This picture, 51KB

Fig. 2-1 Experimental demonstration of the production of a 1 MA plasma current by the beam-driven current and the bootstrap current In this experiment we used both the newly-developed N-NB and the existing P-NB. The N-NB is injected from 7 to 7.6 seconds, and as shown in the middle traces, the plasma loop voltage becomes nearly zero during this time interval, and the whole plasma current is sustained without the aid of a transformer, i.e. no induction current exists (bottom traces).

A requirement for the realization of a steady-state tokamak fusion reactor is the development of an innovative technology to drive a DC plasma current in the tokamak, without using the inductive transformer action of the present-day tokamaks. The basic idea is an optimal use of the "bootstrap current," which is driven spontaneously by the plasma behavior in a high temperature and high pressure plasma. To this end, we need to develop a highly reliable operation and control method to stably sustain a high pressure plasma that is favorable for the generation of the bootstrap current. The injection of high energy neutral particle beams has been used effectively to heat up the plasma core and also to control the driven plasma current. However, in the high energy range (above several hundreds of keV) required for a fusion reactor, the "neutralization" efficiency is important, so we should have a new technology to produce neutral particle beams from high energy negative ions, instead of by using the present existing technology based on positive ions (P-NB). With this in mind, we have developed a new neutral beam injection system based on negative ion technology (N-NB). It is capable of producing 10 MW beams of 500 keV particles. As shown in Fig. 2-1 we have successfully demonstrated on JT-60 that a plasma current of 1 MA, driven by externally injected neutral particle beams and the self-induced bootstrap effect, can be sustained for about 0.5 second, without assistance from the conventional transformer action. This is an important step towards the next stage of the research and development of an experimental fusion reactor.

Reference K. Ushigusa, Steady-State Operation Research in JT-60U, 16th IAEA Fusion Energy Conference, Montreal, Canada, IAEA-CN-64/DP-10 (1996).

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