Relaxation of toroidal plasmas

Equilibria with minimum energy are constructed from a variational principle in which the potential energy of a plasma is minimized, subject to a small set of global invariants. These invariants are preserved exactly, by ideal motions of the plasma, and approximately, by a tearing mode of the m = 1, n = 1 type. Estimates of decay rates for the energy and the invariants are given. A tokamak window with low q and zero current on the boundary is found and shown to be ideally and resistively stable. The reversed-field pinch (RFP) window and the transition region between the two windows is found to be unstable to high-n modes