Patent Number: 048636711
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a system for plasma confinement by magnetic fields, and more particularly to a plasma confinement system having magnetic field generation equipment which is well suited for the highdensity plasma confinement of a nuclear fusion apparatus or the like. The plasma confinement system produces a plasma within a toroidal vacuum chamber and exerts magnetic fields on the plasma so as to confine the plasma. Conventional plasma confinement systems are based on pulse operations. With the intention of the A.C. operation of the system, there has been the idea that primary winding coils are excited by the use of alternating current as disclosed in Japanese Patent Application Laid-open No. 100891/1984. The operation on this occasion, however, has been quite equivalent to the ordinary pulse operation as regards the half cycle of the alternation. In the Tokamak-type confinement system which is a typical conventional system, it is the ballooning instability that forms an obstacle to the future perfection of this system as a nuclear fusion reactor. This instability takes place for the reason that the outward convex part of a plasma swells outwards still more. The system therefore has the problem that the plasma disappears in a short time and cannot be confined for a long time. SUMMARY OF THE INVENTION A general object of the present invention is to provide a plasma confinement system which can confine a plasma for a long time. Another object of the present invention is to provide a plasma confinement system which can eliminate the instability stated before. The above objects are accomplished by changing a magnetic surface shape within a plasma. In order to change the magnetic surface shape, currents to coils for establishing magnetic fields within a poloidal surface may be changed. In case of the Tokamak-type system, a poloidal coil system for holding an equilibrium is wound axisymmetrically. When coils for affording the change to the magnetic surface shape are wound axisymmetrically, also the change in the shape becomes axisymmetric. On the other hand, when the coils for affording the change to the magnetic surface shape are wound non-axisymmetrically, the change in the shape becomes non-axisymmetric, but the basic plasma equilibrium can be held axisymmetric. Usually, the feed of power to the coils has been performed quasi-statically. That is, a single operating state has existed within one operating cycle. With an ordinary change in the shape, a part in which the magnetic line of force is convex outwards cannot be eliminated, and the instability grows. The growth of the instability can be prevented in such a way that the magnetic surface is rotated to rapidly remove the plasma from the unstable position. The rotation of the magnetic surface is achieved by supplying an alternating current to the poloidal coil system with phase shifts. In the toroidal plasma, the magnetic surface is formed by the magnetic field within a poloidal plane which is established by a plasma current flowing in the lengthwise direction of the toroid and the currents flowing in the coils wound in the lengthwise direction of the toroid, namely, the poloidal coils. When the magnetic field established by the poloidal coils forms a quadrupole field, a nearly circular magnetic surface generated by the plasma current is deformed to be elliptical. When the magnetic field established by the poloidal coils is a hexapole field, the section of the plasma is to be triangular. If the magnetic field established by the poloidal coils is an octupole field, the change in the shape of the section comes to have a quadrilateral component. In general, the relationship between a multipole magnetic field and a sectional shape holds. When the multipole magnetic field is the quadrupole, four coils are wound in the lengthwise direction of the toroid and are arranged at intervals of about 90.degree. within the poloidal plane, and the coils adjoining each other are supplied with currents in opposite directions. For the hexapole, six coils are arranged at intervals of 60.degree. , and for the octupole, eight coils are arranged at intervals of 45.degree. . A method of power feed to the coils for attaining the rotating magnetic field is as follows: Alternating currents are supplied to the coils for establishing the multipole magnetic field. The alternating currents to be supplied to the adjacent coils are shifted in phase. A phase difference on this occasion is set at: ##EQU1## Here, 2M indicates the number of the coils, and N indicates the change in the shape to an N-gon. In the configuration of a steady non-rotating magnetic surface, a value equal to N is the lower limit of M. However, in the case where M is equal to N, the phase difference based on the alternating currents becomes 180.degree. in view of the above expression, and the magnetic surface does not rotate. Accordingly, a condition for the rotation is M&gt;N. With the six coils, the hexapole magnetic field can be realized, but the plasma column can be rotated in the case of up to N=2 deformation based on the quadrupole magnetic field, namely, the elliptical section. With the eight coils, the sections of N=2 and 3 deformations can be rotated. Thenceforth, the same applies. Even if the number of the coils is odd, [(the coil number - 1) .div.2]-gonal shape can be rotated in principle. However, the set of even-numbered coils is more advantageous as to the number of power sources and the aspect of wiring because in-phase and anti-phase coils are existent. Since charged particles are bound to the magnetic line of force, the rotation of the magnetic surface drives the plasma column to rotate in accordance with a peristaltic movement in the rotating direction thereof. Regarding the instability of the high-density plasma, the ballooning type wherein on the outermost side of the toroid, the plasma protrudes outwards still more, has the greatest growth rate. According to the present invention, the unstable plasma part is quickly moved to the stable part, so that the instability does not grow thereafter. The growth rate .gamma. of the ballooning instability is approximately indicated by the following expression: ##EQU2## Here, n indicates the density of electrons, v.sub.th the thermal velocity of ions, R the major radius of the toroid, and V a differential operator indicative of a gradient. Letting .omega. denote the rotating angular velocity of the magnetic surface, the period of time in which the magnetic surface rotates between the outer side and inner side of the toroid is .pi./.omega.. Since the growth of the instability may be little meantime, the following may be held: ##EQU3## where ##EQU4## are used for approximation. r denotes the radius of the plasma column. It is indicated by the above expression that the rotating velocity of the magnetic surface is effective when it exceeds the thermal velocity of the ions on the outermost side.