Multipacting suppression of the HOM-damped 166.6-MHz β=1 quarter-wave superconducting cavities at HEPS

Abstract

A higher-order-mode (HOM) damped 166.6-MHz β=1 quarter-wave superconducting cavity has been designed for the High Energy Photon Source project. Dangerous multipacting barriers have to be removed by geometry optimizations in superconducting cavity design while retaining favorable radiofrequency and mechanical properties as well as sufficient HOM damping. Multipacting analysis has been conducted on the cavity by using coupled simulation codes comprising electromagnetic calculations and particle tracking. A circular transition structure between the cavity and the beam pipe was proposed to remove the high-field multipacting band by eliminating the local minimum of the electric field thus preventing the field-induced electrons from being trapped into the potential well. The impacts of the geometric change on the cavities' electromagnetic and mechanical properties as well as HOM damping were examined. The deteriorated impedance of the first monopole HOM can be recovered by a slightly enlarged beam pipe, while the total loss factor has little change. The compact footprint of the cavity module was maintained as required for a limited straight section with a fixed length. Mechanical properties of the cavity were also retained with an improved tuning sensitivity and frequency stability. These constitute a first HOM-damped 166.6 MHz β=1 quarter-wave superconducting cavity with multipacting suppressed.