Rotation of tokamak halo currents

Abstract

During tokamak disruptions, halo currents, which can be tenths of the total plasma current, can flow at the plasma edge along the magnetic field lines that intercept the chamber walls. Non-axisymmetric halo currents are required to maintain force balance as the plasma kinks when the edge safety factor drops to about two in a vertical displacement event. The plasma quickly assumes a definite toroidal velocity va (r) with respect to that of the magnetic kink, vk, where va (r) is set by the radial electric field required for ambipolarity. The plasma velocity, vpl = va + vk, near the edge is influenced by the interaction with neutrals and with the potential in the halo required for quasi-neutrality on open magnetic field lines, and the plasma velocity in the core is influenced by external error fields. When plasma effects dominate magnetic locking, the magnetic kink should rotate at a diamagnetic speed of either the edge or the core. If the magnetic field lines of the halo plasma intercept the wall at locations of very different electrical conductivity, the toroidal rotation of the halo currents can intermittently stall at wall locations of high conductivity. Such stalling is seen in experiments. The toroidal phase difference between the stalled halo currents and the kink, which is expected to rotate smoothly, must satisfy δψ ≤ ± π/2. A concern cited by ITER engineers is that the time varying force of the rotating halo could substantially increase the disruption loads on in-vessel components. © 2012 American Institute of Physics.