Paleoclassical electron heat transport

Abstract

It has been hypothesized that radial electron heat transport in low collisionality, current-carrying magnetically-confined toroidal plasmas results from paleoclassical Coulomb collision processes (parallel electron heat conduction and magnetic field diffusion). In such plasmas the electron temperature equilibrates along magnetic field lines a long length L, which is the minimum of the electron collision length and a maximum effective half length of helical field lines. Diffusing field lines carry this equilibrated electron temperature with them and thus induce a radial electron heat diffusivity M ≃ L/(πR0q) ∼ 10 ≫ 1 times the resistivity-induced magnetic field diffusivity η/μ0 ≃ νe (c/ωp)2. Interpretations of many features of ‘anomalous’ electron heat transport provided by the paleoclassical model are discussed: magnitude and radial profile of electron heat diffusivity (in tokamaks, spherical tokamaks and reversed field pinches), Alcator scaling in high density plasmas, electron heat transport barriers around low order rational surfaces and near a separatrix and a natural heat pinch (or minimum temperature gradient) electron heat flux form. Also, the context (relative to other transport models), regime of applicability and suggestions for experimental tests of the paleoclassical model are discussed.