Electron heat flux and propagating fronts in plasma thermal quench via ambipolar transport

TL;DR

This paper investigates the process of thermal quench in nearly collisionless plasmas, revealing how ambipolar transport limits electron heat flux and leads to propagating fronts that influence plasma cooling dynamics.

Contribution

It introduces a theoretical and numerical analysis of electron heat flux and propagating fronts during plasma thermal quench, emphasizing ambipolar transport effects.

Findings

Thermal quench occurs via propagating fronts originating from the cooling spot.

Slow fronts propagate with ion sound speed, moderating plasma cooling.

Ambipolar transport constrains electron thermal conduction, enabling sustained temperature gradients.

Abstract

The thermal collapse of a nearly collisionless plasma interacting with a cooling spot, in which the electron parallel heat flux plays an essential role, is investigated both theoretically and numerically. We show that such thermal collapse, which is known as thermal quench in tokamaks, comes about in the form of propagating fronts, originating from the cooling spot, along the magnetic field lines. The slow fronts, propagating with local ion sound speed, limit the aggressive cooling of plasma, which is accompanied by a plasma cooling flow toward the cooling spot. The extraordinary physics underlying such a cooling flow is that the fundamental constraint of ambipolar transport along the field line limits the spatial gradient of electron thermal conduction flux to the much weaker convective scaling, as opposed to the free-streaming scaling, so that a large electron temperature and hence pressure gradient can be sustained. The last ion front for a radiative cooling spot is a shock front where cold but flowing ions meet the hot ions.