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

DESIGN OF THE FLAT-TOP RESONATOR FOR THE JAERI AVF
CYCLOTRON
S. Kurashima, N. Miyawaki, M. Fukuda, Y. Nakamura, T. Nara, T. Agematsu, I. Ishibori,W. Yokota, S. Okumura and K. Arakawa
Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan
Abstract

A flat-top acceleration resonator for the JAERI AVF
cyclotron has been designed. The fifth harmonics of thefundamental frequency is used to make overall energy
gain uniform. A cold model test was carried out todetermine optimum parameters of the flat-top resonator.
The design of the flat-top resonator has been modified
using the MAFIA code to minimize a cavity length and tosave output power of an amplifier. The optimum ratios of
the fifth harmonic amplitude to the fundamental one forminimizing the energy spread were evaluated from aradial distribution of the dee voltage.

1 INTRODUCTION
The JAERI AVF cyclotron can accelerate a variety ofion species for the research in biotechnology andmaterials science. A flat-top (FT) acceleration system forthe cyclotron has been designed to minimize the energy
spread mainly for a microbeam production. Themicrobeam is extremely useful for elucidation of thedynamics of cellular repair and the intracellular processof functions such as apoptosis. The present microbeam[1] is produced by using a collimator with an aperture ofabout 10 J.l.m in diameter. The size of the microbeam spot
needs to be decreased to 1 J.l.m to improve .the precisionof the biological experiment. The energy spread of thecyclotron beam is required to be reduced to aE/E = 2 X
10-4 to produce the 1 J.1.ffi diameter microbeam byfocusing the beam with quadrupole magnets.
The cyclotron has a pair of quarter-wavelength (A/4)
coaxial type resonators with a movable-short [2]. A
maximum acceleration voltage is 60 kV in a CW mode.
The diameters of inner and outer tubes of the coaxial
cavity are 300 mm and 1000 mm, respectively. The rangeof the fundamental frequency is 11 to 22 MHz. Harmonicmodes of 1, 2 and 3 are used to cover a wide range ofenergy.
In general, the third- or fifth-harmonics of thefundamental frequency is used for the flat-top
acceleration [3, 4]. The amplitude of the higher
harmonics superimposed to the fundamental waveform isestimated to be 1/N2 times the fundamental voltage,
where the N is the order of the harmonics. We haveadopted the fifth-harmonics to save output power of anamplifier. The fifth-harmonic voltage in the frequency
range of 55 to 110 MHz is required to cover the whole
range of the fundamental frequency.

2DESIGN OF THE FLAT-TOP
RESONATOR

2.1 A Cold Model TestTo determine parameters of the FT resonator, a coldmodel test was carried out [5] using the model of an FT
resonator preliminarily designed for the RIKEN AVF
cyclotron [6]. Figure 1 shows the cross sectional view ofthe model of the FT resonator coupled to the main
resonator with a capacitive coupler (C5) of 150 mm
square. The diameters of inner and outer tubes are 40 mm
and 200 mm, respectively. The gap of C5 was varied
from 6 to 25 mm and the position of a movable-short(LS) from 29 to 388 mm. The induced FT waveforms
were observed successfully at the dee voltage pick-up of
the main resonator when the fundamental frequency weretuned to 11, 13, 15 and 20 MHz. Power dissipations forthe fifth-harmonic frequencies were estimated from theamplitude levels of the signal generator and the pick-up
signal. Assuming that the fundamental voltage was 30 kV,
about 1 kW output power of an amplifier is required toachieve the FT acceleration. The power dissipations werecorrected on the voltage distribution along the-232-50kl A~oll I i or!

III!

Figure 1: Cross sectional view of the main resonatorand the model of theFT resonator. TheFT resonatormodel is capacitively coupled to the main resonatorwith an electrode nlate of 150 X 150 mm 2_

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

acceleration gap of the dee electrode and on frequency
dependence of the pick-up capacitor.

