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

HIGH POWER TEST OF C-BAND PULSE COMPRESSORUSING LOW THERMAL EXPANSION MATERIAL
Mitsuhiro Yoshida†, Tokyo Univ. Tokyo, Japan
Hiroshi Matsumoto, KEK, Tsukuba, Japan
Tsumoru Shintake, RIKEN, Harima, Japan
Abstract
The first high power model of C-band (5712 MHz) RFpulse compressor was developed for the e+e- linear
collider. To obtain high efficiency and stable operation ofthe pulse compressor, we developed thermally stablehigh-Q cavity using Super Invar material, which has ultralow thermal expansion coefficient which is lower than 1.0
x 10-6 /°C.
Since the RF pulse compressor uses the narrow
resonant cavity of highest Q-factor in electron linear
accelerator systems, it is extremely sensitive totemperature variation. To stabilize this cavity, one
solution is to employ a low thermal expansion material to
make the cavity. However, such material usually has poor
electrical conductivity. To maintain high Q-factor, we
made use of copper plated invar material. To improveplated copper property, we applied two different methods,
special electroforming technique (PR process) and HIP
process in addition.
This paper describes the detail fabrication technique of
this type of RF cavity, and present status of high power
test of the RF pulse compressor.

DESIGNElectrical Design
The C-band RF pulse compressor was designed for thee+e- linear collider in Japan (JLC) based on C-band
technology [1]. We adopted 3-cell coupled cavity system[2] to obtain maximum efficiency for the flat output pulse,which is required for multi-bunch beam operation. The
design goal is to compress the pulse width of the 100 MW
input power by a factor of 5, and generate 350 MW peakpower. Figure 1 (left side) shows the pulse form of
normalized input and output power of the RF pulsecompressor.
The RF pulse compressor consists of three cavities asshown in Figure 2. The first and third cavities aredesigned for TE01,15 mode and the second cavity isdesigned for TE01,5 mode. The ideal Q value using Oxigen
Free Copper (OFC) is 185,400 for TE01,15 and 82,600 for
TE01,15, respectively.

Figure 1 shows the energy gain change due to theresonant frequency shift (∆f) of the third cavity. To reach
99% of the maximum efficiency, acceptable resonant
frequency change is only ±25 kHz. This value
corresponds to ∆L = 2.3 µm. To satisfy this permissible
range using only OFC material, we must control thetemperature within 0.3 °C. In order to relax this severe
temperature requirement, we decided to make the energy
storage cavity using Super Invar, which is ultra low
thermal expansion material.

Figure 1: RF pulse compressor performance(simulation), normalized power of the pulse compressor(left side)and power gain dependence on resonant frequencyof the third cavity (right side).
Table 1 shows characteristics of copper and Super Invar
material (Fe:63%, Ni:32%, Co:5%). The thermal
expansion coefficient of Super Invar is 1/40 of the one ofcopper . If we can replace all parts of cavities by SuperInvar, 10°C temperature change is acceptable. Thus using
Super Invar material for the base material of cavities, very
big improvement in temperature stability is expected.
Table 1: Material properties of the super invar andcopper
CopperSuper Invar
ThermalExpansion
Coefficient

16 x 10-6 /°C(Poor)

0.4 x 10-6 /°C(Excellent)ElectricalConductivity

1.7 x 10-8 Ωm(Good)

Poor (improved bymaking copper wallon inner surface)

ThermalConductivity

394 W/m⋅°C(Good)

13.5 W/m⋅°C(Poor)

Thermal and Mechanical Design
At an optimum condition, the microwave energy ismainly stored in the third cavity. Thus the performance ofthe third cavity dominates the compressor performance.
The frequency sensitivity of the third cavity is∆f/∆L=10.8 kHz/µm for the cavity length (L) and ∆f/∆D= 6.6 kHz/µm for the cavity diameter (D).

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

Figure 2 : RF pulse compressor.
Since Super Invar has poor electrical conductivity, it isnecessary to make pure copper plating on inner surface ofSuper Invar for high Q cavity. We applied two methods toform the copper layer; the PR electroforming [3] and HIP
method.
Since the thermal conductivity of Super Invar is fairly
smaller than the one of the copper, we have to be careful
about the heat load density. The thermal loading of thethird cavity is 25 J per one RF pulse, of which 38 % isdissipated in the surface of the cylinder and 31 % each isdissipated in the both end plates. Under 50 pps operation,
thermal loading of the cylinder surface becomes 2.2
kW/m2. At this condition, the temperature rise at the innersurface of Super Invar becomes 3 °C (outer wall is watercooled). If we assume that the average temperature riseinside of the body is 1.5 °C, the frequency shift becomes
only 5 kHz.

