Influence of spatial geometrical curvature on nonlocal electron heat transport in expanding plasmas

Abstract

The electron thermal transport in fluid theory would be inaccurate when the collisionality is not enough, and the Fokker-Planck (FP) simulations are usually employed to resolve the inadequacies. In this paper, the one-dimensional Fokker-Planck code is extended to handle the cylindrical and spherical geometries in which the electron distribution functions are solved in the reference frame of the ion fluid. The FP code is validated in the fluid limit by comparing with fluid (MULTI) simulations. Then, the expansions of plasmas in different spatial geometries are simulated with the FP and fluid codes. As the main characters of nonlocal transport, the electron thermal transport inhibition and preheating are investigated in expanding plasmas. The spherical nonlocal theory can give the thermal transport inhibition and preheating phenomenon, which is exploited to fit the heat flux with variation of fitting parameter . The spherical nonlocal theory will reproduce Spizer-Hrm expression as = 0. Then we analyze the heat flux after the plasma expanding 200 ps with a uniform initial temperature T = 100 eV and density ne= 1 1021 /cm3. By comparing the heat flux computed by spherical nonlocal thermal transport theory and FP simulation, it is found that (n-1)/r term in Eq. (3a) cannot be neglected when the radius is small and the geometrical curvature effect will decrease the nonlocality of transport in outer expanding plasmas. The geometrical curvature effect leads to a smaller thermal transport inhibition and preheating in the expanding plasmas as comparing the spherical case with the planar one. The expansions of plasmas in different spatial geometries are also simulated with the FP and fluid codes under the initial conditions which are similar to the inertial confinement fusion. The same influence of geometrical curvature on nonlocal electron thermal transport are also obtained.