Experimental evidence of thermal smoothing in a double-pulse produced plasma

The smoothing mechanism, that is due to the thermal conductivity in the las er plasma generated by a double pulse, is examined. Plasma preformed by a s econd harmonics prepulse serves as a low density gradient plasma for impact ing the main pulse, the frequency of which was tripled for improving the la ser-target coupling. The effect of the preformed plasma on the thermal smoo thing of the heating pulse, which was split to create two foci on the targe t surface, was explored by varying the time delay between the prepulse and the main pulse. The smoothing effect was monitored by a pair of pinhole cam eras: one viewing side-on and the other one placed at the rear side of the target. Spatially resolved X-ray emission spectra were recorded to determin e the density and temperature distributions in the plasma. The maximum smoo thing effect was observed for the time delay t = 0.4-0.7 ns.