Early hot electrons generation and beaming in ICF gas filled hohlraums at the National Ignition Facility*

In laser driven hohlraum capsule implosions on the National Ignition Facility, supra-thermal hot electrons generated by laser plasma instabilities can preheat the capsule. Time resolved hot electron Bremsstrahlung spectra combined with 30 keV x-ray imaging uncover for the first time the directionality of hot electrons onto a high-Z surrogate capsule located at the hohlraum center. In the most extreme case, we observed a collimated beaming of hot electrons onto the capsule poles, reaching 50x higher localized energy deposition than for isotropic electrons. A collective SRS model where all laser beams in a cone drive a common plasma wave provides a physical interpretation for the observed beaming. Imaging data are used to distinguish between this mechanism and 2$\omega_{\mathrm{p}}$ instability [2]. The amount of hot electrons generated can be controlled by the laser pulse shape and hohlraum plasma conditions. [1] E.L. Dewald, \textit{et. al.,} \textit{Rev. Sci. Instrum.} \textbf{81}, 10D938 (2010). [2] S. Regan et al, \textit{Phys. Plasmas} \textbf{17, }020703 (2010). *This work performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.