  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.

Plasma Physics