The smoothing mechanism, that is due to the thermal
conductivity in the laser plasma generated by a double
pulse, is examined. Plasma preformed by a second harmonics
prepulse serves as a low density gradient plasma for impacting
the main pulse, the frequency of which was tripled for
improving the laser-target coupling. The effect of the
preformed plasma on the thermal smoothing of the heating
pulse, which was split to create two foci on the target
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 cameras: 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 determine the density and temperature distributions
in the plasma. The maximum smoothing effect was observed
for the time delay t = 0.4–0.7 ns.