Intense laser interactions with sprays of submicron droplets

Picosecond laser interaction experiments conducted at peak intensities of 1.5×1017 W cm−2 using a new target medium consisting of a dense spray of 0.5 micron radius ethanol droplets indicate a strong laser–plasma coupling. The laser absorption exceeds that seen in solid targets of greater Z, and remains high over more than four orders of magnitude of intensity. Invariance in the laser absorption with wavelength and polarization is also reported. Together with x-ray spectroscopy studies, absorption measurements have been used to implement nonlocal thermodynamic equilibrium (NLTE) plasma simulations in order to isolate the important features of the droplet heating and explosion dynamics. These simulations show that the interplay of laser heating and energy transport processes is significantly different from those seen in continuous solid target interactions and that a substantial fast electron fraction must be inferred.