2.2 Design of theFT Resonator Using the MAFIA CodeIn order to ·improve the design of the FT resonator,
optimum parameters to downsize the FT resonator itself
and to decrease power dissipation were investigatedusing the MAFIA code [7]. The compactness of theresonator is indispensable condition due to the limited
space for mounting the FT resonator.
The resonance frequencies of the A./4 and 3/../4 modes
were evaluated to determine the reliability of a mesh used
for the calculation with the MAFIA code. Dependenceof the resonance frequencies on the position of the
movable-short is shown in Fig. 2. The calculated
frequencies were consistent with the measured ones. The
power dissipation at the fundamental frequency of 15
MHz was found to be minimized using the FT resonatorconsisting of a 70 mm diameter inner- and a 300 mm
diameter outer-tubes. The power consumption wasestimated to be 80 % of the result in the cold model test.
Figure 3 shows a correlation between the gap of C5 andfundamental (measured)
fumdarnental (MAFIA)

- --•
•

120 l""""....,...,._....,.,...,.......,...,j ~~ -- 3.</4 mode (measured)g

N 100 -'~+... +- • •t~"'-'-·80-•..;

fundamental" _SX__ _..,..
_ _ _(measured).1. - . __>.

1.2~----~--~----~----~-----,:::i 0.8

6.-...

the position of L5 to obtain the fifth-harmonicfrequencies. The maximum length of L5 was limited to
400 mm for the compactness, and the minimum length to
100 mm to mount a power feeder and a frequency
compensation tuner.
In principle, the amplitude of the fifth-harmonic
voltage should be 1125 of the fundamental one to acquirea uniform energy gain when the voltage distribution
along the acceleration gap is uniform. However, theresonator actually has position dependence of the voltage
distribution, which varies with the resonance frequency.
Variations of the fundamental and fifth-harmonicvoltages are shown in Fig. 4. The calculated fifthharmonic voltage along the acceleration gap of the dee atthe fundamental frequency of 21 MHz decreases to 30 %of the muximum voltage at an extraction radius of 92.3
em. On the other hand, the fundamental voltage has alittle decrease. Therefore, the fifth-harmonic amplitude
needs to be optimized for each frequency to uniform theoverall energy gain just before extraction.

0.6"'tiD"' 0.4c"'::>..."' 60 u..
0"-->...__-/\\),._

0.2 -"' 40 <.>

1:

"'c

1:

"'"'

D<200··-· -·-·-·--- --· --·-- -·- -·Figure 2: Comparison of the resonance frequencies
between the calculation and the measurement.

250u;....J200
150
100
500Jo L5 :-:1 csj
DD025 ("')
20 ,......
""

D06060070

5th Harmonic

8090

34035
30050••• • •315 .......
3
10500100Frequency (MHz)

200

400

600

800

1000

Figure 4: Distribution of the fundamental and the fifthharmonic voltage along the acceleration gap at thefundamental frequencies of 11 and 21 MHz.

Position of the Movable Short (mm)e••

11 MHz55 MHz
21 Milz
1115 Mile

Distance from the Center (mm)---'-"••00 0!-'-.......2::-:!0~0..........4~0':"0.......~6-:-00:.""-'~80~0.........1~0~00~-1~20~0.........,1400

450
400
350.......
e 300"'•••110

Figure 3: Parameters of the FT resonator calculated withthe MAFIA. C5 is the gap of the capacitive coupler. L5is the position of the movable-short.

EVALUATION OF THE FIFTHHARMONIC VOLTAGE

In order to evaluate the fifth-harmonic voltage
required to minimize the energy spread of a beam, we
have calculated particle energy using the voltage
distribution obtained by the MAFIA code. Assuming thatisochronism was ideally guaranteed and particles traveledon equilibrium orbits, energy gains of the particles weresummed up until reaching to the extraction radius.
Energies of the particles with different initial beamphases were averaged simply. The fifth-harmonic voltage
providing the minimum energy spread was estimated by aleast-squares method. We have calculated the optimum
fifth-harmonic voltage for each harmonic mode. Thex2 values of the least-squares fitting for 70 MeV W are
shown in Fig. 5. The ratio of the fundamental voltage tothe fifth-harmonic one, VrN5 , giving the minimum x2-233-

