The ffth Efymposium on Accelerator &ience and Technology, Tsukuba, Japan 1993

High Power CW Linac in PN C
S. Toyama, Y. L. Wang, T. Emoto, M. Nomura, N.Takahashi, H. Oshita,
K. Hirano, Y. Himeno
Power Reactor and Nuclear Fuel Development Corporation
4ll02 Narita, Oarai-machi, Ibaraki, 311-13 Japan
I. Sato, A. Enomoto, M. Ono
National Laboratory for High Energy Physics
1-1 Oho, Tsukuba-shi, Ibaraki, 305 Japan
Abstract
Power Reactor and Nuclear Fuel Development
Corporation(PNC) is developing a high power electron linac
for various applications. The electron beam is accelerated in
CW operation to get maximum beam current of lOOm A andenergy of lOMeV. Crucial components such as a high power
L-band klystron and a high power traveling wave resonantring(TWRR) accelerator guides were designed .· andmanufactured and their performance were examined. Thesedesign and results from the recent high power RF tests weredescribed in this paper.

2. STRUCTURE OF HIGH POWER CWELECTRON LINAC
A conceptual RF diagram of PNC accelerator is shown inFigure 2. The accelerator consists of eight normal conducting
accelerator guides which each have TWRR including one
buncher guide.

1. INTRODUCTION
The development of a high intensity CW electron linac tostudy feasibility of nuclear waste transmutation was startedin 1989[ 1]. Its purpose is to establish the high current beam
acceleration technique with more than 50% efficiency. The
specifications of the PNC Iinac are described in Table I. An
accelerating guide and klystron was newly developed in Lband 1249 MHz RF so as to achieve high efficiency. The
accelerator facility building was constructed in August, 1991.
The exterior of the building is shown in Figure I. The facility
has three floors, where there are the accelerator in thebasement, klystrons and its power supply in the first floor.
The base floor has 2.3 m concrete shielding on its roof. The
high power RF test for the components started in 1992. The
study of high current beam operation will be started in 1996
after the injector test in 1995 using partially built accelerator.

Figure 2. Conceptual diagram of PNC accelerator:
G is the electron gun; C, a chopper; PB,a prebuncher; B, a buncher; AC, a acc.guide;K, a klystron.
The injector section consists of an electron gun, aprebuncher, a chopper and a buncher guide. A water cooledchopper slit is designed because of high heat removal of 11kW. Prebuncher system with two standing wave cavities isdesigned to compress phase angle of beam bunch without
large energy dispersion which happens to escape from beamtransportation. Two high power CW klystrons feed RF into
four accelerator guides respectevely. The Iinac accelerates
lOMeV-lOOmA electrons with the duty factor of 20-100%.
A 5 mm<j> impregnated cathode generates electron beam of
200kV and average current lOOmA. It is very important tokeep low beam ernitance in order to avoid beam breakup(BBU) in the case of high current beam acceleration The
high quality LaB6 cathode which can emit stable and brilliant
electron beam will be future candidate for the smaller
emitance electron gun besides of thermionic cathode. Thebuncher guide was designed in constant gradient structure foruniform power dissipation along the buncher cavity wall.
Klystron is driven by 90kV DC power supply to produce 1.2

Figure 1. View of accelerator facility.

-53-2050

MW RF with the efficiency of more than 65%. The 1.2MW
RF power is fed into four TWRR through two 3 dB
directional couplers.
The constant gradient regular accelerator guide and phaseshifter and stub tuner form a recirculating wave guide called
TWRR, which get high accelerating efficiency using travelingwave.
This guide has a efficiency 65% and field
multiplication factor two with full beam loading, which isalmost same· efficiency by standing wave type. The
advantage to employ TWRR rather than standing wave
accelerator guide is as follow; simplicity of cavity structure,larger aperture size bf disk hole, less effort of fabrication, andeasy mechanical separation from the recirculating wave
guide. These make it easy to handle under heavy radiation
field.

46Ol

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1000200

300

400

500

600

Q

700

Power in TWRR [kW]Figure 3 a. RF power dependence of ace. temperature.

N

3. RESULTS FROM RECENTDEVELOPMENTI~

Low power RF characteristic of an accelerator guide wasmeasured in preceding experiment[2]. The maximum RFpower from the klystron was limited to 330 kW byunexpected heat generation from the klystron ceramic
window. High power RF characteristic of a accelerator guideand a klystron output window was examined by using theklystron powered by a 90kV DC power supply in the national
laboratory for high energy physics(KEK).

A. Accelerator guide
The experiment of an accelerator guide is performed witha one TWRR unit and one klystron. The RF power wasloaded into the TWRR unit with CW and long pulse (-4 ms)
aging to squeeze the RF spark and gas emission inside of theTWRR. It took about two weeks to reach the final RF power
level, which power dissipation was equivalent to full beamloading.
Unexpected heating of a phase shifter in the TWRR
occurred when RF power level reached 35 kW. One of thephase shifter choke cylinder temperature increased more than
100 degree. This phenomenon came from incomplete
electromagnetic contact of a finger strip between a stub anda outer cylinder of choke component. It was turned out theRF attenuation of the phase shifter must be reduced less than
0.05 nep. After the phase shifter was replaced by straightwave guide, the accumulated power in the TWRR achieved
800 kW in CW operation. The Q value was 1150 which isin accordance with calculation. The temperature changes andchanges of resonant frequency along TWRR RF power are
summarized in Figure 3 a and b. The following values can be
1. frequency deviation per RF-50.1 kHz/1 OOkW2. temperature deviation per RF
2.1 ·cnOOkW
3. frequency deviation per temperature -23.4 kHz/ ·c.

