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 (${\mathrm{TM}}_{11}$-like, ${\mathrm{TM}}_{12}$-like, and ${\mathrm{TM}}_{13}$-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 high-gradient accelerator PBG structures.
Highlights
Future high-energy accelerators will require highgradient electromagnetic fields to accelerate the electron beam to obtain high luminosity
This work represents the first step in the analysis of the wakefield damping in these high-gradient photonic band-gap (PBG) structures, which in turn will guide the design of future highgradient PBG structures for testing with and without beam
We report the results of PIC simulations of the wake potential in a seven-cell 17 GHz PBG structure based on the structure tested at MIT
Summary
Future high-energy accelerators will require highgradient electromagnetic fields to accelerate the electron beam to obtain high luminosity. By introducing a defect in the center of a triangular lattice of metallic rods, a PBG cavity can be formed which confines a particular operating mode and lets other modes radiate through the lattice This radiative damping decreases the Q factor of the dipole modes, making the wake damp faster. This work represents the first step in the analysis of the wakefield damping in these high-gradient PBG structures, which in turn will guide the design of future highgradient PBG structures for testing with and without beam. We report the results of PIC simulations of the wake potential in a seven-cell 17 GHz PBG structure based on the structure tested at MIT. A Rod spacing, b Iris thickness, t Iris radius, d Cavity length, L TM01 mode frequency Accelerating mode Number of cells
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