The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

RF CAVITY FOR TABLE-TOP PROTON SYNCHROTRON
Z. Fang, K. Endo, K. Egawa, KEK, Tsukuba, Japan
T. Hirashima, Denki Kogyo Co., Aikou-gun, Japan
Abstract
An rf cavity is being developed for a table-top proton
synchrotron with a ring of circumference of 11.9m. The rf
cavity is required to be of a wide bandwidth with thefrequency sweeping from 1.64MHz to 14.26MHz, and ahigh gradient with the maximum acceleration voltage of20kV. The acceleration section consisting of 2 cells of rfcavity loaded with 4 high-permeability magnetic alloy
cores in each cell has been designed. The calculation andtest results of the cavity will be presented.

RF CAVITY STRUCTURE
The rf cavity has been designed and developed with 2cells loaded with 4 high-permeability magnetic alloy
cores in each cell as shown in Fig. 2 and Fig. 3. The coredimension is 397 mm and 170 mm in outer and innerdiameters, and 25 mm in length. The length of each cell is
200 mm, and the total length of the acceleration section is500 mm. A push-pull power amplifier with two tetrodetubes 4CX35,000C will be used to drive the 2 cavities inparallel [7-8].

INTRODUCTION
The table-top proton synchrotron is being developed forproton therapy [1-6] with a ring of circumference of
11.9m. Fig. 1 shows with protons accelerated from 2MeV
to 200MeV within 5ms, the fundamental rf frequency will
increase from 1.64MHz to 14.26MHz, and the required
fundamental voltage in the cavity has the maximal valueof 13kV at the start of acceleration. Since high orderharmonics will also be applied to the cavity to get a flatregion in the cavity voltage signal to increase the capture
efficiency, the rf cavity is designed to produce maximal
gap voltage of about 20kV. Thus the rf cavity with a wide
bandwidth and a high gradient should be designed toobtain a high shunt impedance over the whole operation
frequency range.
f(MHz)

Vcav(kV)

15150
10100
5W (MeV)

Frequency (MHz),
Vcav(kV)

Figure 2: Structure of rf cavity.

W(MeV)
200500001

2
3Time (ms)45Figure 1: fundamental rf frequency, cavity voltage, andproton energy, as functions of acceleration time.
Figure 3: RF cavity.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

EQUIVALENT CIRCUIT AND CAVITY
CHARACTERISTICSu" (KE028- 1)
u" (KE028- 2)u" (KE028- 3)u" (KE028- 4)u" (KE033- 1)
u" (KE033- 2)u" (KE033- 3)u" (KE033- 4)u" (KE033- 5)u" (KE033- 6)u" (KE033- 7)u" (KE033- 8)u" (KE033- 9)u" (KE033- 10)

10000

I1

R1aR2aC1a

L1a

C2aR2bC1b

L1b1000

L2aR1bI2u" , u'Since the cavity wall is connected to the ground and the2 rf cavities are driven by a push-pull amplifier, theequivalent circuit of the rf cavity can be illustrated byusing Fig. 4. Since the rf cavity consists of 4 same units,
the cavity characteristics can be evaluated just with the
impedance of one unit, namely, one quarter of the rfcavity.

u'(KE028- 1)u'(KE028- 2)u'(KE028- 3)u'(KE028- 4)u'(KE033- 1)u'(KE033- 2)u'(KE033- 3)u'(KE033- 4)u'(KE033- 5)u'(KE033- 6)u'(KE033- 7)u'(KE033- 8)u'(KE033- 9)u'(KE033- 10)

C2bL2bFigure 4: Equivalent circuit of rf cavity.

100
110100Frequency (MHz)The impedance of one unit is calculated by:
1Z cav =1+ jωCjω (u ′ − ju ′′) L0
where u = u ′ − ju ′′ is the complex permeability of cores,

and L0 = u0 h ln b = 2 × 10 − 7 × h ln b , where a and b are
2πaa
the inner and outer diameters of cores, and h is the core
length. C is the distributed capacitance for one unit.

