Two-Plasmon-Decay Hot-Electron Distributions from Bremsstrahlung Measurements

Preheat caused by hot electrons from the two-plasmon-decay (TPD) instability has been implicated in the reduction of fuel areal density \textit{$\rho $R} in direct-drive implosions on OMEGA.$^{ }$Hard x-ray measurements have been performed using a four-channel, hard x-ray detector (HXRD) with surrogate targets at an intensity of 5 $\times $ 10$^{14}$ W/cm$^{2}$. A bremsstrahlung model has been developed assuming that the electrons are produced within a cone of half-angle $\Theta $ and exponentially distributed in energy, characterized by $T_{hot}$. The angular-dependent signals of the HXRD channels are consistent with $T_{hot}$ = 120$\pm $20 keV and a half-angle of $\Theta $~= 10$\pm $5\r{ }. The possible origin of such a distribution and its implications will be discussed. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.