The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October2001

Development of PPM-Focused X-Band Pulse Klystron
S. Matsumoto, K. Ohya, S. Tokumoto, Y.H. Chin, Y. Morozumi, H. Mizuno
S. Kazakov *,A. Larionov *, V. Teryaev *, M. Kanno**, H. Urakata **, S. Miyake**
KEK, High Energy Accelerator Research Organization, Tsukuba, Ibaraki, 305-0801 Japan* BINP, Branch of the lntsitute of Nuclear Physics, Protvino, 142284 Moscow Region, Russia** Electron Tubes & Devices Division, Toshiba Corporation, Ohtawara, Tochigi, 324-8550 JapanAbstract
Development of high power X-band (11.42GHz) klystrons
is in progress at KEK, as one of the R&D programs forJapan Linear Collider (JLC) project. Effort has been devoted to develop klystrons with the focusing by periodic
permanent magnet (PPM) in recent years, after almost adecade development work of the klystrons with solenoidal
focusing. Two prototype PPM klystrons have been built
successively and each of them has shown good performance. The second tube produced the output power of 73
MW at 1.4 microseconds pulse length where the RF efficiency was 54%. These values are almost those required inJLC. The design features and high power test results of thetwo PPM klystrons are reported.

1 POWER EFFICIENCY: THE REASONWE ADOPT PPM KLYSTRONS

from that of XB72K for good beam transmission. Thus anew gun design was prepared for the PPM tube.
The focusing field and RF circuit were designed based on
the new gun. As the specification of the first tube (#1), weadopt 50 MW RF power at the pulse length of 1.5 JLS (microseconds) under 450 kV cathode voltage. This design isfairly conservative to be the base design for the successivetubes. To make the magnet design simple, no cooling water
channel was equipped for the PPM circuit of #1 tube.
The performance of #1 tube was very good. The second
tube (#2) was designed by the optimization of RF circuit
to achieve JLC specification: 75 MW RF power at 1.5 JLS
pulse length (480 kV, 55% RF conversion efficiency). Adequate cooling water was distributed in the tube for the operation under high repetition rate. The shape of the magnets
in PPM circuit was redesigned for the water pipes while thefield design was unchanged.

Japan Linear Collider (JLC) is a future electron-positron
collider machine to accelerate the beam up to TeV energy
scale[1]. We have developed at KEK a series of X-band(11.42GHz) klystrons named XB72K[2] as a JLC prototype klystron, which has a strong solenoidal focusing magnet (the maximum field is some 6.5 kGauss). Since a few
thousands of klystron tubes would be operated in JLC main
linacs, the power efficiency of individual tube is a critical issue in the operational cost of the machine. From this pointof view, one cannot neglect the electrical power consumed
in the solenoidal magnet. In fact, the magnet of XB72K· consumes 35 kW, which is almost the same to the average
beam power to produce RF. In order to save this power, useof periodic permanent magnet (PPM) as a focusing magnet had been proposed[3]. Another advantage of the PPM
focusing system lies in its compactness and small weight.
Soon after the proposal, X-band PPM klystrons were built
atSLAC[4].

2DESIGN OF PPM KLYSTRONKEK and Toshiba Co. started a two-year project of Xband PPM klystrons to develop a new prototype of JLC
klystron[S]. It was planned that two klystron tubes werebuilt in 1999 and 2000 (one tube per year).
We do not build a PPM tube by simple replacement of thefocusing system of XB72K since it is not realistic for PPM
to make the field equivalent to the strong solenoidal field ofXB72K. The beam current (perveance) should be lowered

Figure 1: PPM klystron (#2).

2.1

GunThe perveance of the electron gun is 0.8JLK. We used
DGUN code[6] as a design tool. The parameter list of thegun is given in Table 1. The two electromagnets for beam-247-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

tuning are placed in the upstream region of the tube. One
is the bucking coil which controls the magnetic field on thecathode. The beam size is tunable by the cathode field as inTable 1. The other is the matching magnet (coil). It consistsof a pair of short solenoids in series and produces alternate
axial field. It controls the matching of the beam from thegun to the PPM focused region to minimize the beam scalloping in the tube.
The heat expansion of the cathode and its neighbors wasestimated by ANSYS code. We measured the displacementof the cathode by a CCD laser displacement sensor. We
found the measurement agreed well to the result of ANSYS.
The cathode stem was shortened for compensating the expansion. The measured perveance of the gun was very close
to the design value.

