Magnetic coordinates for equilibria with a continuous symmetry

Magnetic coordinates for hydromagnetic equilibria are defined which treat toroidal and ''straight'' helical plasmas equivalently and yet exploit the existence of a continuous symmetry to derive relations between various geometrical and physical qualities. This allows the number of equilibrium quantities which must be known to be reduced to a minimal, or primitive, set. Practical formulas for various quantities required in hydromagnetic stability calculations (interchange, ballooning, and global) are given in terms of this primitive set