High-Power Test of a Traveling-Wave-Type RF-Pulse Compressor
S. YAMAGUCHI, A. ENOMOTO and I. SATO, KEK, National Laboratory for High Energy Physics
1-1, Oho, Tsukuba, Ibaraki, 305 Japan
Y. IGARASHI, Mitsubishi Heavy Industries, Ltd., 10, Oye, Minato, Nagoya, Aichi, 455 Japan
A high-power model of an S-band rf-pulse compressor
utilizing a coaxial traveling-wave resonator has been designed,
manufactured and tested regarding the energy upgrade of thePF 2.5-GeV linac for the KEKB project. An output peakpower of 201 MW was obtained at an input RF power of 45
MW with no serious rf breakdown. The average energy gain
was estimated to be 1.75.

I. INTRODUCTION
An energy upgrade of the PF 2.5-GeV linac up to 8
GeV is underway for the KEK B-factory project with anextension of the linac and a reinforcement of the rf power [1].
For the rf power reinforcement, a new-type rf-pulse
compressor utilizing a TE620-mode coaxial traveling-wave
resonator has been developed in parallel with the SLED [2]application. The advantages of the new-type pulse compressor
are simple structure and low cost.
A cold test using a low-power model was completed in1994, and the expected pulse-compression ability has been
demonstrated [3]. However, since the electric-field lines areperpendicular to the metal surface and the electric field
gradient is extremely high (∼100 MV/m for 45 MW inputpower), high-voltage breakdown may well occur. To test thebreakdown and related problems, such as the radiation of Xrays and temperature rise, a high-power test has been carriedout. The design and fabrication of a high-power model andthe high-power test results are described in this paper.

II. DESIGN AND FABRICATIONThe cavity comprises four parts: an inner cylinder, anouter cylinder, a bottom board and a ceiling board. These fourparts and waveguide were assembled by silver and gold
brazing. The material of the cavity is oxygen-free copper. Thecavity dimensions were just the same as those of the coldmodel. Because the direction of the real current on the metal
surface is normal to that of the lathe's blades, the surface
flatness should be sufficiently smaller than the skin depth toobtain a high Q-value. The inner wall of the cavity wasplaned to less than 0.1 µ m and finished by electrolytic
polishing. A photograph of the high-power model of thepulse compressor is shown in Figure 1.

Figure 1: Photograph of the high-power model set at the test
bench.
The measured rf characteristics of the cavity aresummarized in Table 1 along with the design values. The
decrease in Q0 (unloaded Q-value) is attributed to insufficientelectrical contact between the inner, outer cylinders and thebottom, ceiling boards. The total weight of the cavity,
including support structure, is 179 kg.

Table 1
Electrical characteristics of the pulse compressor.
designed
f0 [MHz]
2.856
59000
Q0
3.8
β
1.00
VSWR ( tuned )1.00
VSWR ( detuned )1.89
MM : energy multiplication factor

measured
2.858
48000
2.7
1.03
1.01
1.75

Fine tuning of the resonant frequency was performedby adjusting the rf frequency during this work. The reflection
from the cavity to the klystron was minimized by adjusting
the stubs set at the ceiling board of the cavity.

III. PERFORMANCE TEST
A. SETUP
A test bench was constructed in order to perform ahigh- power test of the pulse compressor. The layout of thetest bench is shown in Figure 2. The output power from thepulsed high- power klystron ( 45 MW, 2856 MHz, 3.8 µs,

3600
T-junction
1150.0

400

70 dB
BHC

706.5
60 dB
BHC

149
E-bend

(a)

