Higher Order Mode Damping in a Higher Harmonic Cavity for the Advanced Photon Source Upgrade

A superconducting higher-harmonic cavity (HHC) is under development for the Advanced Photon Source Upgrade based on a Multi-Bend Achromat lattice. This cavity will be used to improve the Touschek lifetime and the single bunch current limit by lengthening the beam. A single-cell 1.4 GHz (the 4th harmonic of the main RF) cavity is designed based on the TESLA shape. Two adjustable fundamental mode power couplers are included. The harmonic cavity voltage of 0.84 MV will be driven by the 200 mA beam with a bunch length of >50 ps RMS. Higher-order modes (HOM) must be extracted and damped. This will be done with two silicon carbide beamline HOM absorbers to minimize heating of RF structures such as the superconducting cavity and/or couplers and suppress possible beam instabilities. The HHC system is designed such that 1) most monopole and dipole HOMs are extracted along the beam pipes and damped in the ‘beamline’ silicon carbide absorbers and 2) a few HOMs, resulting from introduction of the couplers, are extracted through the coupler and dissipated in a room temperature water-cooled load. We will present time and frequency domain simulation results and discuss damping of HOMs.