Optimising Production of High Energy Radiation Using Laser Wakefield Acceleration

This thesis presents work from experiments and simulations on characterising and optimising electron and X-ray sources created using laser wakefield accelerators.

The flux, critical energy of the spectrum, source size and divergence of a laser wakefield accelerator driven bremsstrahlung X-ray source were characterised exper- imentally and through simulation. The source had the highest energy photon spec- trum measured from a laser wakefield bremsstrahlung source, with critical energies over 100 MeV. The source is shown to be tunable over the measured characteristics with changes in the plasma density and converter parameters.

A 5% energy spread 1.2 GeV electron beam was experimentally created using a density profile injection mechanism, the highest energy recorded using this injection technique. The density profile formation was investigated using fluid simulations, and the effect of the profile on electron generation was explored using particle-in-cell simulations.
Optimisation of electron beams using a machine learning technique was deployed experimentally in the form of a Bayesian optimisation algorithm. The algorithm was shown to be an effective method of finding a global optimum, and for creating electron beams with different characteristics. Comparisons between optimising the total beam energy and optimising for the energy in a narrow divergence were made. This difference in optimal position in parameter space was shown to be based on pulse evolution. This approach found the global optimum, in a four input parameter space for accelerated charge, in <~ 20 data points. This efficient optimisation of a laser wakefield accelerator will increase the usable time of future devices using this approach.

A new design for a high repetition rate plasma mirror was characterised. The mirror was created from a flowing liquid which refreshes the surface at a rate suitable for operation at ∼1 kHz, compared with other liquid based plasma mirrors operating at ∼1 Hz. The injection of subsequent laser pulses into a staged wakefield accelerator operating at high repetition rate could be achieved with this plasma mirror.