Investigation of a Ridge-Loaded Waveguide Structure for CLIC X-Band Crab Cavity

In conventional crab cavities the TM11 mode is used to deflect the beam. It is necessary to damp all the other modes, namely the accelerating i.e. lower order mode (LOM), same order mode (SOM) and higher order modes (HOMs). In addition to this, as the TM11 mode is not the fundamental mode, it is generally not excited with the highest shunt impedance. This necessitates damping of the high shunt impedance modes to acceptable level. Here we report on the investigation of an alternative design of the X-band crab cavity for CLIC, based on ridge-loaded waveguide. In this type of cavity, the deflecting mode is the fundamental mode and has the maximum shunt impedance. However, the geometry of the cavity is chosen to optimise the ratio of group velocity to shunt impedance to minimise the effect of beam loading. The other modes are excited above the crabbing mode and are damped using wave-guides of appropriate cut-off frequency. Other advantages are: contribution of the higher order dipole modes is negligible and no SOM. However, the higher order monopole mode needs damping.