Evaluation of magnetic coordinates in finite-beta, asymmetric plasmas

Analytical and numerical stability or transport calculations for toroidal plasmas are facilitated by using flux coordinates in which the magnetic field lines are straight. A published method for obtaining these magnetic coordinates, or more specifically the Fourier components of the magnetic field in this coordinate system, has existed for a vacuum magnetic field. A procedure for calculating these components for a finite-..beta.. plasma equilibrium, however, has not previously been available. (Here, ..beta.. is the ratio of plasma pressure to magnetic pressure.) In this report, we describe such a procedure, in which we start with an arbitrary-..beta.. magnetic equilibrium for which nested toroidal flux surfaces exist and obtain the Fourier components of the magnetic field in straight-line magnetic coordinates. We describe the various numerical procedures, referring to published documentation where needed. We then document the codes involved and apply them to a specific toroidally nonsymmetric, finite-..beta.. plasma. 8 refs., 3 figs.