Patent Number: 042644135
Section: summary

The present invention is directed to the production, control and confinement of plasma in systems involving a generally toroidal plasma configuration, and more particularly is directed to improvement of the ratio of thermal energy density to magnetic energy density of such systems. Various devices have been developed for generating, confining, and studying plasmas, which are ionized gases comprising approximately equal numbers of positively charged ions and free electrons at high temperatures. If a plasma is formed from a suitable gas or mixture of gases, such as deuterium or tritium, fusion reactions may occur within the plasma. Such fusion reactions produce energetic charged particles and neutrons. If the proper conditions are realized the energy obtained from the fusion reactions may exceed the input energy and provide useful power, and certain of such plasma devices find utility in connection with the generation and study of plasma relative to the production of such reaction, or relative to other aspects of the study of the physics of plasmas, and the provision and confinement of very high temperature plasmas. In order to provide hydrogen plasmas (including plasmas of hydrogen isotopes deuterium and/or tritium) for study or other utilization of high temperature plasma conditions, it is not only appropriate to confine the plasma in a given region at extremely high temperatures for an appreciable period of time, but also to exclude the plasma from contact with solid container walls. Consequently, a large number of inertial and magnetic and electrical field configurations, and apparatus for producing them, have been developed or proposed in connection with the confining of plasma. One general type of device for plasma confinement comprises 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 of the Tokamak configuration, and such "diffuse pinch" 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, such as those involving plasma configurations which are axisymmetrically elongated in a direction parallel to the major toroidal axis. U.S. Pat. Nos. 3,692,626 entitled "Apparatus for Forming and Containing Plasma", and 3,801,438 entitled "Toroidal Apparatus for Confining Plasma" both to the present inventor, and both assigned to the assignee of the present invention, illustrate plasma confinement apparatus of the toroidal type having a noncircular cross-section in a plane parallel to and intercepting the major toroidal axis. As indicated previously, Tokamak 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 generally provided by electrical current in one or more conductive coils encircling the minor toroidal axis. In this connection, the term "axis" is used herein to include multiple axes or axial surfaces, such that reference to toroidal diffuse pinch systems includes those having a non-circular cross-section. Such systems also comprise means for providing a toroidal electric field to maintain a current flowing in the plasma, generally in the direction of the minor axis, and this plasma current in turn generates a magnetic field component which is poloidal (i.e., the magnetic flux lines 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 conductive coil current may provide resultant 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. In the diffuse-pinch design, the current is distributed throughout the cross-section of the plasma, which accordingly provides a diffuse poloidal magnetic field which is within, as well as encircling, the plasma. The resulting diffuse magnetic field provides for a diffused pinching force in the confining magnetic field substantially greater than the outward pressure of the plasma. One measure of the confinement efficiency of plasma confinement systems is the ratio of thermal energy density of the plasma, to the energy density of the magnetic field in which the plasma is confined. This ratio is conventionally known as the beta ratio, .beta., and when this ratio is defined in toroidal systems with respect of the energy density of the poloidal magnetic field, it is known as the poloidal field beta ratio, .beta..sub.p. It is generally known that a thermonuclear plasma confinement system should demonstrate high .beta. containment for economical power production. Conventional toroidal diffuse pinch Tokamak plasma confinement systems generally have a poloidal field beta ratio which is, generally, less than one. Substantial improvement in the beta ratios of diffuse pinch toroidal confinement plasma devices may be achieved by imposing specific magnetic boundary conditions in plasmas of non-circular cross-section, and in this connection, for example, the utilization of an axisymmetrically elongated plasma of doublet configuration may provide a beta value which is a factor of ten larger than in circular cross-section Tokamak devices. However, the beta values of even the elongated diffuse pinch plasma systems are relatively low in respect of confinement effectiveness, and further improvements in the beta ratios of toroidal plasma confinement systems would be desirable. In this regard, the beta values of conventional Tokamak systems have probably not reached the limiting value dictated by magnetohydrodynamic considerations. Limiting values may instead be imposed by heating methods or transport rates. Furthermore, there are theoretical arguments that the transport rates at lower values of beta are much higher than those at higher beta values. Moreover, some plasma instabilities may become absent at large beta values, thereby providing for improved confinement effectiveness. Thus, provision of toroidal confinement systems with higher beta values could provide for a desirable