Optimisation of PBG-Waveguides for THz-Driven Electron Acceleration

Vint, Andrew and Letizia, Rosa (2021) Optimisation of PBG-Waveguides for THz-Driven Electron Acceleration. PhD thesis, Lancaster University.

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Abstract

Particle acceleration in dielectric structures driven by laser sources has been demonstrated as a viable approach to overcome the limitations of conventional accelerator technology and achieve unprecedented levels of electric field gradients. Among the dielectric laser acceleration schemes possible, the use of THz high energy pulses has gained increasing attention in recent years to provide a solution to the challenging whole bunch acceleration problem typical of the optical frequency regime. Most of the experimental work performed so far in THz driven acceleration is based on dielectric lined waveguides. This thesis investigates the use of photonic crystal technology to assess their potential in supporting THz acceleration. Photonic crystals have already been extensively investigated as optical scale accelerating structures. The use of photonic crystals over conventional dielectric lined metal walls allows for higher breakdown resistance which in turn offers the potential of higher accelerating gradients. For this reason, photonic crystals are a potential future technology for high-gradient, low-footprint particle accelerators. Here, their properties are exploited in novel metal/dielectric structures to provide efficient acceleration at THz. In particular, a methodology for the design and optimisation of photonic crystal-based waveguides, also known as photonic bandgap waveguides (PBG-Ws), for THz electron acceleration is presented. The effects of broad and narrow pulse bandwidths on the effective accelerating voltage are considered, with particular attention on providing a design methodology to tune central frequency of operation. The effects of tunability, coupling, wall-fields, and beam-driven excitation are numerically investigated and compared to the dielectric lined waveguide. Advantages and limitations of two main geometries of PBG-based waveguides are provided. It was found that 1D PBG-based waveguides lead to better accelerating voltage overall, while 2D PBG-based waveguides are interesting for very narrowband coherent Cherenkov source applications. In addition, the design methodology here presented is not limited to photonic crystal-based waveguides as shown by results obtained for the dielectric lined waveguide and can contribute to the design of future THz acceleration dielectric structures.