The theory of plasma relaxation is described and developed. Turbulence, allied with a small resistivity, allows the plasma rapid access to a particular minimum-energy state. This process involves reconnection of magnetic field lines in a manner that destroys all the topological invariants of ideal plasma so that only total magnetic helicity survives. Although this mechanism, and the equations describing the relaxed state, are similar in all systems, the properties of the relaxed state depend crucially on the topology—toroidal or spherical—of the container and on the boundary conditions. Consequently there are several different types of relaxed state, each with its own special characteristics, which are derived and discussed. The measurements made on many experiments, including toroidal pinches, OHTE, multipinch, and spheromaks, are reviewed and shown to be in striking agreement with the theoretical predictions.

DOI:https://doi.org/10.1103/RevModPhys.58.741