A Field-Reversed Configuration (FRC) is a device developed for magnetic fusion energy research that confines a plasma on closed magnetic field lines without a central penetration.
One approach to producing fusion power is to confine the plasma with magnetic fields. This is most effective if the field lines do not penetrate solid surfaces but close on themselves into circles or toroidal surfaces. The mainline confinement concepts of tokamak and stellarator do this in a toroidal chamber, which allows a great deal of control over the magnetic configuration, but requires a very complex construction. The Field-Reversed Configuration offers an alternative in that the field lines are closed, providing good confinement, but the chamber is cylindrical, allowing easy construction and maintenance.
A Field-Reversed Configuration is formed in a cylindrical coil which produces an axial magnetic field. First, an axial bias field is applied, then the gas is pre-ionized, which "freezes in" the bias field, finally the axial field is reversed. At the ends, reconnection of the bias field and the main field occurs, producing closed field lines. The main field is raised further, compressing and heating the plasma and providing a vacuum field between the plasma and the wall.
The toroidal field is zero or at least relatively small. A closely related concept where the strength of the toroidal field is similar to that of the poloidal field is the spheromak. Field-reversed configurations and spheromaks are together known as compact toroids.