HOM Damping Coupler Design for the 400-MHz RF Dipole Compact Crab Cavity for the LHC HiLumi Upgrade

Crab cavities are adapted as the baseline design for the LHC HiLumi upgrade to achieve head-on beam-beam collisions for further improvement in luminosity. A 400-MHz compact RF dipole crab cavity design was developed by a joint effort between Old Dominion University and SLAC under the support of US LARP program. This design has shown very favorable RF parameters and can fit into the available beamline spacing for either vertical and horizontal crabbing schemes. A niobium prototype cavity based on such a design has been manufactured for vertical test. In addition, there are stringent wakefield requirements that needed to be met for such a cavity in order to preserve the quality of the circulating beams. In this paper, we will discuss different damping schemes for such a compact design and present the HOM coupler designs to meet the damping requirements.