n; is the ion density in the plasma. In
this ion source, the ion density seems to be about
1 x 1012 (cm- 3 ). The ion-plasma frequency is estimated about 200 MHz. The frequency of modulated voltage for the direct fast beam chopping isabout 2.2 MHz which is a negligible value from theion-plasma frequency. The H- ion source is operated in pulse mode (200 p.sec x 20 Hz) and the fast
chopped H- beam produced from the ion sourceis accelerated up to 30 keV at the test stand. The
H- production efficiency was changed by the modulated voltage of converter bias with this circuit,and the direct fast chopped H- beam was produced successfully.

Result and discussion

3.2

Production of direct fast chopped Hbeam

The circuit diagram of the rf modulated power
supply for the converter is shown in Fig. 2. The
converter bias is modulated with the high voltage
pulse triggered by the clock pulse of rf frequencyof the booster synchrotron . The characteristi c ofthe output voltage for the input voltage of thematching circuit as a function of the rf frequency
is shown in Fig. 3. In the region of this experiment (about 2.2 MHz), the matching circuit is designed to make the unique ratio of input voltage
to output voltage and not to cause any resonance.
The converter is shielded from the plasma by theion sheath. However the ion sheath is broken by
applying the rf voltage beyond the ion-plasma frequency. The ion-plasma frequency, w;, is given by,w;=(1)

Acceleratio n of the direct fast
chopped H- beam

The power supply for the rf modulation of converter bias is installed in the high voltage stationof the Cock croft-Walton type pre-accelera tor. A
direct fast chopped H- beam extracted from theion source is accelerated up to 750 ke V, injected
into the 40 MeV linac and then injected into the500 MeV booster synchrotron ofthe KEK-PS. The
wave form of the direct fast chopped H- beammeasured by the Faraday cup after the 40 MeV
linac is shown in Fig. 4. About 94% of the maximum H- beam current is suppressed by the modulation of converter bias voltage[6].
3.3Longitudin al emittance of the direct
fast chopped H- beam in the booster
synchrotro nThe direct fast chopped H- beam is accelerated
at the linac and injected into the booster synchrotron. The wave form of each bunched H--231-

JAERI-Conf 95-021

I==:c:==:c= =:;J:~_..-·, 1.o 1 --,::=::::::::.J-o-owidth 40J1s with modulation

Oj

Rl' 26.2kV

width 40J1S without modulation RF 26.2kV Mesh in-6- width 40i.s with modulation
Rl' 19.9kV

0.8 1-·············.!!

0.6 f-·············+··············t·················!·················!················~

0.0 0.

.

.

:-..;;

510152025~ :·: ~....J.:::.::.:::::.:r:::·:::·:::::r::·::::::::::
1. j i j ji

Delay time from injectiont (msec)

Fig.5: The longitudinal eniittace as a function of
the injection time.
Fig.4: An example of the waveform of the fast
chopped H- beam measured by the Faraday cup
at 40 MeV beam line. (vertical axis:2 mA/div.,
horizontal axis:l JLSec/div.) About 94% of themaximum H- beam current is suppressed by theconverter bias modulation.
beam is observed during the accelerating period in
the booster synchrotron, and the bunch length including 90% of the bunch height is measured. The
longitudinal emittance is derived from the bunch
length of each bunch in the direct fast chopped Hbeam injected into the booster synchrotron. Fromthe phase equation[7], the longitudinal emittance,€1, is given by,T€1= .6.E · 2'(2)where .6.E, r are the energy spread for the energy of the synchronous particle and the bunch
length of 90% of the bunch height, respectively.
The longitudinal emittance as a function of thetime delay from the particle injection is shownin Fig. 5. The particle number injected into thebooster make to be almost equal in each experiment; about 4x 10 11 ppp. In non-chopped Hbeam directly from the ion source, the beam intensity is decreased by the carbon mesh. The longitudinal emittance with the direct fast chopped
H- beam is almost equal to that without chopped
H- beam directly from the ion source. Using thedirect chopped H- beam, the beam loss of theleak from the rf bucket could be suppressed by agood matching between rf bucket and linac beam.
From this figure, the longitudinal emittance depends upon the amplitude of the rf voltage, nevertheless the particle number is equal in each experiment.

4Summary

The direct fast chopped H- beam is produced
from the ion source and injected into the 500 MeV
booster synchrotron of the KEK-PS. And it isconfimed that the longitudinal emittance depends
upon the amplitude of the rf bucket.
The authors would like to thank Prof. Yamane
and PS group for valuable comments.
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Accelerator Conference, Washington, pp.31753177.
[3] K. W. Ehlers and K. N. Leung, Rev. Sci. Instrum., 51, pp721-727(1980).
(4] R. L. York and R. R. Stevens, Jr., AlP Conf.
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[5] Y. Mori,A. Takagi, K. Ikegami and S. FUkumoto, AlP Conf. Proc. No. 158, pp378383(1986).
[6) K. Shinto, A. Takagi, M. Kinsho, K. Ikegami,Z. Igarashi, S. Machida, M. Yoshii and Y.
Mori, Submitted to Proc. of 1995 IEEE Particle Accelerator Conference, Dallas.
[7] J. L. Duff, Cern Accelerator School General
Accelerator Physics (CERN 85-19), pp.125143.

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