Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

R&D OF WIDEBAND RF SYSTEM FOR COMPACT MEDICAL PROTONSYNCHROTRON
Z. Fang, K. Endo, K. Egawa, S. Yamanaka, KEK, Tsukuba, Japan
T. Hirashima, T. Fusato, Y. Cho, Denki Kogyo Co., Aikou-gun, Japan

Abstract
The compact proton synchrotron is being developed forthe medical radiotherapy. The rf acceleration system isrequired to be of a wide bandwidth with rf frequency
sweeping from 2.0MHz to 17.8MHz during theacceleration time of 5ms and a high gradient withmaximal cavity voltage of 10kV. The prototype of the rf
system has been developed and the high power test is
being performed. The R&D of the rf system and the test
results will be presented in detail.

impedances of test results and the calculation results areshown in Fig. 3. It is shown that the test results agree withthe calculation results very well. The gap capacitance isabout 24pF, and the complex permeability of cores is
u = 1288 f −0.81 − j 3161 f −0.81 , where f is in MHz.

1 INTRODUCTION
The compact proton synchrotron is being developed forthe medical radiotherapy [1-2]. According to the latticedesign of the machine, the proton synchrotron is operated
in pulse mode with the beam acceleration time of 5ms, inwhich the bending magnet attains 3T and the proton beamenergy reaches 200MeV [3-5]. The circumference ofsynchrotron is only 9.54m, so the beam acceleration
system is required to have frequency sweeping from
2MHz to 17.8MHz, and maximal rf cavity voltage of
10kV, as shown in Fig. 1. Therefore, the key features ofthis rf system are the relatively wide bandwidth and thehigh acceleration gradient. The rf system has been
designed and developed as shown in Fig. 2, and the above
requirements have been successfully achieved.

500

Vacc(kV)
Phase(Deg)

20

50
40

1530

1020

51002
3Time (ms)4Z(Cg=24pF)(calculation)200

Z_gap_av.(test)

100
510Frequency (MHz)15a) Impedance amplitude
90

Phase(Cg=24pF)(calculation)

60

5Phase (Deg)

1300

0004000Phase (Deg)f (MHz), V (kV)600
Impedance (Ohm)f(MHz)
Vcav(kV)

Figure 2: RF cavity and power amplifier.

Figure 1: Required rf frequency, beam accelerating
voltage, cavity voltage, and acceleration rf phase, asfunctions of acceleration time.

Pha_gap_av.(test)

300-30-60

2 RF CAVITY-900

The rf cavity consists of 2 cells with gap located in thecenter of each cell, and the 2 cells of rf cavity are driven
by a push-pull power amplifier in parallel. There are 4
high-permeability magnetic alloy cores loaded in each cell,and the cavity length is 400mm. The impedances at thetwo gaps have been measured. The average value of51015

Frequency (MHz)b) Impedance phase
Figure 3: Test and calculation results of cavity gap
impedance.

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Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

3 POWER AMPLIFIER
In the rf system, two tetrode tubes 4CX35,000C areused to form a push-pull amplifier to drive the two cells ofrf cavity in parallel, as shown in Fig. 4. Each side of thecavity gaps is directly connected to the anodes of the twotubes through the DC blocking capacitors of 0.1uF.
V_plate
160u

0.1u

0.1u

I_plate

51p

4CX35,000C

R1_a

R2_a

L1_a

L2_a

Cg_1

Cg_2

is obtained smaller than 1.15 in the whole operation
frequency range, as shown in Fig. 5. A 2kW preamplifier
is used to drive the main amplifier.

4HIGH POWER TEST

We have taken the data of input power for getting therequired gap voltage. Since there are 2 gaps in the rf
cavity, the required gap voltage is half of the summation.
Thus the maximal required gap voltage is 5kV. Fig. 6
shows the experiment results. From the test results, wecan see that the rf system can successfully produce therequired gap voltage.

0.042u10864
20-2-4-6-8-10

∆20

V_gap
100u

Ek_a10R1_b100u0.032uEs_a2kR2_b

L2_b

L1_b0.033u

Eg_a-150

160u

0.1u
0.55u

51p

0.1u

4CX35,000C

V_grid
0.042u

0.55u

Vin50

440p

20

1.414:1

110p

5000p

321005020

4Pin (dBm)
Vgap(kV) (required)
Vgap(kV) (test)Pin (dBm)

0.02u
Ep10k5Vgap (kV)440p

2051015Frequency (MHz)200.55u
100u

Ek_b
10100uFigure 6: Input power for getting required gap voltage.

