JAERI-Conf 95-021

2p2-3
High Power Test of a Damped Cavity for High-Brilliant
Synchrotron Radiation Source
T. Koseki, M. Izawa*, S. Tokumoto*, K. Shinoe, Y. Kamiya, T. Nagatsuka**,
T. Miura***, K. Sato***, T. Naba***, Y. Ohnishi*** and S. Fujii***
Synchrotron Radiation Laboratory, The Institute for Solid State Physics (/SSP),The University of Tokyo, Tanashi, Tokyo 188, Japan*Photon Factory, National Laboratory for High Energy Physics,Tsukuba, Ibaraki305, Japan** Hitachi-Zosen Corporation, Taisyo, Osaka 551, Japan***Toshiba Corporation,Tsurumi, Yokohama 230, Japan
Abstract
The high power model of the damped-cavity whichbeing developed at ISSP and Photon Factory (PF) wasfabricated The high power conditioning of the cavity wascarried out. The input power of 150 kW was obtained
without any severe problems.
I. INTRODUCTION
We have developed the damped-structure RF cavity for
two low emittance electron/position storage rings. One is athird-generation VlN and SX synchrotron radiation source
which is a future project of the University of Tokyo. The
storage ring has a beam energy of 2.0 GeV, a circumferenceof about 400 m, an emittance of less than 5 nm-rad [1,2].
The other is a high brilliance configuration of the PF
storage ring [3]. In these storage rings, coupled-bunch
instability due to higher-ordermodes (HOM's) in RF cavity
is a serious problem when stable and high beam current isrequired The damped cavity, we have studied, has large
beam duct, a part of which is made of a SiC microwave
absorber. The HOM's propagating out from the cavity
through the beam duct are expected to be damped by the SiC

The design parameters of the damped-cavity aresummarized in Table 1. The nominal operating voltage ofthe cavity system is 1.5 MV for both the PF ring and theVlN-SX ring. The number of the cavity to be installed inthe storage rings is four for the PF ring and three for theVlN-SX ring. For the VlN-SX ring, the nominal gap
voltage per cavity is 0.5 MV, requiring the power of about
33 kW to be dissipated in the cavity. The design of 140 kW
wall loss has large safety margin and operational flexibility.
The R&D of the damped-cavity is now focusing on ahigh power test of the cavity. The specification of the highpower model and recent status of its high power
conditioning are presented here.
II. HIGH POWER MODEL
The high power model cavity was manufactured at
Keihin Product Operations of Toshiba Corporation [7].
Figure 1 shows the cross sectional view of the high powermodel.
Coupler port

part.

We fabricated two cold models of the cavity andmeasured their RF characteristics in low power level. As theSiC absorber, we chose CERASIC-B (made by Toshiba
Ceramics Co. Ltd.) which was fabricated by sintering in anargon atmosphere under normal pressure. The low power
measurement showed the SiC beam ducts strongly reduce
the Q-values of HOM's in the cavity [4, 5, 6]
Table 1: Parameters of the cavity.
RF frequency
Shunt impedance
UnloadedQ
Coupling coefficient (B)Maximum wall loss

500.1 MHz7.68 MQ
44000
>2
140kW

Blank-flange port
632

Figure 1: Cross section of the high power model.

-239-

JAERI-Conf 95-021

The main part of the cavity is mack of classl-OFHC
copper which was treated with Hot Isostatic Press (HIP).
The layout of water cooling channel is also shown in Fig. 1.
The water flow of 200 //min is available with a pressure
drop of 4 kgf/cm2 •The cavity has two beam ports and four side ports for an
input coupler, a tuning plunger and two blank-flanges. Utight seal gaskets are adopted as RF contacts between theport franges and the attached equipments.
A thermal structure calculation with two dimensionalmesh was carried out [7]. The analysis assumed a water flowof 1401/min andinlet watertemperature of20 ·c. The edgeof nose cone gave the highest power density in the cavity.
For 160kW total power dissipation, peak power density
around the nose cone was calculated to be 30 W/cm2• Then,the temperature of the inner wall of the cavity was 50 - 60 ·c
and the edge of the nose com was about 70 ·c. The frequency
shift caused by the thermal deformation was expected to be-250kHz for the fundamental mode.
Figure 2 shows the input coupler of the cavity. Thecoupler was newly designed basing on the coupler which isused for the 508 MHz APS cavity of the TRISTAN ring.
We changed the shape of the end of coaxial line andoptimized the positions of the short plates of rectangular
waveguide and coaxial line in order to obtain low-loss of
500 MHz microwave [8].

