The 9th Symposium on Accelerator Science and Technology, Tsukuba, Japan 1993

High Power Test of a 500 MHz RF Cavity
K.Satoh, T.Yoshiyuki, T.Rizawa*, T.Miura, Y .Hirata**, Y.Yoshiwara and H.Yonezawa***
Toshiba Corporation
Keihin Product Operations
2-4 Suehiro-cho, Turumi-ku, Yokohama, 230 Japan
M.Kwon, H.S.Youn and Y.S.Kim
Pohang Accelerator Laboratory, POSTECH
P.O.Box 125 Pohang, Kyungbuk, 790-600 Korea
Abstract

A prototype RF cavity for the Pohang Light Source(PLS) was manufactured and was tested with an input rfpower up to 70 kW. The resonant frequency and the effective shunt impedance of the cavity are 500.082 MHz and 8.6
MQ, respectively. At the end of the test, the vacuum pressure inside the cavity improved to 5x10-9 Torr with an input
rf power of 60 kW. The performances of the cavity satisfied
the requirements of the storage ring of the PLS.
I. INTRODUCTION

For the storage ring section of the PLS which is underconstruction [1], four single cell RF cavities will beapplied. The power dissipation on the cavity wall is inexcess of 60 kW for each cavity. A good vacuum condition of I Q-9 Torr order is required at the maximum input
power level. Toshiba Corp. manufactured a prototype cavity
in 1992 at Keihin Product Operations and performed a highpower test from November to December at Nasu ElectronTube Works [2]. The purpose of the test was aging and conditioning of the cavity and to confirm the cavity performanceswith a high input rf power. Another high power test wasalso carried out at POSTECH with their rf system in February, 1993. Other four cavities which will be installed in thestorage ring are now being manufactured.
Design features, rf characteristics, setups of the highpower test and test results are presented in this paper.
II. DESIGN AND PERFORMANCE

Figure 1 shows a cross section of the cavity. The shapeof the inner surface is identical with RF cavities of the Photon Factory (PF) storage ring. The prototype cavity is madeof OFHC-classl and the inner surface is mechanically finished with a surface roughness of lJ..lm (Rmax). Water cooling channels are shaped on the outerwall of the cavity andthe flow rate of the water is 100 1/min with a pressure
drop of about 2 kg/em'.
The cavity is equipped with six ports which are an input
coupler port, a tuner port, two higher order mode (HOM)
damper ports and two beam ports. Herico flex type copper
c-rings are used as rf contacts between equipments, such as
an input coupler, and the port flanges. All the ports except
for the beam ports are cooled directly by water channels
formed in the port walls. Diffusion bonding techniques areapplied for manufacturing the ports [3]. Experiences ofimprovements on the PF cavities indicated that an effective
cooling for ports is one of the important factors to realize
high power operations [4]. In addition to these main ports,
two small ones for an rf monitor and a viewing window
are provided.
To feed an rf power up to 100 kW (including a beamloading), an input coupler having an air cooled cylindricalceramic window is applied. The vacuum side of the ceramic
window is coated with TiN.
A plunger type tuner with a diameter of 70 mm is used
for compensating a thermal detuning and a beam loading.
Two dummy blocks are prepared for the HOM damper
ports because HOM dampers will not be adopted at thefirst stage of the ring operation. The dummy block length
was adjusted to realize the designed resonant frequency.
The performances of the cavity as well as design values
are listed in Table 1.
III. SETUP AND PROCEDURE OF HIGH POWER TEST

Figure 2 shows a schematic diagram of the setup of thehigh power test. A 400 1/sec ion pump (Varian) was used
for pumping the cavity at Toshiba. In the test atTable 1 Main parameters of the cavity
Design500.082
Resonant frequency (MHz)60
Input rf power (kW) :wall loss6;8
Effective shunt impedance (MQ)
Unloaded Q-value :before aging;;;; 37000:after aging
max. 3
Coupling coefficient of coupler
1.8
Tuning frequency range (MHz)
Base pressure (Torr) :before aging 5XlQ-9:after aging
2x1 o~gPressure with 60 kW (Torr)

Achieved
500.08270
8.6 (Qu=39100)
35700 (23°C)
39100 (23°C)
max. 3.2
1.9
5Xl Q-95Xlo~•o5Xlo~ 9 ;reducing* Heavy Apparatus Engineering Laboratory** Energy System Group*** Nasu Electron Tube Works-243-PORT

HOM

DAM_PI!RPORTFigure 1. Cross section of the PLS prototype cavity.