PPM

Output:~:~__ ______ [_____ ::_; _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ , __ _-4000

i --::--- :::: :~:~~:~:;:::::~:~~~--:___________ :_ __________ , __ -- -- ' -- -- _; -- -- --,-- -- -- ;0:::~~~'~~:--------1----------@~~z(mm)Figure 3: The design focusing magnetic field on the axis.

2.3

The material of the magnets is Nd-Fe-B (Sumitomo Special
Metal Co. NEOMAX 35SH, 39SH). The period of the PPM
is 30 mm and the maximum field strength on the beam axis
is 3.2 kGauss. The stopband voltage of this PPM focusing
structure[?] is about 30 kV.
Figure 2 shows the observed beam transmission rate fromthe gun to the collector under the design matching coilcurrent. The rate is 100% above the threshold voltage of
rv250 kV, which is much higher than the stopband voltage.
Simulation shows that below the threshold, the beam particles are lost due to large scalloping (the beam mismatches
to the PPM field).
1~~~~~~~~~~~~~~!~: : r~r r 1: r ::0. 2

Matching
Section

-2000
-3000

Permanent Magnet Circuit

Fooiu

Cathode4~~~~~~~~~~~~~rr~~~~~ _::1 ::::::::::;:::::::_:: j:Table 1: Parameters of the gun.
CathodeVoltage
480kV
Current
266A
Loading< 10Ncm2
Diameter
61mm
Max. Field on whole electrode< 230kV/cm
cathode192 kV/cm
anode
224kV/cm
Cathode Field
0 rv 20 Gauss
Beam Radius¢4.8 rv 6.8 mm

2.2

As the modulated beam is bunched in the downstream of
the tube, the beam tends to expand in the radial (r) direction.
The magnetic field is gradually increased in (about 1/3 of)the downstream to focus the bunched beam well.
The uni-directional magnetic field along z direction is located over the output cavity. The magnet surrounding theoutput cavity needs two cuts since two output waveguides
stick out in opposite directions. The field is not cylindrically symmetric due to the cuts. We designed the magnet
as the radial component, Bro to be zero on z axis. Otherwise the beam should be deflected in a transverse direction
and this may trigger off a parasitic oscillation.

RF Circuit

The klystrons have five cavities between the input and output cavity: Two gain cavities upstream and three bunchingcavities downstream. In order to obtain a robust structure
against the parasitic oscillations, all these five cavities are
made of lossy metal to damp out effectively the higher order modes in the cavities (Monel= Ni-Cu alloy for #1 whilestainless steel for #2). The drift tubes are stainless steel todamp the trapped modes between the cavities.
The output cavity of the PPM tubes is a traveling wave
type. We modified the design of traveling wave structure
installed on #10 of XB72K[8] for the PPM tubes. The
shape of the coupling ports to the output waveguides wasredesigned to increase the output power. The original structure is characterize as its relatively low surface electric field
and TE 111 (-like) mode is detuned away from the operation
frequency for the suppression of the parasitic oscillation.
The modification of the couplers does not spoil these characters.
The output cavity is copper-made. The higher ordermodes in the output cavity, other than the TE111-like, aredamped by the output waveguides. The two waveguides areconnected to the cavity not aligned to each other but slanted.
The field of higher order modes runs away into the waveguides.

--------l----------j------------j-----------r---------r----------l-----------

3 mGH POWER TEST

Oc_.__.~-'---'---~.__c_,_~_.__._~--'-'-~.__._j~~'---'--''--'--'--'

100150200 250 300 350 400
Cathode Voltage (kV)

Figure 2: Observed beam transmission rate.

