Shockfronts as Model Targets in Laser-Plasma Interaction Experiments,

Abstract : We have used a shockfront, produced in a small, electrothermal shocktube filled with 40-60 torr D2, as a model target for studies of the interaction of intense laser light with hot, dense, inhomogeneous plasmas. The CO2 laser pulse (lambda = 10.6 micrometer; 100-500 psec FWHM; typical energy 100 mJ) is tightly focussed, at an oblique angle, onto the shockfront axis, and a laser-Schlieren shockfront detection system fires the laser at the instant the Mach-5 shockfront passes into focus. The shockfront, whose motion is negligible on the time scale of the laser pulse is ionized by optical breakdown. When the laser pulse and the shockfront are properly synchronized, we observe energetic electrons (40-140 keV) to be emitted in a narrow cone centered on the shocktube axis. Our measurements of the angular distribution and energy spectrum of these electrons are in agreement with the predictions of the theory of resonance absorption of laser light by plasmas.