Axisymmetric Mode Stability in Tokamaks with Reversed Current Density

The ”current hole ” tokamak regimes with nearly zero or negative toroidal current in the central region feature good plasma confinement properties and may be naturally attained in the advanced tokamak (AT) regime. Resistive n = 0 stability of the circular cylinder with reversed current was investigated in [1]. Non-linear MHD modeling of the toroidal plasma evolution including off-axis current drive, show that the n = 0 activity leads to cyclic reconnection events which clamp the central current density at approximately zero [2]. These results could be due to the non existence of axisymmetric equilibria with nested flux surfaces [3]. In this paper we investigate the stability properties of reversed current density equilibria with axisymmetric islands. The stability analysis of equilibrium configurations with islands requires the devel-opment of new numerical tools capable to deal with arbitrary topology of the magnetic surfaces. In particular, the most general and promising approach is to renounce the idea of using of magnetic coordinates and projections of the plasma displacement vector onto magnetic surfaces. For that purpose, a code for ideal MHD stability calculations on un-structured triangular grids was developed. The new code is coupled to an unstructured grid MHD equilibrium code [4] capable to treat equilibria with axisymmetric magnetic islands. The code was verified against the KINX code results [6] for free boundary n = 0 insta-bility. Preliminary results for reversed current equilibria with islands show a possibility of ideal internal n = 0 instability – a novel property not possible for tokamak plasma with nested flux surfaces. 1. Ideal MHD stability of force-free configurations: the problem formulation For stability modelling without use of special coordinates and magnetic surface projec-tions the original form of plasma potential and kinetic energy functionals can be used