Instabilities driven by electron velocity and temperature gradients in tokamak scrape-off layers with non-uniform parallel currents

Abstract

A study of instabilities driven by gradients in the parallel electron velocity and electron temperature in magnetized plasmas bounded by a conducting surface is presented. The parameters and general conditions considered are relevant for the scrape-off layer of magnetically confined plasmas. The inclusion, for the first time, of a finite current and a current gradient results in unstable modes which are quite different from the modes driven solely by the electron temperature gradient. In the parameter regime corresponding to high power discharges with attached plasmas, where the typical scale length of the velocity gradient is much shorter than that of the temperature gradient, the velocity gradient is the dominant effect. The anomalous thermal diffusivity, from mixing length theory, obtained including the velocity gradient is larger than that obtained including only the temperature gradient.