Field-Reversed Configuration
Encyclopedia
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
Fusion power
Fusion power is the power generated by nuclear fusion processes. In fusion reactions two light atomic nuclei fuse together to form a heavier nucleus . In doing so they release a comparatively large amount of energy arising from the binding energy due to the strong nuclear force which is manifested...

 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
Tokamak
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus . Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape...

 and stellarator
Stellarator
A stellarator is a device used to confine a hot plasma with magnetic fields in order to sustain a controlled nuclear fusion reaction. It is one of the earliest controlled fusion devices, first invented by Lyman Spitzer in 1950 and built the next year at what later became the Princeton Plasma...

 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
Spheromak
A spheromak is an arrangement of plasma formed into a toroidal shape similar to a smoke ring. The spheromak contains large internal electrical currents and their associated magnetic fields arranged so the magnetohydrodynamic forces within the spheromak are nearly balanced, resulting in long-lived ...

. Field-reversed configurations and spheromaks are together known as compact toroid
Compact toroid
Compact toroids are a class of toroidal plasma configurations that are self-stable, and whose configuration does not require magnet coils running through the center of the toroid. They are studied primarily in the field of fusion energy, where the lack of complex magnets and a simple geometry may...

s.

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