Patent Number: 042630975
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 the driving of a plasma current in such systems. Various apparatus has been developed for 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 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 diffuse pinch systems may include such systems having a noncircular crosssection. Conventional toroidal plasma 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 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 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. However, such inductive plasma current generation utilizes pulsed current in the primary winding, and can only sustain an ohmic current in the plasma for brief time periods before the peak current in the primary is reached and begins to decay. Such factors limiting the time duration of inductively produced plasma current are a substantial disadvantage and have the effect of limiting the performance and operating parameters of toroidal plasma systems. However, steady state operation of toroidal plasma systems is a recognized goal in the development of plasma technology and substantial effort in the art has been directed to non-inductive methods which might provide the capability for steady-state operation. Substantial effort has also been directed to methods for heating of plasmas. Techniques currently being considered for providing auxiliary heating in toroidal plasma apparatus include high energy neutral beam injection, radio frequency heating and bootstrap current. [T. Ohkawa, Nuclear Fusion 10, 185 (1970); Messjaen, et al., Nuclear Fusion 15, 75 (1975); D. Wort, Plasma Physics 13, 258 (1971); Kadomtsev, et al., Plasma Physics and Controlled Nuclear Fusion Research (Proc. 5th Int. Conf. Madison, 1971) 2 IAEA 110 (1971); Bickerton, et al., Nature, Phys. Science 229, 110 (1971); patents and papers referred to herein are hereby incorporated in this specification by reference]. Due to long-range electromagnetic interactions between charged particles and external electromagnetic fields, there exists a host of collective motions (waves) in the plasma [T. H. Stix, "The Theory of Plasma Waves," McGraw-Hill, New York (1962)]. The existence of these waves provide a means for coupling of external electromagnetic energy such as radio frequency (r-f) electromagnetic wage energy into the plasma. Various of the plasma waves which may have utilization in respect of plasma heating, in ascending frequency, are: alfven waves, ion cyclotron waves, lower hybrid waves, and electron cyclotron waves. In connection with Alfven wave plasma heating, for frequencies below the ion cyclotron frequency, .omega.&lt;.omega..sub.ci, there are two modes with the dispersion relations EQU .omega..sup.2 =k.sub..perp..sup.2 V.sub.A.sup.2 EQU .omega..sup.2 =k.sup.2 V.sub.A.sup.2 where V.sub.A =.sqroot.B.sup.2 /4.pi.m.sub.i n.sub.i is the Alfven speed, k is the wave number and .perp. denotes the component perpendicular to the magnetic field, and m.sub.i, n.sub.i are the mass and density of the plasma ions. The plasma motion of the first mode is an incompressible shearing motion and the wave is called the shear Alfven waive or torsional Alfven wave. The plasma motion in the second mode is a compressional one with a phase velocity faster than both that of the shear mode and the sound speed. The wave is called a compressional Alfven wave or fast magnetosonic wave. For typical fusion grade plasmas, the frequency of the shear Alfven wave is less than 1.0 MHz and the vacuum wave length is the order of several meters. An r-f coupling structure may comprise coils surrounding the plasma and located inside the metallic vacuum vessel, in order to prevent shielding of the r-f field. In a confined plasma, the shear Alfven wave has a continuous spectrum for a given wave number which may be fixed by the coil structure. According to conventional theory, at the resonant layer x.sub.n where the driving frequency matches the local shear Alfven frequency .omega.