Studies on higher order modes damper for the 3rd harmonic superconducting

To investigate the higher order mode (HOM) damping in the higher harmonic cavity for Shanghai Synchrotron Radiation Facility(SSRF) when using HOM absorbers, simulations have been done for changing the position and the length as well as the thickness of ferrite of HOM damper. The best values under which the Q value of HOMs can be greatly lowered and the impedance of harmonic cavity will be below the impedance threshold have been decided. INTRODUCTION The SSRF is a 3.5GeV synchrotron light source that has been constructed in Shanghai, China. The emittance of the SSRF storage ring is designed very low while the current remains 300mA. The beam lifetime is one of the important aspects for SSRF, high harmonic cavity usually be used to increase the lifetime, and the longitudinal parasitic coupled-bunch instabilities can be suppressed by the hhc cavity [1]. A third harmonic cavity is a common choice to increase the lifetime without deteriorating brightness, and in several light sources, it has been operated successfully such as ALS [2], NSLS [3], BESSY-II [4]. The Shanghai Superconducting Cavity Key Laboratory undertook the design and fabrication of this passive third harmonic superconducting cavity. Table 1: Parameters of a single cell HHC cavity for SSRF frequency 1499.69 MHz fundamental mode TM010 cavity voltage nominal 0.5 MV cell length 100 mm slope angle 90 beam pipe radius 41.5 mm iris circular radius 16 mm equator radius 90.1 mm The damped structure for the 3rd harmonic superconducting cavity has large beam pipe, Most of the HOMs go through this pipe and are absorbed by HOM absorber. The HOM absorber for SSRF requires sufficient HOM damping, power handing capability, UHV (ultra high vacuum) compatibility, free of particulates. We choose the ferrite C48 that be made by Countis. THRESHOLD OF THE MULTI-BUNCH INSTABILITY From the accelerator physics of the SSRF, we want to pay more attention to minimizing the impedance that can cause the single or coupled bunch instability. The growth rate of the instability should be less than synchrotron radiation damping. The threshold of longitudinal multi-bunch instability should be [5]