INPUT COUPLER OF SUPERCONDUCTING CAVITY FOR KEKB Y.Kijima*, S.Mitsunobu, T.Furuya, T.Tajima, KEK, High Energy Accelerator Research Organization T.Tanaka, Furukawa Electric Co Abstract The high power coupler is one of the key components of the superconducting cavity for high current operation The input coupler for the KEKB superconducting cavity is of coaxial type. It has been operated on a beam exceeding 380 kW. We conditioned the input coupler for the next four cavities up to 300kW of input power, while applying a DC bias voltage of up to 2kV on the inner conductor. This RF processing with bias voltage was found to be effective in suppressing multipacting. 1 INTRODUCTION KEKB is an asymmetric electron-positron collider for B-meson physics, consists of two rings, an 8 GeV high energy ring (HER) and a 3.5 GeV low energy ring (LER). Four superconducting cavities have been operated under a beam of 500mA in the HER. The cavity is supplied with a power of 270kW. During this summer, additional four cavities will be installed in the HER to increase the beam current to 1.1 A. The conditioning procedure of the coupler was studied at the HER and also at the AR-ring in 1995. This paper describes the preparation and aging process of the couplers used on the four additional superconducting cavities. 2 INPUT COUPLER FOR KEKB The RF system at the KEKB is required to accelerate the large beam current stably. The input coupler needs to feed a power exceeding 270kW. Table 1 gives the parameter of the SCC coupler. Figure 1 shows the structure. Table 1. Parameters of the SCC coupler SC cavity parameter Frequency 508.887 MHz Voltage 1.5 MV/cell R/Q 93 Number of cells 8 Coupler Loaded Q 7 *10 4 Power to beam 270 Figure 1. Input coupler for the KEKB The window of the input coupler has a choke structure to decrease the electric field at the gold braze of the ceramic. This is almost the same as in the 1MW klystrons used at TRISTAN, which have demonstrated long-life operation at high power. The gap of 3 mm of the choke was changed to 4 mm to reduce the field strength around the ceramic window by a 20%. The ceramic window is a 10mm thick coaxial disk made of 95% purity alumina. The surface of the window on the vacuum side is coated with TiNxOy to reduce the coefficient of the secondary electron emission. The inner conductor side of the window is cooled by water and the outside conductor is cooled by air . The impedance of the coaxial parts is 50 ohms. The inner conductor is made of electropolished copper, and the outer conductor is made of stainless-steel plated copper by pyrophosphoric acid. The coaxial pipe has fins outside, cooled by 8 l/min He gas flow at 4 K operation. To suppress multipacting and make the aging process effective, doorknob transition sections are used to supply a bias voltage of 2kV to the inner conductor. An electric insulator made of two layers of 0.125 mm thick polyimide kW Proceedings of EPAC 2000, Vienna, Austria 2040