Grad-Shafranov Equilibria with Negative Core Toroidal Current in Tokamak Plasmas

Numerical Grad-Shafranov (GS) equilibria with negative current density in the plasma core are computed which do not impose any particularly chosen models for the pressure and current-density profiles. This flexibility allows the profiles to be tailored so that an island unfolds in the low-field side, even for elongated plasmas, thus sustaining the negative-current core against outward forces. Among other topological results, reversed GS equilibria are also shown to be necessarily non-nested, except for the cylindrical and other very special degenerate, hence structurally unstable cases