JAERI-Conf 95-021

PA20
An Untuned Type RF Cavity using Multiple Power Feeding
J.I. HIROTA, M. KATANE, K. SAITOU*, M. TADOKORO* F. NODA*'
Y. IWASHIT A** , A. NODA ** and M. INOUE **'Hitachi Works, Hitachi Ltd., Hitachi, Ibaraki 317, Japan***Hitachi Research Laboratory, Hitachi Ltd., Hitachi, Ibaraki 319-12, Japan
Accelerator Laboratory, Institute for Chemical Research, Kyoto University,Uji, Kyoto 611, JapanAbstract
A ferrite loaded untuned type RF cavity has been fabricated and tested for a compact proton synchrotron dedicated for medical use. Using a new power-feed method
named as multi-feed coupling, a gap voltage of more thanlkV has been achieved with the generator power of 1.5kW
in the frequency range from 1.5MHz to 10MHz. These
values satisfy the requirement of a few hundred volts forthe medical synchrotron. The multi-feed coupling has
been confirmed to be effective by high power tests. The
temperature rise caused by power loss in ferrite cores wasless than 25 degrees up from the room temperature at 2hours after the start of the power feeding when the generator power was lkW. This cavity can therefore be operated
only by the forced air cooling system.

2. Multiple Power Feeding
An untuned type RF cavity is characterized as asimple RLC resonant circuit, in which R, L and C correspond to the resistance of the cavity, the inductance of themagnetic materials ( ferrite cores) to obtain a wide operating frequency mnge, and the capacitance of the accelerating gap, respectively. In the direct coupling which is theusual method of power feeding, RF power generated by
the power source is fed into the inner conductor directly
and returned to the source through the outer conductor.
The cavity voltage Vd is given as

vd = J2Pizdl.j=

1. Introduction
Nowadays a compact proton synchrotron has been
hoped to be used efficiently for treatment of tumors. We
proposed a medical compact proton synchrotron which
consists of combined type magnets with the circumferenceof about 23m[l]. In the compact synchrotron, due to its
short circumference, the cavity voltage required in an acceleration process is relatively lower than that of a large
synchrotron, though a wider operating frequency range isneeded. Based on this condition, an untuned type RF cavity in which the tuning procedure of resonant frequency is
not necessary, has been adopted as an accelerating system
for the synchrotron[2]-[3].
Untuned type RF cavities have already been constructed in severallaboratories[4]-[7]. These cavities consist of a quarter or a half wavelength coaxial resonator andmagnetic materials with large permeability. Power loss
caused by the imaginary part of the complex permeability
in the magnetic materials plays an important role in obtaining a wide operating frequency range. However, ingeneral, this effect makes it difficult to get a high accelerating voltage in the untuned type RF cavities. We havedeveloped a new power-feed method named as multiple
power feeding(multi-feed coupling) so as to increase theaccelerating voltage over a wide frequency range compared with the conventional direct or push-pull power
feeding. In section 2, the principle of the multi-feed coupling is described in brief. The construction of the highpower model cavity and experimental results of the multifeed coupling are mentioned in section 3.

~ ~> 2 Pg I zd I

2 (1(1 >where P, P , Zd and S are the net power, the generator
power, the ~unt impedance of the cavity and the value ofvoltage standing wave ratio(VSWR), respectively. The Z
depends only on the inductance L of the ferrite cores be~cause their permeabilities are large enough to get the wide
operating frequency mnge. As Zd increases, a large impedance mismatching between the cavity and the powersource occurs and almost all of the genemtor power is reflected back to the power source. The Vd cannot be in--79-n

Ideal transformer 1: 1Fig. 1. Equivalent Circuit in Multifeed Coupling.

JAERI-Conf

95-021

creased because of the decrease in the net power fed into
the cavity. The reflection power becomes too large to operate the power source under this condition. This effect isthe main cause of lower accelerating voltage in theuntuned type RF cavity.
In order to reduce the reflection power and increase
the cavity voltage, the impedance mismatching must be
improved keeping the cavity impedance higher. A new
power-feed method, hereafter we call multiple power
feeding (multi-feed coupling), was developed to solve thisproblem. Figure 1 shows the equivalent circuit in multifeed coupling. The cavity and the generator are divided
into the same number of sub-circuits as the loaded ferrite
cores. Assuming n the loaded number of the ferrite cores,
the cavity is represented by the series connection of n subcircuits whose impedance is one-nth of that of the direct
coupling. The coupling impedance between the cavity andthe power source can therefore be decreased to one-nth
while the to~l impedance is equal to that of the direct coupling. In this scheme, the reflection power is much reduced and the cavity voltage is increased by the series connection of the sub-circuits. The cavity voltage in themulti-feed coupling V is given by.
:.:....
Photo 1. Photo of the high-power model cavity.

m=n

2!:9:. I zdl
4 S/ n(1+ S! n) 2 n n(2)where Zgiven byis equal to the impedance of the sub-circuitZJn. If the VSWR is large enough, S»n> 1, Vm

can be Iii times larger than Vd' However, if the VSWR Sis nearly equal ton, S=n>l, Vm can be lif/2 times larger
than Vd' Hence, in the real cavity, Vm is expected to bebetween Iii and lif/2 times of Vd' In this analysis, mutualinductances between the ferrite cores are ignored.

