The Reversed-Field Pinch: From Experiment to Reactor

In the reversed-field pinch (RFP), the plasma is confined in an axisymmetric toroidal configuration by a combination of toroidal (Bφ) and poloidal (Bθ) fields with Bθ ≫ Bφ outside the plasma and Bθ ≃ Bφ within the plasma. The essential property of the RFP equilibrium is that it is a near-minimum-energy relaxed state that the plasma finds naturally; the spontaneous generation of reversed toroidal field (dynamo process) is a consequence of this relaxation, and, if the plasma current is maintained, the field generation continues so that the configuration exists as a quasi steady state. Since such equilibria have little free energy to drive instabilities, the system has good stability properties, and, theoretically, confinement at high beta is possible; the plasma current can be sufficiently high to allow the possibility of plasma ignition in deuteriumtritium by ohmic heating alone. Experimentally, values of poloidal beta typically 10% or more are usually observed, and conditions have been found in which the temperature is found to scale approximately linearly with the plasma current up to 0.5 MA; maximum temperatures of the order of 0.5 keV have been observed. This temperature scaling corresponds to a current dependence of the energy confinement time of I3/2Φ. Ohmically heated RFP reactors are discussed with emphasis on improved designs with increased power density up to values comparable to those in fission systems. Such compact reactors efficiently utilize normal copper coils and operate at relatively high wall loading. Compact reactors appear to offer significant advantages f or fusion power generation, and it is further shown that the RFP, because beta can be high and confinement is mainly by the poloidal field, offers advantages compared to other systems for compact reactor embodiments. These features also favor unique solutions to the impurity-control problem by the use of high-coverage pump limiters or toroidal magnetic-field divertors. One consequence of the relaxed-state equilibrium is the strong coupling between the poloidal and toroidal circuits through the plasma, offering the possibility of a low-frequency, low-amplitude oscillating-field current drive. The plasma-physics and plasma-engineering basis for the reactor is discussed, and a reactor design point established by extrapolation from the existing physics data base, along with a brief account of reactor optimizations and technological considerations, is given.