NONLOCAL ELECTRON-TRANSPORT IN LASER CREATED PLASMAS

The classical linear Spitzer-Harm formula has been shown to lead to an overestimation of the electron heat flux in laser-plasma interaction experiments. We briefly review the classical theory of heat transport in a plasma, and give a simplified demonstration of the Spitzer-Harm formula. The electron heat conductivity is calculated for a large value of the ion charge Z. Correction due to a finite value of Z is evaluated with a simplified electron-electron collision operator. We then show that in a steep temperature gradient, the collisional mean free path of the electrons that transport the energy may be larger than the scale length of the temperature gradient. In this case the Spitzer-Harm formula overestimates the actual heat flux in the main part of the temperature gradient, and predicts a too small heat fur slightly away from the location of the large temperature gradient. A nonlocal macroscopic formula, which is a sort of convolution of the Spitzer-Harm heat flux by a delocalization function, is shown to accurately describe the electron heat flow in both smooth and steep temperature gradients. This nonlocal formula for the heat flow is analytically justified. A selection of slightly different delocalization functions proposed in the literature is compared to the original one and to the results of Fokker-Planck calculations of the heat flow.