The Effect of Cross-Beam Energy Transfer on Two-Plasmon Decay in Direct-Drive Implosions

Mitigation of cross-beam energy transfer (CBET) in direct-drive implosions was shown to increase the hot electrons generated by two-plasmon decay. Reducing the diameter of the laser spots by 30{\%} significantly reduces CBET and the laser absorption was measured to increase from 75{\%} to nearly 90{\%}.\footnote{D. H. Froula \textit{et al}., Phys. Rev. Lett. \textbf{108}, 125003 (2012). } The reduced CBET leads to higher intensity at the quarter-critical density surface, increasing the hot-electron production by a factor of $\sim {\kern 1pt}7.$ Adding a thin layer (0.6 to 1.1 $\mu $m) of Si to the target ablator reduced the hot-electron fraction by a factor of $\sim {\kern 1pt}2.$ Spatially resolved Thomson-scattering measurements show an $\sim {\kern 1pt}15\% $ increase in the electron temperature and an increase in the Si fraction at the quarter-critical surface when the Si layer is added. Three-dimensional laser--plasma interaction simulations of hot-electron production using the code \textit{LPSE} show that in addition to the reduced gain (smaller ${IL_{\mbox{n}} } \mathord{\left/ {\vphantom {{IL_{\mbox{n}} } {T_{\mbox{e}} }}} \right. \kern-\nulldelimiterspace} {T_{\mbox{e}} })$, the observed reduction in hot electrons results from increased electron--ion collision frequencies and reduced Landau damping of ion-acoustic waves.\footnote{R. K. Follett\textit{ et al.}, Phys. Rev. Lett. \textbf{116}, 155002 (2016).\par } This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.