Applications o f Hot Isostatic Pressing (HIP) for a High-GradientAccelerator Structure
Hiroshi Matsumoto, Mitsuo Akemoto, Hitoshi Hayano, *Atsushi Miura, Takashi Naito, and Seishi Takeda
KEK, National Laboratory for High Energy Physics
Oho 1-1, Tsukuba-shi, Ibaraki-ken 305, Japan*The Graduate University for Advance Studies
KEK, National Laboratory for High Energy Physics
Oho 1-1, Tsukuba-sh^ Ibaraki-ken 305, Japan
Abstract

In order to reduce the dark current from disk edges in atraveling wave accelerator structure at field gradients of up to80 MV/m, a new type o f disk has been proposed which
comprises of a part made of Titanium around the beam holearea and a part of made OFHC at the outer ward. These twoparts are joined by a hot isostatic pressing (HIP) technique.
HIP causes diffusion bonding at temperatures ranging from
700 to 850 °C at an isostatic Ar gas pressure of 1,200
kgf/crn2 in a pressure vessel. All samples have shown vacuum
leak rate of less than 10"9 Torr-l/sec and a tear strength ofabout 6.4 kgf/mm^ in the reaction zone between the twometals.
I. INTRODUCTION

increase the upper limit of RF breakdown, we designed a newdisk comprising two metals, OFHC and Titanium(Ti-6A1-4V).
Since the secondary electron emission coefficients of Titanium
is less than unity, the beam hole of a disk has been designed
to be made of Titanium. We applied diffusion bonding by hot
isostatic pressing to join Titanium to OFHC. We made thecoaxial columnar blocks with OFHG (outer) and Titanium(inner) by this technique. This paper describes the preliminary
results concerning the characteristics of HIP diffusion bonded
OFHC-Titaniom metals.
II. EXPERIMENTAL PROCEDURES
A. H o t Isostatic Pressing

Hot isostatic pressing (HIP) is a thermomechanical process
for materials that makes use of an applied gas pressure in orderto achieve both high density and diffusion bonding in thetreated material. HIP generally subjects a material to pressures
as high as 2,000 kgf/cm2 and temperatures up to 2,000 °C ina pressure vessel. The source of heat is a furnace locatedwithin the pressure vessel, which hold temperatures well
below the melting point of the material being processed. Thismethod can be used for a wide range of materials such as
metals, ceramics and composites.

The accelerating gradient of the Japan Linear Collider (JLC)
is 100 MV/m for the X-band main linac and 50 MV/m for theS-band injector and the preaccelerator. We have alreadyachieved the acceleration of an electron beam of 0.9 A and apulse width of 0.2 |isec by utilizing a 0.6 m-long travelingwave structure at a gradient of 85 MV/m with a Ijisec RF
pulse duration at 50 Hz [1]. At this gradient, however, the darkcurrent is very large from disk edges and breakdown occursfrequently, at 70 MV/m, however, fairly stable beam
acceleration has been achieved. It seems to be feasible to B . M aterials Characteristics
operate the accelerating structure at this gradient for practical
HIP was applied to the diffusion bonding o f OFHC
use after a reasonable processing period. In this sense, 50
cylinders and Titanium bars. The chemical compositions ofMV/m operation at the S-band is quite promising.
Experimental studies on the upper limit of the electric field the samples are listed in Table 1 . The composition of thestrength in conventional disk-loaded structures and single OFHC is very high-quality type and that of Titanium
cavities have been reported from several laboratories [2] [3], corresponds to the Japanese Industrial Standards(JIS) type.
From these studies, many factors have been found which must
Table 1be discussed concerning the RF breakdown phenomenon with
Chemical cornposition of OFHC cylinders
a dark current, such as the surface finish, micro dust, electron
CHEMICAL COMPOSITION (%)
multiplication and vacuum conditions. However, the MATERIALPB
2NBICDffiOPSSETE
fundamental mechanism of RF breakdown is not yet clear. AsOFHC5<1<1<1<1328<1<1
described in references [4] and [5], electron multiplications atCHEMICAL COMPOSITION (%)
HONFeCAlV
disk edges is one o f the main reasons for limiting theTi-6A1-4V*880 .1 8
0.0 1 70 .2 3
0.0 1 96 .3 44 .1 3
accelerating gradient, other factors such as surface cleanness,
surface finish and vacuum conditions were found not to be * ppm (x lO '6)
serious problems [6]. In order to reduce the dark current and toC. Fabrication Methods

Samples o f OFHC cylinders and Titanium bars weremachined from columnar blocks。The surface finish (RMS) of
the inner wall of OFHC is less than 0.5
and the one ofTitanium bar is less than 0.2 [im. The dimensions of thesesamples are listed in Table 2.
Table 2Dimensions of samples
OFHC
0 110 土 0.1mm
0 40+0.05/+0.0mm
110 土 0.1 mm<0.5 |im
INNER WALL ■OUTER DIAMETER
INNER DIAMETERLENGTHSURFACE FINISH(RMS)

Ti-6A1-4V
0 40 -0.05/-0.0mm

110 土 0.1 mm< 0.2OUTER WALL

These metals were cleaned by acetone, the Titanium bars
were then set inside of OFHC cylinders. Each sample wasplaced into a completely leak-tight soft-iron capsule. Thesecapsules were then pumped down to less than 10つ Ton* andsealed-off.
The results at three different temperatures, (100, 750 and800 °C) were obtained in an HIP cycle within a pressurevessel. The isostatic pressure was held at 1,200 kgf/cm2 bypure Ar gas for 2 hours. The above parameters for HDP-cycle(temperature, pressure and holding time) are based on both
metallurgical and economical considerations. A schematic
view of the HIP-cycle is shown in Figure 1.

