Tokamaks with Reversed Current Density: Stability of Equilibria with Axisymmetric Islands

Tokamak regimes with nearly zero or negative toroidal current in the central region (current hole) feature good plasma confinement properties and may be naturally attained in the advanced tokamak (AT) regime. On the other hand, quasi-steady-state AC plasma current operation in tokamaks relies upon the existence of current reversal equilibria with zero net toroidal current. In both cases a system of axisymmetric magnetic islands exists in the equilibrium configurations. Recent papers on the reconstruction of current hole equilibria consistent with experimental data [1] and AC tokamak operation experiments [2] provide examples of experimentally relevant equilibria without nested flux surfaces. The question of the stability of such configurations has received little attention so far. The great variety of topologies of these islands poses non trivial challenges for its numerical resolution. In particular, the common use of structured, magnetic surface-aligned grids is practically prevented. This paper deals with the axisymmetric stability using a newly developed code [3, 4] based on finite elements on an unstructured triangular grid. We show its ability to treat arbitrary island topologies.