Impact of Non-Maxwellian Electron Distribution Functions on Crossed-Beam Energy Transfer

Energy transfer between crossed laser beams is an important process in both the direct- and indirect-drive approaches to inertial confinement fusion (ICF), and unreliable predictions in numerous contexts have raised questions as to the validity of models. Typically, those models require state variable inputs (i.e., $n_e$, $T_e$, and $T_i$) that are computed in radiation-hydrodynamic simulations, which assume Maxwellian electron distribution functions (EDF). However, laser plasma heating is predicted to distort the EDF away from Maxwellian\footnote{A. B. Langdon \textit{et al.}, Phys. Rev. Lett. {\bf 44}, 575-579 (1980).}. Here, measurements of the complete Thomson scattering spectrum indicate the presence of super-Gaussian EDF's that are consistent with existing theory\footnote{J. P. Matte \textit{et al.}, Plas. Phys. & Cont. Fus. {\bf 30}, 1665 (1988).}. In such plasmas, ion acoustic wave (IAW) frequencies increase monotonically with super-Gaussian exponent\footnote{B. B. Afeyan \textit{et al.}, Phys. Rev. Lett. {\bf 80}, 2322-2325 (1998).}. To match experiments that measured power transfer between crossed laser beams mediated by IAW's, accounting for the measured non-Maxwellian EDF is required\footnote{D. Turnbull \textit{et al.}, in review (2019).}. This effect is estimated to decrease energy transfer in indirectly-driven hohlraums at the National Ignition Facility by $\approx27\%$; this will reduce (and may eliminate) the \textit{ad hoc} saturation clamp that has previously been used to match observables like shape, thereby improving the predictive capability of integrated modeling.