POSTECH, a 220 IIsee ion pump with NEG module(Perkin Elmer) was applied. An ionization gauge waslocated in a duct between the beam. port and the ion pump.
Interlock trigger levels were 1Xlo-' Torr and 10 kW forthe vacuum pressure and the reflected rf power, respectively. An arc sensor was also used at POSTECH.
Figure 3 shows the cavity under preparing the test.
Before starting the high power test at Toshiba, the cavity was baked at 150 oc for 15 hours (flat top period).
We started the high power test (aging) with an input rf
power of several hundreds watt and increased ·the power
level gradually so that the vacuum pressure was kept
below 1X1 (17 Torr.
At POSTECH, a baking process was also performed at150 oc for 20 hours and the high power test was very carefully progressed as keeping the vacuum pressure below
1X1o-' Torr.
·The aging periods amount to 80 hours at Toshiba and45 hours at POSTECH.
U.TSTIOl

to InDLOCI~8184 K<!774(YALYO RIISO •t POSTICIIl

N. TEST RESULT

A. Aging processFigure 4 shows the input rf power over the test period.
The rf power was occasionally turned off due to the vacuum pressure or the reflected power interlock during thefirst 30 hours at Toshiba. After then we could reach 60
kW. However, during the period from 32 to 40 hours, frequent arcing accompanied with pressure rise limited theinput power. We could not identify the arcing positions
due to a restricted viewing field of the lTV camera. After
this period, aging progressed relatively easily with less arcing. During the last period of the test at Toshiba, the input
rf power reached 70 kW and could be sustained at the power level for several hours. When the cavity was disassembled after the test, it was found out that two rf contacts
inserted between the HOM damper ports and the dummy
blocks were partially melted. The section diameter of thecontacts was increased because we supposed that a poor pressures for the contacts could cause the abnormal arcing and·the excessive temperature rise.
During the first 20 hours at POSTECH, the high power
test was inte:nupted frequently due to the vacuum interlock. However, it turned out that this troublesome situation originated in noise problems. After modifying thegrounding system, we could easily achieve the input rf power of 60 kW (the maximum power of the Valvo YK1250
klystron) without severe problems. Through the test period at POSTECH we detected only several times of arcing,some of which occurred at the ceramic window of the coupler. After the test, we confirmed that there were no damages on the rf contacts between the HOM damper ports andthe dummy blocks.
The unloaded Q-value improved to 39100 which was
88% of a Q-value calculated by using SUPERFJSH.

70Figure 2. Setup of the high power test.

-.

60.::::. 50-"' 40-"'... 2030

A:'92-1112

at TOSHIBA

C1.

8:'93-2

at POSTECH

c100I0

2030

40

50

60

7080

Time (h)Figure 4. Input rf power over the test period.

B. Vacuum Pressure

Figure 3. The prototype cavity preparing the high power
test at POSTECH.

During the first stage of the aging process, the vacuum
pressure could rise to 10·7 Torr order even with a smallinput rf power around few kW. The vacuum condition wasimproved as more time was spent for aging.
Figure 5 shows the pressure rise (the pressure subtracted the base pressure) as a function of the input rf power for
four different period (A- D) of the aging process. The-244-

1E-07-:r----------------,
A: 30hr-AGING at TOSHIBA
8: 70hr-AGING at TOSHIBA
C: 20hr-AGING at POSTECH

D: 30hr-AGING at POSTECH... 1E-08

E: PF CAVITY (half year use) ::::·-··-:,.. ··•.........................................................
.•.,.._.B......

....

t:.

,"'

------ _.-- '~o,"'...

-,'..""......"' 1E-09_-- -..,. ....

