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

摘要

The surface current on the plasma-vacuum interface during a disruption eventinvolving kink instability can play an important role in driving current intothe vacuum vessel. However, there have been disagreements over the nature oreven the sign of the surface current in recent theoretical calculations basedon idealized step-function background plasma profiles. We revisit suchcalculations by replacing step-function profiles with more realistic profilescharacterized by strong but finite gradient along the radial direction. It isshown that the resulting surface current is no longer a delta-function currentdensity, but a finite and smooth current density profile with internalstructure, concentrated within the region with strong plasma pressure gradient.Moreover, this current density profile has peaks of both signs, unlike thedelta-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 separatedinto 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 isargued that consideration of both types of current density is important and canresolve past controversies.