Patent Number: 043057837
Section: description

One of the more difficult aspects of high temperature plasma devices is the confinement of the plasma, which is ionized gas. This can be accomplished by the now well-known tokamak device. It has a toroidal containment vessel for containing the gas and the plasma. Twisting magnetic fields are created within the toroidal vessel to confine the plasma and keep it from striking the walls of the toroidal vessel. These fields include toroidal and poloidal components as produced by the flow of electric current. The manner of creating such fields is illustrated conceptually in FIGS. 1 and 2, and a generalized and simplified form of tokamak device is illustrated in FIG. 3. In FIG. 1 is illustrated the means for producing the toroidal magnetic field component. Electrical current is applied over conductors 10 to toroidal field coils 12. The current in these coils links a toroidal space 14 and hence generates a toroidal magnetic field 16 therein, as indicated by the arrows. In FIG. 2 is illustrated the means for producing the principal poloidal magnetic field component that is necessary for stable confinement. In this device, the poloidal field 18, as indicated by the arrows, is induced by toroidal current 20 in the plasma 22. In practice electric current in equilibrium field coils outside to torus generates an additional poloidal magnetic field which modifies the principal poloidal field to control the position of the plasma. As generalized, a conventional tokamak device, as illustrated in FIG. 3, combines the features of FIGS. 1 and 2 to provide a high level of plasma stability. As there illustrated, current from a power source 24 is applied over the conductors 10 to the toroidal field coils 12 which are disposed around a toroidal liner 26 which contains and defines the toroidal space 14 in which the plasma 22 is created. Equilibrium field coils 28 are supplied with electrical current from a source not illustrated to position the plasma 22 within the liner 26. Ohmic heating coils 29, also supplied with electrical current from a source not illustrated, induce current in the plasma 22 to ionize the gas, heat the plasma, and generate the poloidal magnetic field illustrated in FIG. 2. In FIG. 4 is illustrated a preferred form of the invention for producing the toroidal magnetic field. It is thus a form of the device shown in stylized form in FIG. 1. In this preferred embodiment of the present invention, a pressure vessel 30 forms a reservoir filled with liquid metal 32. A toroidal liner 34 is supported within the liquid metal 32 by struts 36 extending to the vessel 30. The pressure vessel 30 is formed of material, such as stainless steel, capable of withstanding relatively high internal pressure while not being attacked by the environment, notably the liquid metal 32. While various other metals are effective for certain purposes, liquid lithium is preferred for the liquid metal 32, particularly for deuterium-tritium plasma devices, for lithium is suitable for moderating resultant neutrons and acts to breed tritium fuel by reaction with the neutrons: EQU .sub.3 Li.sup.6 30 .sub.0 n.sup.1 .fwdarw..sub.1 H.sup.3 +.sub.2 He.sup.4. The liquid metal also acts as a coolant, being circulated by a pump 38 through a heat exchanger 40 by way of conduits 42. The toroidal liner 34 is preferably formed of electrically insulating material and may have equilibrium field coils 44 and ohmic heating coils 46 embedded therein to provide an appropriate poloidal magnetic field and appropriate ohmic heating in the usual fashion. Alternatively, these coils 44 and 46 may be supported in the liquid metal 32. The toroidal liner 34 defines a toroidal space 48 in which gas is confined for producing plasma. The liner 34 separates the liquid metal 32 from the toroidal space 48 and thus forms a bubble of gas in a pool of liquid. The ohmic heating coils 46 are energized in a conventional manner to ionize the gas and produce the plasma. The plasma is positioned by the action of the poloidal equilibrium magnetic field and is stabilized by a toroidal field produced by current passed through the liquid metal over a conductive path 49 linking the toroidal space 48. Such current is passed through the liquid metal 32 between conductive feed plates 50 and 52, the feed plates 50 and 52 having an insulator 54 interposed therebetween to cause the current flow to link the space 48. Current is supplied to the conductive plates from a power supply 56. To confine and heat the plasma well, it is desirable to provide a high toroidal magnetic field. This requires extremely large electrical currents through the liquid metal, which is a good electrical conductor. It is difficult to provide such large electrical currents at low impedance efficiently. Furthermore, in order to provide uniformity, it is desirable that the currents be generated in a manner evenly distributed azimuthally around the torus. Specific preferred power supplies 56 for so generating the current are shown in FIGS. 5, 6 and 7. The power supply illustrated in FIG. 5 is an equatorial homopolar generator 58. A homopolar generator operates on the same principle as a conventional generator of electrical current, namely that when a conductor is moved across a magnetic field, current is generated orthogonally to both the direction of motion and the direction of the magnetic field. The difference is that in a homopolar generator the magnetic field does not vary along the direction of conductor motion. Homopolar generators are characteristically