title: Investigations Into Dual-Grating Dielectric Laser-Driven Accelerators
Creators: name: Wei, Y
Publisher: name: University of Liverpool Repository
Year: 2018

description: Dielectric laser-driven accelerators (DLAs) utilizing the large electric fields from commercial laser systems to accelerate particles with high gradients - in the range of GV/m - have the potential to dramatically reduce the size and cost of future particle accelerators. Dual-gratings are one of the candidates for DLAs. They can be mass-produced using currently-available nanofabrication techniques, due to their simpler structural geometry compared to other types of DLA. So far, dual-grating structures have experimentally demonstrated accelerating gradients of 300 MV/m, 690 MV/m, and 1.8 GV/m for relativistic electron acceleration and gradients of 220 MV/m and 370 MV/m for non-relativistic electron acceleration. These demonstrations pave the way for implementing an on-chip particle accelerator in the future. This thesis presents detailed studies of dual-grating DLAs, including geometry optimizations, beam quality and energy efficiency studies, wakefield and particle-in-cell (PIC) simulations, as well as the fabrication and experimental preparations beyond the current state-of-the-art DLAs. In order to identify the optimum geometry, various numerical studies are carried out using the CST and VSim simulation codes, to maximize the accelerating factor and the accelerating efficiency for dual-grating structures, and results from both codes are compared. The beam parameters of the future Compact Linear Accelerator for Research and Applications (CLARA) facility are then used to load an electron bunch into an optimized 100-period dual-grating structure, where it interacts with a realistic laser pulse. The emittance, energy spread, and loaded accelerating gradient for modulated electrons are then analysed in detail. Results from simulations show that an accelerating gradient of up to 1.15 GV/m, with an extremely small emittance growth, 3.6%, can be expected. Two new kinds of schemes are also investigated in this thesis to improve the electron energy efficiency for dual-grating DLAs. One is to introduce a Bragg reflector, which can boost the accelerating field to generate a 70% higher energy gain compared to bare dual-grating structures, from PIC simulations. As a second scheme, pulse-front-tilted operation for the laser beam is proposed, to extend the interaction length and thereby increase the greater energy gain by more than 138% compared to normal laser illumination, from PIC simulations. When both schemes are combined together for dual-grating DLAs, the energy gain generated is increased by 254% compared to normal laser illumination on bare dual-gratings. This thesis also presents numerical studies for a THz-driven dual-grating structure to accelerate electrons, including geometry optimizations and detailed studies of wakefield and THz-bunch interaction. Such scaled structures can be fabricated using conventional machining techniques, which makes future demonstrations easier than for DLAs. Finally, the fabrication techniques for laser-driven dual-grating structures are studied, with researchers in Paul Scherrer Institut (PSI). A dual-grating structure with a channel depth of 7.6 μm and excellent alignment has been successfully fabricated using a monolithic method. The preliminary experimental studies carried out using the electron beam from the Swiss FEL facility are also presented in this thesis, and the future challenges for dual-grating DLAs are discussed.