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

HIGH POWER TEST OF A 500 MHZ RF CAVITY OF A CW MICROTRONFOR INDUSTRIAL APPLICATIONSYo Makita, Tetsuya Nakanishi, Yuehu Pu, Hirofumi Tanaka, Akihiko Maruyama, Tae Hyun Kim,
Chihiro Tsukishima, Shunji Yamamoto and Shiro Nakamura,
Mitsubishi Electric Corp., Advanced Technology R & D Center,
8-1-1 Tsukaguchi-honmachi, Amagasaki, Hyogo 661-8661, Japan
Abstract

Table 1: Parameters of the CW microtron

A 500 MHz RF cavity of the CW microtron wasfabricated and high power test has been carried out. Thecavity consists of 2-cells with re-entrant nose cones andcoupling slots. Inductive Output Tubes (IOTs) are used as
RF power sources. PC based control system with NI-PXI
system was adopted for tuner feedback, remote control
and measurements. The input power of 40 kW was
successfully obtained.

EnergyAverage beam current
Average beam power
Injection energy
The number of turns
RF frequency
Wall loss of the RF cavity

1 INTRODUCTION
High power electron beams are necessary for industrial
applications: X-rays irradiation and electron irradiation.
Accelerators operated with a continuous wave (CW)
mode are of considerable practical interest for industrial
applications from a point of view of supplying a highintensity beam [1-3]. We have been developing a CW
microtron with a 500 MHz RF cavity [4, 5]. Figure 1
shows a schematic drawing of a 5MeV microtron. Basicparameters are shown in Table 1.
For a conventional microtron, an S-band cavity iswidely used. Although, it is not suitable to use this type ofcavity due to a heat problem in case of applying for highintensity machines. Due to the problem above, a 500 MHzcavity, which is frequently used to an electron storage
ring, was adopted. Currently, we have already done thedesign, the fabrication and power tests of the RF cavity.
This paper describes the result of the high power test.

5 MeV
10 mA
50 kW
80keV
6500 MHz
40 kW

2 DESCRIPTION OF
RF CAVITY AND RF SYSTEM
Figure 2 shows a cross sectional view of the RF cavity.
The cavity consists of 2-cells with re-entrant nose conesand coupling slots. The selected resonant frequency of500 MHz is mainly based on the consideration of input
RF power and the total size of the accelerator.
The cavity is designed to operate in the π-mode. Nose
cones and rounded shape of the cavity design wereapplied to obtain the higher shunt impedance. An
optimum gap length was calculated since the injectionenergy is 80 keV (β=0.5), and the velocity of an electron
beam changes with every turn. We have done the RF
analysis by using SUPERFISH and MAFIA codes. A
thermal and structural analysis was done by using
ANSYS code. The frequency shift due to the thermal
deformation, temperature rise and the stress distribution
was estimated [6, 7].
The material of the cavity is class 1 oxygen free copper(OFHC) so that high electrical and thermal conductivity
and low out-gassing rate can be obtained. The design
parameters of the cavity are shown in Table 2.
Table 2: Parameters of the RF cavity
Number of cells
2Cell length
299 mmOuter diameter
510 mmBore diameter
65 mm
Gap length
170 mm
Gap volt. /2cell
1 MV
Unloaded Q value
37400

Figure 1. Schematic drawing of the 5 MeV CW microtron

At the operation of 5MeV-50kW, an electron beam
consumes 50 kW and a wall loss of the RF cavity
consumes approximately 40kW by the RF analysis.
Therefore, an RF power supply with 500MHz-100kW isrequired for the RF system. We used two 500MHz-50kW

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

Inductive Output Tubes (IOT, CPI Model K2H50), which
are commonly used for broadcasting systems, as a 100
kW power source by combining outputs from the IOTs bya magic tee. A coaxial line instead of a waveguide wasadopted to make the system compact.

(a)

(b)

Figure 4 shows the feedback system. The operations
were controlled by a PC based control system using
National Instrument (NI) PXI system. For remote controlof the RF power source, a digital I/O module, NIPXI6533, was used. Monitoring of forward and reflected
power, tuner position, temperature and the cavity pressure,and feedback control of the tuner were done by ADC andTTL pulse train by using NI-PXI6040E module. A simple
program to control the tuner position was made byLabVIEW, which keeps the cavity on resonance.
As for interlocks for the system, signals of reflected
power, vacuum pressure and arc detectors located at theceramic window of the input coupler were hard-wired
into the input RF signal of the power supply. The vacuum
interlock was set to turn off the RF power supply at 5x105
Torr to avoid high power operation with bad vacuum
which may cause severe problem to the ceramic windowand so on.
Ethernet

