Characterization of laser-driven proton acceleration with contrast-enhanced laser pulses

In this thesis, various novel aspects of laser-driven ion acceleration with contrast-enhanced laser pulses have been characterized. The maximum ion energies' dependence on the pulse energy and the foil thickness was investigated in a campaign at the POLARIS laser using a plasma mirror for contrast enhancement. The steepest increase of the ion energies depending on the pulse energy was measured for a 5 nm thin foil and linear polarization. Above a certain pulse energy, the onset of the foil's transparency correlated with a stop of the ion energies' increase. Consequently, additional enhancements of the temporal intensity contrast (TIC) and higher laser pulse intensities are required to exploit ion acceleration with such thin foils. The ring-like beam profile formed by protons with low kinetic energy, which originated from submicron thick plastic foils, was characterized. Simulations support the explanation that such structures are a consequence of the proton density's spatial distribution during the acceleration with the target normal sheath acceleration mechanism (TNSA). These findings deepen the understanding of ion acceleration with thin foils and may help to distinguish features of other acceleration mechanisms in the beam profile from those attributed to TNSA. In an experiment with water microdroplets, the effects of the laser's TIC and the incidence angle in the polarization plane were investigated. It was found that both parameters have a significant influence on the kinetic energy of the accelerated protons. An optical probe laser was used to observe the plasma expansion on a picosecond timescale. A correlation between the expansion and the maximum proton energy was found. The proton beam profile exhibited a reproducible net-like pattern depending on the irradiation geometry as well. The results show that the use of microdroplets irradiated with frequency-doubled laser pulses and optically probed gives new insights into laser-plasma interaction.



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