A field-reversed configuration (FRC) has been proposed for a magnetic mirror — a solenoidal electromagnet configured and operated in such a way as to effect at least partial confinement of a plasma. Magnetic mirrors had been investigated for potential use as plasma-confinement devices in nuclear fusion reactors, and had been largely rejected for that use because, as explained below, they allow too much plasma to escape. The proposed FRC is intended to increase the degree of confinement achievable by a mirror magnetic field of a given flux density and/or to reduce the flux density needed to obtain a given degree of confinement. (Whether the increase in effectiveness of confinement would be sufficient to justify the use of magnetic mirrors in fusion reactors remains to be seen.)
The figure depicts baseline and FRC versions of a typical magnetic mirror at one end of a fusion reactor. In the baseline version, the magnetic mirror is only partially effective in that a substantial amount of plasma escapes along the magnetic field lines at the outer end. Efforts to reduce the rate of loss of plasma by increasing the mirror magnetic field have been unsuccessful because the required mirror magnetic-flux density is unrealistically high. Other measures intended to reduce the width of the throat through which the plasma escapes have been only partially successful; these measures include the use of multiple mirrors and of various types of thermal barriers.
In the terminology of plasma science as applied to a magnetic mirror like the one shown in the figure, "field-reversed configuration" ("FRC") refers to a family of compact toroidal magnetic-field/plasma formations in which an azimuthal electron current flows and gives rise to a poloidal magnetic-field component that can be strong enough to reverse the polarity of the magnetic induction along the cylindrical axis. FRCs have been investigated previously, but not for the use proposed here.
According to the proposal, the FRC would reduce the loss of plasma by at least partly plugging the throat of the magnetic mirror. The requirement for design and operation of the magnetic-mirror magnets would be reduced to one of constricting the throat (as defined by the magnetic-field lines) just enough to prevent expulsion of the FRC plug; it should be considerably easier to satisfy this requirement than to attempt to reduce the loss of plasma by applying a magnetic field strong enough to constrict the throat severely.
Further research will be necessary to identify the best technique for effecting the FRC. One notable technique, pioneered at the University of Washington, involves the superposition of a magnetic field that rotates at a frequency between the electron and ion gyrofrequencies. Under some circumstances, such a magnetic field can give rise to a stable, persistent FRC.
This work was done by William J. Emrich, Jr., of Marshall Space Flight Center.