Figure 2. Cross sectional view of the RF cavity.
(a)side-cut view, (b)axial-cut view

NI PXI controller

A 500 MHz Power Supply
Remote Control(PXI-6533)

2 HIGH POWER TEST

Remote Control PC

2.1 Set-upSince the high power conditioning was done without
beam so that the coupling coefficient of the input coupler
was adjusted to 1.0. A 500 l/sec turbo molecular pump
was attached to the vacuum port and two ionization
gauges were placed at vacuum gauge ports. The flow ofcooling water of each cell was set to 50 l/min, and 30l/min for other components such as tuning plungers, thecoupler and the coaxial line. A ceramic window at thecoupler was cooled by forced air flow. The temperature
distribution of the outside wall of the cavity and the watertemperature were monitored by thermo couples. Figure 3
shows the set-up of the high power conditioning.

A 500 MHz RF cavity
Coaxial line

Tuner control(PXI-6040E)

Measurements(PXI-6040E)

Potentiometer Stepping motor control
Motor driver

Phase detector
Detector
Thermo couple

Control room

Tuner control
Measurement of the dissipated power
Measurement of the cavity temperature

Experimental hall

Figure 4. The schematic view of the control system.

2.2 High Power Conditioning
Figure 5 shows the whole progress of the conditioning.
At first, the RF power was fed at a power level of several
hundreds of watts. Then, the input power was gradually
increased in order to keep the cavity pressure below 5x105
Torr. The conditioning progressed smoothly without any
serious problems. After conditioning for 62 hours an input
power of 40 kW and reflected power 2.2 kW wereattained. There is a lower power conditioning donearound the running time of 35 hours. This is due toleakage of cooling water around the coupler occurred. We
have carefully ensured that no severe trouble was found inthe coupler.
The vacuum pressure without input RF power, before
starting the test, was 8.6x10-7 Torr. The vacuum pressureof the cavity after the completion of the conditioning,
without RF power, was 6.2 x10-8 Torr as shown in Fig. 5.

Figure 3. The RF cavity placed in the microtron and acoaxial line for RF power feed.

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

a-5

1010-6SS
Sb
C4:31.6 °C

C3:34.4 °C

10-75010-8

Cavity pressure

Temperature (°C)

40

C2:32.4 °Cc

Cavity input power

30(Treference=25°C)

20

C1:30.7 °C

10001020304050Running Time (hours)

6070Figure 5. The progress of the high power conditioning.
The cavity input power and the cavity pressure are plotted
as a function of the conditioning time.
Table3 shows the measured and calculated temperaturerise in each point during 40 kW operation. Thermalanalysis was done by ANSYS code with a half cell model.
A symmetric boundary condition was applied along theline marked with “S” in Fig. 6. A heat load distribution on
the inner surface of the cavity was calculated bySUPERFISH. Three dimensional structures such as acoupler port and coupling slots were neglected. Twogroups of water channels, C4 and C1-C2-C3, were taken
into account since we designed that C1, C2 and C3 areserially connected and C4 flows in parallel. Temperaturein each channel shows the average water temperature.
Detailed description is given in Ref. [7].
The calculation result and measured positions are given
Fig. 6. The measured temperature rises were in agreement
within a factor of two.
Table 3: Measured and calculated temperaturerise of the cavity at 40 kW operation.
Calculation (°C)
Position Measurement (°C)
a10.9
12.2
b4.5
7.9
c11.1
14.3
d9.7
-Fig. 6 Temperature distribution of the cavity for total
power dissipation of 40 kW calculated with ANSYS code.
Measured position are given in a, b and c.

3 CONCLUSION
We have done the high power test of the 500 MHz RFcavity. The input power of up to 40 kW demonstrated thecapability of the cavity. PC based control systems such asa tuner feedback of the cavity, a remote control of a power
supply and other monitoring systems are established andoperated without any problem. The measured
temperatures rises were in agreement within a factor oftwo. The acceleration beam test will be performed by theend of this year.

REFERENCES[1] J.M.Bassaler et al., Nucl. Instr. Meth. in Phys. Res. B
68 (1992), p.92.
[2] A. J. Stirling, Proc. 5th European Particle Accelerator
Conf., Barcelona (1996).
[3] H. Kwon et al., Proc. Particle Accelerator Conf., NewYork (1999) p. 2558.
[4] H. Tanaka et al., Int. J. Appl. Electromagnetics Mech.
14 (2001/2002) p.177.
[5] T. Nakanishi et al., in these proceedings.
[6] Y. Pu et al., Nucl. Instr. Meth. in Phys. Res., to bepublished.
[7] Y. Makita et al., Proc of the 13th Symp. on Acc. Sci.
and Tech., Osaka (2001) p. 244.