Hard-X-Ray Measurements of 10.6-μm Laser-Irradiated Targets

The first measurements of high-energy x-ray emission ($h\nu \sim 30-300$ keV) by high-$Z$ microballoon targets irradiated at $5\times {10}^{14}<\varphi <2\mathrm{}\times {10}^{16}$ W/${\mathrm{cm}}^2$ by 10.6-μm laser light are reported. An exponential spectrum with a slope $k{T}_H\sim 250$ keV provides the best fit to spectrometer data at ${\varphi }_l={10}^{16}$ W/${\mathrm{cm}}^2$. The hard-x-ray yield indicates that a substantial fraction, probably between 10% and 100%, of the absorbed laser energy is converted to hot electrons. The slope $k{T}_H$ is proportional to the fastest ion energy.