K-Photon and Thermal X-Ray-Emission Measurements from Planar Copper Foil Targets Irradiated by High-Intensity Laser Pulses

K-shell x-ray-spectroscopy measurements of small-mass copper foil targets ($>$20 $\times $ 20 $\times $ 2~\textit{$\mu $}m$^{3})$ irradiated by \textit{I$\lambda $}$^{2} >$ 10$^{18}$ Wcm$^{-2}$\textit{$\mu $}m$^{2}$ laser pulses are presented. K$_{\beta }$/K$_{\alpha }$ variations with increasing energy density using $<$0.5-kJ, 5-ps laser pulses are fully characterized. K{\-}photon yields and bulk-electron temperatures calculated by 3-D numerical target-heating simulations are in good agreement with the experimental measurements. The first observation of a transient ``double-flash'' of x-ray radiation (1 to 2 keV) with increasing energy density indicates a finite-time, thermal-plasma response during the rapid isochoric heating phase. Time-resolved, K-photon emission measurements indicate decompression effects are minimal. This work was supported by U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.