Fast-Electron Temperature Measurements in Laser Irradiation at $10^{14}$ W/cm$^2$

The temperature $T$ of the fast electrons in planar-target irradiation using 2-ns UV pulses at $10^{14}$ W/cm$^2$ was measured on the OMEGA EP laser using the bremsstrahlung radiation [hard x-ray (HXR)] and the K$_{\alpha }$ radiation from high-$Z$ signature layers. The HXR was measured by a nine-channel filter spectrometer [hard x-ray image plate (HXIP)]. Two types of experiments used the K$_{\alpha }$ radiation. The first used a thick Mo (or Ag) target and the ratio of K$_{\alpha }$ emitted toward the front and the back of the target, measured and simulated by a Monte Carlo (MC) code. The ratio decreases with increasing $T$ (since K$_{\alpha }$ is emitted deeper in the foil and therefore absorbed less on the way back out). The second type used a target composed of five consecutive-$Z$ layers (Nb, Mo, Rh, Pd, Ag) and K$_{\alpha}$ lines emitted from the back (highest-$Z)$, measured and simulated by the MC code. For higher temperatures, the K$_{\alpha }$ energy decreases more slowly with $Z$. All of these measurements agree with each other. However, a three-channel scintillation photomultiplier system systematically yields higher temperatures. This indicates a higher-energy radiation component that is not detected by the HXIP because of the sharp drop in image plate (IP) sensitivity. Extending the HXIP detection to higher energies (using K$_{\alpha }$ fluorescence, for which the IP sensitivity is high) is planned. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.