Bremsstrahlung measurements for fast electron characterization at OMEGA EP

Fast electrons in 1-3 MeV energy range are required for efficient core heating in Fast Ignition concept. An experiment to infer the fast electron spectrum from the bremsstrahlung x rays has been performed at the OMEGA EP using two pulse lengths, 1 ps at 150 J and 10 ps at 1000 J. The EP beam with the intensity of $\sim $ 5$\times $10$^{18}$ W/cm$^{2}$ was focused normal onto a planar Al target with a buried Cu layer. The bremsstrahlung x-ray emission was measured with a stack of image plates and differential filtering at the front and rear sides. The Cu K$\alpha $ from the tracer layer was measured using an absolutely calibrated HOPG crystal spectrometer. Preliminary results show that the measured laser-to-electron conversion efficiency and energy spectrum are similar at 1 and 10 ps shots. A detailed data analysis and consequences on FI will be presented. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and DE-FG-02-05ER54834 (ACE).