Proceedings of the 25th Linear Accelerator Meeting in Japan (July 12-14, 2000, Himeji, Japan)[14A-01]

THE NEW FABRICATION METHOD FOR A PERIODIC PERMANENT MAGNET(PPM) C-BAND (5712 MHZ) KLYSTRON
H. Matsumoto*), T. Shintake, Y. OhkuboA), H. TaokaA), K. OhhashiB) and K. KakunoB)
High Energy Accelerator Research Organization (KEK)
Oho, Tsukuba, Ibaraki, 305-0801 JAPANA)
Toshiba Co.: 1385, Shimo-Ishi-gami, Ohtawara, Tochigi, 324-8550 JAPANB)
Shin-Etsu Chemical Co., Ltd.: 1-5, Kitago 2, Takefu, Fukui, 915-8515 JAPANAbstract
PPM focusing is quite an attractive scheme, since it eliminates the focusing solenoid (and its power supply) fromthe klystron. However, one technical difficulty lies in how to integrate the magnetic-circuit, which is relatively complicated, into the vacuum tube. We have solved this problem by introducing a new fabrication technique of metal-to-metal
diffusion-bonding by hot-isostatic-pressing (HIP). After simply stacking alternatively the disks made of magnetic
stainless-steel (Mag-SUS) and oxygen-free-copper (OFC) without any brazing-material, they were bonded together bypressing with a high-pressure (1200-kgf/cm2) at a high temperature (800-°C) for 2 hours. Machining this bonded block
produces the drift-tube for the PPM-klystron. We chose the neodymium magnet because of its high maximum
BH-product (>40 MGOe), while still having relatively low cost. A 50-MW class C-band PPM klystron is under fabrication. Already magnetic field measurement has been performed, and the axial-field profile was confirmed to be withintolerance. A high power test is scheduled in July 2000.
1 INTRODUCTION
Hardware R&D on the C-band (5712-MHz)
RF-system for the Japan Linear Collider (JLC) started in
1996 at KEK. For Phase-I (500-GeV C. M. energy), theJLC will use 3500 klystrons of 50-MW output power [2],[3]. No laboratory has ever built such a large-scale accelerator, and nobody has any experience in fabricating sucha large number of accelerator components. Therefore, it isvery important to design the machine to provide the following key features: high reliability, high machine-availability and good maintainability, lower construction cost, reasonably high power-efficiency and operationally ease.
We have already developed three 50-MW class klystrons in the three years 1996-1998. All of them successfully generated rf output power as high as 50-MW ormore. After long test runs (>5000 hours each), we confirmed performance as being acceptable for the JLC-I
project [3].
The rf power efficiency of existing high-power klystrons is usually less than 50%, which is the main limit
for the total power-efficiency of the RF-acceleration system. PPM focusing is a very attractive scheme, since it
improves the system power-efficiency by eliminating theelectrical power for the focusing solenoid, and also improving the beam-to-rf power conversion efficiency.
The PPM focusing scheme was first applied to ahigh-power klystron by D. Sprehn [4] in 1996 at SLAC in
the course of R&D for the X-band NLC. They demonstrated stable operation at an output power level over50-MW and a beam-to-rf power-efficiency as high as
60%. In spite of this promising result, industrialization ofthis type of tube seems to be not easy, because (1) wehave to integrate the relatively complicated magnetic circuits into the vacuum tube; at least 20 pole-pieces arerequired in one tube and (2) the pole-pieces are usually*)

