Laser Interaction in Long-Scalelength Plasmas.

Absorption of a short-pulse, high-intensity Nd-laser beam vacuum irradiance of 10 to the 14th power to 10 to the 15th power Wsq cm by preformed plasmas of different density scalelengths is investigated. Increased effects of plasma instabilities are found at longer scalelengths. The amount of backscattered light increases with plasma scalelength and limits the absorption fraction at the longest scalelength. The onset of suprathermal electron production, deduced from observations of energetic 20 to 50 keV x rays, occurs at lower laser irrradiance for longer scalelength plasmas. A correlation between energetic x rays and 3 omega sub 02 emission suggests that the suprathermal electrons are produced by a plasma instability at quarter-critical density. At higher intensities there is evidence for severe perturbations of the preformed plasma and for self-focusing of the incident beam. Keywords Long scalelength Laser plasma interaction Stimulated brillouin backscatter Laser absorption.