The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Construction of the Flat-Top Acceleration System in the RIKEN AVF Cyclotron
Shigeo KOHARA", Akira GOTo•, Osamu KAMIGAITO", Masayuki KASE", Naruhiko SAKAMOTO",
Shinichi WATANABEb, and Takeshi KATAYAMN·b•The Institute of Physical and Chemical Research (RIKEN)
bCenter for Nuclear Study, Graduate School of Science, University of Tokyo (CNS)a,b2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan

Abstract
The flat-top acceleration system in the RIKEN AVF
Cyclotron was constructed and installed with the main
resonator in orcb- to improve the extraction efficiency andenergy spread of the beam in August 2001. The flat-top
accelerating voltage is generated by a superimposition of thefundamental-frequency and 3rd-harmonic-frequency voltages.
The fundamental-frequency range is from 12 to 24 MHz; the3rd-harmonic-frequency is used, from 36 to 72 MHz. Resultsof the test performed with the flat-top acceleration system are~cribed.

5th-harmonic frequency. So we have decided to ad:>pt the
3rd-harmonic frequency mode instead of the 5th-harmonicfrequency mode.

2 Structure of the resonator
The cross-sectional views of the main resonator and flattop resonator are shown in Fig. 1. The size of the flat-topresonator is 1 m in length, 50 mm in outer diameter of theinner conductor of the coaxial resonator and 250 mm ininside diameter of the outer conductor of the coaxialresonator.
Movable shorting plate

1 Introduction

Flat-top resonato1'

The RIKEN Ring Cyclotron (RRC) can acceleratevarious kind<; of ions ranging from proton to uranium in awide energy region [1]. One of the two injectors of the RRC
is the AVF Cyclotron, which is used for ions mainly fromproton to light heavy ions like Argon [2]. The AVF
Cyclotron has two resonators, each of which is the coaxial
quarter-wave-length type with a dee angle of 83°. A
grounded-cathode tetrode amplifier is capacitively coupled toeach resonator with a fixed vacuum coupling capacitor. Its
maximum output power is 20 kW. The rf system of the AVF
Cyclotron has been working well after its completion in
March 1989 [3]. The flat-top acceleration system in theAVF
Cyclotron has been planned to obtain better quality beam.
The result of the cold model test for the 5th-harmonicfrequency mode was reported elsewhere [4]. As the result, theflat-top accelerating voltage was generated in the whole
frequency range. But the power loss of the resonator was notmeasured when the rf characteristic measurement of the 5thharmonic-frequency mode was carried out. The rf
characteristics of the resonator were measured again in detailin order to ~ign a power amplifier after the construction ofthe real resonator was planned. In general, the power loss canbe less for the 5th-harmonic frequency than for the 3rdharmonic frequency, since the required amplitude of the 5thharmonic-frequency voltage is about 1125 of thefundamental-frequency voltage, while that of the 3rdharmonic-frequency voltage is about 119 of the fundamentalfrequency voltage. However, we found that the power loss forthe 3rd-harmonic frequency was actually less than that for theMain resonatorI
Movable 1
shortingI
plateI Voltage
1 pickup

L3

Dee50

100 emDee-voltage pickup

Fig. 1 Cross-sectional view of the flat-top acceleration
system for the RIKEN AVF Cyclotron.
The flat-top accelerating voltage is generated on the deeby aqjusting both the movable shorting plate (L3) and themovable coupling capacitor (Cc). The strokes of the movable
shorting plate and the movable coupling capacitor are 400
mm and 50 mm, respectively. The size of the coupling
capacitor is 150 mm in wi<.ih and 150 mm in height. The
capacitive feeder is. used to match the input impedance of theflat -top resonator to the impedance of the feeder ( 50 Q ). The
stroke of the capacitive feeder is 30 mm. The resonator istuned automatically with the capacitive tuner by detecting
incident and reflected waves on the feeder line, The stroke of-235-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

the capacitive tuner is 50 mm. A ceramics insulator is used
for support of the inner conductor of the coaxial resonator aswell as vacuum seal. Total coolant water of each flat-topresonator is 9 1/min. A photograph of the flat-top resonatorinstalled on the existing main resonator is shown in Fig. 2.

