Design of Electromagnetic Bandgap Cavities for High-Gradient On-Axis Coupled-Cavity Linear Accelerators

The design of suitable electromagnetic bandgap (EBG) cavities has been performed by means of a hybrid numerical/analytical approach implemented via a homemade code with the aim of optimizing a novel accelerating structure for proton linear accelerators (linacs). In particular, a 3-GHz proton linac tank with on-axis coupled EBG cavities closed with full-end cells has been optimized. The proton beam input energy is 27 MeV. The performances of the proton linac EBG accelerating cavities have been compared with the performances of 27-MeV 3-GHz side-coupled proton linac accelerating cavities in terms of typical linac figures of merit. The use of EBG cavities allows to increase the transit time factor (by about 8%). Moreover, the peak surface electric field is strongly reduced (by about 65%), paving the way to the design of very high accelerating gradient microwave proton linacs. Furthermore, the wakefields of the EBG structure have been compared with those of the SCL structure, showing that the EBG structure provides effective damping of the transverse wakefields.