Figure 3: The fixed tuner block.
Figure 4 shows the fabricated high power model. Theunloaded Q value of the accelerating mode was measured tobe 40000 at 500.1 MHz. The coupling coefficient B was2.35, while the value expected from the low-power
measurement was 2.27 [6].

707 5!

~~~~
Loop
Outer conductor

mL-$I

Nozzle for air cooling --- 'Ijt= :lWR1500 waveguide

Figure 2: The newly designed input coupler.
The tuning plunger of the cavity is the same type as usedin the PF cavity. The blank-flange, also called fixed tuner, isa flange with cylindrical block to pad the port of the cavity.
Figure 3 shows the fixed tuner block. The center of theblock was hollowed out and a viewing port was attachedthere. The cavity has two blank-flanges (horizontal andvertical). The blocks of the blank-flanges are projecting 10mm from inner surface of the cavity.

Figure 4: The high power model.

ill. HIGH POWER CONDffiONING
The high power test of the cavity has been carried out at
the high power test bench of the Photon Factory. SiC-240-JAERI-Conf 95-021

beam-duct was not attached to the cavity in this test. Before
the high power test, the cavity was prebaked at 150 OC for 24hours at Keihin Product Operations of Toshiba
Corporation.
In the high power test, a 300 l!sec turbo molecularpump was attached to the cavity and an ionization gauge wasplaced between the beam port and the turbo pump. The
vacuum pressure before input RF power was 5x 10· 8 Torr.
The high power conditioning started with an input
power of a few hundreds watt and the power level was slowly
increased by keeping the vacuum pressure below 5x 10"6Torr. The reflected RF power signal was used as an interlock
trigger. The tuning plunger was controlled forcompensating a thermal detuning of the cavity.
After 20 hours conditioning, the input power wasreached to 60 kW. The vacuum pressure was 5x 10·8 Torr
without RF power. In order to try higher power
conditioning under the lower vacuum pressure, the cavity
was baked at 150 T for 24 hours. After the baking, thepressure was drop to 3x 10·9 Torr, and then pulse
conditioning at 100 kW with 10% duty was carried out for
12 hours. After that, cw conditioning was restarted.
The cavity has three viewing ports to observe discharge
phenomena around input coupler, tuning plunger and nose
cone. Small glowing points and occasional arcing weregrow with increasing input power. However, as the highpower conditioning progressed, the number of theseglowing points decreased at the same level of input power.
160.------------------ ------------.
Cavity input power

140-120~a::;1003g_------""'\,_-::> 80
0.

c:

15 ~

_£60>

10 ~

<:5 40llmin and22 OC (inlet temperature). When 140kW is applied,
the reflection power of 23 kW is expected from the value ofcoupling coefficient, B = 2.35. However, the measured
reflection power was lower than the expected value. The
reason of this discrepancy has not been clarified yet.
We have also fabricated the SiC beam-duct for highpower test. The duct is composed of Al duct with ICF253
flanges and CERASIC-B duct which is inserted in the Al
duct by the method of shrink fit. The high power test of the
SiC beam-duct has been successfully carried out as describedin Ref. [9].
IV. ACKNOWLEDGMENTS
We would like to thank to all staff members of SRLaccelerator group and of Light Source Division of thePhoton Factory for their support and useful discussions.
V. REFERENCES[1] Y. Kamiya et al., Proc. 4th EPAC, Loncbn, 1994, p.
639.
[2] VUV -SX Accelerator Group, ''VUV-SX High BrillianceLight Source", in these proceedings.
[3] M. Katoh et al., Rev. Sci. lustrum., 66 (1995) 1892.
[4] T. Koseki et al., Proc. 4th EPAC, Loncbn, 1994, p.
2152.
[5] T. Koseki etal., Rev. Sci. lustrum., 66 (1995) 1926.
[6] T. Koseki, M. IzawaandY. Kamiya, "Developmentofa Damped Cavity with SiC Beam-Duct" to be published inProc. 1995 PAC, Dallas, 1995.
[7] T. Miura et al., Proc. 20th linear Accelerator Meeting inJapan, 1995, p.200.
[8] T. Nagatsuka, T. Koseki, Y. Kamiya, M. Izawa andY.
Terada, "A Design of Input Coupler for RF cavity" to be
published in Proc. 1995 PAC, Dallas, 1995.
[9] M. Izawa et al., Rev. Sci. Instrum., 66 (1995) 1910

0.

205.g0'-------1 0&!

~

TimeFigure 5: An example of high power test.
Up to now, total conditioning time has been 60 hours.
The input power of more than 150 kW was obtained without
any severe problems. Figure 5 shows an example of input
power, vacuum pressure and reflection power. At an input
power of 140 kW, the maximum temperatures of cavity
wall was measured to be 47 OC with the cooling waterof200-241-