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3 Cold test of the resonatorRadio-frequency characteristics of the resonator were
measured with low-level signals by using a network analyzer.
The resonant frequencies with the flat-top resonator being
coupled to the main resonator were measured In thismeasurement, the signal was fed into the flat-top resonatorthrough the capacitive feeder and was monitored by the deevoltage pickup. The measured position of the movable
shorting plate and the movable coupling capacitor of theflat-top resonator are shown in Fig. 3 as a function of the
3rd-harmonic-resonant frequency. It was found that the 3rdharmonic mode was realized below 63.3 MHz, but not in thewhole required-frequency range. The measured Q-values of thefundamental and the 3rd-harmonic modes as a function of theresonant frequency are shown in Fig. 4 (a) and (b),respectively. Figure 5 shows the measured transmission(S21) of the flat-top resonator of the 3rd-harmonic mode. In
the measurement, the input impedance of the flat-topresonator was matched to the impedance of the feeder and thedee-voltage pickup was calibrated The estimated power loss
and the required voltage of the 3rd-harmonic mode are shownin Fig. 6, respectively. The power loss of the 3rd-harmonicmode was estimated by the measured value of thetransmission and the required voltage. The frequency-change
rate measured of the capacitive tuner was about 1 %. The
measured result shows that the performance of the No. 2
resonator is better than that of the No. 1 resonator. The
differences of the measured results between the No.1 and theNo.2 resonators are under investigation.

Fig. 3 Position of the movable shorting plate and coupling
capacitor as a function of the 3rd-harmonic-resonant
frequency. Squares and circles represent the position ofthe shorting plate and the coupling capacitor of theNo.1 resonator, respectively. Diamonds and triangles
represent the position of the shorting plate and thecoupling capacitor of the No.2 resonator, respectively.
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3rd-harmonic-resonant frequency (MHz)Fig. 4 (a) Q-value of the fundamental mode as a function ofthe resonant frequency. (b) Q-value of the 3rd-harmonicmode as a function of the resonant frequency. Circles
represent the Q-values of the No. 1 resonator andtriangles represent the Q-values of the No. 2 resonator.

-236-

4 Power test
In the first step, the power test of the 3rd-harmonic mode

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

was carried out to ~timate the required output power of the?rd-harmonic amplifier. The output power of the fundamental
amplifier was not fed into the main resonator in this power
test. The parameters of the flat-top resonator were set eachposition as shown in Fig. 3. The required output power ofthe 3rd-harmonic amplifier is shown in Fig. 7. The required
power was estimated by the measured dee voltage and output
power of the 3rd-harmonic amplifier and the required voltage.
The voltage of the 3rd-harmonic mode was generated below
57.3 MHz in the No. 1 resonator. The power test of the No.
2 resonator was not carried out because its 3rd-harmonic
power amplifier was breakdown.
In the next step, the power of the fundamental and the
3rd-harmonic frequencies were fed into the resonator at thesame time to generate the flat-top voltage on the dee. The
typical waveform of the flat-top voltage is shown in Fig. 8.

5 Conclusion
The flat-top acceleration system in the RII<EN AVF
Cyclotron was manufactured and installed with the main
resonator. The length of the flat-top resonator is 1 m and thestroke of the movable shorting plate is 400 mm. In the test,
the flat-top voltage was generated below about 60 MHz. For
the frequencies higher than 60 MHz, we plan to retest bychanging (reducing) the size of the coupling capacitor (Cc).
A beam test employed the flat-top acceleration system will
be performed in the near future to obtain better quality beam
after the power test of the No. 2 resonator will be carried out.
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3rd-harmonic-resonant frequency (MHz)u~ ~~-50[""'=~ 3a:; ·c0 02"'1l·S _gFig. 7 Required output power of the 3rd-harmonic amplifier.

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3rd-harmonic-resonant frequency (MHz)Fig. 5 Transmission of the flat-top resonator of the 3rdharmonic mode. Circles represent the transmission ofthe No. 1 resonator and triangles represent thetransmission of the No. 2 resonator.

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Fig. 8 Typical waveform of the flat-top voltage measured bythe dee-voltage pickup. The fundamental frequency was14.05 MHz and the 3rd-harmonic frequency was 42.15
MHz. The fundamental voltage was 29 kV .

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3rd-harmonic-resonant frequency (MHz)Fig. 6 Estimated power Joss and the required voltage of the3rd-harmonic mode. Triangles represent the estimatedpower loss of the No. 1 resonator. Circles represent theestimated power loss of the No. 2 resonator. Squares
represent the required voltage of the 3rd-harmonicmode.

[1] Y. Yano, "Status report on RII<EN Ring Cyclotron",
Proc. 12th Int. Conf. on Cyclotrons and theirApplications, Berlin, Germany (1989) 13[2] A. Goto et al., "Injector AVF cyclotron at RIKEN", Proc.
12th Int. Conf. on Cyclotrons and their Applications,
Berlin, Germany (1989) 51[3] S. Kohara et al., RIKEN Accel. Prog. Rep., 23, (1989)
110[4] S. Kohara et al., "Model Test of a Resonator for Flat-TopAcceleration System in the RIKEN AVF Cyclotron", Proc.
12th Symp. On Ace. Sci, and Tech., (1999) 227-237-