Study of preplasma properties using time-resolved reflection spectroscopy

The aim of this work was to develop a new diagnostic method to probe preplasma properties in laser-plasma interaction experiments, using the time-resolved measurement of the laser pulse reflected by the plasma. Its spectral change over time can be attributed to the motion of the critical-density position of the plasma, which can be correlated with the preplasma properties that are present at the beginning of the interaction. 2-D particle-in-cell (PIC) simulations showed a correlation between the blue shift of the spectrum at the temporal beginning of the laser pulse and the expansion velocity of the preplasma, which can be used to derive the corresponding electron temperature. In addition, a correlation between the acceleration of the reflection point into the plasma and the density scale length has been observed. This has also been confirmed by an analytical description of the holeboring velocity and acceleration, which has been developed to include the effect of the preplasma scale length. To verify this method, two experimental campaigns were performed at the PHELIX laser system, while employing different temporal contrasts using so-called plasma mirrors. The experimental observations matched the predictions made by the numerical simulations. By comparing the maximum red shift of the spectrum with the results of the analytical description, the scale length of the preplasma was determined to be (0.18+-0.11) m and (0.83+-0.39) m with and without plasma mirror, respectively. At last, two further experimental campaigns to improve laser-ion acceleration at PHELIX were carried out. First, by increasing the laser absorption during the interaction using a p-polarized laser pulse and second, by increasing the laser intensity. The latter led to the generation of protons with a maximum energy of up to 93 MeV, for a laser intensity in the range of 8e20 W/cm^2, resulting in a new record for the laser system PHELIX.