Progress in the design of a damped and tapered accelerating structure for CLIC

Two of the main requirements for CLIC 30 GHz accelerating structures are an average accelerating gradient of 150 MV/m and features which suppress long-range transverse and longitudinal wakefields. The main effects that constrain the design of a copper structure are a surface electric field limit of about 300 MV/m, from evidence produced by the CLIC high-gradient testing program, and a pulsed surface heating temperature rise limit estimated to be of the order of 100 K. The interplay between maximum surface electric field, maximum surface magnetic field, transverse-wakefield suppression and RF-to-beam efficiency has been studied in detail. Several structures with a 110° phase advance and rather constant peak surface electric field distributions have been designed. Different damping-waveguide geometries and waveguide-to-cavity couplings are compared.