Alfven MHD spectrum distortions caused by the presence of magnetic islands*

The presence of magnetic islands can alter the MHD ideal spectrum with respect to that traditionally calculated for equilibria that have nested magnetic surfaces everywhere. Here, we use SIESTA [1], a 3D MHD equilibrium solver  capable of finding ideal MHD equilibrium solutions without assuming the existence of nested toroidal magnetic surfaces everywhere in the plasma volume, to investigate these changes for selected tokamak and stellarator configurations. The Eulerian coordinates that SIESTA uses for pressure and magnetic field allows for the direct computation of the local Hessian of the problem  (i..e, the derivative of the force with respect to the local displacement) for the converged equilibrium solution. The study of its eigenvalues and associated eigenvectors will be used to quantify  changes in the ideal MHD Alfven spectrum such as gap width and  the existence of gap modes.  

[1] Hishman SP, Sanchez R and Cook, CR. Physics of Plasmas 18, 062504 (2011).