An FRC (field reversed configuration) is an elongated plasma ellipsoid conducting an azimuthal current which reverses the direction of an externally applied magnetic field. The resultant field provides for toroidal plasma confinement without requiring a toroidal vacuum vessel or coil set. This type of magnetic configuration is called a compact toroid, and has an ideal geometry for a fusion reactor.
A WHITE PAPER ON FRC DEVELOPMENT
The ultimate objective of fusion research is the application of fusion energy in a manner acceptable to society. This concerns not only its economic benefit, but also safety and environmental issues. The field reversed configuration (FRC) may be the ideal tool for confining a fusion plasma, especially one with low-hazard advanced fuels. This document expresses the views of a worldwide community of fusion energy scientists on near-term directions for research on the FRC.
The FRC is a variety of compact toroid that occupies a unique position in the parameter space of magnetically confined plasmas. It differs from other toroidal systems in possessing the following attributes: no mechanical structure in the center of the torus; no appreciable toroidal field; an engineering beta near unity; no rotational transform; all the equilibrium current (except possibly a seed current) maintained by classical diamagnetism; and a scrape-off layer exhausting outside the coil system. FRCs range from small gyro-orbit fluid-like plasmas to large-orbit ion ring-dominated plasmas. Because of their peculiar attributes FRCs offer the possibility of a step change in reactor attractiveness. In addition, FRC research adds unique insight into the physics of other fusion systems such as tokamaks, and offers a means of exploring fundamental plasma physics questions unrelated to fusion.
Review panels have repeatedly called for fusion system improvements in order to project economical fusion energy. However, even improved tokamaks may not overcome the shortcomings of low power density, high complexity, large unit size, and high development cost. Among alternative concepts based on low-density magnetic confinement, the FRC offers arguably the best reactor potential because of high power density, simple structural and magnetic topology, simple heat exhaust handling, and potential for advanced fuels. The unit size of FRC reactors may be smaller than those based on the tokamak. Low magnetic field and a simple structure also lead to lower costs. These advantages might be accentuated if an innovative reactor design such as a liquid wall vessel could be adopted. The enormous potential payoff as a reactor justifies a broad and sustained program on FRC sustainment, stability, and confinement.
Several FRC-related facilities are in operation around the world as well as other small theory efforts. Favorable results from theory and experiments have raised hopes for ultimate development into a practical fusion system. Parameters achieved include densities ranging from 5x1013 to 5x1015 cm3, temperatures up to 3 keV (ions) and 500 eV (electrons); and b ~ 0.75-0.95. Noteworthy achievements include: formation by q-pinch, counter-helicity spheromak merging, and by rotating magnetic fields; simulation of large-orbit ion ring injection and trapping; stabilization of rotational instability; detection of global internal modes; tilting mode theory; global translation and acceleration along a guide field; identification of transport anomalies; and demonstration of the convective nature of energy loss.
In view of the foregoing, five action items are recommended. (1) FRC research should be continued and expanded both as an adjunct to mainline fusion research and as a stand-alone alternative fusion concept. (2) Existing FRC-related resources should be effectively utilized in an expanded program: including both facilities and the intellectual capital established in institutions and individuals with a strong commitment to FRCs. (3) New FRC facilities or upgrades of existing facilities should be considered on the merits of how they address the directions offered in this document. This should include consideration of a jointly-operated international FRC research facility. (4) Researchers and institutions with a history of activity on the tokamak should be encouraged to broaden their research to include FRC theory, diagnostic development, and systems studies. (5) Vigorous international collaboration on FRC research should be encouraged, including, at the least, annual workshops and long-term exchange visits.
University of Washington
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