105.5

409.5

H-bend1627.307

1400

dummy
load

60 l/s ion pump

105.5

T-junction dummy
load

KlystronH-bend1m

Figure 2: Layout of the test bench.
50 pps ) was fed into the cavity, and the output power fromthe cavity was divided between a 3-dB hybrid and two Tjunctions, and absorbed by four high-power water loads (1 µs,50 pps, 100 MW) developed by Nihon Koshuha Co. The rf
power was monitored for forward and backward waves before
and after the cavity. The vacuum was evacuated by two ion
pumps ( 60 l/min ) and the pressure was monitored by cold
cathode gauges. The cavity temperature was monitored using
a thermistor thermometer. For the interlock to stop theklystron, vacuum pressures were used as well as the coolingwater for the klystron and modulator. The flow rate of thecavity cooling water was 80 l/min ( 30 ± 0.2 °C ).
B. RESULTS
The rf conditioning was carried out while monitoring
the vacuum pressure and rf power. The base pressure was ~10 -6 Pa after a bakeout at 100 °C for 4 hours; the pressure
was kept below 2 x 10-4 Pa during rf operation.
After 250 hours rf conditioning, an output peak powerof 201 MW was obtained for an input RF power of 45 MWwith a pulse width of 3.8 µs. An example of the output wave
form from the pulse compressor is shown in Figure 3. The(b)Figure 3 : Output wave form, (a) detuned, (b) tuned.
average energy multiplication factor (M) was estimated to be
1.75 from the pulse shapes for the tuned and detuned
conditions, assuming that the output power decreases linearlywith time :M=( 45.2 + 16.9) / 2= 1.75 .
10.1

The temperature rise of the cavity was 1.0 °C for an
input power of 45 MW, 25 pps.
Figure 4 shows the dose-equivalent rate ofbremsstrahlung X-rays as a function of the input rf power
measured by ionization chamber at a point 2 m away from thecavity center without shielding. The extrapolated value of thedose rate for an input power of 45 MW, 50 pps is about 100µSv/h. The measured maximum energy of the X-ray was 1.8
MeV for a 40 MW input.

(3.94)100(0.42)(3.42)

13050 pps

y [mm]

RF25 pps

ω t=-60
ωt=-3010

ωt=0Pin = 46 MW

H [ µSv/h]

ωt=30
ω t=600(0.41)-10(3.29)(0.00)z [mm]

153

1

Figure 5: Typical trajectories of field-emitted electrons ina waveguide; field gradient is 130 MV/m and the phasevelocity is 1.46c. The numbers in parenthesis are the kinetic
energy of the incident electrons.
0.1
1020304050

P [MW]

Figure 4: Dose-equivalent rate vs. input rf power. The pulse
repetition rates are 25 and 50 pps.

IV. DISCUSSIONSThe high-power capability of the new-type rf pulse
compressor was demonstrated experimentally in this work.
To apply this pulse compressor to the PF-linac, however,there are two problems. One is that the energy multiplication
factor (M) is slightly smaller than that required for the KEKB
injector linac. The other is that the radiation-dose rate is toohigh to use this device in the klystron gallery from a radiation
protection point of view (the maximum permissible dose rateis 20 µ Sv/h). Figure 5 shows the simulated trajectories offield-emitted electrons in a simple model using a tracking
code for electrons in a traveling-wave field [4]. It is shownthat some electrons are incident on the metal surface withvery high kinetic energy, and that multipactoring-like motion
is not observed. If we use a quite "clean" and flat surface
metal, the amount of the radiation, which is caused by thebombardment of field-emitted electrons, will be reduced. It is
not so easy, however, to obtain such a high-quality surface,especially at the mass production base.

To overcome these problems, such an electromagnetic
mode with a much higher Q-value (unloaded) than this cavity
and with no component of electric field perpendicular to themetal surface should be selected. Design work for the cavity
that satisfies these criteria is now in progress.

V. ACKNOWLEDGEMENTS
The authors would like to express thanks to the RFgroup of the PF-Linac for preparing the rf source (klystron,modulator and rf driver) and Mr. K. Kakihara for setting up
the vacuum system. We also acknowledge to Dr. S. Ban andMr. H. Nakamura of the Radiation Safety Center formeasuring the X-ray energy spectrum.

VI. REFERENCES[1] A. Enomoto, et al., " RE-FORMATION OF THE PF
2.5- GeV LINAC TO 8 GeV", Proc. of 1994 Linac
Conference, Aug. 21-26, Tsukuba, Japan pp. 184-186 .
[2] Z. D. Farkas, et al.,"SLED:A Method of Doubling
SLAC's Energy", Proc. 9th Int'l Conf. on High EnergyAccelerators, SLAC pp. 576-583 (1974).
[3] S. Yamaguchi, et al., "DEVELOPMENT OF AN RFPULSE COMPRESSOR USING A TRAVELINGWAVE RESONATOR", Proc. of 1994 Linac Conference,Aug. 21-26, Tsukuba, Japan, pp. 478-480.
[4] S. Yamaguchi : "Simulation Studies on High-GradientExperiments," LAL/RT 92-18 (1992).