Thermal filamentation in plasmas with nonlocal thermal conductivity

Abstract

Thermal filamentation and self-focusing of laser light has been shown to be significantly altered in typical inertial confinement fusion (ICF)-type laser plasmas because of heat flux inhibition associated with nonlocal electron thermal conduction. The conductivity becomes appreciably nonlocal when the temperature scale length is less than about 100 electron mean-free paths. In contrast to previous flux-limiter models, this mechanism is also important for small temperature perturbations such as those associated with thermal filamentation, where the heat flux can be orders of magnitude less than the free-streaming value. Using a three-dimensional time-dependent laser–plasma propagation code modified to include nonlocal electron thermal conductivity, the scaling and behavior of nonlocal thermal filamentation are investigated. Both unsmoothed and optically smoothed laser beams are studied. The simulation results are contrasted with earlier results generated with the classical Spitzer–Härm conductivity, and compared to a perturbation analysis. The filamentation length predicted by the perturbation analysis for unsmoothed and smoothed beams compares favorably to the simulations. This theory is used to predict the behavior of optically smoothed beams in large inhomogeneous plasmas that cannot be adequately resolved by simulation.