Thin-foil targets were irradiated with high-power ($1\le P_L\le 210\mathrm{TW}$), 10-ps pulses focused to intensities of $I>{10}^{18}\mathrm{W}/{\mathrm{cm}}^2$ and studied with $K$-photon spectroscopy. Comparing the energy emitted in $K$ photons to target-heating calculations shows a laser-energy-coupling efficiency to hot electrons of ${\eta }_{L\mathrm{\text{−}}e}=20\pm 10\%$. Time-resolved x-ray emission measurements suggest that laser energy is coupled to hot electrons over the entire duration of the incident laser drive. Comparison of the $K$-photon emission data to previous data at similar laser intensities shows that ${\eta }_{L\mathrm{\text{−}}e}$ is independent of laser-pulse duration from $1\le {\tau }_p\le 10\mathrm{ps}$.

• Received 10 August 2009

DOI:https://doi.org/10.1103/PhysRevLett.105.235001