A novel plasma-wall instability and the distribution of halo currents in tokamaks

It is recognized that tokamak plasma disruptions and vertical displacement events, with the attendant appearance of 'halo currents', are a threat to future experiments such as ITER. Halo currents, flowing between the plasma and the wall, can develop large spatially localized components. Here, this is ascribed to a new instability that can occur in a composite circuit of a magnetized plasma and a solid conductor. The presence of the conductor divides the current perturbation into topologically distinct stable and unstable composite plasma-wall circuits. The plasma paths of such circuits are subject to hydromagnetic motions, which alter circuit geometry and conductivity while self-consistently preserving toroidal and poloidal periodicity. A simple prototype model is developed that is solved numerically and analytically to illustrate the geometrical aspect of the mechanism. The heterogeneity of the true plasma-wall system is shown to introduce considerable complexity. The basic concept may underlie a wider class of instabilities and waves.