=k.sub..parallel. V.sub.A X.sub.n (subscript .parallel. denotes the component parallel to the magnetic field phase mixing occurs and wave energy is damped [Chen and Hasgawa, Phys. Fluids 17, 1399 (1974); J. Tataronis, J. of Plasma Phys. 13, 87 (1975)]. Alfven wave heating mechanisms might presently be considered to potentially involve transient time magnetic pumping, electron Landau damping, ion viscous damping or some nonlinear process, but are not presently fully understood or fully utilized in plasma systems. The development of effective plasma heating systems utilizing Alfven wave interaction would provide for realization of potentially favorable characteristics of this form of heating, including the relatively low frequency of the waves and the conventional availability of relatively inexpensive power sources for this frequency range. Furthermore, losses between an r-f generator and the plasma can in principle be made very small (e.g., less than 10%) in respect of r-f-Alfven wave coupling. However, disadvantages of conventional Alfven wave utilization include the requirement for protection and cooling of the coils within the metallic vessel and possible large impurity production. Furthermore, because the frequency range is below the ion cyclotron frequency range, Alfven wave excitation may induce enhanced plasma loss. Conventional Alfven wave heating techniques have not been thoroughly tested on tokamaks, although low power experiments have been conducted. As the frequency .omega. approaches the ion cyclotron frequency .omega..sub.ci, the shear Alfven wave becomes an ion cyclotron wave with frequency .omega..sub.ci and is left-hand polarized (in the same sense of ion gyration). As .omega. increases beyond .omega..sub.ci, this wave disappears due to the ion cyclotron damping. On the other hand, the compressional mode has a high percentage of right-hand polarization and is only weakly damped by the ion cyclotron damping for frequencies at the ion cyclotron frequency and disappears only at the electron cyclotron resonance. In the frequency range above the ion cyclotron frequency, it can also heat electrons via transient time magentic pumping. Since the ion cyclotron wave is heavily damped and its propagation region is generally confined to the high magnetic field side of the resonance, it is not readily adapted for heating in tokamak type machines in which little space is available in the high field region. However, the compressional wave propagates around the torus. Experiments on the ST Tokamak at a frequency near the second harmonic 2.omega..sub.ci have demonstrated good heating, and similar results have also been obtained in Tokamak TO-1 with a loop exciter [Ivanov and Kovan, Proc. IAEA Conf. (Tokyo) Vol I (1974) p. 231]. Fast magnetosonic wave experiments are currently being carried out on the TFR Tokamak as well. For typical fusion grade plasma, the frequency may be in the 10 MHz range and the vacuum wavelength may be on the order of about a meter. The excitation structures in such a system may comprise coils surrounding the plasma or large wave guides with a size of one-half meter or so. Wave guide coupling is considered favorable as the wave guides can also be used as pumping ports. In a plasma with two ion species, the fast magnetosonic wave can also parameterically excite the two ion Buchsbaum modes [J. Adam et al., paper A3-2 in IAEA-CN-33 (1974)] which may result in nonlinear heating [Sperling and Perkins, Phys. Fluids 17, 1857 (1974)]. Favorable characteristics of this form of heating include the fact that the r-f power is available and not too expensive. The power may be fed into the plasma system by means of waveguides which might also be used for pumping ports. The method has been tested on ST Tokamak and the theoretically predicted eigenmodes were observed, with wave generation efficiencies of up to 90%. At .omega..sub.o =.omega..sub.ci, the ion temperature was doubled (.DELTA.T.sub.i =100 eV) with 20% heating efficiency. No deleterious effects on confinement due to the r-f field was observed. Even in the linear region, plasma heating is reasonably good and the physics of heating is simple. Disadvantages of conventional ion cyclotron wave heating proposals include the eigenmode frequency change as the density varies, requiring wave generation systems with sufficient bandwith to follow an eigengrequency. Moreover, as demonstrated in r-f experiments on the ST Tokamak, there may be significant production of impurities, (possibly resulting from the low field and low current required to obtain resonance for the ST Tokamak) and the heating of particles on ion "banana" orbits that hit the wall. While such problems may presumably be alleviated in larger tokamaks in which r-f heating might be specifically designed to heat only the plasma core, such systems have not been developed. The lower hybrid plasma wave has the dispersion relation ##EQU1## The lower hybrid wave is a slow electrostatic wave, and for a fusion grade plasma, its frequency made be in the range of a few GHz, which is the upper limit below which relatively inexpensive power systems are conventionally available. The vacuum wavelength may be in the range of 10 cm. In order to have access to the resonant region without first crossing a region where it becomes evanesent, the wave must have a wave number parallel to the field line satisfying the following