Laser interaction in long-scale-length plasmas

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Abstract
Absorption of a short‐pulse, high‐intensity Nd‐laser beam (vacuum irradiance of 1014 to 1015 W/cm2) by preformed plasmas of different density scale lengths is investigated. Increased effects of plasma instabilities are found at longer scale lengths. The amount of backscattered light increases with plasma scale length and limits the absorption fraction at the longest scale length. The onset of suprathermal electron production, deduced from observations of energetic (20 to 50 keV) x rays, occurs at lower laser irradiance for longer‐scale‐length plasmas. A correlation between energetic x rays and 3ω0/2 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.