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Descrizione
We present the results of an experiment at the POLARIS laser system (at $1030\,$nm) using an off-harmonic optical probe, in which the laser-plasma interaction with water micro-droplets was investigated. In contrast to experiments with thin foils, the spherical symmetry of droplets facilitates a direct imaging of the plasma expansion process using shadowgraphy. In the experimental setup, a jet of water was broken up into droplets with a diameter of $20\,$µm. The POLARIS main laser pulses were focused onto the water droplets to intensities of $4\cdot10^{19}\,$W/cm$^2$. The plasma expansion process was probed in a temporal window between $0\,$ps and $258\,$ps after the arrival of the main laser pulse. The strong light emission from the laser-induced plasma at the fundamental and second harmonic frequencies was suppressed by using an off-harmonic probe with a bandpass filter. The probe pulses were generated with a synchronized NOPA (µJ-level pulse energy and $750\dots 950\,$nm bandwidth). The plasma emission was further reduced by a polarization and a spatial filter. A detailed analysis of the shadowgraphy images allowed us to estimate the plasma expansion velocity of the front $(v_\text{front} = 1.27(6)\,$µm/ps$)$ and rear side $(v_\text{rear} = 0.77(5)\,$µm/ps$)$ at the beginning of the expansion process.