450

The high power test was done at #3 station of KEK ARSouth Experimental Hall. The modulator produces themaximum voltage of 550 kV at 1. 7 JLS flat-top pulse. Themaximum repetition rate is 50 Hz.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

The #1 tube was tested form July to September 2000. Thetest was done under low rate (> 5 Hz) due to the limit ofcooling ability of the tube. We have never seen any signalof parasitic oscillation during the test. The tube produced
the maximum power of 68 MW at 514 kV.
After the 3-month test, the tube got modified to improve
its RF performance. The "Taper" RF windows[9] were replaced to the "Mixed-Modes"[lO] for the run with long RFpulse. The gain cavities were replaced and the frequencyof output cavity was tuned properly to improve the efficiency. The modified klystron, #1A, was tested from January to March 2001. We confirmed the efficiency was improved from #1. No parasitic oscillation was found either.
Table 2 summarizes the results of #1 and #1A.

450

[J45!+=40>.
u·o35420440

460

480500520

Cathode Voltage (kV)

Figure 4: Power and efficiency of #1, #1A and #2.

Table 3: High power test result of #2 and JLC specification.
Achieved
JLC I Design735275Power(MW)
(503)
(443)
(kV)
@Voltage
545255
Efficiency (%)
1.4
1.5
1.5
Pulse Length {p,s)3.625150/50
Rep-Rate (Hz)I~30400

The #2 klystron was tested from April to June 2001. We
operated the tube under 25 Hz most of the time. It was confirmed soon by a pilot run with short RF pulse that the efficiency was improved by about 5% compared to #1A. We
have found no parasitic oscillation. After the pilot run, westarted the run with full RF pulse (1.5 J-LS). We reached 50
MW at the full pulse length. The ceramic in the RF window
was damaged on a way from 50 to 75 MW output. The rate
was down to 3.6 Hz and we reached 73 MW at 1.4 J-LS. The
test result of #2 tube is shown in Table 3.

I55

tilTable 2: High power test result of #1 and #lA.
#1A
#1
Design666854>50
Power(MW)
49
47
44>50
Efficiency (%)
0.4
1.5 0.3
1.5
Pulse Length (J-LS)
525
Rep-Rate (Hz)

I60

ISUMMARY

We have built two PPM klystrons during 1999 to 2000. The
performance of these PPM tubes was very good. In Figure 4, we summarize the measured output power and efficiency of #1, #1A and #2 vs. the cathode voltage. The performance is improving from tube to tube, although we have
not reached to a JLC klystron yet. We will restart the test of#2 tube after the RF window gets repaired.
Photos and descriptions on the X-band klystron tubes developed at KEK are found at [ 11].

5 ACKNOWLEDGMENT
The authors would like to acknowledge to H. Nakayama
for his help in designing of magnets. We are grateful toT. Kohda of Sumitomo Special Metal Co. and K. Ogata ofTokin Co. for their special efforts in design and fabricationof the permanent magnets and the electromagnets respectively. Thanks are also to K. Takata, V. Balakin, M. Akemoto and N. Toge.

6 REFERENCES[1] JLC Group, JLC Design Study, KEK Report 97-1.
[2] H. Mizuno, "X-Band RF System for JLC", Proc. of RF96
(KEK Proceedings 97-1), Hayama, Kanagawa, Japan, 1996.
[3] D. Sprehn, et al., "PPM Focused X-Band Klystron Development at the Stanford Linear Accelerator Center", ibid.
[4] G. Caryotakis, "Development of X-Band Klystron Technology at SLAC", Proc. of 1997 PAC, Vancouver, B.C., Canada,1997. ; E. Jongewaard, et al., "The Next Linear Collider
Klystron Development Program", Proc. of EPAC 2000, Vienna, Austria, 2000.
[5] Y.H. Chin, "Status ofX-Band Power Source Development forJLC", Proc. of 18th HEACC 2001, Tsukuba, 2001.
[6] A gun simulation code developed at BINP.
[7] G.R. Brewer, Chap. 3 in "FOCUSING OF CHARGED PARTICLES" Vol. II, Ed. by A. Septier, Academic Press, 1967.
[8] Y.H. Chin, et al., "The 120MW X-Band Klystron Development at KEK", Proc. ofEPAC98, Stockholm, Sweden, 1996.
[9] Y. Otake, et al., "Design and High-power Test of a TEll-mode
X-Band RF Window with Taper Transition", Proc. of 1995
PAC, Dallas, Texas, 1995,[10] S. Kazakov, KEK Preprint 98-120.
[11] URL http://lcdev.kek.jp/-249-