Conceptual Design Of An Ideal Variable Coupler For Superconducting Radiofrequency 1.3GHz Cavities

TL;DR

This paper proposes a novel over-moded variable coupler design for 1.3GHz SRF cavities, emphasizing high efficiency, thermal isolation, and minimal mode conversion, optimized through analytical and numerical methods.

Contribution

It introduces a new over-moded variable coupler design for SRF cavities that achieves high match and thermal isolation, with detailed analytical and numerical optimization.

Findings

Achieves S11 variation from 0 to -20dB at 7 kW CW power.

Ensures minimal high-order mode conversion, nearly full match.

Provides effective thermal isolation between 2K and 300K sections.

Abstract

Inspired by the development of over-moded RF component as an undulator, we explored another over-moded structure that could serve the variable coupling for SRF purpose. This application is to fulfill variation of S11 from 0 to -20db with CW power of 7 KW. The static heat loss in the coupler is trivial from calculation. An advantage of this coupler is that the thermal isolation between the 2K and 300K section is considerable by vacuum separation. Within this coupler, only a single propagation mode is allowed at each section, and thus, the fact that no energy is converted to high order mode bring almost full match without loss. The analytical and numerical calculation for a two window variable coupler is designed and optimized. A RF power variation is illustrated in the scattering matrix and coupling to cavity is also discussed.