Higher order mode coupler for the circular electron positron collider

Abstract

A conceptual design report (CDR) for the Circular Electron Positron Collider (CEPC) was published in September 2018. CEPC will use a 650 MHz radiofrequency (RF) system with 240 two-cell cavities. The collider will be a double ring with shared cavities for Higgs operation and separate cavities for W and Z operations. The higher order modes (HOMs) excited by the intense beam bunches must be damped to avoid additional cryogenic loss and multi-bunch instabilities. This study examined the impedance budget and HOM damping requirements for the CEPC ring to keep the beam stable, and a coaxial HOM coupler was considered for extracting the beam-induced HOM power. A double-notch coupler was chosen because of its wide bandwidth for the fundamental mode. Each cavity has two detachable coaxial HOM couplers mounted on the cavity beam pipe with a HOM power handling capacity of 1 kW. This paper presents the design process of the HOM coupler: the RF design, thermal analysis, and mechanical aspects. To validate the design, a prototype of the HOM coupler and a coaxial line test bench were fabricated and tested, and the test results agreed well with the simulation results. The HOM damping results for a real 650 MHz two-cell cavity were also measured and compared with the damping requirements.