Patent Number: 042630967
Section: description

FIG. 1 illustrates a toroidal plasma device 10. The device includes a large toroidal containment vessel 12 containing and confining gas. The containment vessel 12 may be more than 6 meters high, with a shape and other relative dimensions as shown in the drawings. The plasma is created by a poloidal field, established by E-coils 14. When the E-coils 14 are energized, they produce a time varying magnetic flux linking the vessel 12. The electric field induced by this flux variation initiates and maintains the toroidal discharge current required for plasma confinement and ohmic heating. F-coils 16 control the magnetic configuration of the discharge, confining it generally to the shape, position and dimensions of the vessel 12. The F-coil system establishes the magnetic boundary conditions for the plasma. It is essentially a passive system with energy being added to overcome resistive losses in the F-coils. Around the vessel 12 are toroidal B-coils 18, which establish an azimuthal magnetic field for stable plasma confinement. To achieve the required plasma temperature, auxiliary neutral beam heating may be provided in addition to the ohmic heating provided by the E-coils 14. To this end, high energy neutral particles may be injected tangentially into the vessel 12 through injection ports 20. The operation of the described coils, together with the neutral beam heating, produces a plasma of ions magnetically confined in the vessel 12. The magnetic confinement maintains the plasma sufficiently out of contact with the first wall 22 forming the inner wall of the vessel 12 so that the plasma is not cooled to destruction by the first wall 22. A blanket 24 surrounds the vessel 12. Coolant gas is circulated through the blanket 24 from a conduit 26. Cool gas is introduced into the conduit 26, and heated gas is withdrawn from a conduit 28. In order that the plasma may be generated at relatively low pressures, the vessel 12 is constantly pumped out by vacuum pumps through ports 30 and conduits 32. A radiation shield 34 limits the escape of harmful radiation. As is evident from FIG. 1, it is easy enough to remove the outer E-coils 14, as by raising or lowering them to get them out of the way. The B-coils 18 are another matter. As is also evident from FIG. 1, the B-coils 18 impede access to the containment vessel 12. Access is facilitated by the toroidal coil system of the present invention, a preferred embodiment of which is illustrated in FIGS. 2 and 3 which illustrate generally one half of the coils 18 of a toroidal coil system according to the present invention. The coils 18 are comprised of the composite of linking coils 40, 42, 44, 46 and 48 and unlinked coils 50, 52, 54 and 56. The linking coils are formed of first sections or legs 40-1, 42-1, 44-1, 46-1 and 48-1, respectively, which pass through the open central portion of the containment vessel 12 and second sections or legs 40-2, 42-2, 44-2, 46-2 and 48-2 which complete the respective linking coils to link the containment vessel. The unlinked coils are formed of first C-shaped sections or legs 50-1, 52-1, 54-1 and 56-1, respectively, and second C-shaped sections or legs 50-2 52-2, 54-2 and 56-2 joined to respective first C-shaped sections at their open ends with their bights spaced apart. The linking coils 40, 42, 44, 46 and 48 are relatively fixedly disposed to link the reaction vessel 12 with the first sections 40-1, 42-1, 44-1, 46-1 and 48-1 distributed substantially evenly around the central opening of the vessel and with the second sections 40-2, 42-2, 46-2 and 48-2 grouped together at a single azimuthal position. A corresponding group of linking coils is included in the other half of the toroidal coil system, providing two groups of linking coils 180.degree. apart. The unlinked coils 50, 52, 54 and 56 are movable. They are removably mounted in the device 10 with the respective second sections 50-2, 52-2, 54-2 and 56-2 closely adjacent respective second sections of the linking coils, with the vessel 12 disposed between the open ends of the respective first and second C-shaped sections. The bights of the respective coils are spaced from one another around the periphery of the vessel 12 by approximately multiples of 360.degree./N where N is the effective number of coils, 10 in the embodiment shown in FIGS. 2 and 3. Thus, the bights of the second coil sections 52-2 and 54-2 are spaced from the bights of first coil sections 52-1 and 54-1, respectively, by about 36.degree., and the bights of second coil sections 50-2 and 56-2 are spaced from the bights of first coil sections 50-1 and 56-1 by about 72.degree.. In this way, the first sections of the movable, unlinked coils are substantially evenly distributed about the periphery of the vessel between the positions of the first sections of the linking coils. Current is applied to the fixed linking and movable unlinked coils from the usual power supplies, with current being passed in the opposite sense in adjacent second sections of linking and movable coils so that their respective magnetic fields offset one another. The number of turns and current magnitude in respective coils are made such as to make the magnetic field strength at the location of the second sections of the linking coils approximately the same as at the first sections of the movable coils, thus providing a uniform toroidal magnetic field with a relatively small ripple. In the embodiment illustrated, all linking and unlinked coils are provided with substantially the same number of turns and substantially the same current is applied, so that the magnetic field occasioned by second sections 50-2, 52-2, 54-2 and 56-2 of the unlinked coils substantially neutralizes the field occasioned by the adjacent second sections 40-2, 42-2, 46-2 and 48-2 of the linking coils, effectively leaving the field occasioned by second section 44-2 unneutralized. This effectively makes first sections 40-1, 42-1, 46-1 and 48-1 of the linking coils the return path for current in the first sections 50-1, 52-1, 54-1 and 56-1 of the unlinked coils, so that there are effectively five coils linking the vessel 12 evenly distributed over each half of the vessel. While a particular preferred embodiment of the invention has been shown and described, various modifications can be made within the skill of the art without departing from the scope of the present invention. For example, FIG. 1 illustrates a tokamak plasma device in which the present invention may be utilized. FIG. 1 is intended to illustrate a generalized device, whether or not the B-coils are movable as in the present invention and irrespective of the number of coils. In fact, the number of B-coils in FIG. 1 is not the same as the number of coils in the preferred embodiment of FIGS. 2 and 3, which is effectively a 10 coil system. By the appropriate placing and energizing of linking and unlinked coils according to the present invention, substantially any number of B-coils may be used, as may be desired to achieve a particular field ripple. It should also be noted that very large repulsive forces may be produced between adjacent legs of unlinked and linking coils, requiring appropriate structural restraints as are conventional for magnetic devices creating such large repulsive forces, such restraints providing the required mechanical strength without interfering adversely with the magnetic and electrical properties of the coils.