On the inward drift of runaway electrons during the plateau phase of runaway current

TL;DR

This paper investigates the inward drift of runaway electrons during the plateau phase in tokamaks, revealing that decelerating runaways naturally drift inward due to phase space dynamics, with a simplified model matching observed displacement timescales.

Contribution

It introduces a phase space dynamic model explaining runaway electron inward drift and provides an analytical description of the displacement during the plateau phase.

Findings

Runaway electrons drift inward when decelerating.

The drift is non-linear and affects the magnetic axis.

Displacement timescale matches observations.

Abstract

The well observed inward drift of current carrying runaway electrons during runaway plateau regime after disruption is studied by considering the phase space dynamic of runaways in a large aspect ratio toroidal system. We consider the case where the toroidal field is unperturbed and the toroidal symmetry of the system is preserved. The balance between the change in canonical angular momentum and the input of mechanical angular momentum in such system requires runaways to drift horizontally in configuration space for any given change in momentum space. The dynamic of this drift can be obtained by taking the variation of canonical angular momentum. It is then found that runaway electrons will always drift inward as long as they are decelerating. This drift motion is essentially non-linear, since the current is carried by runaways themselves, and any runaway drift relative to the magnetic axis will cause further displacement of the axis itself. A simplified analytical model is constructed to describe such inward drift both in ideal wall case and no wall case, and the runaway current center displacement as a function of parallel momentum variation is obtained. The time scale of such displacement is estimated by considering effective radiation drag, which shows reasonable agreement with observed displacement time scale. This indicates that the phase space dynamic studied here plays a major role in the horizontal displacement of runaway electrons during plateau regime.