Laser Driven Plasma Instabilities at Moderate Laser Irradiances

A systematic investigation is in progress on Second Harmonic (SH) and Three-Half Harmonic (3ω/2) emission generated by the interaction of a 3 ns 1.064 μm Nd:YAG laser beam with underdense plasmas produced from thin plastic targets at irradiances up to 5x1013 Wcm-2. 2-D time resolved imaging of the interaction region in SH light has been performed for a detailed characterization of SH radiation sources in the plasma. The onset of a SH emission regime dominated by Filamentation Instability is clearly evidenced by these images of the interaction region and confirmed by the anomalous scaling of SH power with incident laser power. The dependence of SH emission on target position has also been measured revealing an unexpected behaviour which we believe is linked to the effect of local beam geometry on Filamentation Instability growth rate mechanisms. A simple model of 3ω/2 emission is discussed which takes in account refraction of Two Plasmon Decay (TPD) produced electron waves. This model shows a strong dependence of 3ω/2 spectra on longitudinal density gradients. In contrast with 2ω, transverse density gradients do not contribute appreciably to 3ω/2 emission. We report on a new experiment performed to test the model and the effective reliability of 3ω/{ti2 as a diagnostic tool to estimate electronic plasma temperature.