Laser transport and backscatter in low-density SiO2 and Ta2O5 foams

Abstract

Experiments using a single 527 nm 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 5 mg/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 2 mg/cc SiO₂ foam (n_e/n_c = 0.15), the same model over-predicts the propagation distance by ∼40%. Existing analytical theories on propagation through subcritical foams were able to account for this difference. The laser heat wave propagated ∼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 2 mg/cc SiO₂ foam shot was ∼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 ∼ 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.