In tokamak type magnetic confinement fusion devices, the spiral magnetic field lines created by the toroidal coil current and the plasma current confine high-temperature plasmas up to several hundred million degrees centigrade. In the operation of a conventional tokamak, the plasma current is generated by electromagnetic induction; the change of the central solenoid (CS) coil current (primary current) induces the plasma current (secondary current). However, the maximum CS coil current limits the duration time of plasma current, and therefore only intermittent operation is possible (Fig. 2-1).
A fully noninductive operational method of generating and sustaining current without using CS coils has been developed in the large tokamak device JT-60 (Fig. 2-2). With this method, only the poloidal coils, for adjusting the plasma position, and rf wave injection were used to generate the plasma and ramp up the plasma current. High-power neutral beams were injected, which generated the beam driven current and increased the plasma temperature. The high plasma temperature has improved the current drive efficiency of both the beam driven current and rf driven current with respect to the injected power. The large pressure gradient due to the intensive heating has also generated the bootstrap current.
Intensive studies on the above operation have resulted in the full noninductive generation and sustainment of high-temperature plasma (70 million degrees centigrade) (Fig. 2-3). About 90% of the plasma current is bootstrap current and the performance of the plasma confinement is equivalent to the target value of a future fusion reactor. These experimental results offer a very promising prospect for the continuous operation of fusion plasmas.
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