LOM and HOM damping study in a superconducting deflecting cavity for ALS at LBNL

Superconducting deflecting cavities can be used in synchrotron light source to generate subpicosecond X-ray pulses while the impedance of the lower order modes （LOM） and higher order modes （HOM） in the cavity should be kept below an accepted level to avoid beam instability. These modes can be damped by adding waveguide on beam pipe. Detailed simulation of Q in CST Microwave Studio is introduced and experiment results on an aluminum model cavity with damping waveguide are reported to make a comparison.

HOM damping requirements and measurements for CEPC 650-MHz 2-cell cavity

Introduction The circular electron-positron collider(CEPC)will use a 650-MHz RF system with 240 two-cell cavities for the collider.The collider is a double ring with shared cavities for Higgs operation and separate cavities for W and Z operations.The higher-order modes(HOM)excited by the intense beam bunches must be damped to avoid additional cryogenic loss and multi-bunch instabilities.Materials and methods To get the real damping results,two prototypes of HOM coupler have been fabricated and installed on the 650-MHz two-cell cavity.The HOMs have been verified by bead pulling method.A test bench with two 2-cell cavities is used to measure the real damping results and study HOM propagating properties for a cavity string.Conclusion In this paper,the impedance budget,HOM damping and HOM power requirements for the CEPC collider ring are given.The damping results measured for the fundamental mode and HOMs seem good compared with the simulated results.The absorbing efficiency of the absorber and the extraction power efficiency of HOM couplers were also achieved.

A coaxial HOM coupler for a superconducting RF cavity and its low-power measurement results

A resonant buildup of beam-induced fields in a superconducting radio frequency （RF） cavity may make a beam unstable or a superconducting RF cavity quench. Higher-order mode （HOM） couplers are used for damping higher-order modes to avoid such a resonant buildup. A coaxial HOM coupler based on the TTF （TESLA Test Facility） HOM coupler has been designed for the superconducting RF cavities at the Proton Engineering Frontier Project （PEFP） in order to overcome notch frequency shift and feed-through tip melting issues. In order to confirm the HOM coupler design and finalize its structural dimensions, two prototype HOM couplers have been fabricated and tested. Low-power testing and measurement of the HOM couplers has shown that the HOM coupler has good filter properties and can fully meet the damping requirements of the PEFP low-beta superconducting RF linac.

Preliminary study of the higher-harmonic cavity for HLS-Ⅱ

A higher-harmonic cavity will be used to increase the beam lifetime and suppress coupled-bunch instabilities for Hefei Light Source-II. In this paper, results simulated by the particle-tracking model confirm that tuning in the harmonic cavity may suppress the parasitic coupled-bunch instabilities. The factors calculated for lifetime improvement are larger than 2.5. The 3rd and 4th harmonic cavities have been designed. In particular, the absorbers and antenna couplers are applied in harmonic cavities to damp the higher order modes. Finally, the 4th harmonic cavity similar to the Duke＇s RF cavity will be used for HLS-II.