Calculation of wakefields in a 17 GHz beam-driven photonic band-gap accelerator structure

Abstract

We present the theoretical analysis and computer simulation of the wakefields in a 17 GHz photonic band-gap (PBG) structure for accelerator applications. Using the commercial code CST PARTICLE STUDIO, the fundamental accelerating mode and dipole modes are excited by passing an 18 MeV electron beam through a seven-cell traveling-wave PBG structure. The characteristics of the longitudinal and transverse wakefields, wake potential spectrum, dipole mode distribution, and their quality factors are calculated and analyzed theoretically. Unlike in conventional disk-loaded waveguide (DLW) structures, three dipole modes (TM 11 -like, TM 12 -like, and TM 13 -like) are excited in the PBG structure with comparable initial amplitudes. These modes are separated by less than 4 GHz in frequency and are damped quickly due to low radiative Q factors. Simulations verify that a PBG structure provides wakefield damping relative to a DLW structure. Simulations were done with both single-bunch excitation to determine the frequency spectrum of the wakefields and multibunch excitation to compare to wakefield measurements taken at MIT using a 17 GHz bunch train. These simulation results will guide the design of next-generation highgradient accelerator PBG structures.

High power experimental studies of hybrid photonic band gap accelerator structures

“…The gradients and breakdown probabilities achieved by the metallic PBG structures were found to be comparable with those of conventional disk-loaded waveguide (DLWG) cavities [16,17]. Wakefield damping has been studied in simulation for metallic PBG structures including both room temperature cavities and superconducting cavities [13,18]. A superconducting PBG cavity for damping high order modes in a superconducting structure has recently been successfully tested [19].…”