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

A new, fast, and accurate numerical algorithm to assess stability against i deal ballooning modes in general three-dimensional magnetic configurations of interest to controlled thermonuclear fusion is described. The code for b allooning rapid analysis (COBRA) performs this assessment by solving an eig envalue problem in the form of a linear second-order ordinary differential equation along magnetic field lines in the configuration. An initial approx imation for the eigenvalue is obtained from a fast second order matrix meth od. In COBRA, this approximate eigenvalue is further refined using a variat ional principle to obtain fourth-order convergence with the mesh size. Rich ardson's extrapolation is then applied to a sequence of eigenvalues to esti mate the exact eigenvalue using the coarsest possible mesh, thus minimizing the computational time. (C) 2000 Academic Press.