CORE PERMEABILITYFirstly 4 cores, and later 10 cores, totally 14 cores,
are manufactured and tested. The measurement results ofpermeability of the 14 cores are shown in Fig. 5. We cansee there is a large discrepancy of permeability between
these cores. For example, at 1.6MHz, |u| can be from 2214
to 2842, namely, ±12% around 2528. At 15MHz, |u| canbe from 388 to 469, namely, ±9.5% around 428.5. Fromthese 14 cores, the best 8 cores with relatively higher
permeability are chosen out and used in our cavity.
Furthermore, considering the balance of the impedance ofeach unit in the rf cavity, a core with a relative higher
permeability and another core with a relative lower
permeability are combined to form a pair. Thus 4 pairs are
obtained and filled in the rf cavity.

Figure 5: Measurement results of permeability of 14cores.

TEST RESULTS OF CAVITY IMPEDANCE
The impedances of the 4 units have been measured asshown in Fig. 6. It is shown that the impedances of the 4units are very close, that is, a very good balance has beenobtained. The impedance amplitude of each unit is about215Ω at 1.6MHz, and 268Ω at 15MHz. The test results of
the average value of impedance of the 4 units have been
compared with the calculation results, as shown in Fig. 7.
It is shown that the test results agree with the calculation
results very well. The distributed capacitance for each unit
is about 35pF. Also as shown from Fig. 7, the rf cavity isof a very good performance of frequency response, with ahigh shunt impedance over the whole operating frequency
range.
For the cores used in this cavity, the measured averagevalue of core permeability is:
ffu ′ = 2409 × ( ) −0.8 and u ′′ = 6047 × ( ) −0.85
0 .50.5
where f is in MHz.
The high power test of the rf cavity is planned in thecoming months.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Impedance (Ohm)

400

Z_1a

Z_1b

Z_2a

Z_2b

SUMMARY
The rf cavity has been designed and developed to be ofa wide broadband and high gradient. The test results agreewith the calculation results very well. And a high power
test of the rf system will be performed soon.

300

REFERENCES
20010000510Frequency (MHz)

15Figure 6: Test results of impedance amplitude of 4 units
in rf cavity.
Z_amp(c=35.pF)

Z_av.(Test)

Impedance (Ohm)

400
300
200
100
00510Frequency (MHz)15a) Impedance amplitude
phase(c=35.pF)

Pha_av.(Test)

90Phase (Deg)6030
0-30-60-900510
Frequency (MHz)15b) Impedance phase
Figure 7: Test and calculation results of average value of
impedance of one quarter of rf cavity.

[1] K. Endo et al, Compact Proton Synchrotron,
Proceedings of the 4th Symposium on Acceleratorand Related Technology for Application (ARTA2001), Japan, 11-14, Oct. 2001.
[2] K. Endo et al, Compact Proton and Heavy Ion
Synchrotron for Cancer Therapy and Bio-Science,
Proceedings of the 13th Symposium on Accelerator
Science and Technology (SAST 2001), Japan,426-428, Oct. 2001.
[3] K. Endo et al, Compact Proton and Carbon Ion
Synchrotrons for Radiation Therapy, Proceedings ofthe 8th European Particle Accelerator Conference(EPAC 2002), France, 2733-2735, Jun. 2002.
[4] K. Endo et al, Development of High Field Dipole andHigh Current Pulse Power Supply for Compact
Proton Synchrotron, Proceedings of the 2003 Particle
Accelerator Conference (PAC 2003), USA, May
2003.
[5] K. Endo et al, Development of Compact Proton
Synchrotron for Radiation Therapy, Proceedings ofthe 5th Symposium on Accelerator and Related
Technology for Application (ARTA 2003), Japan,Oct. 2003.
[6] K. Endo et al, Development of High Field Pulse
Magnet and Power Supply for Proton Therapy, thisconference.
[7] Z. Fang et al, RF Cavities and Power Amplifier for
the Compact Proton Synchrotron, Proceedings of the2003 Particle Accelerator Conference (PAC 2003),USA, May 2003.
[8] Z. Fang et al, Development of the RF System for theCompact Proton Synchrotron, Proceedings of the 5th
Symposium on Accelerator and Related Technology
for Application (ARTA 2003), Japan, Oct. 2003.