Design of High Gradient Accelerating Structure for CLIC

A new CLIC main‐linac accelerating‐structure design, HDS (Hybrid Damped Structure), with improved high‐gradient performance, efficiency and simplicity of fabrication is presented. The gains are achieved in part through a new cell design which includes fully‐profiled rf surfaces optimized to minimize surface fields, and hybrid damping using both iris slots and radial waveguides. The slotted irises allow a simple structure fabrication in quadrants with no rf currents across joints, a reduced number of pieces per structure (only 4) and a reduced surface requiring precise machining. Further gains are achieved through a new structure optimization procedure, which simultaneously balances surface fields, power flow, short and long‐range transverse wakefields and rf‐to‐beam efficiency. The optimization of a 30 GHz structure with a loaded accelerating gradient of 150 MV/m results in a bunch spacing of eight rf cycles and 31 % rf‐to‐beam efficiency.