Magnetic islands and perturbed plasma equilibria

The confinement of plasmas in toroidal magnetic configurations is sensitively dependent on the quality of the magnetic surfaces. Existing numerical techniques for studying islands in plasma equilibria are computationally demanding and require far more computer power than equilibrium codes that presuppose the existence of perfect surfaces. Here new techniques are developed so equilibrium codes that assume perfect surfaces can be used with an ideal magnetohydrodynamics (MHD) stability code to find the modifications to the plasma shape that are required to make the assumption of perfect surfaces an accurate approximation. The method also allows one to determine what magnetic field distributions that are produced by the coils must be controlled to have good surfaces. Methods for calculating the island widths produced by external currents are also given. For an error-field-induced 1/1 island and the natural 5/5 islands in a finite-plasma-β Wendelstein 7–X variant [C. Nührenberg, Phys. Plasmas 3, 2401 (1996)], the island widths resulting from such an MHD calculation have been found to be approximately the same as the corresponding ones from a vacuum field analysis