Investigating Electron Dynamics in Ultra-Short Relativistic Laser-Solid Interaction - An Experimental Study via X-ray and Particle Spectroscopy

Ultra-short, ultra-relativistic laser–solid interactions generate energetic electrons that mediate the trans-fer of laser energy into a wide range of secondary processes, including ion acceleration, x-ray emission, and dense plasma heating. This thesis presents a comprehensive experimental investigation of electron acceleration and transport in this regime, conducted at the DRACO petawatt laser facility at Helmholtz-Zentrum Dresden-Rossendorf using high-contrast, femtosecond pulses at relativistic intensities. A multi-diagnostic approach – combining angularly resolved electron spectrometry, spectrally resolved x-ray 1D imaging, and proton spectrometry – was applied to directly correlate laser and target parameters with electron behavior and secondary emissions.

Four systematic studies were performed, varying key parameters relevant to electron dynamics: laser intensity, target thickness, laser incidence angle, and target surface morphology. Benchmarking experiments established scaling laws for electrons, protons, and x-rays, revealing sub-ponderomotive electron temperatures and intensity-dependent changes in energy redistribution within the target. The thickness scan yielded a volumetric reconstruction of the plasma temperature distribution from spectrally resolved x-ray 1D imaging – a novel experimental result. This result shows that low-temperature radiation originates from an extended volume, whereas high-temperature plasma is confined to a shallow layer at the front surface but spreads transversely well beyond the laser spot size. Varying the incidence angle demonstrated the role of the sheath-field geometry in modulating electron refluxing, and structured-target experiments identified geometry-dependent performance enhancements, particularly for tubular structures on planar targets.

The results provide the first systematic, multi-diagnostic benchmark dataset for this interaction regime, demonstrate the importance of sheath-field shaping in controlling electron refluxing, and highlight geometry- and alignment-sensitive pathways for enhancing laser energy coupling. These insights are directly relevant to optimizing laser-driven ion and x-ray sources, and to informing designs for high-energy-density and fusion-relevant experiments.