Patent Number: 043308643
Section: summary

The present invention is directed to methods and apparatus for confining and controlling plasmas, and, more particularly, to such methods and apparatus for maintaining high temperature toroidal plasmas. Various methods and apparatus have been developed for generating and confining plasmas, which are ionized gases consisting of approximately equal numbers of positively charged ions and free electrons at high temperatures. One general type of device for plasma confinement consists of an endless, closed tube, such as a toroid, with a geometrically coextensive, externally imposed magnetic field (e.g., a toroidal magnetic field) in which magnetic lines of induction extend around the toroid generally parallel to its minor axis. Such a magnetic field is conventionally provided by electrical currents in one or more conductive coils encircling the minor axis of the toroid. Illustrative of such devices are the toroidal diffuse pinch plasma confinement devices of the Tokamak configuration, and such devices may be generally referred to hereinafter as tokamak devices or systems. The toroidal configuration may be advantageously employed with plasmas and plasma confinement systems of noncircular cross section either with respect to planes perpendicular to the minor axis or the major axis such as those involving plasma configurations which are axisymmetrically elongated in a direction parallel to the major toroidal axis. In this connection, U.S. Pat. Nos. 3,692,626 and 3,801,438 illustrate plasma generation and confinement apparatus of the toroidal type having a noncircular cross section in respect of a plane parallel to and intercepting the major toroidal axis. As previously indicated, toroidal systems for the containment of high-temperature plasmas comprise means for providing a strong, toroidal magnetic field in which the plasma is to be embedded, and which is conventionally provided by electrical current in one or more conductive coils encircling the minor toroidal axis. The term "axis" is used herein to include multiple axes or axial surfaces, such that reference to toroidal systems may include such systems having a noncircular cross section such as utilized in the various Doublet devices of the assignees of the present invention. Toroidal plasma systems, which are hereinafter generally referred to as tokamak systems, may also comprise means for providing a toroidal electrical field to produce a current flowing in the plasma, generally in the direction of the minor axis, and this plasma current in turn may generate a magnetic field component which is poloidal (i.e., the surfaces of constant magnetic flux are closed about the minor toroidal axis). The combination of the poloidal magnetic field produced by the plasma current, with the toroidal magnetic field produced by the toroidal coil current, is suitable for providing helix-like magnetic field lines that generally lie on closed, nested magnetic surfaces. The plasma is accordingly subjected to confining, constricting forces generated, at least in part, by the current flowing in the plasma. The resulting magnetic field provides for a diffused pinching force in the confining magnetic field which may be substantially greater than the outward pressure of the plasma. The generation of a current in the plasma may conventionally be provided in various ways, such as by providing current in an inductive primary coil configured such that the plasma serves as the secondary coil of a transformer system. Such inductive current further provides for inductive, ohmic heating of the plasma, and systems for additional heating of tokamak plasmas are known in the art. Experimental results in a variety of tokamaks indicate the importance of operation at high density in the achievement of good plasma confinement [Alcator Group in Plasma Physics and Controlled Nuclear Fusion Research (Proc. 5th Int. Conf. Berchtesgaden 1976), Pulsator Group in Plasma Physics and Controlled Nuclear Fusion Research (Proc. 5th Int. Conf. Berchtesgaden 1976) IAEA-CN-35A6], and for a given magnetic field, operation at higher density requires the achievement of higher values of .beta.. Substantially higher values of .beta. than obtainable for circular tokamaks may be provided in tokamaks with vertically elongated cross sections. The provision of such elongated, noncircular cross section plasmas, however, involves added complexity (e.g., in connection with shaping the plasma and stabilizing axisymmetric modes) and good plasma shaping conditions involve relatively flat plasma current profiles or the placement of field-shaping coils very close to the plasma [Doublet IIA Group in Plasma Physics and Controlled Nuclear Fusion Research (Proc. 6th Int. Conf. Berchtesgaden 1976) IAEA-CN-35/A10-3; Chu, M.S., et al., in Plasma Physics and Controlled Nuclear Fusion Research (Proc. 6th Int. Conf. Berchtesgaden 1976) IAEA-CN-35/B11-1]. However, for a fusion reactor system, energy conversion considerations involve the placement of a blanket between the plasma and the field shaping coils, which removes the coils sufficiently far from the plasma that acceptable control of the plasma configuration may require an unrealistically flat current profile. The utilization of a poloidal magnetic divertor as a means of impurity control involves similar considerations. Considerations of plasma shaping and stabilization of axisymmetric modes require very accurate control of the separatrix position, particularly when the amplitude and/or profile of the plasma current is evolving. Impurity control by means of poloidal divertors is accordingly most difficult during the early phase of the discharge which unfortunately is the time period in which most of the impurity contamination is believed to take place in present-day tokamaks. Accordingly, improved field shaping systems would be desirable for present generation tokamaks and perhaps ultimately essential for reliable, economical reactor operation. Accordingly, it is an object of the present invention to provide improved field shaping methods and apparatus for tokamak plasma systems. It is a further object to provide such methods and apparatus which may be adapted to a new field shaping concept that is compatible with reactor design considerations and provides added stability to axisymmetric plasma modes and which have improved plasma shaping and control characteristics. It is a further object to provide such methods and apparatus which may be adapted for use in poloidal divertor systems.