SIESTA: A scalable iterative equilibrium solver for toroidal applications

A new solver for rapidly obtaining magnetohydrodynamic (MHD) equilibria in toroidal systems in the presence of islands and stochastic regions is described. It is based on the Kulsrud-Kruskal MHD energy minimization principle. To carry out this minimization, small displacements are made around a convenient set of curvilinear coordinates obtained from a nearby three-dimensional equilibrium that assumes nested surfaces. Because the changes of the magnetic fields and pressure are small, corresponding to small changes in the initial magnetic and kinetic energies, solutions for the linearized perturbations can be used to rapidly and iteratively find lower energy states with magnetic islands. A physics-based preconditioner is developed to accelerate the convergence of the iterative procedure to obtain an ideal MHD equilibrium with broken magnetic surfaces (islands).