110p

5000p

0.55u
Es_b2k0.032u

500.033u

Eg_b-150

SWR

Figure 4: Circuit of rf system.
1.2
1.18
1.16
1.14
1.12
1.1
1.08
1.06
1.04
1.02
1SWR #1
SWR #2

Figure 7: Waveform of gap voltage with rf frequency
sweeping from 2.0MHz to 17.8MHz and amplitudeenvelope of 5cos(90πt)kV.

024

68

1012141618Frequency (MHz)Figure 5: SWR of two input matching circuits for tubes.
The input and output capacitances of the tube are
440pF and 51pF, respectively. All-pass network is applied
to the input circuit for matching the tube’s input
capacitance [6]. The parts of the all-pass network havebeen adjusted carefully, and the SWR of tube input circuit

The above data of the input power for obtaining therequired gap voltage is used as the initial input data of therf control system based on a DDS signal generator. The
high power test with the rf control system has been
performed with feedback off, and a waveform of gap
voltage with rf frequency sweeping from 2.0MHz to
17.8MHz and amplitude envelope of 5cos(90πt)kV has
been successfully obtained, as shown in Fig. 7. Soon wewill do more experiments carefully to carry out thewaveform clearer.
Also we have taken the data of maximal gap voltage
produced by the rf system. The test results of maximal
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Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

gap voltage and its ratio to the required gap voltage areshown in the curves with Li=0 in Fig. 8 and Fig. 9. It
shows that from 13MHz to 17MHz, almost no margin
exits between the maximal voltage and the requiredvoltage.
14

Vgap(kV) (required)
Vgap_max(Li=0)
Vgap_max(Li=1.9uH)

12
Vgap (kV)1086where C = Ct + C g . And the formula shows that thefrequency response of rf system will be improved byinserting a small inductor (Li). We have taken the data ofmaximal gap voltage again, and the test results are show
in the curves with Li=1.9µH in Fig. 8 and Fig. 9. The
above experiments show that the performance of the rf
system is successfully improved, exactly agreeing withthe calculation results. Now a relatively wide band rf
system has been carried out with much higher cavity
voltage than required in the whole operation frequency
range. Fig. 11 shows the obtained maximal acceleration
gradient as function of frequency. The maximal gradientreaches 60kV/m at 5.91MHz.

42600510Frequency (MHz)

1520Gradient (kV/m)

0Figure 8: Obtained maximal gap voltage as functionof frequency.
Vgap(kV) (required)
V_max(Li=0)/V_required
V_max(Li=1.9uH)/V_required

Vgap (kV)

32.5102
1.55

1020Cg
LFigure 10: Equivalent circuit with an inductor
inserted between the cavity and amplifier tube.
In order to improve the frequency response of rf system,
an inductor (Li) of 1.9µH is inserted between the cavityand amplifier tube, as shown in Fig. 10. In this case, theratio of cavity voltage to rf current becomes:
R 2 + ω 2 L2[1 − ω LC − ω C L (1 − ω LC )] + [ωR(C − ω C C L )]222tig

22gt

15

20REFERENCES

R210Frequency (MHz)5 SUMMARYVCt

5The rf system for the compact proton synchrotron has
been developed. And the high power test has beensuccessfully performed to achieve the required gap
voltage. More tests of the rf system with the low-level
control system will be performed.

Figure 9: Ratio of obtained maximal gap voltage tothe required gap voltage as function of frequency.

I20

Figure 11: Obtained maximal acceleration gradientas function of frequency.

01015Frequency (MHz)Li3000.554003.515050

104
V_max/V_required

20V=I

Gradient(kV/m) (Li=0)
Gradient(kV/m) (Li=1.9uH)

70i[1] K. Endo et al, “Compact Proton and Carbon Ion
Synchrotrons for Radiation Therapy”, EPAC 2002,
2733-2735, France.
[2] K. Endo et al, “Development of High Field Dipole andHigh Current Pulse Power Supply for Compact
Proton Synchrotron”, PAC 2003, 1071-1073, USA.
[3] K. Endo et al, “Development of Compact Proton
Synchrotron for Radiation Therapy”, ARTA 2003,
51-54, Japan.
[4] K. Endo et al, “High Field Pulse Dipole andQuadrupole Magnets for Compact Medical Pulse
Synchrotron”, APAC 2004, Korea.
[5] K. Endo et al, “Compact Synchrotron forRadiotherapy Based on Pulse Technology”, ARTA
2004, 7-10, Japan.
[6] Z. Fang et al, “Test Results of RF Cavity and Power
Amplifier for Compact Medical Proton Synchrotron”,
APAC 2004, Korea.
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