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

value, shifts toward the smaller side when the initial
beam phase acceptance l<l>ol increases.
The permissible range of the fifth-harmonic voltage toreduce the energy spread to L\EIE = 2 X 10'4 becomeswider for smaller initial phase acceptance. The optimum
voltage ratio is 11.86 for the beam phase acceptance ofl<!>ol :s; 6 ° RF, 11.68 for l<!>ol :s; 8 ° RF, 11.45 for l<!>ol :s;
10 ° RF. The calculated voltage ratio is about a half ofthe predicted value of 25. The energy spread within L\E/E= 2 X 10'4 can be achieved for the initial beam phase
acceptance of l<!>ol :s; 10 ° RF. Figure 6 shows thedependence of the VtN 5 on the fundamental frequencies
for each harmonic mode. The voltage ratios for theharmonic mode 2 and 3 are larger than the harmonicmode 1. The turn number of the harmonic mode 1 is 550,
which is more than twice of the turn number of theharmonic mode 2 and 3. The energy gains in the larger
radius region decrease due to the noticeable drop in the/,.,. "/I/

•

<f//'

4. 0 10 -·/2. 0 10 -·

11.5

12

12. 5

Voltage Ratio (V /V 5)
Figure 5: The x2 of the least-squares method obtained for
70 MeV W. The optimum voltage ratio VtN 5 wascalculated for the initial beam phase acceptance of l<l>ol :s;
6 o RF, l<l>ol :s; 8 o RF and I<Pol :s; 10 o RF.

fifth-harmonic voltage distribution as shown in Fig. 4.
The harmonic mode 1 is more sensitive to the effectbecause of the large turn number. In the higher frequency
region, the effect is clearly identified, since the voltage
drop becomes steeper.

REFERENCES[1] T. Kamiya, et al,. "Development of an AutomatedSingle Cell Irradiation System Combined with A
High-energy Heavy Ion Microbeam System," Proc.
7th Int. Conf. on Nuclear Microprobe Technologyand Applications, Bordeaux, France, (2000) to bepublished.
[2] Y. Kumata, et al., "A Modified RF System of JAERIA VF Cyclotron," Proc. 13th Int. Conf. on Cyclotron
and Their Applications, Vancouver, Canada, 526(1992).
[3] B. Bischof, "The RF-System of the FlattopAcceleration Structure in the SIN 590-MeV-RingCyclotron," IEEE Trans. Nucl. Sci. NS-26, 2186
(1979).
[4] J. L. Conradie, et al., "A Flat-Top Acceleration
Systems for the NAC Light Ion Injector Cyclotron,"
Proc. 14th Int. Conf. on Cyclotron and TheirApplications, Cape Town, South Africa, 249 (1995).
[5] · S. Kurashima, et al., "Design of the Flat-TopAcceleration System for the JAERI A VF Cyclotron,"
Proc. 15th Int. Conf. on Cyclotron and TheirApplications, East Lansing, USA, (2001) to bepublished.
[6] S. Kohara, eta!., "Model Test of a Resonator for FlatTop Acceleration System in the RIKEN A VF
Cyclotron," Proc. 12th Symp. on Accelerator Scienceand Technology, Wako, Japan, 227 (1999).
[7] The MAFIA collaboration, User's Guide MAFIA
Version 4, CST GmbH, LauteschliigrerstraBe 38, D64289, Darmstadt, Germany .

...........
>......_>........ 20.0-..,.
0IX~ 15.00>'"·

10. 0 ..............~...........""'"""..............J-;...........................~............
101214161820

Fundamental Resonance Frequency (MHz)22Figure 6: Dependence of the optimum voltage ratio on
the fundamental frequencies. The optimum voltage ratios
were calculated for each harmonic mode. The initial
beam phase acceptance was !<Pol :s; 8 ° RF.

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