decrease in transport rates which limit plasma energy confinement. The desirability of providing toroidal confinement systems with a high value of beta may be further indicated, in historical perspective, with reference to the development of mirror machine plasma confinement systems. In this connection, the approach of starting from a very low plasma density and low beta value in the so-called Baseball configuration (in which Joffe bars resemble the seams of a baseball) turned out to be very difficult (as reported in "Plasma Production and Confinement in the Baseball II Mirror Experiment" by O. A. Anderson et al, paper D-5-2 of the Proceedings of the 5th Conference on Plasma Physics and Controlled Nuclear Fusion Research, Tokyo, Japan, Nov. 11-15, 1974), while the relative success of the 2XII experiments has demonstrated the advantage of starting at a moderately high value of beta (as reported in "Plasma Containment in 2XII" by F. H. Coensgen et al., paper D-2-1 of the Proceedings of the 5th Conference on Plasma Physics and Controlled Nuclear Fusion Research, Tokyo, Japan, Nov. 11-15, 1974). Also illustrative of the toroidal plasma confinement systems are the higher mass density systems known as toroidal theta pinch devices, in which an electrical current is provided in the theta, or azimuthal direction (around, or encircling, the minor toroidal axis). The resulting magnetic field is in the zeta, or axial direction (along, or in the same direction as, the minor toroidal axis). Conventional theta pinch devices tend to be fast-pulsed systems which have current flow around the plasma column within a thin surface layer of the plasma, producing a magnetic field which surrounds the plasma, but which does not provide magnetic flux lines within the plasma. These conditions provide for a "sharp" pinch in which the confining force is exerted generally from the conducting zone at the exterior surface of the plasma, rather than throughout the plasma as in "diffuse" pinch configurations. Conventional theta pinch devices have an advantage of being adapted for higher density plasma confinement at relatively high beta values (.beta..sub.p may be greater than 1) but have relatively poor magnetohydrodynamic stability. Conceptually, the problem of providing toroidal plasma confinement configurations with a respectable .beta.-value may be considered to be approachable from two extremes. One approach is to attempt to improve the .beta.-value of conventional diffuse-pinch devices, which generally have a poloidal beta value less than 1. At the other end of the spectrum is the approach by utilizing as a starting point a conventional theta pinch device configuration having a relatively high poloidal beta value of more than 1, and attempting to improve the magnetohydrodynamic stability by partial sacrifice of the high beta values. For various reasons, including the historical considerations previously referred to, if toroidal diffuse pinch plasma confinement systems are to eventually be developed with more respectable beta values, the latter approach should receive preferential consideration. The appropriate system would be capable of providing information as to the magnetohydrodynamic limits on the beta values. Moreover, as indicated previously, such a confinement system may avoid the anomalous transport associated with a low beta value. In addition, such an approach may provide a high density, short pulse type of confinement system. However, an appropriate confinement system must satisfy the magnetohydrodynamic equilibrium and stability conditions throughout the discharge period, and conventional systems have not satisfied such conditions. In this connection, the Belt pinch experiment (reported in "The Belt Pinch II Experiment with Improved Shock Heating" by O. Gruber et al., Proceedings of the Seventh European Conference on Controlled Fusion and Plasma Physics (1975), Volume I, page 43) is an example of a noncircular plasma cross-section, high beta Tokamak system which fails in satisfying the magnetohydrodynamic conditions. In the Belt experiments, a toroidal plasma with an elongated cross-section was produced initially by a rapid pulse of toroidal magnetic field and toroidal electric field. As in the theta pinch configuration, this produces a plasma current which is on the surface. However, when the current sheath relaxes so that the plasma carries current throughout its cross-section as is characteristic of diffuse-pinch confinement, the plasma configuration becomes less elongated and the plasma hits the insulating wall near the median plane. No provision was made to keep the equilibrium with the same elongation independent of the current distribution. Moreover, the Belt confinement system fails to meet safety factor stability conditions; the initial theta pinch plasma configuration and the final diffuse-pinch configuration have quite different safety factor (q) profiles. The final configuration has the minimum value of q on the axis, while the initial configuration has the minimum value of q at the edge of the plasma. The transitory configurations as the current sheath relaxes, or diffuses, thus have the minimum of q in the body of the plasma. It is known that a plasma configuration with q-minimum within the plasma, at a location other than on the minor axis, is unstable. In accordance with the present invention these short-comings in respect of toroidal discharge systems employing a sharp pinch-diffuse pinch transition may be remedied so that a plasma configuration may stay stable throughout the discharge period. It is a further object to provide toroidal plasma confinement systems having improved beta ratios of plasma energy to magnetic field confinement energy.