Staged cooling of a fusion-grade plasma in a tokamak thermal quench

TL;DR

This paper investigates the staged thermal quench process in tokamak disruptions, revealing new physics scalings of electron temperature decay due to suppressed parallel thermal conduction in collisionless plasmas.

Contribution

It introduces a novel understanding of the four-stage thermal quench in tokamaks, highlighting the role of suppressed parallel electron thermal conduction in collisionless conditions.

Findings

Main temperature drop occurs with nearly collisionless core plasma.

Electron temperature scales as T_{e ext{parallel}} ext{ } ext{ } ext{ } ext{ } t^{-2}.

Cooling time scales with ion sound wave transit time.

Abstract

In tokamak disruptions where the magnetic connection length becomes comparable to or even shorter than the plasma mean-free-path, parallel transport can dominate the energy loss and the thermal quench of the core plasma goes through four phases (stages) that have distinct temperature ranges and durations. The main temperature drop occurs while the core plasma remains nearly collisionless, with the parallel electron temperature $T_{e\parallel}$ dropping in time $t$ as $T_{e\parallel}\propto t^{-2}$ and a cooling time that scales with the ion sound wave transit time over the length of the open magnetic field line. These surprising physics scalings are the result of effective suppression of parallel electron thermal conduction in an otherwise bounded collisionless plasma, which is fundamentally different from what are known to date on electron thermal conduction along the magnetic field in a nearly collisionless plasma.