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

RF Acceleration System for Medical Synchrotron
Kazuyoshi SAITO, Hideaki NISIDUTI, Jun-ichi HIROTA* and Mamoru KATANE*Hitachi Research Laboratory, Hitachi Ltd. 7-1-1 Omika, Hitachi, lbaraki 319-12, Japan* Hitachi Works, Hitachi Ltd. 3-1-1 Saiwai, Hitachi, lbaraki 317, JapanAbstract

2. RF Cavity

We have developed a FINEMET-core loaded untuned
RF cavity using multifeed coupling. The operationfrequency range was widened to 0.5-8.5 MHz, where thevoltage standing wave ratio (VSWR) was lower than 2. The
accelerating gap voltage higher than 1.5 kV was achieved
with the cavity length of 550 mm and a supplied RF powerof 2.4 kW. We have also developed an RF control systemusing a direct digital synthesizer (DDS). The performanceof the RF control system was checked with a signal
generator to simulate beam motions such as radialdisplacement of the central orbit, dipole and quadrupole
oscillations of the bunch.

A photograph of the fabricated untuned RF cavity ispresented in Fig. 1. The design specifications aresummarized in Table 1. The RF cavity has a single
accelerating gap structure which consists of two quarterwavelength coaxial resonators loaded with 12 FINEMET
cores. The length of the RF cavity is 550 mm. The cavity
impedance is designed as 700±150 n in the operation
frequency range of 0.5-8.5 MHz.

1. Introduction

Recently, the need for cancer treatments has been
increasing more and more, reflecting the situation that morethan one fourth of all deaths in Japan are due to various
kinds of cancers. We have been developing a medical
synchrotron for cancer therapy which accelerates proton andheavy ion beams. The medical synchrotron is operated in a
hospital by a few operators who are non-experts onaccelerators. Compactness, low cost and simple operation
are therefore required. One of the complexities in its
operation comes from control of the RF cavity. We selected
an untuned RF cavity, since its resonant frequency does notneed to be tuned to the frequency of supplied· RF power.
The control system of the RF cavity is therefore simple andan expensive power supply for a bias current to adjust
permeability of loaded magnetic cores is unnecessary.
Because complicated bias windings around the magnetic
cores are also dispensable, the RF cavity becomes simpleand compact.
The untuned RF cavity requires a Q-value lowenough (Q<l) to cover a wide revolution frequency range of
0.5-8.5 MHz and an accelerating gap voltage higher than 3
kV to accelerate heavy ions up to 400 MeV/u. We selected
FINEMETO as the material employed for the magnetic
cores to realize the wide operation frequency range and thehigh accelerating gap voltage. FINEMET is a Fe-based soft
magnetic alloy composed of ultrafine grain structure,developed by Hitachi Metals, Ltd. It has high complex
permeability of (800,1500) around 3 MHz, high Curie
temperature of 570 °C and high saturation flux density of
1.35 T. Excellent stability against excitation and heat-up
by a high RF power has already been confirmed with atesting cavity [1].
In this paper, results of performance tests arepresented of a fabricated untuned RF cavity dedicated tomedical synchrotron, together with those of RF control
system using a direct digital synthesizer (DDS).

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Table 1 Design specifications.
operation frequency

0.5- 8.5MHz> 1.5 kV (( > 3 kV ))cavity imp!l(iance
700 ± 150 n (58± 13 ntcore)quarter wavelength coaxial resonator x 2
cavity length
SSOmm
cavity structure
outer conductor diameter
S80mm
inner conductor diameter
1SOmm
gap
lengthSOmm
accelerating gap
installed number
1
core material
FINEMET ( Fr- 3M)
toroidal ringouter diameterSOOmmcore shape
inner diameter
280mm
26mm
width
core loaded number
12200 W/core (( 800 W/core ))core power loss
total 2.4 kW (( 9.6 kW ))core coolingforced air-<lOOlinll
power feedim~
multifeed couolin2
gap voltage(( ... )) means tbe possible value.

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

A solid-state amplifier is employed as an RF powersource instead of a vacuum tube amplifier to reduce theburden of maintenance. Impedance matching between thecavity, feeding line and the power source is indispensable toproduce a high voltage at the accelerating gap. Although
the impedances of the power source and the feeding line are50 0, the cavity impedance is designed as about 700 0 tomoderate dissipated RF power. Multifeed coupling [2,3] hasbeen adopted to realize impedance matching between thecavity and feeding line. The cavity is coupled to the feedinglines with magnetic loops, each of which is wound around
one FINEMET core. The cavity impedance is set to about
50 Q per one FINEMET core to match the impedance ofindividual feeding lines.
Figure 2 shows the frequency dependence of thevoltage standing wave ratio (VSWR) measured by thereflection method with a network analyzer. The VSWR waslower than 2 in the frequency range from 0.5 to 8.5 MHz.
This meant power reflection from the cavity was less than
10 % and the cavity impedance matched well the impedanceof individual feeding lines over the operation frequency
range. On the other hand, a ferrite-loaded untuned RF cavity
did not give such a good impedance matching over the wide
frequency range without a parallel resistor across theaccelerating gap [4].

results of the testing cavity [1], the operation at the supplied
RF power of 800 W/core should be possible, owing to thestability of the FINEMET cores against excitation and heatup by a high RF power. Then, the accelerating gap voltage
higher than 3 kV can be obtained.

o 2.4 kWa 1.2 kW2.5
2.0- - FINEMET······-·· Ferrite

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20Q .......

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2t -r- r- -:46810Frequency ( MHz )12Figure 3 Result of the high power test.
The test was carried out by constant power
feeding and also under AVC operation.

