Reduction of the hot electron energies with increasing pulse rise time in CO2-laser–target interaction

The effect of increasing the laser pulse rise time on the hot electron energy distribution is studied using one or three prepulses near or on the rise of a main CO2-laser pulse. For some slower rising pulses, the pressure from a hot and long gradient plasma balances the ponderomotive laser force in such a way that the hot electron energies are significantly reduced while the overall resonance absorption fraction remains almost the same. The experimental results are modeled with a 1-D hydrocode using multigroup fast electrons and a dependence of the absorption and the hot electron temperature on the density gradient scale length at the critical density. Qualitative agreement is obtained over a wide range of laser pulse conditions and rise times.