Surface Currents Associated with External Kink Modes in Tokamak Plasmas during a Major Disruption

TL;DR

This paper investigates the surface currents during kink mode disruptions in tokamak plasmas, revealing that realistic profiles lead to smooth, structured current densities with both signs, clarifying previous theoretical disagreements.

Contribution

It introduces a more realistic model of surface currents with finite profiles and separates them into convective and residual parts, resolving past theoretical controversies.

Findings

Surface current profiles are smooth and structured, not delta functions.

Current density peaks of both signs are observed within the plasma.

Separating currents into convective and residual parts clarifies previous disagreements.

Abstract

The surface current on the plasma-vacuum interface during a disruption event involving kink instability can play an important role in driving current into the vacuum vessel. However, there have been disagreements over the nature or even the sign of the surface current in recent theoretical calculations based on idealized step-function background plasma profiles. We revisit such calculations by replacing step-function profiles with more realistic profiles characterized by strong but finite gradient along the radial direction. It is shown that the resulting surface current is no longer a delta-function current density, but a finite and smooth current density profile with internal structure, concentrated within the region with strong plasma pressure gradient. Moreover, this current density profile has peaks of both signs, unlike the delta-function case with a sign opposite to, or the same as the plasma current. We show analytically and numerically that such current density can be separated into two parts, with one of them, called the convective current density, describing the transport of the background plasma density by the displacement, and the other part that remains, called the residual current density. It is argued that consideration of both types of current density is important and can resolve past controversies.