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~100:

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II AVC 1.1 kV!
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04
68Frequency ( MHz)

210Figure 2 Result of the low power test.
Comparison with a ferrite-loaded
untuned RF cavity is shown.
Figure 3 shows the frequency dependence of theaccelerating gap voltage measured with two voltage dividers
connected to both sides of the gap. The wide operation
frequency range was also conftrmed in the high power test.
The accelerating gap voltage higher than 1.5 kV wasattained with a supplied RF power of 2.4 kW (200W/core).
Surface temperatures of the FINEMET cores were monitoredwith thermocouples. There should be little differences
between the measured surface temperatures and the internaltemperatures, because of good thermal conductivity of theFINEMET cores. The maximum temperature rise was 25°C from the room temperature of 20 °C. Therefore theeffectiveness of air cooling with two blowers wasconfirmed. The accelerating gap voltage is now limited
only by the available RF power. Based on the experimental

3. RF Control System
We have developed an RF control system employing
a DDS as the reference signal generator for the RF voltage
applied to the cavity, to realize stable and reproducible
operation of the synchrotron. The excellent performance of
the RF control system was already reported at HIMAC [5].
The DDS realizes the RF control system operating in a wide
frequency range with low FM noise and highly stable andaccurate center frequency. RF frequency can be set withsuch resolution that the frequency error at the injectionenergy becomes -lo-6, which is small enough comparedwith the momentum spread of the injection beam of -lo-3.
The injection beam can be captured stably in the RF bucketwith high efficiency.
The RF control system employs the following four
feedback loops: automatic voltage control loop (AVC loop),
radial beam position control loop (L\R loop), dipole andquadrupole bunch-oscillation suppression loops (L\<I>, 2fsloops). The AVC loop adjusts the cavity gap voltage so as
to follow the designed value. The RF frequency is correctedwith the L\R and L\<I> loops so as to reduce displacement of
the beam orbit and to damp the dipole oscillation. Thecavity gap voltage is slightly modulated with 2fs loop forthe suppression of the quadrupole oscillation.
The performance of the feedback loops was checkedwith a signal generator to simulate the beam motions: radialdisplacement of the central orbit, dipole and quadrupole
oscillations of the bunch. The signal generator realized thebeam transfer function from the cavity gap voltage to theoutput of beam monitor, and closed the feedback loops even
without a real beam and a beam monitor. Results of theperformance tests were shown in Figs. 3-6, in which each-198-The 11th Symposium on Accelerator Science dnd Technology, Harima Science Garden City, 1997

-feedback loop was independently exainitled, although a
combination test was possible. (1) The AVC loop realized.
the voltage regulation within 5 %. (2) The 4R loop reduced
the orbit displacement from 60 mm to lower than 1 mm.
(3) The A~ loop suppressed the dipole oscillation of 5.7
kHz completely.. (4) The 2fsloop damped the quadrupole
oscillation of 8 kHz rapidly. These results meet thespecifications required for the RF control system of a
medical synchrotron.

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0t4. Discussion.

.

We have confnmed the performance of a FINEMETcore loaded untuned RF cavity using multlfeed coupling.
For a heavy ion synchrotron, two cavities should beinstalled to produce the accelerating gap voltage higher than
'3 kV with the cavity length of 1.1 m and the supplied RF
power of 4.8 kW. For a proton synchrotron, only one
cavity should be installed to generate the accelerating gap
voltage of 1 kV without suffering beam loading effectduring debuncbing process. If a rapid-cycling synchrotron isadopted, the circulating beam current is much lower
although the required accelerating gap. voltage is ratherhigher. Several cavities should ·then be installed. For
example, four cavities give the accelerating gap voltage
higher than 6 kV with the cavity length of 2.2 m and thesupplied RF power of 9.6 kW.

time ( 1Oms/div )Figure 4 Performance of AR feedback loop.

F.B.
OFF4.0 MHz

Acknowledgment

F.B.
ON

----50 kHz - - - 1....

time(a) Output Signal ofPhase Detector(b} Frequency Spectrum of
Beam Intensity Signal

Figure 5 Performance of A~ feedback loop.
The upper and lower figures show
the signals when the feedback was
OFF and ON, respectively.

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The authors greatly appreciate the. opportunity tocollaborate with Dr. Y. !washita, Prof. A Noda, and Prof.
M. Inoue of Kyoto University, to develop a ferrite-loaded
untuned RF cavity for a medical proton synchrotron.
Experiences obtained from the collaboration promoted thiswork a great deal.

References[1] K. Saito, 1.1. Hirota and F. Noda, "An Untuned RF
Cavity Loaded with. Fe-based Nanocrystalline
FINEMET Cores", Beam Science and Technology,
Kyoto Univ., 2 (1997) 15-19.
[2] Y. !washita, ''Ferro-magnetic Material Loaded Untuned
RF Cavity for Synchrotron", Jpn. J. Appl. Pbys. 36(1997) L727-L728.
.
[3] J.I. Hirota, K. Hiramoto and M. Nishi_, "A Ferrite
Loaded Untuned Cavity for a Compact Proton
Synchrotron", Proc. of the IEEE Particle AcceleratorConf., 1995, Dallas, USA, pp. 1770-1772.
[4] 1.1. Hirota, M. Katane and K. Saito, "An UntuDed TypeRF Cavity Using Multiple Power Feediq", Proc. ofthe lOth Symp. on Acceleratm' Science and TechnoloJY,
199S, Hitachbuaka, Japan, pp. 79-81.
[S) M. Kanazawa et al., ''Beam Test of the Acceleration
System with the DDS in IUMAC Synchrotron", Proc.
of the IEEE Particle Accelerator Conf., 1997,Vancouver, Canada, to be published.

Figure 6 Performance of 2fs feedback lOOp.
1) FINEMET is a registered trademark of Hitachi Metals,

Ltd.

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