A field-reversed configuration (FRC) is a type of plasma device studied as a means of producing nuclear fusion. It confines a plasma on closed magnetic field lines without a central penetration. In an FRC, the plasma has the form of a self-stable torus, similar to a smoke ring. FRCs are closely related to another self-stable magnetic confinement fusion device, the spheromak. Both are considered part of the compact toroid class of fusion devices. FRCs normally have a plasma that is more elongated than spheromaks, having the overall shape of a hollowed out sausage rather than the roughly spherical spheromak. FRCs were a major area of research in the 1960s and into the 1970s, but had problems scaling up into practical fusion triple products. Interest returned in the 1990s and , FRCs were an active research area. History The FRC was first observed in laboratories in the late 1950s during theta pinch experiments with a reversed background magnetic field. The first studies were at the United States Naval Research Laboratory (NRL) in the 1960s. Considerable data were collected, with over 600 published papers. Almost all research was conducted during Project Sherwood at Los Alamos National Laboratory (LANL) from 1975 to 1990, and during 18 years at the Redmond Plasma Physics Laboratory of the University of Washington, with the large s experiment (LSX). Later research was at the Air Force Research Laboratory (AFRL), the Fusion Technology Institute (FTI) of the University of Wisconsin-Madison, Princeton Plasma Physics Laboratory, and the University of California, Irvine. Private companies now study FRCs for electricity generation, including General Fusion, TAE Technologies, and Helion Energy. The Electrodeless Lorentz Force Thruster (ELF) developed by MSNW was an attempt to design a space propulsion device. ELF was a candidate in NASA's NextSTEP advanced electric propulsion program, along with the X-3 Nested-Channel Hall Thruster and VASIMR before MSNW dissolved. Applications The primary application is for fusion power generation. The FRC is also considered for deep space exploration, not only as a possible nuclear energy source, but as means of accelerating a propellant to high levels of specific impulse (Isp) for electrically powered spaceships and fusion rockets, with interest expressed by NASA. Comparisons Producing fusion power by confining the plasma with magnetic fields 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