relationship: ##EQU2## Therefore, properly-phased wave guides are required for the coupling [Brambilla, M., in "Symposium on Plasma Heating in Toroidal Devices," Varenna, Italy (1974) p. 113]. According to the linear theory the incident wave, traveling in the magnetized plasma with a gradually increasing density, will convert into a slow electrostatic mode in the vicinity of the lower hybrid resonant layer [Stix, T. H., Phys. Rev. Lett 15, 878 (1965); Piliya, A. D., and V. I. Fedorov, Sov. Phys. JEIP 33, 210 (1971), and 30, 653 (1970]. The converted, short-wavelength electrostatic mode may heat the plasma by either linear ion Landau damping or cyclotron damping, and nonlinear parametric processes may also be involved. In the nonlinear parametric processes, the incoming wave may parametrically excite a short-wavelength lower hybrid mode plus either a backward ion cyclotron wave, an ion Bernstein mode, or an ion quasi-mode [Sperling, J., and C. Chu, "Sherwood Annual Theory Meeting," Madison, Wis. (1976); Berger, R. L., and F. W. Perkins, Phys. Fluids 19, 406 (1976); Rogister, A., and G. Hasselberg, Phys. Fluids 19, 108 (1976); Porklab, M., Phys. Fluids 17, 1432 (1974)]. Such instabilities may lead to large internal electrical fields and anomalous heating of the plasma. The amount of energy fed into the electrons and ions depends upon the angle of propagation of the daughter waves. The electrons are heated in the parallel direction while ions are heated in the perpendicular directions. Similar results have been observed experimentally. [Kitsenko, A. B., et al., Nucl. Fusion 13, 557 (1973)], and recent results on the ATC and Alcator systems indicate plasma heating by nonlinear processes. The Tokamak FT-1 shows strong collisional absorption in the vicinity of the resonance with possible parametric heating for the ions [Golant, V. E., et al., Proc. of IAEA Conf. (Tokyo), Vol. I, (1974) p. 231]. Favorable characteristics of the lower hybrid resonance frequency are that the frequency is much higher than the ion cyclotron frequency, so that the field-induced diffusion may not be as detrimental as experiments might presently indicate. Moreover, the heating is localized and is suitable for profile control purposes. However, the accessibility condition is not completely understood at present. There are some indications that nonlinear effects could be important in wave propagation. However, heating by nonlinear processes is hard to control; the heating mechanism is not well understood, and resolution of uncertainties concerning the complex mechanisms involved by conventional lower hybrid resonance heating proposals will require additional experimental evidence and theoretical studies. In respect of conventional proposals for r-f plasma heating, it is recognized that the higher the frequency and the shorter the wavelength of the wave, the less deleterious it is on plasma confinement. High frequency waves with frequencies near the electron cyclotron frequency .omega..sub.ce or the second harmonic 2.omega..sub.ce, can be absorbed by electrons via electron cyclotron damping. These waves have a vacuum wavelength in the millimeter range. Conventional approaches for coupling and injection are relatively simple, and the effect of such waves on plasma confinement may be beneficial [Alikaev, V. V., et al., MATT-TRANS-120 (1976)]. Commercially available power sources at the indicated wavelengths are undesirably expensive for economical fusion grade plasma heating, but commercial development of high power, millimeter wavelength sources to provide high power, single tubes is presently being carried out. Radio frequency heating has various potential advantages over neutral beam injection techniques. The conversion rate of wave energy into thermal energy can potentially be made considerably larger than the slowing-down rate of energetic ion beams. Radio frequency heating is less sensitive to charge-exchange; therefore, impurity generation by high energy neutrals may be greatly reduced. Furthermore, the penetration of neutral beams is more difficult with increasing machine size and higher plasma density. While wave energy penetration may also be more difficult under such conditions, wave heating may be more flexibly chosen to heat the bulk or the tail of the distribution function. However, despite the potential advantages of r-f heating there are disadvantages with respect to various of the conventional radio frequency heating approaches, and new developments in radio frequency plasma heating technology would be desirable. Furthermore, such conventional r-f technology does not provide for the maintenance of a continuous plasma current for magnetic field generation.