Prediction of nonlinearly saturated tearing mode islands with an equilibrium code

We demonstrate that it is possible to use an MHD equilibrium code to directly predict the nonlinear saturation of tearing modes without resolving the dynamics and without explicit dependence on the plasma resistivity. This is achieved by exploiting the stepped-pressure equilibrium code [1], which can describe equilibria with magnetic islands and magnetic field-line chaos, and by identifying the appropriate constraints under which tearing modes evolve [2, 3]. Calculations are carried out in a slab and for a cylindrical tokamak and the predicted values of wsat are shown to reproduce the theoretical scaling at small values of ?' and the scaling obtained from resistive MHD simulations at large ?'.