Design of a Higher Order Mode (HOM) coupler for suppression of Multipactor in a 325 MHz SSR cavity

This study presents the design and analysis of a Higher Order Mode (HOM) coupler for a 325 MHz Superconducting Spoke Resonator (SSR) cavity, aimed at suppressing multipacting discharge — a critical limitation in high-performance accelerator cavities. Building upon a previous simulation study that demonstrated the potential of higher-order quadrupole modes in mitigating the multipacting discharge, this manuscript details the design of a probe-type HOM coupler using CST Microwave Studio (CST MWS). The design objectives include maximizing HOM coupling efficiency across a broad frequency range while ensuring effective rejection of the fundamental accelerating mode, achieved through a capacitive coaxial coupler integrated with a parallel LC filter. The coupler's performance is characterized by external quality factor (Q _ext) simulations, demonstrating tunable coupling via antenna insertion depth adjustment for the selected quadrupole modes. Power loss analysis, bandwidth considerations, and thermal load estimates are provided to guide operational parameters and cryogenic requirements. The present results provide a basic foundation for implementing the HOM-based multipacting suppression in SSR cavities, thereby enhancing the overall accelerator performance.