COBRA: an optimized code for fast analysis of ideal ballooning stability of three-dimensional magnetic equilibria

Abstract A new, fast, and accurate numerical algorithm to assess stability against ideal ballooning modes in general three-dimensional magnetic configurations of interest to controlled thermonuclear fusion is described. The code for ballooning rapid analysis (COBRA) performs this assessment by solving an eigenvalue problem in the form of a linear second-order ordinary differential equation along magnetic field lines in the configuration. An initial approximation for the eigenvalue is obtained from a fast second order matrix method. In COBRA, this approximate eigenvalue is further refined using a variational principle to obtain fourth-order convergence with the mesh size. Richardson's extrapolation is then applied to a sequence of eigenvalues to estimate the exact eigenvalue using the coarsest possible mesh, thus minimizing the computational time.