An improved method of calculating flux coordinates (Q1118505)

The twisty nature of stellarator geometry can be ``hidden'' from many calculations by basing a coordinate system on the twisty flux surfaces. A particularly good choice for this flux coordinate system, developed by \textit{A. H. Boozer} [Phys. Fluids 23, 904-908 (1980; Zbl 0432.76099)], has straight field lines and a Jacobian that depends only on the magnitude of the magnetic field, B. Boozer's original method of generating the coordinate system had difficulties in determining the correct harmonics when the rotational transform, ł (the number of times a field line twists around the torus in the short direction for each transit in the long direction), is rational (ł=n/m) and when the transform per field period is large. With this improved method, two field lines are used to calculate Boozer's flux coordinates on a given flux surface. This splits the spectra so that adjacent toroidal mode numbers are assigned to different spectra, and the correct amplitudes of overlapping harmonics can be determined.