Design of a HOM coupler for a damped cavity at the Photon Factory storage ring

We designed a higher order mode (HOM) coupler with a rod-type antenna to reduce HOM's impedances of trapped modes in a damped cavity. The antenna is followed by a standard coaxial line. A tapered silicon-carbide (SiC) load is fixed at the end of the coaxial line. The HOM coupler is attached to an opening for the fixed tuner which is used to detune the resonance frequencies of trapped modes to avoid the coupled-bunch instabilities. The shape of the SiC load and the insulator for the inner conductor of the coaxial line is optimized so that an input SWR is as small as possible by using the computer code HFSS. The input SWR was obtained to be less than 1.7 over the wide frequency range between 0.7 to 1.4 GHz. The low power measurement showed this type of HOM coupler sufficiently damped the HOM's in the cavity. The fabrication of the high power model based on this design is in progress.