The 11th Symposium on Accelerator Science dnd Technology, Harima Science Garden City, 1997

Design of a Resonator for a Flat-Top Acceleration System in the RIKEN A VF Cyclotron
Shigeo KOHARA, Yoshitoshi MIY AZAWA, Osamu KAMIGAITO, and Akira GOTO
The Institute of Physical and Chemical Research (RIKEN)
Wako-shi, Saitama 351-01 , Japan

Abstract
A resonator for a flat-top acceleration system inthe RIKEN AVF cyclotron is designed to improve theextraction efficiency and the energy spread of a beam. In ortlerto generate the flat-top accelerating voltage qn the dee, anadlitional resonator or a transmission line is capacitively
coupled to the AVF resonator with a coupling capacitor. The
flat-top accelerating voltage is obtained by thesuperimposition of the fundamental frequency and the fifthharmonic-frequency voltages. Length of the adlitional
resonator is 90 em and capacitance of the coupling capacitor
30 pF. The frequency range of the AVF resonator is from 12
to 23 MHz. Structure and rf characteristics of the resonator
designed for the flat-top acceleration system is described.

1 Introduction
The RIKEN Ring Cyclotron (RRC) can acceleratevarious kinds 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 Ar[2]. The rf system of the
AVF cyclotron has been working well after its completion inMarch 1989[3].
Resonator

Fig. 1. Layout of the AVF cyclotron.

A layout ofthe A VF cyclotron is shown in Fig. 1.
The AVF cyclotron has two resonators, each being of thecoaxial quarter-wave-length type with a dee angle of 83 °. Thefrequency range is from 12 to 23 MHz. The required
maximum accelerating voltage is 50 kV. In general, theamplitude of the fifth-harmonic-frequency voltage required isabout 1/25 of the fundamental-frequency voltage. Therefore,
the frequency range and maximum voltage of the fifthharmonic-frequency system are estimated to be from 60 to
115 MHz and 2 kV, respectively, when the voltage
distribution is flat. A movable shorting plate has been used
for the coarse tuning and a capacitive tuner for automatic finetuning in the AVF resonator. The stroke of the movable
shorting plate is 2 m. The frequency-shift range of thecapacitive tuner is about 500kHz. A grounded-cathode tetrode(4CW50,000E) amplifier is capacitively coupled to the AVF
resonator with a fixed vacuum coupling capacitor (16 pF). Its
maximum output power is 20 kW.

2 Structure and rf characteristics
A cross-sectional view of the resonator designed
for the flat-top acceleration system is shown in Fig. 2. It
consists of AVF and adlitional resonators to generate theflat-top accelerating voltage on the dee[4]. The adlitional
resonator is employed to excite a higher harmonic resonancein the AVF resonator with the frequency five times of thefundamental frequency without shift of the fundamentalfrequency. The additional resonator consists of a transmission
line and a coupling capacitor (Cc). The fifth-harmonicresonant frequency is determined by acljusting both theposition of the movable shorting plate (LS) and the coupling
capacitance of the aditional resonator after the fundamental
frequency of the AVF resonator is set by the movable
shorting plate (Lavt).
The rf characteristics of the resonator wascalculated with the transmission-line approximation. Figure
3 shows the positions of the movable shorting plates of theadlitional and AVF resonators to obtain the fifth-harmonicresonant freq.Iencies. The most upper position of themovable shorting plate of the adlitional resonator is 50 em
when the coupling capacitance is 30 pF. Shunt impedances
calculated as a function of the fifth-harmonic resonant
frequency are shown in Fig. 4. The ratio of the voltage at the-194-

The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

extraction position to that of the injection position of theAVF cyclotron is from 0.85 at 60 MHz to 0.49 at 115 MHz.
Movable shorting plate
Additional resonator

AVF resonator,------------~!

