Laser Transport And Backscatter In Low-Density Sio2 And Ta2o5 Foams

Experiments using a single 527nm wavelength beam interacting with sub- and supercritical density SiO2 and Ta2O5 foams examined laser propagation and backscatter from laser-plasma instabilities such as Stimulated Brillouin Scattering (SBS). Two densities of each material were examined, and multiple diagnostics were used to characterize the propagation and backscatter. For 5mg/cc SiO2 (n(e)/n(c) = 0.375), the laser propagation distance was well approximated by treating the foam as a gas. However, for the 2mg/cc SiO2 foam (n(e)/n(c) = 0.15), the same model over-predicts the propagation distance by similar to 40%. Existing analytical theories on propagation through subcritical foams were able to account for this difference. The laser heat wave propagated similar to 1/2 as far in Ta2O5 than SiO2 foams with similar electron density. We showed that this difference is due to the increased radiation losses in the higher Z foam. The fraction of backscattered light scales linearly with incident laser intensity for the range of intensities examined. Ta2O5 foams had significantly lower levels of backscatter (1-3%) than the SiO2 (4-8%), which is consistent with estimates of large Landau damping due to the presence of the oxygen atoms. The measured fraction of SBS backscattered laser energy for a 2mg/cc SiO2 foam shot was similar to 4 times lower than predicted by simulations assuming a gas-like foam. We found that we needed to assume increased ion heating such that T-i/T-e similar to 1.2-1.5 in the plasma to agree with the measured SBS reflectivity. Analytical models of laser-heated foams predict preferential heating of the ions as has been observed in previous experiments.