AN ALUMINIUM RF CAVITY FOR THE RING CYCLOTRON PROJECT
T. Saito, H. Tamura, A. Shimizu, M. Inoue, T. Itahashi and I. Miura
Researh Center for Nuclear Physics, Osaka University
Ibaraki, Osaka 567, JapanABSTRACT
An aluminium RF cavity was constructed for theRCNP ring cyclotron and preliminary power test wasdone. The RF cavity was successfully excited by apower amplifier which has been developed for thecavity. Cavity voltage above 200kV was achieved in thepreliminary test.
INTRODUCTION
The RCNP ring cyclotron project has been outlinedelsewhere.! Fig. 1 shows the layout of the ring
cyclotron. The single gap H101 mode cavity is equippedwith a movable tuner plate to cover the proposedfrequency range. This cavity has frequency independent
voltage distribution and be able to produce phase
compression ratio of around 3. Detailed structures ofthe cavity were determined on the basis of modelstudies. The characteristics of the RF system aregiven in Table 1.
MECHANICAL DESCRIPTION OF THE CAVITY
Schematic drawing and photograph of the cavity isshown in Figs. 2 and 3. Walls of the cavity are madeof 50mm thick pure aluminium (Al070) , and welded eachother. The cavity is reinforced by ribs made of
anticorrosion aluminium alloy (A5052) • Walls will becooled with demineralized water. The mechanicalstructure itself is rigid enough and needs no additional support. A stockr the tuner plate which are
shown in Figs. 1 and 2 are covered with oxigen freecopper. The copper of the stock and the tuner plate issustained by stainless steel and aluminium structures.
An anti stock electrode is also made of oxgen freecopper. A space between the stock and the anti sotck
electrode forms acceleration gap. The gap is 100 mm at
the injection radious and 250 mm at the extraction one.
The tuner plate slides on the stock to vary theFig. 2.

Fig. 1.

Layout of the ring cyclotron.

Table 1
Characteristics of the RF system
RF frequency
Harmonic No.
Phase compression ratio
RF peak voltage
RF powerNo. of cavities

Schematic drawing of the cavity.

108020 "'33 Mhz
4' 6' 8' 12'V3500 kV
200 kW/cavity
22m

is larger than that of copper by 30%. The maximum RFpower loss of the cavity is estimated to be 200 kW for
500 kV at 33 MHZ.
MHz
1001f==============--

60

40
H101Fig. 3.

Photograph of the cavity
0resonant frequency. A rack and pinion system rotates
the tuner plate around a shaft. The rack is set on thestock. The pinion is driven by a stepping moter which
is set inside the tuner plate. Cooling water of thetuner plate and power of the stepping motor aresupplied turough the shaft. Water cooling tubes are
soldered inside the tuner plate and the stock.
Electrical contact between the stock and the tuner
plate is performed by silver-plated Be-cu contact
springs shown in Fig. 4. Th.e springs are 0.6 mm inthickness. Copper-clad aluminium plates are welded atcontact between the cavity and copper components. Thecavity will be evacuated down to lxlo-5 Pa b~ a 22 in.
cryogenic pump and a 10 in. diffusion pump.

246810121416 x10 4Pulse

Fig. 5. Resonant frequencies of the cavity vs.
position of the tuner plate.
Qx1o'1.8

1.6

1.4

1.2

f2~0----~2~5----~3~0----~35 MHzFig. 6.

Q value of H101 mode resonance vs. frequency.

V (RELATIVE)

1.0.----------------------.

Fig. 4.

0.8

Photograph of Be-cu contact springs.

• 213.4MHz

0.6
X 330.9MHz

RF CHARACTERISTICS OF THE ~VITY

0.4

The final RF characteristics were investigated on
a 1/10 scale model study. Several modifications weremade to correct the frequency range, power reduction
and voltage distributions for a new version of theproposal of the ring cyclotron project3. Resonant
frequencies for various modes of the cavity wereinvestigated as shown in Fig. 5. There are sevenresonances below 100 MHz. The resonant frequency rangeof H101 mode is 22 "'33 MHz. Fig. 6 shows measured Q
value of H101 mode. Because of lower conductivity ofaluminium canparing with that of copper, RF power loss

0.2150Fig. 7.

109

200

250

300

350

R

Voltage distributions of the 1/10 scale model.

POWER TESTThe relative gap voltage of the 1/10 scale model
were measured with a perturbation method. The measured
voltage distributions along acceleration gap are shwonin Fig. 7 for two resonant frequencies. The voltage
distributions show radially increasing shape and nofrequency dependence. These voltage distributions canproduce phase compression ratio of around 3. The
voltage distributions are able to be varied with theradial trimmer shown in Figs. 1 and 2.
RFThe cavity was successfully excited by theamplifier at 22.5 MHz, 25.2 MHz and 30.5 MHz. After 50hours ageing, the cavity generates voltage more than
200 kV. The RF power is supplied with pulse mode
operation. Figs. 10 and 11 show typical wave form ofcavity voltage under and above multipactoring voltage,respectively. The vacuum of the cavity was 2 ~a xlo-4
Pa above multipactoring voltage. Output RF power at
25.5 Mhz, 180 kV was 59 kW. The full power test of thecavity is in progress.

POWER AMPLIFIERAn RF amplifier was developed to excite the cavity
up to 500 kV. A tetrode (RCA 4648) is used for theamplifier. Maximum output RF power of the amplifier is
200 kW. An input circuit of the amplifier consists ofa quarter wave-length coaxial line with a sliding
short. The input circuit is damped by 50$1 terminating
resistor. An inductive power feeder is adopted for acoupling between the power amplifier and the cavity.
An output capacitance of the power tube is 85 PF, and aself inductance of the couling loop is ~0.5 ~H. A
coupling circuit was designed to cancel effects ofthese reactances. Fig. 8 shows an equivalent circiut
for output circuit of the amplifier. The characteristics of the H101 mode resonance are expressed inlumped constants. c, Lm and Cc are adjusted to obtain
desired resonance frequency and plate inpedance with awell centered circle on complex plane4. A water cooled
vacuum variable capacitor is used for Cc. Photographof the power feeder is shown in Fig. 9.

I---------------.--------~CcLs

Fig. 10. Typical wave form of the cavity voltage under
mul tipactoring.
Upper; G1 voltage: 100 V/div.
Lower; Cavity voltage: 18 kV/div.
Horizontal: 50 ~s/div.
f = 30.5 MHzR(f)v,cVz

I4648

CAVITY

Fig. 8. Equivalent circuit of the output circuit ofthe RF system.

Fig. 11. Typical wave form of the cavity voltage above
multipactoring voltage.
Upper; Anode voltage: 20 kV/div.
Lower; Cavity voltage: 75 kV/div.
Horizontal: 0.5 ms/div.
f = 25.5 MHZ

REFERENCESl)

2)

Miura et al., RCNP Ring Cyclotron, in theseproceedings.
A. Shimizu et al., An Aluminium RF Acceleration
Chamber for the Ring Cyclotron, in theseI.

proceedings.

Fig. 9.

3)

Photograph of the power feeder.

4)

110

I. Miura et al., A High Energy Version of RCNP
Ring Cyclotron, Proc. lOth Int. Conf. on Cyclotrons
and Their applications, East Lansing (1984).
T Saito et al., RCNP Annual Report (1981) p.227.