Movable shorting plate

Therefore, the required fifth-harmonic-frequency voltage at
115 MHz is estimated to be 1/17 of the fundamental. frequency voltage; its maximum voltage is to be 3 kV. Powerlosses are estimated to be 2,900 W at 58.5 MHz, 560 W at 80
MHz, and 690 W at 115 MHz when the voltage is 3 kV.
Each movable shorting plate is used for coarse
tuning and each capacitive tuner for fine tunirig. Frequency
shift calculated as a function of capacitance of the capacitive
tuner of the additional resonator is shown in Fig. 5.

8
Dee

Capacitive tuner
16 pF
4CW50,000E~

I

6Cc4
Dee

2
Capacitive tuner

00~-L~~~~--~-i~

IOOcm

50

so 60 70 80 90 100 11 0 120

Dee voltage pickup

Fifth-harmonic resonant frequency (MHz)Fig. 2. Cross-sectional view of the resonator for the flat-top
acceleration system.

4-;00,-...

]Sc,u'...-'

gjl~nso2SO"'~

20040

s0 0~

15030"'0o..s115MHzl:li)U1000!=:'--'.... 4-;t::>
0 C<:l,....._"'.QS.0 .....

:I:,.1<1

N.._.

100

20

0:a~

""

se:>-.

~.S:>

so;:I"'<t:0"!=: 0400

£ 200

o.S

:.a"'0p,.

050

600

uOf.J;.

10800

¢.::

C<:l C<:l-~~

p,.

1200

:> !=:.........

..d".O
4-<"00"0!=: o:s
00 ,.c::....

0,.c::...:l0 Q-o~"'>~
o~4-;

0.....
C<:l,.-....

-~~,.c::OFig. 4. Shunt impedance calculated as a function of thefifth-harmonic resonant fre<pency. The coupling
capacitance (Cc) is 30 pF.

60708090100 110 120

00

Fifth-harmonic resonant frequency (MHz)Fig. 3. Position of the movable shorting plates of theadditional and AVF resonators calculated as a functionof the fifth-harmonic resonant frequency. The coupling
capacitance (Cc) is 30 pF.

2345

67

Capacitance of the tuner (pF)
Fig. 5. Fre<pency shift calculated as a function of thecapacitanCe of the capacitive tuner of "the adlitional
resonator.

-195-The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

3 Power feeding

References

A power amplifier for the fifth-harmonicfrequency will be a solid-state wide-band amplifier (WBA),
whose output impedance is 50 0. A capacitive divider isused to match the input impedance of the additional resonatorto the output impedance of the wide-band amplifier. The
capacitive divider consists of a coupling capacitor (C I) and avariable capacitor (C2). To get a good impedance matching,
the capacitance of the variable capacitor is estimated to befrom 90 to 120 pF when the coupling capacitance is 3 pF.

[I] Y. Yano, "Status report on RIKEN Ring Cyclotron," in4 Conclusion
Voltage of the fifth-harmonic· frequency wasobtained on the dee without shift of the fundamental
frequency, when capacitively coupling an additional resonatorto the AVF resonator with a coupling capacitor. Therefore, aflat-top accelerating voltage can be generated on the dee by
feeding rf powers synchronously to the respective resonatorswith fundamental-frequency and fifth-harmonic-frequency
amplifiers. The required frequency range was found to becovered with a fixed coupling capacitor of 30 pF.

Proceedings of the 12 Int. Conf. on Cyclotrons and theirApplications, (1989) 13.
[2] A Goto et al., "Injector AVF cyclotron at RIKEN," inProceedings of the 12 Int. Conf. on Cyclotrons and their
Applications, ( 1989) 51.
[3] S. Kohara et al., "Status of the rf ~ystem for the injector
AVF cyclotron," RIKEN Accel. Prog. Rep., 23, ( 1989) II 0.
[4] J. L. Conradie, "Improved proton beam quality findintensity from a 200 MeV cyclotron system," National
Accelerator Centre, South Africa, Dissertation presented forthe degree of Doctor of Philosophy in Physics at theUniversity of Stelenbosch, (1992).

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