IIHfK

Figure 2. Optical micrograph showing forged OFHC (left) andHIP-OFHC (right) at a temperature of 800 °C and an isostaticpressure of 1,200 kgf/cm2 for 2 hours.
B. Reaction Zone
The width of the HIP reaction zone in three samples ranged
from 7.3 to 20 |im, corresponding to the applied HIP
temperatures. These reaction zones were etched and measured
by a scanning electron microscope (Figure 3).

Pressure Vessel

M IflfUIflfTitanium was observed. The micro structures of an HIP
sample and a forged OFHC are shown in Figure 2. A forged
OFHC generally involves a large number of micro pores thesize of a few pm at the grain boundary. Figure 2 shows thatthe micro pores disappeared in HIP-processed OFHC.
Moreover, HIP-processed OFHC is much more homogeneousthan forged OFHC. Titanium is free from porosity, even in aforged material.

HeaterIt]
Temperature
800 °C

/

[1]
[2]
[3]

圖 画 圏 _ _ 画

TmeFigure 1.HIP diffusion bonding process.
IIL EXPERIMENTAL RESULTS
A. Micro structures
The quality o f HIP was studied with metallographicalmeasurements (optical microscope and scanning electronmicroscope) and mechanical tests. At three different HIP
temperatures (700,750 and 800 °C) with an isostatic pressureof 1,200 kgf/cm2 for 2 hours, all samples took on a fine,homogeneous micro stricture. In experiments at these threedifferent temperatures no grain growth of the whole OFHC andFigure 3. Scanning electron micrograph o f HIP-processed
OFHC-Titanium sample.
Figure 3 shows the reaction zone between two materials,indicating that the quality is high, no porosities and impurities
can be seen. The x-ray analysis of the reaction zone in Figure
3 shows the contents of OFHC and Titanium within thereaction zone.

C. Mechanical Properties

A drawing with the dimensions of the cavity is shown inFigure 6.

The quality of HIP was studied using both mechanical tests
and metallographical measurements. The tear strength wasmeasured using plates with a thickness of 2 mm that satisfy
JIS standard tear test specimens (Figure 4). Five specimens
were machined from each sample. The reaction zone waslocated in the middle of the gauge length.
60
28.2

UNIT: mmFigure 6. Titanium-bonded disk structure.

丨OFHCY. SUMMARY
t= 2

diffusion zone
UN IT: mmFigure 4. JIS standard tear test specimen, rop view.
We obtained an average tear strength of 6.4 kgf/mm2 forthe three different HIP temperatures. The minimum value ofthe strength was 4.9 kgf/mmへ Figure 5 shows the tearstrength of all the specimens, it does not depend on the HIP
temperature. All of the test specimens were fractured inside of
the reaction zone where the concentrations of OFHC andTitanium were equal.
The vacuum leak rate of the reaction zone was well blow
10~9 Torr®l/sec.

8700

750

X~b IwcT—lgzzz^^^bblcc一^i24r jlb6'一e h Hl
N l lsCS
H(je/)ovc

800

HSP TEMPERATURE (DEG. C)

Figure 5. Tear strength according to the JIS standard test

IV. DESIGN OF DISK STRUCTURE
A. Calculation o f Q
A new disk structure was designed for a conventional diskloaded structure comprising HIP-bonded OFHC and Titaniummetals. The calculated results o f the Q value using theMAFIA code was 129(XX) for the 2k/3 mode. This is about
14% lower than that for a normal cavity made of only OFHC.

An HIP diffusion bond of OFHC and Titanium was
successfully achieved at temperatures ranging from 700 to 800°C and a pressure of 1,200 kgf/cm2 for 2 hours. The micro
pores in both OFHC disappeared with HIP processing. Thequality o f the reaction zone is high and is free from both
porosities and impurities. The minimum tear strength of thereaction zone is 4.9 kgf/mm^. The vacuum leak late ofreaction zone is less than 10"9 Torr-l/sec. The calculated Qvalue of a Titanium bonded disk is 12,000. These results
indicate that a Titanium bonded disk can be applied to highgradient accelerator structures.
YL ACKNOWLEDGMENTS
HIP was performed with NKK corporation. Also, theanalysis of samples was performed by joint work involving
KEK and NKK corporation. The authors wish to thank theNKK group for corporation led by Mr. Hoshiaki Terao as wellas many fruitful discussions.
VII. REFERENCES[ 1 ] H. Matsumoto et al” “ High Gradient of the S-band
Electron Linac” , Linear Accelerator Conference, pp.
629-631, September 1990.
[2] G. A. Locw and 1.W. Wang, “ RF Breakdown and FieldEmission，9, SLAC-PUB-4845, January 1989.
[3] E. Tanabc, “ Breakdown in High-Gradient AcceleratorCavities, IEEE Trans. NS-30, No. 4, pp 3551, 1983.
[4] S. Takeda et al” “ High Gradient Experiment byAccelerator Test Facility for Japan Liner Collider” ,Linear Accelerator Conference, pp. 144-146, September
1990.
[5] S. Takeda et a l9“ High Gradient Experiments by ATF*,
Particle Accelerator Conference, May 1991.
[61 J. W. Wang, “ Some Problems on RF Breakdown in
Room Temperature, A Possible Criterion” ，SLAC/AP51，April 1986.