Stability and optical diagnosis of laser wakefield accelerators

This thesis presents experimental and simulation work contributing to the stability and control of laser wakefield accelerators, as required for their successful use in further applications.

Experimental results of laser wakefield acceleration in a gas jet target with a density transition produced by a razor blade in the flow are reported. Modifications to the target setup are correlated with variations in the plasma density profile diagnosed via interferometry. The shot-to-shot fluctuations of the target plateau density, peak density and down-ramp position are characterised. Simulations of the effects of density profile variations reveal that peak density fluctuations independent of the plateau density dominate the variations in electron beam energy (15 %) and charge (9 %). The results suggest that blade motion is more detrimental to stability than gas pressure fluctuations, and that early focusing of the laser may improve stability. The injection dynamics are shown to be sensitive to the length scale of the density ramps even within the regime of sharp density transitions, as a result of transverse effects.

The post-interaction laser spectrum is measured as a proxy diagnostic for the electron dynamics in a laser wakefield accelerator using ionisation injection in a tailored gas cell. The unguided laser energy identified from the signal in the input wavelength range is indicative of laser-plasma coupling and the transmission below 900nm is indicative of pump depletion and correlated with electron beam energy. The injection and acceleration dynamics are shown to be highly sensitive to the vacuum laser focus. Simulations reveal that the balance between self-focusing and geometric diffraction modifies the position of the peak a0 and that the earliest position of effective focus results in the highest injected charge. The strength of the wakefield is shown to be determined by the time evolution of the laser a0 and not by the instantaneous laser conditions.