H. Matsumoto, 0298-64-5213, hiroshi.matsumoto@kek.jp

made of iron which
is troublesome forthe brazing process
due to its thermal
expansioncoefficient being lower
than copper, and theironeasily
becomes rusty.
We have solved
these problems by
developing a new
fabrication techniquefor metal-to-metal
diffusion-bonding by
hot-isostatic-pressing (HIP). Figure 1
shows
thefirstC-band PPM klystron now underFig. 1: Cut away view of the first C-bandat(5712 MHz) 50 MW-class PPM type fabrication
TOSHIBA
Co.
Inklystrons (TOSHIBA E3747).
this paper, we will
describe the basic design of the C-band PPM klystron andthe new fabrication technique for the magnetic circuit.
2 PPM KLYSTRON DESIGNIn order to minimize the required R&D items, wedecided to upgrade the existing design of our C-band klystron (No.3-tube). Thus we kept the same design for theelectron gun, the output cavity (traveling-wave structure)and the beam collector, limiting the new design to thedrift-tube part in order to implement the PPM scheme.
2.1 Electrical Design
We used the FCI-code [5] to find the optimum magnetic-field profile. The simulates the beam dynamics in−102−the drift-tube using a PIC-method, which takes into account the space-charge field, interaction with RF-cavities,
and the external focusing PPM-field. The electricalparameters for the design are listed in Table-1.
Table 1: Main parameters for the PPM klystron.
Frequency
Output rf power
RF pulse width
Beam voltage
Beam current
RF power efficiency
Perveance (10-6)
Drift tube radius(up/down stream)
Beam radius (up stream)
Brillouin field (relativistic)
Peak field (on axis, upstream)
Plasma wavelength (λ)
PPM pitch (λ)
Cutoff voltage (α=66)
Permanent Magnet 1)
Residual Induction: Br
Coercive force: Hc
Pole-piece 2)
Note:5712 MHz50 MW
2.5 µsec
350 KV317 A
48 %1.53 A/V1.5
7.5/9.0 mm
5.0 mm
1.9 kG
2.0 kG
82.3 mm30 λ.PPM
23 kV
Neodymium N40A
1.2 Tesla
11 k-Oe
Mag-SUS:
Fe+14%Cr+C (20-ppm)
1) Shin-Etsu Chemical Co., Ltd., type N40A.
2) NKK Co., special order material.

is magnetized. Handling the fully magnetized neodymium magnet without shields is sometimes risky due to thevery strong attractive force. But since the pole-piece acts
as a magnetic shield, we can safely handle the magnetized
module, and easily mount it onto the klystron body.
Figure 3 shows the magnetic-field patterns from thesimulation. Since there is a rather long path through theMag-SUS disks, which are relatively wide, a large amountof the total field flux is bypassed between them, and only
a fraction remains which can reach into the beam pipe.
While this design results in an inefficient field usage, itdoes generate a fairly axial-symmetric field in the beampipe. This is quite an important feature for achieving stable beam transport. Additionally, the thick body serves asa radiation shield for the neodymium magnet, and provides space for the cooling water channel and enoughmechanical rigidity to support the gun parts.

2.2 Design of PPM Focusing System
We chose "Mag-SUS" for the pole-piece instead ofpure iron. The principle chemical composition isFe+14%Cr, which avoids the rust problem. The carbon
content is controlled to be below 20-ppm; this is important in preserving the ferrite-phase after the heat-cycles
from the HIP and conventional brazing processes.
We chose a neodymium magnet (Nd2Fe14B) because of its excellent performance. Fortunately it has become lower in cost, because it is widely using in
high-tech devices like computers, electro-mechanical devices, transportation and consumer electronics.
The magnet module is made of a half-ring shaped
permanent magnet sandwiched between two ironpole-pieces and an aluminum guard ring as shown in Figure 2. The permanent magnet is nickel plated to avoid
degradation of field property due to oxidation. After assembling the parts in one module using "Epoxy-Bond", itFig. 3: Cutaway view of the PPM structure and the magnetic-field
pattern.

In order to verify the quality of the permanent
magnets and the magnetic circuits, field measurement wasperformed before welding on the electron gun and beamcollector. All of the magnet modules were mounted on theklystron body as also shown in Figure 2. Figure 4 shows
the measured field profile along the axis. The large positive swing around z=550 mm corresponds to the output
cavity of the three-cell traveling wave structure. The
measured data is within tolerance of the expected profile.

Fig 4: Measured field profile along the axis at the center. (Bz, r=0)

3 FABRICATION OF PPM CIRCUIT
Left: each magnet comprises of 2 of halfrings. Right: assembled PPM magnets
comprises of 24 modules.
Fig. 2: PPM magnet mounted on the main body of klystron.

