Achieving optimal laser-proton acceleration through multi-parameter interaction control

Relativistic laser-driven plasmas can be the source of energetic proton beams and have received increasing attention due to their high potential as compact and cost-efficient medical particle accelerators for radiation therapy. As such, exploring viable routes to scale the maximum proton energy to the medically relevant regime remains the subject of ongoing efforts in the Field. This endeavor is inherently linked to the discernment and control of seminal aspects of the acceleration process, ranging on vast temporal and spatial ranges due to highly variable plasma densities and laser intensities within one single interaction. This thesis investigates laser-proton acceleration on various physical scales and the influence of realistic laser pulse parameters, to ultimately find an optimum regime for stable proton beam production with highest particle energies. Experimental studies following this objective were primarily conducted at the high-power titanium:sapphire laser system Draco 150 TW at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). Efficient on-demand control of the temporal laser pulse history was established in the form of a plasma mirror filter combined with on-shot temporal pulse contrast characterization based on an advanced spectral interferometry diagnostic. This allowed for experiments with variable pulse contrast, thus providing additional handles for proton source optimization and additionally, extending the selection of applicable interaction targets to lower thicknesses and densities. Studies with novel target
technologies such as ultra-thin liquid crystal films and solid hydrogen jets were performed, each at optimized acceleration conditions, resulting in excellent proton beams with high energies and particle numbers that promise to be highly scalable with increasing laser intensities. Elaborate diagnostic suites in combination with numerical simulations delivered an improved picture of the acceleration process, which generally remains difficult to assess experimentally on the microscopic spatial and ultrafast temporal scale. As an important result, the onset of relativistic target transparency was observed for ultra-thin liquid crystal films, an operation regime
that may deliver increased proton energies when optimized. Proton acceleration results from the hydrogen jet agreed well with predictive particle-in-cell simulations, thus establishing a test bed for closely linked experimental and numerical studies into advanced acceleration mechanisms, as are for example associated with target transparency. Furthermore, an unexpected proton beam structuring effect was discovered that can play a significant role in experiments with transparent or very small targets. Formerly unrecognized by the community, this effect leads to the extension of spatial and temporal interaction scales beyond the initial proton acceleration in the laser focus, that need to be considered for appropriate interpretation of proton profile signatures.