/:~~ -~ ~ .;.(.;~, ... "' / ./..··____ _. / ./ "'-, E. . . . . .• , · : : :..· : : : : : : : : : : : : : :...

.;:

•••

"·--"""""··"".,....,..··"' .....

2010•••••••• 0 •••••••••••••••••••••••••••3040

706050

RF Pore r (kW)Figure 5. Pressure rise as a function of the rf power.

0(31,..., 100lE-l'lt__, FASTC. Temperature Rise and Frequency Change

HOH--------------~------~-----

Figure 7 shows the temperature changes (the temperature subtracted the base temperature) of the various cavity
parts as a function of the input rf power. In addition tothe parts shown in the figure, the temperature of many
portiollS including the tuner port and the HOM damper
ports were monitored. We observed no remarkable temperature rise which could disturb the high power operation.
The tuner position was recorded during the test. From
the results, the frequency changing rate as a function ofthe input rf power was deduced to be about 3.2 kHz/kW
with the cooling water flow late of 100 1/min. It agreed
well with a result of a calculation performed by using
SUPERFISH and COSMOS.

AFTER BAKING
HzO

1/ CO_...;J.tdt _...L . . . ~-HzL ••0 01.,..., 1 0 0

IE ·f'l!'J'.I!J___________ _FAST

curve:C suggests that the effect of aging partiallyremained though the inner surface of the cavity was once
exposed to the air after the test at Toshiba and was kept
with dry N2 gas for nearly two months. A data of the PFcavity is presented in the figure for reference (curve:E)[4] .
This data was taken after a half year usage of the cavity in
the PF storage ring and a pulse aging (peak :100 kW, duty:10 %) had been carried out for 24 hours [5].
. At the end of the test at POSTECH, the vacuum pressure with an input rf power of 60 kW was 5X10·9 Torr
and was still improving .
The results of the residual gas analysis during thetest at Toshiba were presented in Fig.6. After the baking,the partial pressure of H10 as well as the total pressure
was reduced but still remained at higher level than thatof CO. After the aging, the level of 110 became lowest
among the main three elements of the residual gas.

HOM--------------~------------AFTER AGING
HzOt- - _t •V. CONCLUSION
The prototype RF cavity for PLS had been tested with
the input rf power up to 70 kW. The high power performance of the prototype cavity satisfied the requirementsof the PLS storage ring. Now four RF cavities which
will be installed in the storage ring are being manufactured. These cavities have the same design as the prototype one but a longer baking time shall be applied inorder to reduce further the partial pressure of 110. The
high power test is going to start from September in 1993
also at Nasu Electron Tube Works.

~co.uJ.- _J;_ ------ _. . . ----------- . . --Figure 6. Results of residual gas analysis.

30
<325~

...

... 20~~~

A: Ceramic Windn
8: Cavity Side
C: Cavity Center
0: Coupler Port
E: Water (Outlet)... 15;;;:;;10~,_...
i!!!

--8;;••·'·;,,;;,.:. "''''~c . ..

c..>":;;

Acknowledgments~--~~~.~~~~.. -·.,,..-··~::~:::·.:~.·.·~:.·~::~.~... ..
;:.~·::·:·:·---------- ~'-D. .... .........

010

204030RF Ponr (kW)5060

Figure 7. Temperature changes of the various parts.
A : subtracted the room temperature.
B-E : subtracted the water inlet temperature.

70

The authors would like to thank Drs. H.Kobayakawa,
S.Tokumoto, M.Izawa and S.Sakanaka of Photon Factory,
KEK for their valuable advises and helps on this work.
VI. REFERENCES[1] Design Report of Pohang Light Source,
Pohang Accelerator Laboratory, POSTECH, Jan. 1992.
[2] M.Kwon et al.; PLS Report TR/SR(RF)-93-001, Feb.
1993.
[3] H.Suzuki eta!., Proc. of Particle Accelerator Conf.,
San Francisco, U.S.A., May 1991.
[4] S.Tokumoto eta!., KEK Internal 88-6, August 1988.
[5] S.Tokumoto, private communication, June 1993.

~245~