Observation of enhanced x-ray emission from long-pulse-width laser-produced plasmas

Abstract

Experiments investigating the pulse-width dependence of thermal x-ray conversion efficiencies (hν<1.5 keV) in laser-heated gold plasmas are described. The results show that the instantaneous ratio of the emitted x-ray flux to the laser energy deposition rate increases throughout a 4-ns laser pulse. The studies were carried out using single arms of the 10-beam Nova laser facility. The irradiance was ∼4–5×1014 W/cm2 in the target plane as we varied the laser pulses’ FWHM from 2 to 4 ns. The laser pulses were nominally flat-topped and contained between 1 and 2 kJ of (1)/(3) μm light. Time-resolved plasma radiation was monitored with a broadband, streaked x-ray spectrograph that has a roughly 30-ps time resolution and channels that are roughly 100–150 eV wide. One-dimensional numerical models run with the lasnex code produce a conversion efficiency that is nearly constant throughout the laser pulse. We discuss various approximations made in the one-dimensional models and conclude that none of them are a likely explanations for the increase of conversion efficiency with time. A preliminary two-dimensional model of a disk heated by a 3-ns pulse shows that the conversion efficiency increases throughout the pulse. The increase is due to soft x rays emitted from outside the laser spot. Further experiments and modeling will be carried out to assess these two-dimensional effects.