Copper Surface (HIP)
Another candidate to form copper layer on the invar isHIP (Hot Isostatic Pressing) technique. HIP process has
many advantages;• Suitable for mass production,• Lower fabrication cost (especially in massproduction),• Diffusion bonding is very strong, thus it becomes
possible to braze all components after HIP,
• OFC-class 1 material can be used. This leads tohigh reliability and no defects.
HIP process for this bonding was done under 800 °C,
1000 atm for 2 hours.

MANUFACTURING
Invar Material with Cylindrical Shape
Because of the hardness of Super Invar material, it ishard to forge and machine on turning lathe. To solve thisproblem, we applied the casting method to make directly
the cylinder from Super Invar. The ordinary cast products
have, however, impurities and defects, which cause
troubles in electroforming process and under ultra highvacuum. Therefore we selected the high quality cast
Super Invar which has low carbon, high purity and few
defect developed by Nihon Chuzou Company.

Copper Surface (PR electroforming process)
The PR (Periodic polarity Reverse) copper
electroforming process was recently developed by
Mitsubishi Heavy Industry Company. The copper surface
formed by this process becomes very pure copper. It issuitable to make high-Q cavities operated at ultra highvacuum condition. The process time is fairly long, since
we have to grow the copper layer very slowly to avoid
impurity contamination, and results higher cost.

Figure 3: Triple layer pipe (OFC-Super Invar-OFC)
using HIP method.

Assembling
The machining process for final adjustment and theassembling process using TIG welding were done at
TOYAMA Company. This company has skill in treating
precise components under ultra high vacuum. We canquickly perform the repetitive process to tune the resonant
frequency of cavities, since the company has almost allbasic machine tools near from the assembling place andthe working environment is enough clean to assemble theRF pulse compressor.

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

Figure 4: Pulse compressor after TIG welding
Figure 4 shows the RF pulse compressor which isassembled using TIG welding. In this assembly, the mostserious problem is how to tune the resonant frequency of
the first and the second cavities which have no tuner,
because the cavity length is shrunk irregularly after TIG
welding. To tune these cavities, we prepare anotherwelding guide to adjust the cavity length using low
controlled current of the TIG welding.

Performance of cold test
Table 2 shows the comparison of electricalcharacteristics between design value and measured valuesof two pulse compressors. From these measured values,
we can expect to obtain over 95% of the designed power
gain.
Table 2: Electrical characteristics
Second one
Design value First one
f1(MHz) 5712.00
5712.215712.05
f2(MHz) 5712.00
5711.88
5711.93f3(MHz) 5712.00
Adjustable
Adjustable
k120.0012
0.001120.00114k23
0.000690.000590.00075
QL322729
Concerning about the thermal stability, the resonant
frequency drift by the temperature change becomes
13kHz/°C. This value is lower than 1/6 of ordinary copper
cavity system, and is low enough to operate easily such ahigh-Q cavity.

HIGH POWER TEST

developed two RF pulse compressors, two mode
converters for rectangular TE10 to cylindrical TE01 mode,a 3 dB hybrid, two -3 dB attenuators using slow wave
structure. And 4 ion pumps were arranged for fast agingprocess.
Before the high power test of the RF pulse compressor,
we tested the high power test stand without the pulse
compressor with the input RF power of 40 MW.

Current Status
We are now operating the RF pulse compressor at theinput RF power of 35 MW of 2.5 µs and the compressed
RF power of 105 MW of 0.5 µs. Figure 6 shows the pulse
waveform of the input and output power of the RF pulse
compressor.
Output: 105MW
Input: 35MW

Figure 6: Pulse waveform of input and output power ofthe RF pulse compressor.
Under the present circumstances, the compressedpower gain becomes only 3. This value is a little bit lower
than the target gain of 3.5. This low gain caused by thedetuning of the resonant frequency of the first cavity is
due to the deformation by the force of the vacuum flange.
And the actual operating time is almost 2 months because
the inverter power supply has frequent troubles in theother days.

SUMMARY
We developed the RF pulse compressor using Super
Invar material, which has ultra low thermal expansion
coefficient. We successfully achieved the resonant
frequency change by temperature less than 13 kHz/°C for
the whole pulse compression system. We are currently
working on the high power test to reach the target output
power of 175 MW, 220 ns, 50 pps.

REFERENCES

Figure 5: High power test.

Components
Figure 5 shows the high power test stand of the RF
pulse compressor. For this high power test, we newly[1] http://c-band.kek.jp/[2] T.Shintake et al., “A New Pulse-Compressor Using
Multi-Cell Coupled-Cavity System”, EPAC96, Sitges,
June10-14, 1996, KEK Preprint 96-71.
[3] K.Tajiri et al., “PR copper electroforming method forJHF”, Proceedings of 25th Linear Accelerator
Meeting in Japan, 2000.