3. High Power Experiments.
3.1 High-Power Model Cavity
In low power level, the effects of multi-feed coupling have already been verified[2]-[3). To confirm RF
and thermal characteristics in the multi-feed coupling, thehigh-power model cavity has been fabricated and tested.
The cavity has been constructed with a double re-entrant
coaxial resonator and Ni-Zn ferrite cores manufactured byHitachi Metals Ltd. The outer and inner diameters are
550mm and 160mm, respectively. The lengths of the cavity and the accelerating gap, vacuum-sealed by ceramic
duct, are400mm and 20mm, respectively. Photo 1 shows
the high-power model cavity. The dimensions of the ferrite cores installed in the cavity are 500mm and 280mm inouter and inner diameters, respectively and 25.4mm inthickness with the complex permeability of about(1000,100) at 5MHz. The number of ferrite cores is 8 toincrease the cavity voltage. In the multi-feed coupling, thesame number of independent RF-power amplifiers areemployed. The RF power is fed into the cavity through theone-turn coil wound on each ferrite core in such a way thatthe magnetic flux is generated in the same direction. At
first, low power tests were performed to measure VSWR
using a network analyzer. The experimental results showthat the cavity can be operated from 1MHz to 10MHz
which satisfies the proposed specification of the operating
frequency range from 1.57MHz to 8MHz[1].

3.2 Measurement of Cavity Voltage
Figure 2 shows the block diagram of measurements.
The cavity voltage induced at the accelerating. gap wasmeasured with a voltage divider connected to the direct
coupling loop. The voltage divider consists of capacit~rsof lpF and IOOpF, giving the dividing ratio of 1/100. Figure 3 shows the dependence of the cavity voltage on thegenerator power and the operating frequency. The generator power P means the total power toward the cavity. Upto 2kW, R~power can be fed into the cavity stably in themulti-feed coupling. The cavity voltages at the frequencyof 2MHz, 4MHz and 8MHz are indicated with blackened
circles, squares and open circles, respectively. It is ?bvious that the cavity voltage of more than 1kV was achieved
with almost flat property in the required operating frequency range from 1.5 to 8MHz. The experimental result
satisfies the voltage of 500V required for a medical compact proton synchrotron. Figure 4 shows the frequency
dependence of the cavity voltage normalized by the. value
in the case of the direct coupling. Measurements with the-80-JAERI-Conf 95-021

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FerriteCores~E> 1 .....~. . . . .!. . . . . !. . . . . 1...........1...........1..........!. . . .

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Sealt··········t·········-r·······~xpe1me~t ····

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i ••••iii~ 20 ,_ .......... t··· ... ·····t············t········•···········t···········t··········

E=':110 01II1••• -,lll~ 15 ,_ ........ t;••·r········t··········t·. ········t-·········t······..··

Pa 10 -·········'···········l............l...........l............l...........l..........
•1 Room Temperature 21 degree
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•i::

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•Eiii:...... 5 -··········t·····.. Generator Power 1kWt··········~0...

•5 °o'0ii::

i60 80 100 120 140Time(min.)Fig. 5. The temperature rise underPg= 1kW.
2040Acknowledgment

1The present work is supported by Grant-in-Aid forScientific Research from Ministry of Education, Scienceand Culture of Japan.

E•2MHzJl ·1I·r.Z. ········
500 -········0 ··•·i······················t···········c··~.jt.lV..
::
18(1.)-1000 -···················1...................... ......... .....•..................

ei

76h

1500.....,...,...,....,...,....,...,~,....,....,....,...,....,..'"T'""''...,...,....,...,

@

65

4

32v25~~~~~~~~~~~~~

E-<1i1f(MHz)Fig. 4. The frequency dependence of thevoltage ratio normalized by the direct
coupling.

feed power of more than 200W were not carried out in thedirect coupling, because the waveform of the cavity voltage was deformed. The figure presents the measured values at P =200W. Blackened circles and solid line indicate
the explrimental results and the calculated values, respectively. The latter is obtained from eqmnion (2) by using theVSWR measured by the low power experiments. Measured and calculated values are in good agreement. Meanvalue of the voltage ratio is 1.8 which can be explained byequation (2) substituting n=8 and S=13. It also confirmed

E

1 --!-- calculation. . . . !1. . . . . 100

Power Source
Fig. 2. The block diagram of highpower measurement.

>~EE>........!.... .!. . . . •. . . . .T.......... T-.......... 1.. ········~········-1s8 .
0.5c8~~........~··········t···· ......+..........~ ..·········=·--········i··········.C.········

20 8t1HzReferences

2000
1500
1000
Pg(W)
Fig. 3. The generator power and frequencydependence of the cavity voltage.
500

the effects of the multi-feed coupling.
The cooling system consists of only two forced aircooling fans which are attached to the lower part of theouter conductor. The temperature rise of the cavity walland ferrite cores was measured under P = 1kW, and thehighest values are plotted in figure 5. i'he temperaturereached the equilibrium value, namely, 25 degrees up fromthe room temperature, at 2 hours after the start of thepower feeding. The result shows the cavity can be driven
only by the forced air-cooling.

[1] A. Noda et al., Contribution to this symposium.
[2] J.I. Hirota et al., "A Ferrite Loaded Untuned Cavity fora Compact Proton Synchrotron", Proc. of the 1995 ParticleAce. Conf.
[3] J.I. Hirota et al., Bull. Inst. Chern. Res., Kyoto Univ.,
73 No.l(l995) pp.41-49[4] J.P. Blewett et al.,"Radio-Frequency System Part IVAccelerating Unit", Phys.,Rev. Vol.24 (1953) pp.795-800[5] K. Muto et al.,"Ferrite Loaded Untuned RF Cavity for
Synchrotron", Proc. of the 1989 Particle Ace. Conf.
[6] S. Ninomiya,"Conceptual Design of a Non-Resonant
Accelerating System for Low-Energy Proton and HeavyIon Accelerators", KEK Report 92-2 (1992) A[7] "Conceptual Design of a Proton Therapy Synchrotron
For Lorna Linda University Medical Center", Fermilab.,(l986)-81-