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0I 00

200

300

400

500

600

700

Power in TWRR [kW]Figure 3 b. RF power dependence of resonant frequency.
The result is in good agreement with the result from lowpower experiment carried out in preceding experiment, whichmeans there is no local heating inside the accelerator guide infinal use. The field multiplication factor M for TWRR wasthree, which was reasonable from the expectation bycalculation with no beam loading. The choke structure ofphase shifter should be modified in next development.

B. Klystron window
The maximum power of the klystron is limited to 330 kW,
because the temperature of ceramic window increased up to
90 degree, which is one third of the critical points ofdestruction by thermal stress. So the modified output window
was designed and three different pieces of pill-box type
windows were tested with TWRR unit replaced theaccelerator guide. The materials, dimension and VSWR ofeach piece are summarized in Table 2. The high power test
was done by increasing RF in TWRR with the careful watch
by video camera for light emission from ceramic surface andby infrared-thermometer for the surface temperature. The
aging of these windows is done by CW and long pulse (dutyfactor 5-20% at 50 Hz). The results from three test windows
are shown in Figure 4. The beryllia window which has long
dimension was able to endure the 1.7 MW CW RF power.
The temperature increase is 48 degree and there was no glow
discharge by multipactoring effect on the ceramic surface.
Only some light spots was observed from the small dust

attached to the ceramic surface. The alumina window which
has shorter dimension was broken down at power of 0.5 MW
in CW. The crack was caused by thermal stress. The test of
higher power with long pulse was carried out to look for thepossibility to produce more than 1.2MW from klystron withlong pulse operation. The experiment was done by using 2
ms and 4 ms pulse and duty factor of 10% and 20 %. Therewas no multipactoring bellow 4 MW and weak light emission
was observed above 4.25 MW.

4.SUMMA RY
The high power CW electron Iinac is designed using normal
conducting TWRR and high power CW klystron to accelerate
electron beam with 10MeV-100mA at peak current. The
efficiency of acceleration is designed to get more than 65% atfull beam loading and 65% efficiency for klystron. The highpower component test was carried out to check the design
parameters. It was turned out that an accelerator guide andklystron output window are able to show sufficiently enoughperformance for high power CW operation, however thephase shifter in TWRR should be modified to get lessattenuation.

BREAKDOWN

140

LONG PILL·BOX WINDOWWITH THE ALUMINA CERAMIC~ 120I-

5. ACKNOWLEDGEMENT~ 1000

This study is collaborated between PNC and KEK. Theauthors sincerely would like to acknowledge the member ofKEK, especially, Dr. S. Fukuda for his advice and discussionof klystron window, Dr. S. Yoshimoto for his help anddiscussion of DC power supply and RF system, and Dr. H.
Nakanishi for his help of construction of cooling system for

STANDARD PILL·BOXWINDOWWITH THE BERYLLIA CERAMIC·e....cu 80Cl)()

604020LONG PILL· BOX WINDOW· WITH THE BERYLLIA CERAMIC

4.00

800

1200

1600

high power experiment.
2000

6. REFERENCES

POWER In Pill Box Window (kW)[1] S. Toyama et al, "Transmutation of Long-lived Fission
Product (137Cs,90Sr) by a Reactor-Accelerator system,"
Proceedings of The 2nd International Symposium on
Advanced Nuclear Energy Research, (1990)[2] A. Enomoto et al ,"Development of the CW Electron
Linac Using Traveling Wave Resonant Ring,"
Proceeding of European Particle Accelerator
Conference, (1992)

Figure 4. Temperature increase of test windows.
The result agrees with the characteristic of field decrease
and reduction of VSWR by the design and suggests that theklystron will be able to produce more than 1.2MW RF after
changing the design of the window.

Table I. Parameter of The PNC Linac
operation mode

cw

cw

pulse
Resonant ringtransmission(no load)transmission(load)
multiplication(no load)
multiplication(load)
Klystron(target)
power
beam voltage
micro-perveancegain
efficiency
modulationGeneralpulse width
repetition rateenergy
current
total length
Accelerator section
typemodefrequency
gain(max)

0.8ms
250pps
10MeV 18MeV
100mA 55mA
20m
traveling wave
constant-gradient2rt/31249MHz
1.4 MV/m2.0MV/m

Table 2.

No.
AB

0.946
0.850
3.0
2.0
4.1MW
147kV
0.8
55 dB
50 dB
50%
65%
modulating anode

1.2MW
90kV

Parameters of window test

material length of pill-box dia. of pill-box
190.5mm
402mm
BeO
193.7mm
412mm
Al 20 3VSWR
1.04(measured)1.30(measured)-55-pulse

phase length
751.6deg.
777.8deg.