JAERI-Conf 95-021

PA49
Development of a Single-Bunch Selector
for the Riken Ring Cyclotron
N. Inabe, M. Kase, I. Yokoyama, A. Goto, andY. Yano
The insitute of Pysical and Chemical Reaearch (RIKEN)
Wako-shi, Saitama 351-01, Japan

Abstract

Continuum beam fromthe ECR ion source

We have developed a single-bunch
selector with fast repetition rate (1 MHz) for theRIKEN Ring Cyclotron. The system is compact
due to use of a beam chopper for the beam withlow velocity. Moreover by operating the buncher
for injection in the A VF cyclotron with a subharmonic mode, a purity of the single-bunchedbeam was raised up. The obtained purity of thebeam is more than 99.7 %.
1. Introduction

A single-bunched beam is often demanded
to measure a time spectrum in very short range (< 1ms). To fulfill the demand a single-bunch selector
has been developed. In the first step a beam
chopper was installed at the injection line of theA VF cyclotron (AVF). We could get the almost
single-bunched beam. A purity, that is a ratio of
the current of the desired bunch to total one
inciuding undesired bunches, is about 90 %.
However the purity is not enough for theexperiment to measure the time spectrum with a
low background. In the next step we tried to raiseup the purity by operating the buncher with subharmonic frequency of the A VF.
In similar cyclotron, the single-bunchedbeam is normally produced at downstream of thecyc-lotron where the beam energy is relativity sohigh. For the case the purity is independent of
extraction of the beam but the system is very large
and needs a high power. On the other hand, forthe present method a single-turn extraction isneeded. As the beam can be extracted from theA VF and the RIKEN Ring Cyclotron (RRC) 1), thepresent method can be applied in our facility.
Since single-bunched beam can be produced at low
energy the whole system can be small and of low
cost
2. Principle
Figure 1 shows the method for productionof the single-bunched beam. The single-bunchedbeam can be produced by sweeping undesired
bunches.

Beam picking up by thechopper
Acceptable bunchesof the AVF cyclotronwith the buncher
Single Bunch Mode___ ( 113 mode)_(Normal mode)!""1Ii""lr·:Ir·~r·:

Ir·r

Single bunched beamfrom the AVF cyclotron( !::_! swept bunch )Fig. 1. Production scheme of the singlebunched beam in the method.
One way is to sweep the beamtransversally using the chopper with parallelplate electrodes. In the present method sweeping is done for the continuum beam with a low energy
from the ion source (extraction voltage< 10 kV). A
base of the single-bunched beam is formed by theremaining part which is not swept. It has tails
inevitably as shown in Fig. 1. Origins of the tailare a traveling time that the beam is passing
through the electrodes with low velocity as well
as a rise and fall time of the voltage supplied toit. In order to produce the pure single-bunchedbeam the tails should not overlap with theacceptance of the next bunch to the desired one.
However, as it is difficult to shorten the tails fortechnical problems there is a limitation to produce
the pure single-bunched beam in this way.
Another way is to sweep the beam
longitudinally by operating the buncher with thesub-harmonic mode. In this way, a phase
acceptance becomes wide for a bunch synchronized
to the A VF and narrow for the other bunches. As
the result, the intensity of the synchronized bunch
is much stronger than that of the others. By use of
the synchronized beam as the desired one theintense single-bunched beam can be produced
although the purity is not so high.
By combination of the two ways, clearly,
overlap between the tail and the acceptance of
the undesired bunch becomes small. There might
be no limitation for production of the pure singlebunched beam in the combined way. In principle,
if the acceptance of the desired bunch can becomes- 165-JAERI-Conf 95-021

wide enough to include the tail, the singlebunched beam with the purity of 100 % can beproduced.
3. Devices
The electrodes of the beam chopper connect
with a DC power supply via a switching module.
The repetition rate of the switching module is1MHz that is demanded from a typicalexperiment.
A schematic drawing of the circuit of theswitching module is shown in Fig. 2.

Switching Module ""'We used the buncher that has beenalready installed.2) In order to operate the subharmonic mode we must use the buncher with low
frequency (4- 8 MHz). For the purpose we made a
new frequency divider that an amplifier and aphase is controllable. A wave made by thedivider is sent to the buncher via the wide-band
amplifier. A shape of the voltage is sin-curve.
For the single-bunch operation, the range of phase
to be compressed is wider than that for the normal
one. In this case, even the sin-shape can compress
the phase efficiently because the sin-shape isapproximately a straight line for the main regionof the phase to be compressed.

HV 0 "500V

200 kn4. Performance Study and Result
A performance study of the single-bunchsystem was carried out for the 7.45 keV H2+ beamfrom the PIS which was accelerated to 7
MeV /nucleon by the AVF with an RF frequency of
16.3 MHz and to 135 MeV /nucleon by the RRC
with an RF frequency of 32.6 MHz. The setup is
shown in Fig. 3.