3.1 Diffusion bonding
In order to transfer the magnetic flux to the beamfrom the permanent magnet, which is mounted outside ofthe klystron, we need a periodic magnetic-circuit. And inorder to integrate the magnetic components into the kly-

stron body, usually made of OFC copper, we decided touse the HIP process [6] as follows:(1) Prepare disks made of "Mag-SUS" and OFC withpurity better than 99.99%.
(2) Stack them alternatively, without any brazing material.
(3) Put them in a vacuum capsule made of OFC asshown in Figure 5, and seal the end caps with anelectron-beam welder (EBW) in vacuum.

To do HIP, great care has to be taken with regard tosurface roughness and cleanness of all materials to bebonded. A surface flatness of ±25-µm and a roughness ofaround ±0.6-µm is good enough, both can be easily obtained using today's standard turning lathe.
A big technical advantage in using HIP is that one
can obtain perfect metal-to-metal bonding even if the pairof materials has different properties [6]. In the present
case, the thermal expansion coefficients are quite different(Mag-SUS is 1.2 x 10-5, and the copper is 1.7 x 10-5);
even so the bonding process is still performed very wellunder HIP. We believe that the external pressure applied
from the Ar-gas keeps the materials positioned stably
form during the HIP process, and suppresses any deformation associated with the difference in thermal expansions (possibly plastic-flow in copper also helps thisprocess). A schematic view of the HIP process is shownin Figure 6.

Fig. 5: PPM components before assembly. They are stacked together and then inserted into the vacuum capsule.

3.2 Experimental results(4) HIP process at a 1200-kgf/cm2 pressure and 800-°C
temperature for 2 hours in Ar-gas. The process isshown schematically in Figure 6.

After HIP, the bonding zone at the interface between
Mag-SUS and copper was found to be 3-µm in thickness.
The bonding zone showed a tensile strength in the rangeof 213-221 N/mm2, which is comparable to the strength ofbulk copper. The tensile strength of the bonding zone will
gradually increase with lengthening of the HIP and thebaking cycle.
After a 10 minute 1000 °C and 40 hour 500 °C
heat-treatment in a hydrogen furnace, no leak was foundusing a He-leak detector at a sensitivity better than 10-10
Atm・cm3/sec. There was no evidence of dimensionalchange after the heat cycle. An assembled PPM module
ready for brazing is shown in Figure 7.
4 CONCLUSIONS

Fig. 6: HIP diffusion bonding process.

(5) Remove the capsule with a turning lathe, then drill
the beam hole at the center, four water channels. Finally machine the rf-cavity and thewaveguide port. The resulting module is shown inFigure 7.

Fig. 7: The diffusion bonded PPM modules, after machining an
input rf-cavity, an iris, the beam hole and four water channels.

(6) Braze the modules together using brazing wire placed
radially at each rf-cavity.

The first C-band PPM klystron which uses theunique fabrication technique described here is under fabrication. Its first high power test is scheduled in July
2000.
REFERENCE[1] Y. Ohkubo et. al., “The C-band 50 MW Klystron Using Traveling-Wave Output Structure”, LINAC98, Aug. 1998, Chicago, IL,USA, KEK Preprint 98-140.
[2] T. Shintake, “C-band Accelerator Development for Linear Collider
and Industrial Applications” (invited talk), 23th Linear Accelerator
Meeting in Japan, Sep. 1998, Sapporo, JAPAN, KEK preprint
98-146.
[3] H. Matsumoto, T. Shintake et. al., “R&D of The C-band RF-system
Development for e+e- Linear Collider in Japan”, 12th Accelerator
Science and Technology, Oct. 1999, RIKEN, JAPAN, KEK preprint 99-83.
[4] D. Sprehn, G. Caryotakis, K. Eppley, R.M. Phillips, “PPM Focused
X-Band Klystron Development at the Stanford Linear Accelerator
Center”, (SLAC). SLAC-PUB-7231, Jul 1996. 9pp. 3rd International Workshop on RF Pulsed Power Sources for Linear Colliders(RF 96), Hayama, Japan, 8-12 Apr 1996.
[5] T. Shintake, “Klystron simulation and design the Field Charge
Interaction (FCI) cord”, Nuclear Instrument and Methods in Physics
Research, A 363 (1995) 83-89”.
[6] H. Matsumoto et. al., “Applications of the Hot Isostatic Pressing(HIP) fir High Gradient Accelerator Structure”, PAC’99, May 1991,SF, USA, KEK preprint 91-47.