Mechanism of runaway electron beam formation during plasma disruptions in tokamaks

TL;DR

This paper proposes a new physical mechanism for the formation of runaway electron beams during tokamak plasma disruptions, involving stochastic magnetic fields and specific magnetic surface confinement, with implications for understanding and controlling RE beams.

Contribution

It introduces a novel mechanism linking MHD modes and magnetic surface confinement to RE beam formation during disruptions in tokamaks.

Findings

Runaway electron beams form inside intact magnetic surfaces between q=1 and low-order rational surfaces.

The RE beam current exhibits a helical structure with dominant m/n=1/1 component.

Estimated disruption timescales using collisional models align with observed data.

Abstract

A new physical mechanism of formation of runaway electron (RE) beams during plasma disruptions in tokamaks is proposed. The plasma disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is conjectured that the runaway electron beam is formed in the central plasma region confined inside the intact magnetic surface located between $q=1$ and the closest low--order rational magnetic surfaces [$q=5/4$ or $q=4/3$, \dots]. It results in that runaway electron beam current has a helical nature with a predominant $m/n=1/1$ component. The thermal quench and current quench times are estimated using the collisional models for electron diffusion and ambipolar particle transport in a stochastic magnetic field, respectively. Possible mechanisms for the decay of the runaway electron current owing to an outward drift electron orbits and resonance interaction of high--energy electrons with the $m/n=1/1$ MHD mode are discussed.