This thesis concerns experimental work in the field of laser wakefield acceleration, with a focus on the diagnosis and optimisation of the electron beam quality.

The density length parameter space of a 5TW, 0.25J laser driven wakefield accelerator was characterised. Measurements of the electron beams and x-ray pulses were reported, and optimal parameters for various metrics were found. Beam-driven acceleration was identified as the mechanism that produced energies above 211MeV, and the peak x-ray brilliance was 4.2+/-0.8E20 ph/s/mm^2/mrad^2/0.1%BW^-1. Both the electron energy and the x-ray brilliance are significantly higher than literature values using comparable laser powers. Separately, the parameter scans were used to measure an extended dephasing length of the laser-accelerated beam, attributable to semi-localised depletion of the driving laser pulse, and measure
the pulse evolution rate and injection length as a function of plasma density, which was found to be slower than would be expected when only considering the longitudinal evolution.

An emittance diagnostic was developed using a beam mask and electron spectrometer. This was used to measure the spectrally resolved normalised emittance of GeV beams, produced by ionisation injection in a gas jet using a 165TW, 7.4J laser. Average emittance values as low as 4um were measured, which are the lowest emittances recorded using a beam mask technique in the literature, at energies that are close to an order of magnitude higher than other beam mask methods.


The effect of density ramps and plasma mirrors on electron beam divergence was measured in the context of staged wakefield acceleration, using a 242TW, 11J laser. Termination of an acceleration stage with a plasma mirror was found to increase total beam divergence from 3.38+/-0.07mrad to 6.13+/-0.13mrad, and the effect was observed to persist at high energies, up to 2.2GeV. Using simulations and numerical models, the presence of the density ramp was shown to have a divergence-reducing effect with a magnitude that matched the experiment. The 10^3 tesla magnetic fields generated in plasma mirrors were investigated using simulations, and the effect of these fields on the electron beam was quantified. Compared to normal incidence, a 45 degree angle of the plasma mirror to the beam axis was found to reduce the integrated magnetic fields inside the mirror, with beneficial effects on electron beam emittance.