of much lower impedance than conventional generators and produce direct current. In FIG. 5 the homopolar generator 58 is shown in transverse section through the major axis of the toroidal liner 34. The generator 58 is circularly symmetrical about that axis and is mounted equatorially of the toroidal space 48. The generator 58 includes a homopolar rotor 60 mounted in any convenient fashion for rotation about the major axis of the liner 34. Upper and lower field exciting coils 62 and 64, which are preferably superconductive, are driven by a current supply, not shown, to produce a magnetic field indicated by B flowing transversely of the rotor 60. When the rotor 60 is rotated about its axis (into the plane of the drawing as shown in FIG. 5), direct current is induced in the feed plates 50 and 52, flowing through the plate 50 to and through the liquid metal 32 linking the toroidal space 48, and thence back through the plate 52 to the generator 58. Because the tokamak device and the homopolar generator 58 are circularly symmetrical, the current is evenly distributed azimuthally around the torus, hence producing a uniform toroidal field. Brushes 66 connect the plates 50 and 52 to the respective poles of the rotor 60. Because the currents are very great, it is desirable to use brushes of particularly good conductivity. Such brushes may be liquid metal brushes, as in the form of pools of mercury. The rotor 60 may be driven in any conventional manner, as through gears. Preferably, however, it is driven by a hydraulic turbine, turbine blades 67 being fastened on the outer surface of the rotor. Alternatively, the rotor 60 may be driven as the rotor of an induction motor by means of the rotating magnetic field of an adjacent stator. In FIGS. 6 and 7 is shown a related power supply in the form of magnetohydrodynamic (MHD) generator 68 which is much like the homopolar generator but uses flowing liquid metal instead of the rotating solid rotor. In FIG. 6 and MHD generator 68 is shown in section through the major axis of the toroidal liner 34. The generator 68 may be generally circularly symmetrical, like the homopolar generator 58, but it is preferably comprised of a number of separate sections each beginning and ending as shown in FIG. 7. Liquid metal is circulated by a driving means 69 through a conduit or conduits 70 defined by the feed plates 50 and 52 and by insulating wall members 72. The fluid is introduced into the conduits 70 through respective curved inlet conduits 78 and leaves through respective curved outlet conduits 80. The inlet and outlet conduits 78 and 80 are made from electrically insulating materials to reduce eddy current losses. The liquid is conveniently driven by a driving means 69 formed of a high pressure pneumatic accumulator with throttle valves for power modulation for pulsed operation, or by pumps for steady operation. The exciting field is provided by upper and lower exciting coils 74 and 76, just as in the homopolar generator, to produce a radially transverse magnetic field B. Movement of the conductive liquid metal through the conduit or conduits 70 then generates direct current through the feed plate 50, thence through the liquid metal 32 in a path linking the toroidal space 48, and back through the feed plate 52. The result is the same as with the homopolar generator 58. In operation of the tokamak system of the present invention, plasma is created in the toroidal space 48 by introducing appropriate gas filling therein and applying current in a known manner to the ohmic heating coils 46. This may be in a known back-bias to zero mode. The plasma may then be maintained in position in a known manner by applying appropriate current to the equilibrium field coils 48. The present invention permits the application of very high currents in excess of, for example, 10.sup.7 A) through the liquid metal 32 around the toroidal space 48 and hence a relatively high toroidal magnetic field so as to confine the plasma. At the same time, because the conductor is liquid, internal stresses are automatically alleviated, being transferred to the pressure vessel 30, which is made of structural material and is preferably spherical for maximum strength. The power supply 56 must be a low impedance, high power source, preferably providing current to the liquid metal 32 substantially evenly distributed azimuthally around the major axis of the toroidal space. While preferred embodiments of the invention have been shown and described, various modifications may be made therein within the scope of the invention. For example, the containment vessel 30 may take other shapes. The ohmic heating coils 46 and the equilibrium coils 44 may be disposed differently and may be driven in a number of known ways. The power supply 56 may take other forms. Other materials may be used. As the liquid metal 32 is subject to large magnetohydrodynamic convective cells means, such as baffles, may be used to reduce the size of the cells, when necessary or desirable; however, the poloidal field provides some stabilization and damping. It should also be noted that details of well-known components of tokamak devices have been omitted from the drawings in order that the essential parts of the invention may be more easily shown and understood. The present invention provides a relatively high toroidal field with a relatively small overall device. The smaller size of the device may result in lower cost, and the higher field confines the plasma to a smaller volume, increasing the interaction between the plasma particles.