Design and optimization of fundamental power coupler for 1.3 GHz superconducting cavity

Abstract Superconducting (SC) cavities are preferable over normal-conducting cavities for high duty factor or Continuous Wave (CW) operation. In SC cavities, the required Radio Frequency (RF) power is fed through a special kind of fundamental power coupler which makes use of both coaxial as well as waveguide. This kind of design utilizes advantages of both the types of coupler, which make them suitable for high power operation inside cryo-module. In this paper, fundamental power coupler is designed for TESLA shape cavity having 9-cells and resonating at 1.3 GHz frequency. A geometrical factor Q ext i.e., quality factor of external circuit is used to optimize coupling and hence the dimensions of co-axial coupler. The estimation of Q ext is done by using Pascal Balleyguier method and CST Microwave Studio. A special kind of transition called as “Door-knob Transition” is also designed for efficient power transmission from waveguide to co-axial. Further, the multipacting calculations are done using scaling laws to check the operational limit to the RF power. Simulation methods are validated experimentally using a copper cavity and coupler model.