Laser Wakefield Acceleration Using Few-millijoule Laser Pulses at Kilohertz Repetition-Rate.

Abstract

Compared to conventional sources, electrons produced by a laser plasma based accelerator have some unique properties owing to the much higher acceleration gradient that can be sustained in plasma and the inherent synchronization with the driving laser pulse, making them a potentially useful source for developing tools for ultrafast time-resolved studies. This past decade has seen significant advances in the field of laser driven plasma accelerators, which can now generate electron beams with few femtosecond durations and up to GeV energies. One of the main issues with plasma accelerators has been their shot-to-shot reproducibility and stability. In addition, experiments to date have been carried out at low-repetition rate. For many potential applications, increasing the repetition rate from a few hertz to kilohertz or higher will be required. This thesis describes both experimental and numerical work aiming at the development of a wakefield electron source and applications at kilohertz repetition-rate using few-millijoule pulses. We first present a simple yet robust optical pulse compression technique utilizing ionization nonlinearity. A self-compressed 16 fs pulse was measured from an original 36 fs pulse containing a few millijoules of energy, which can be beneficial for driving laser wakefield acceleration. Electron acceleration using uncompressed multi-millijoule laser pulses (8 mJ, 32 fs) was studied both in experiments and with particle-in-cell simulations. The wakefield acceleration experiments described in this thesis are the first of their kind at kHz repetition rate and the first to use a relatively low peak-power (0.3 TW) laser system. Generation of sub-relativistic electron beams from laser wakefield acceleration was demonstrated using this high-repetition rate system. An adaptive optimization method was implemented to improve the performance of laser wakefield acceleration through coherent manipulation of the wavefront of the driving laser pulse, enabled by the stability and high-repetition rate. The structure and dynamics of the plasma wave can be subsequently controlled, leading to more than one order of magnitude improvement on the total beam charge and divergence. Finally, the feasibility of using wakefield electrons for ultrafast studies was investigated through proof-of-principle electron diffraction experiments and probing the ultrafast dynamics of a non-equilibrium laser produced plasma.