Reflection and scattering from CO₂-laser-produced plasmas
An experimental study of the interaction between 10.59 μm C02-laser radiation and the plasma produced by that radiation on plane, massive targets in a vacuum, is described. The laser used was a TEA double discharge device, giving an in vacuo irradiance in the regime of 1012 W/cm2 with 50 ns FWHM gain switched pulses. The laser oscillator cavity was operated in both stable and unstable configurations. Particular emphasis was placed on the full diagnosis of the scattered light and experiments were, therefore, designed to investigate the spatial, temporal and spectral characteristics of this unabsorbed fraction. The irradiation conditions under which the experiments were performed were investigated by analysis of the far field energy distribution. High absorption of the incident radiation was inferred from the low reflectivity level, the fraction of the incident radiation scattered out- side the solid angle of the focusing lens being accounted for, using a novel collecting mirror, or by the use of calibrated burn paper. Thus a total reflection was measured for the first time. A simple model was developed to enable the classical absorption contribution to the total absorption to be estimated, and results indicated that inverse bremsstrahlung absorption would be an important process for the conditions pertinent to the interac- tion. The contribution of anomalous absorption and scattering processes was estimated. Predicted cold electron temperatures and density scale lengths from the model, agreed well with those determined experimentally without the need to invoke flux inhibition. The spectral composition of the backscattered radiation in the region of the fundamental was investigated, and measured spectra were successfully interpreted by considering the stimulated Brillouin scattering process, and the Doppler effect of the expanding plasma corona. In particular, the depen- dence of the parametric growth rate on electron density was investigated by numerical solution of the ion acoustic wave dispersion relation. The angular distribution of the backscattered radiation was determined using annular apertures in the backscattered beam and the distribution found to be non-isotropic. Interpretation was based upon the resonance absorption process in a non-planar plasma geometry and invoked the retro reflecting nature of the stimulated Brillouin scatter mechanism. To investigate the effect of wave refraction in the underdense plasma (exhibiting a continuous density profile), a compact and readily used ray-tracing routine was developed and applied to plasmas with planar, rippled and simple concave or convex iso-density contours.