Electromagnetic simulations and properties of the fundamental power couplers for the SNS superconducting cavities

The Spallation Neutron Source (SNS) makes use of superconducting cavities for the acceleration of negative H ions in the main linac. Two types of 6-cell Niobium cavities are used in the superconducting portion of the linac: 33 /spl beta/=0.61 cavities and 48 /spl beta/=0.81 cavities. Each cavity is powered via a coaxial fundamental power coupler (FPC) of a simple yet robust design. The electromagnetic design of the main components of that coupler has been modeled and some of those properties have been measured experimentally. Modeling includes impedance matching of the window and of the waveguide to coaxial doorknob transition; coupling of the coupler fields to the cavity fields; and multipacting behavior of the coaxial line and window. Various aspects of design, simulation, and testing on the coupler and cavity are presented.

1 INTRODUCTION

Coaxial Fundamental Power Couplers (FPC) with coaxial ceramic windows will be used with the superconducting 6-cell cavities in the SNS main linac [1]. The basic coupler design is derived, with slight modifications, from the one KEK developed for 508.8MHz, due to its simplicity in design and dependability at high power operation [2]. For higher operating RF frequency in the SNS linac, the mechanical dimensions of the critical parts of the coupler have been scaled proportionately. The fundamental power couplers for the SNS superconducting linac cavities will have to satisfy the following specifications:

Frequency: 805 MHz

Peak power: 550 kW (SNS klystrons forward power)

Duty factor: 7.8 %, (60 pps, 1.3 msec pulses)

External Q: