Patent Number: 042773067
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 the purity of high temperature toroidal plasmas. Various methods and apparatus have been developed for generating and confining plasmas, which are ionized gases comprising approximately equal numbers of positively charged ions and free electrons at high temperatures. One general type of device for plasma confinement comprises an endless, closed tube, such as a toroid, with a geometrically co-extensive, 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 or 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" as 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 assigness of the present invention. Toroidal plasma systems, which are hereinafter generally referred to as tokamak systems, may also comprise means for providing a toroidal electric 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 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 toroidal cell 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 more continuous heating of tokamak plasmas are known in the art. An important difficulty in the initial formation and sustained maintenance of a high temperature plasma is the problem of excluding impurity atoms from the plasma, and the substantial and potentially disabling plasma energy losses which result from the presence of such impurities. The impurities in a plasma generally originate from two principal sources. First, contaminants such as oxygen, nitrogen or carbon may be absorbed on the chamber walls surrounding the plasma zone, and driven therefrom by the conditions which are employed to initially form the plasma. The other principal source of contaminants results from the bombardment of the chamber wall material itself by energetic plasma particles and radiation. Further, fusion reactions may generate high energy particles which will increase the problem of contaminants from wall bombardment. Suitable vacuum techniques and high temperature baking may be employed to minimize the adverse effect of absorbed contaminants, but the problem of contaminants produced by bombardment and erosion of the chamber walls have provided substantial difficulties. Complicated magnetic divertor systems have been designed in an effort to overcome the problem in some types of plasma confinement systems, but such divertors are expensive, complex and have various other disadvantages. Conventional divertors are intended to skim off the most contaminated plasma near the wall and are generally structured orthogonally to the minor toroidal axis, such that partical diversion occurs radially outwardly of the minor axis in a cross sectional plane through the plasma. Such poloidal divertors, such as that of the Poloidal Divertor experiment, are designed essentially to bury hot protons into a divertor plate away from the main plasma, to differentially pump away any sputtered atoms before they reach the main plasma, and to ionize sputtered atoms produced by hot neutral hydrogen at the wall facing the main plasma, and carry the ionized atoms to the divertor plate. Conventionally, a magnetic configuration with a separatrix encircling the divertor coil is employed to neck down the passage between the divertor region and the main plasma for differential pumping in order to perform these functions, but such divertor coils must be in the vacuum and also must be shielded from fusion neutrons in a reactor. Such considerations make a plasma confinement system or a plasma fusion reactor very complicated and expensive. The presence in the plasma of impurities, such as those originating from the walls of the chamber surrounding the plasma, leads to undesirable energy loss in the form of radiaton, which in turn has a deleterious effect on the formation and/or maintenance of a high temperature plasma. These energy losses arise because the contaminants generally have a higher atomic number than hydrogen, and the type of electronic excitation, ion recombination and bremsstrahlung radiation losses produced by their presence in a hydrogen plasma (i.e., hydrogen, deuterium, tritium and mixtures thereof) become increasingly deleterious with increasing atomic number of the contaminant. The problem of plasma contamination by impurities may be further aggravated in toroidal confinement systems by the toroidal geometry itself. In toroidal plasma configurations, the inward flow of contaminant impurity ions from the chamber zone surrounding the plasma is enhanced by Pfirsch-Schluter effects. In this regard, each different contaminant ion species has a characteristic flow pattern in which an upward (or downward depending on the polarity of the toroidal magnetic field) flow with respect to the major axis, due to the toroidal drift in the toroidal configuration, is followed by the return flow along the magnetic field line. However, because of the difference of electric charge between the various species of impurity ions, the flow velocities of the different species are correspondingly different. The resulting collisional friction between the different species will disrupt the otherwise established pattern and result in the enhanced inward motion of impurity ions from the surrounding zone into the plasma. [P. Rutherford, Princeton Plasma Laboratory MATT-1039 (1974)]. Methods and apparatus without the undesirable internal coils of poloidal divertors have been devised which utilize an axisymmetric source-sink system for impurity flow control or reversal [K. Burrell, "Effect of Particle and Heat Sources on Impurity Transport in Tokamak Plasmas," Phys. Fluids, 19, 401 (1976); T. Ohkawa, Kakuyugo-Kenkyu, 32, 61 (1974)], but further developments in respect of impurity control are desirable. Accordingly, it is an object of the present invention to provide a method and apparatus for control of impurity ions in toroidal plasma systems without the use of internal coils. It is a further object to provide such a method and apparatus which may be relatively simple, which do not unduly disrupt the delicate dynamics of the toroidal plasma confinement systems, and which may be used with existing toroidal systems without prohibitively substantial structural or design alterations.