Characterisation of self-guided laser wakefield accelerators to multi-GeV energies

Abstract

This thesis details experimental and theoretical work in the field of self-guided laser wakefield accelerators, characterising various aspects of the machine related to the driver laser and electron beam generation. The spectral changes to the laser pulse driving a laser wakefield accelerator were characterised. It was found that the spectral blueshift is directly correlated to the length of the plasma cavity. Spectral phase changes of the driver pulse were measured to dramatically alter the interaction. Positive second order spectral phase was measured to increase electron beam energy, its charge and the spectral blueshifting undergone by the driver pulse. The suppression of self-injection in laser wakefield accelerators operating in the highly non-linear bubble regime was observed. The use of ionisation impurity to pre-inject electrons into the plasma cavity was measured to alter the fundamental properties of the electron beam. Through particle-in-cell simulations it was shown that this effect arises from the repulsive electrostatic force from the beam load, preventing sufficient transverse momentum gain of sheath electrons. Record electron beam energies of nearly 3 GeV were measured in the self-injecting, self-guiding regime of laser wakefield accelerators. These results were obtained at higher than expected plasma densities and are thought to be a direct result of increased energy coupling due to the use of a much longer main focussing optic. Very stable injection in the self-injection regime was observed allowing for experimental measurements of peak accelerating field within the bubble. The field E = (590 ± 180) GV/m is the highest value of the electric field reported. The efficacy and long-term stability of self-guided, self-injecting laser wakefield electron acceleration was evaluated. Highest sustained laser energy to electron beam energy conversion efficiency of nearly 3% was measured. It was also shown the self-injection yields higher overall efficiencies than ionisation induced injection. Stability of injection and acceleration over more than half a thousand consecutive shots was studied and found to be directly dependent on the stability of the driving laser.