Leading pulse

Fig. 2 Schematic drawing for the circuit of theswitching module
The switching module has two transcoupled switches with repetition rate of 1 MHz(HV 1000 P, N), which are bought from DEI inc inUSA. One is for the charge-up of the electrodeand the other for the discharge. To avoid anelectrical oscillation between two switches andthe electrode, we inserted several resisters there.
Two pulses for the switches are made by a pulse
generator located at upstream of the switchingmodule. One is a leading pulse which is sent tothe switch for discharge and the other is adelayed pulse which is sent to the one for charge.
The difference of time when each pulse arrives at
each switch corresponds to the duration for thebase of the single-bunched beam. Characteristicsof the pulse made by the switching module aresummarized in Table 1. The choppers are
installed on the two injection line. One is from theECR ion source (ECRIS) and the other is from thepolarized ion source (PIS).
Table 1. Characteristics of the pulse made by
the switchin_g module.
Voltage
0-500V
Repetition rate
<1MHz
Rise time
15ns
Duration time100-250ns

Fig. 3 Setup for a performance test of thesingle-bunch system.
The duration that the voltage between theelectrodes was switched off was 150 ns. Voltage ofthe electrodes was 470 V. The variable delay
connected with the switching module was adjusted
so as to get the purest single bunched beam for theRRC. Time structures of the beam after the A VF
and RRC were measured by using a time of flight(TOF) between a reduced RF signal and a timing
signal of micro channel plates (MCP's) with atarget 3). In this study we operated the buncher
with the normal mode because the frequency
divider had not been installed by that time.
Figure 4a shows a typical example of theTOF spectrum of the AVF. The main peak in Fig.
4a is due to a single bunched beam to be extracted.

- 166-

JAERI-Conf 95-021

The peak next to the main one is due to the next
bunch of the main one. The existence of the othertwo peaks means that the same bunch inside theA VF is extracted with two-turns. Production
mechanism of those peaks is explained in Fig. Sa.
The result of the many peaks in the spectrum
might be caused by the adjustment of the variabledelay that is suitable to the RRC. In fact, using
the system we obtained the single bunched beam asshown in Ref. 1 by adjustment of the variabledelay for the A VF and by single-turn extraction
from the A VF.
Figure 4b shows a typical example of theAVF (h=2)

NTime Structure of AVF

r--~.~M~d~c,-----Two-turns extraction of one bunch

BandD:
Two-turns extraction of another bunch

TOF-t- TRRC =t= TRRC ---t- TRRC =t= TRRC --:.

DeflrctoLl=_ TRRC~·/ ~·n

B~~~h'· Another turn inboth AVF and RRC"(~Another\,RRC (h=5)

1'0I\150200 250 300
Channel350

4004504TRRC - : - -f- 2TRRC

I-5TRRC -4TRRC

Another turn inboth AVF and RRC

oL~-~~~-~~u..u~-~~~
100

Originated from next bunch

Nr-____T_im_e_St_ru_ct_ure_of_RR_c_ _ _ _ __150/

TRRC RF period of RRC ( = 0.5"Ta0 )NextBunch

0Another /.turn in RRC

Originated from main bunch•turn in AVF

TOF SPECTRUM OF BEAM FROM AVF
200~,- - - - - - - - - - - - - - - - - - - - ,"--,,_Fig. 5 Time structure of the extracted beams for thetwo bunches with two-turns extraction from the
AVF and the RRC. The case for the AVF is shownin a) and that of RRC in b).

500

TOF SPECTRUM OF BEAM FROM RRC
10000lr-----------------,

6000

E sooo
8"
4000

2000

50

100

150

200

250 300
Channel

350

400

450

500

Fig. 4 Examples of time spectra of thesingle-bunched beam. From the A VF in a) andfrom the RRC in b) T_avf means the RF period ofthe AVF and T_rrc, of RRC.
TOF spectrum of the RRC. The main peak in Fig.
4b is made by the single bunched beam to beextracted. The peaks next to the main one are dueto the next bunches of the main one. The two small
peaks correspond to another turn of the RRC ( rightside ) and that of the A VF ( left side ). The purityof the single bunched beam is - 80 %. Production
mechanism of those peaks is explained in Fig. Sb.
As shown in Fig. Sb, in principle, the other turn of
the AVF gives a bad effect to the single bunchedbeam extracted from the RRC but the peak of theturn is not so strong. This might be due to badtransmission of the RRC for the tum.
Recently, the whole system including thenew controller was used to make a single-bunchedbeam for the deuteron beam from the ECRIS which
was accelerated to 4 MeV /nucleon by the AVF
with an RF frequency of 12.3 MHz. For the beam
we used the buncher with an frequency 4.1 MHz.
The purity of the single-bunched beam is 7 % byusing the only buncher with the sub-harmonicmode. Using the whole system we obtained thebeam with the purity of 0.3 %. The current of thebeam was about 100 enA.
References
A. Go to et. al., RIKEN Accel. Pro g.
1)
Rep., 27, p6 (1993)
K. Ikegami et. al.,2)
Prog. Rep., 22, p213 (1988)

RIKEN Accel.

T. Kawama et. al., RIKEN Accel.
3)
Prog. Rep., 27, p135 (1993)- 167-