Fig. 2-8 Non-inductive current driven by high-frequency electromagnetic waves Experimental data of driven current and current drive efficiency are shown versus the input power of microwaves. Current drive product: ICD ¥ ¥ Rp (driven current x average electron density x plasma major radius) Current drive efficiency: current drive product defined above/input power (1019 MA ¥ m-2/MW)

Fig. 2-9 Plasma-driven bootstrap current Plasma-driven bootstrap current increases with the pressure of confined plasma.

Fig. 2-10 Experimental demonstration of quasi-steady-state operation of a reactor-grade plasma in JT-60 Various plasma parameters required for a steady-state tokamak fusion reactor are obtained simultaneously and maintained in an integrated manner for about 0.7 seconds, demonstrating the basic physics concept of the steady-state fusion reactor.

To realize a fusion power reactor of the tokamak-type that is capable of supplying continuous and stable electric power for practical use, a method of continuous, steady-state operation of the tokamak plasma must be developed. In JT-60 we have, from the start, assigned a high priority to the research of this problem. The followings are the key targets of R&D: (1) To drive plasma current non-inductively, or without using transformer action, in contrast to present-day tokamaks, (2) To control the confinement of high pressure plasmas, and (3) To control the exhaust of high heat flux and energetic particles from the plasma. A non-inductive current drive is achieved by applying external, high-frequency electro-magnetic waves and/or high energy particle beams to the tokamak plasma. Good confinement of high pressure plasma and its control are essential to attain a high fusion product as described previously (Fig. 2-4 & Fig. 2-5) and to obtain a significant amount of "bootstrap current" driven by plasma action in the high temperature and high pressure plasma. The bootstrap current saves the externally driven non-inductive plasma current and is expected to be an essential part of the plasma current in a steady-state tokamak reactor. To maintain the reacting core plasma stable in the reactor, extra-heat and impurity particles should be removed efficiently from it (control of exhaust). This is manageable as demonstrated so far using a "divertor", a component composed of a specially deformed magnetic field configuration at the outer edge of the plasma, and divertor plates, that serve as dumps for impurity particles and extra-heat from the reactor-core plasma. As shown in Fig. 2-8 we have demonstrated a non-inductively driven plasma current of 3.6 million amperes by using microwaves in JT-60. A current drive efficiency showing how many amperes are driven in the plasma by a unit input power of microwaves has almost reached a level required for an experimental reactor like ITER. Figure 2-9 shows that the bootstrap portion of current increases as the pressure of the confined plasma increases. All these results support our basic research line toward the steady-state operation of a tokamak. Figure 2-10 shows experimental results from JT-60 that demonstrate a quasi-steady-state operation of the tokamak plasma with good overall performance nearly equivalent to the design values of SSTR (a reference design of a Steady-State Tokamak Reactor at JAERI). The plasma current is fully driven non-inductively by particle beam injection and the bootstrap current. The confinement performance is almost at the same level as that of the SSTR design value. The bottom trace with many spikes shows the luminescent radiation from deuterium atoms around the divertor plates, indicating that the exhaust of heat and particles from the plasma is not excessive and that a good confinement is consequently maintained.

Reference T. Kondoh et al., Large Current Drive Experiments Using Simplified Multijunction Lower Hybrid Wave Launcher in JT-60U, J. At. Energy Soc. Jpn. 37 (2), p.124-132 (1995).

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