RF CAVITY OF KEK-PF STORAGE RING

Y. Yamazaki and K. Takata
National Laboratory for High Energy Physics

The synchrotron radiation loss in the KEK-PF storage ring with thewiggler will be 513 keV per turn for the 2.5 GeV electron. The peak cavity
voltage of 2.84 MV is necessary to obtain a reasonable quantum life time.
Here, the accelerating frequency is 499.99 MHz and the harmonic number h is312. A set of four re-entrant single cell cavities will be used to produce
this cavity voltage. Each of four klystrons will feed the power to eachcavity.
The computer program SUPERFISH 1 )was used to design the inner structureof the cavity~' 3 > The shunt impedance R h of the cavity was approximately
optimized (the R h used here includes th~ transit-time factor). The calculated
results were R hs= 9.87 ~and Q = 44,000 for each cavity made of copper.
From now on wesassume the Q-value of the actual cavity becomes 90 % of thecalculated value. Then the klystron power Pg = 125 kW generates the necessary
cavity voltage 0.71 MV in the beam-loaded cavity (500 mA). The coupling
coefficient of the input coupler should be 2.1 to minimize the reflectedpower. The wall loss is 60 kW for each cavity. The detailed RF parameters
are presented in Ref. 3.
Model cavities were made of aluminum prior to making a final design. In
the model cavity which is axially symmetric and, thus, corresponds to thecalculated one the resonant frequency and field distributions of the fundamental (accelerating) TMO mode were in excellent agreement with the SUPERFISH
calculation. Then, the 120 rom¢ opening was bored for the input coupler. The
coupling coefficient S = 0 to 3.0 was obtained by rotating the coupling loop
situated at 31 mm from the cavity surface. The coupling loop with the .~;:,ening
reduced the resonant frequency by 1.0 MHz. This frequency shift was taken
into account in the final design of the cavity.
The tuner is necessary to make an appropriate detuning of the cavity
depending upon the strength of the beam loading. It is also used to compensate
the temperature increase effect and the machining inaccuracy. The tuning
plunger with 70 rom diameter inserted into the cavity by 40 rom increased thresonant frequency by 1.3 MHz, which is enough for the above purpose.
The ceramic vacuum window for the input coupler is situated at thetransition unit from the wave guide to the coaxial line. The similar structure was already used in the 180 kW klystron, and gave rise to no problem.
The final design of the cavity is shown in Fig. 1. Additional two
openings are bored for the higher-order mode damping couplers. The necessityof the damping couplers is discussed in Refs. 4 and 5.
The cavity is now under construction. Three main components of the cavity
body are machined from OFHC copper separately. Cooling channels are milled
into each component and inner surfaces are then machined. The three components
thus prepared are silver-brazed each other. Here, water cooling jackets andnickel-coated stainless transition pieces are also silver-brazed to the-225-

cavity body. Finally stainless vacuum flanges are helium-arc welded to thetransition pieces.
1)
2)
3)
4)
5)

K. Halbach and R.F. Holsinger, Particle Accelerators l (1976} 213.
K. Batchelor andY. Kamiya, KEK Report, KEK-79-25, 1979.
Photon Factory Design Handbook,§ 4 (1979).
Y. Yamazaki, K. Takata and S. Tokumoto, KEK Report (to be published).
Y. Yamazaki, K. Takata and S. Tokumoto, present Proceedings.

Fig. 1

RF cavity for the KEK-PF electron storage ring. Twoopenings will be used for the higher-order mode damping
couplers.

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