Patent Number: 048636711
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, an embodiment of the present invention will be described with reference to FIG. 1. The system comprises a toroidal vacuum chamber 1 which is substantially axisymmetric, a coil 2 which generates a magnetic field in a toroidal direction mainly within the vacuum chamber 1, current transformer coils 3 which drive current in the toroidal direction in order to generate and maintain a plasma column 7 within the vacuum chamber 1, and coils 41, 42, 43 and 44 which generate magnetic fields for holding the equilibrium of the plasma 7, as well as magnetic field coils 51, 52, 53, 54, 55, 56, 57 and 58 which serve to cause the rotation of a magnetic surface 81. Electric power is supplied by power sources 61, 62, 63 and 64 for feeding the individual coils 41-44 and 51-58. The current transformer coils 3 serve to induce a magnetic flux change on the center axis of the system. Although they are illustrated as having the air-core setup, it is the same as in the conventional Tokamak system that iron cores may well be employed. The current transformer coils 3 are connected in series with the power source 62, and are fed with pulses. The equilibrium field coils 41, 42, 43 and 44 serve to establish a magnetic field perpendicular to the plane of a plasma ring, and are disposed to be symmetric in the vertical direction. The number of the equilibrium field coils is not always four. In addition, the waveform of power feed from the power source 63 is substantially the same as the waveform of a plasma current, and the coils 41-44 are connected in series. These are the same as in the conventional Tokamak system. The toroidal coil 2 is connected and fed with power similarly to that of the conventional Tokamak system. Although the eight rotating coils 51-58 are illustrated, an elliptical magnetic surface 81 can be rotated even by six coils. A method of power feed in the case of rotating the elliptical magnetic surface with the eight coils will be described. The elliptical magnetic surface 81 is established by a quadrupole magnetic field. In establishing the quadrupole magnetic field, forward and reverse currents of two cycles around the plasma column are necessary. If the eight coils are arranged at substantially equiangular intervals, alternating currents which have a phase shift of: EQU 2.times.360.degree./8=90.degree. between the respectively adjacent coils is supplied. Then, the elliptical magnetic surface is rotated 1/2 revolution in one cycle of the alternation. In order to rotate a triangular magnetic surface by the use of the eight coils, the following measure may be taken: Since a triangular deformation is established by a hexapole magnetic field, alternating currents having a phase difference of: EQU 3.times.360.degree./8=135.degree. between the respectively adjacent coils are supplied, whereby the magnetic surface rotation of 1/3 revolution is achieved in a cycle of the alternation. The relationship among the number of coils, the sectional shape and the magnitude of revolution becomes as follows: Letting the number of coils be 2M, and the sectional shape be an N-gonal shape, the phases of the respectively adjacent coils may be endowed with a difference of: EQU 360.degree..times.N/(2M). If this value is 180.degree. or greater, a magnetic field which attains the desired N-gonal shape at every point of time cannot be generated, or the rotating direction reverses meaninglessly. To avoid this drawback, N must be smaller than M, and the smallest number for the deformation is N=2 corresponding to the elliptical deformation. The magnitude of revolution per alternating cycle for the N-angled shape change is 1/N, and does not depend upon the number of coils. In general, the number of coils may be odd as well. In this case, 2M in the above expression may be substituted by (2M'+1). The power source 64 for the magnetic surface rotating coils supplies alternating currents of various phases. FIG. 2 shows the arrangement of the power source and the method thereof for feeding power to the coils. The power source 64 is composed of a signal generator 65, phase shifters 66, 67 and power amplifiers 68, 69. The signal generator 65 produces an alternating waveform. As the waveform, a sinusoidal wave promises the smoothest operation, but another alternating waveform such as a triangular wave, square wave or saw-tooth wave may well be used. The phase shifters 66 and 67 shift the phases of signals therebetween, making it possible to supply the coils 51-58 with currents of waveforms whose phases shift in succession. In the present example, the phases are shifted every 90.degree. between the adjacent coils in order that the elliptical magnetic surface to-be-changed may be rotated by the eight coils. Therefore, the coils 51 and 55, 52 and 56, 53 and 57, and 54 and 58 become the same phases, respectively. Among these four sets, the set of the coils 51 and 55 and the set of the coils 53 and 57, and the set of the coils 52 and 56 and the set of the coils 54 and 58 become the opposite phases with the phase shifts of 180.degree., respectively. Accordingly, the phase shifters 66 and 67 may provide the signals having the phase shift of 90.degree., and any other phase shifter is unnecessary. In principle, the phase shifter 66 may be directly connected so as to afford null phase difference. In the case where the set of eight coils is arranged at the equal intervals and where the elliptical magnetic surface is rotated, the two phase shifters 66, 67 and the two amplifiers 68, 69 suffice in this manner. The arrangement of connection in the case of generating and rotating a triangular magnetic surface by the use of eight coils is shown in FIG. 3. The phase difference between the respectively adjacent coils is 360.degree..times.3/8=135.degree., so that the coils 51 and 55 have a phase shift of 540.degree., namely, the opposite phases. The coils 52 and 56 and the coils 51 and 55 have opposite phases therebetween, respectively, and the former set lags just 135.degree. over the latter set. In a power source 70, accordingly, the combinations of phase shifters 72-75 and amplifiers 76-79 to be connected to a signal generator 71 are required in four sets. In general cases, the phase shifters and the amplifiers which are equal in number to the coils are sufficient. The alternating current coils may well be non-axisymmetric. An example is a shape in which the coils turn helically around the vacuum chamber 1 as shown in FIG. 4 where the coils are numbered 51'-58'. In this case, the shape of the plasma column 7 does not become axisymmetric, but it becomes a shape twisted at the same pitch as that of the helix. When the helical coils are supplied with alternating currents having phase differences, the twisted plasma column can be rotated. By constructing the plasma confinement system as in the foregoing embodiment, the plasma column can be rotated, so that the unstable plasma part is quickly moved to the stable part, and the instability of the plasma can be eliminated. In addition, heat load concentration ascribable to a diverter flux is simultaneously solved by the rotation of a heat flux inflow position during the rotation of the magnetic surface. More specifically, the rotating coils are arranged so that a magnetic neutral point may be formed during the formation of the deformed magnetic surface, and the neutral point is rotated simultaneously with the movement of the whole magnetic surface during the feed of the alternating currents. Then, the diverter heat load is distributed on the entire inside surface of the chamber and is not locally concentrated. When the magnetic surface is rotated, the magnetic neutral point (separatrix configuration) is simultaneously rotated. Therefore, an impurity particle flux to be separated and emitted can be prevented from continuing to enter a fixed position. When the plasma is rotated, centrifugal forces act on particles, and heavy ions move toward the exterior of the plasma. The centrifugation is effective when the thermal velocities of ions exceed the rotating velocity of the plasma. In the nuclear fusion plasma, solely heavy hydrogen and tritium play the principal roles, and all the other particles are impurities. Since heavy impurities have low thermal velocities, the rotation of the magnetic surface can serve the separation and emission of the impurities more effectively. In other words, the impurity inflow flux can be expanded over the entire inside surface of the chamber. Since the mean inflow flux per unit area decreases to about 1/5, the lifetime of the chamber against sputtering becomes 5 times longer. According to the present invention, the period of time for which a plasma column faces the outer side of a toroid accounts for about 1/2 of the whole period of time, so that the growth rate of instability becomes 1/2. Accordingly, the energy confinement time is approximately doubled. In a nuclear fusion apparatus conforming to the system of the invention, the merit appears as a reactivity increment, and the invention is effective to raise the efficiency of the overall plant.