Scaling hot-electron generation to long-pulse, high-intensity laser–solid interactions

Experiments have been performed to determine the effect of laser-pulse duration and energy on hot-electron–generation efficiency at high intensity. Thin copper foil targets were irradiated with 1 to 2100 J, 1 to 10 ps pulses focused to intensities >1018 W/cm2. The target volume was varied from 75 × 75 × 3 μm3 to 600 × 600 × 50 μm3 to access a range of bulk thermal-electron temperatures up to several hundred electron volts. Comparison of K-photon spectroscopy measurements from these targets with electron transport and radiation-generation calculations indicates that the energy conversion efficiency into hot electrons is 20 ± 10%, independent of laser-pulse duration and energy.