Fig. 2-4 Progress in improvement of tokamak plasma performance expressed in terms of the fusion product Best data have been observed for a short period of time (i.e. transiently) JT-60 (JAERI) JET (EU) TFTR, D III-D (USA) JT-60SU (JAERI, under design) SSTR (JAERI, reference design value) ITER (International cooperation, design value) Break-even plasma condition ; fusion output power balances external input power. Self-ignition condition ; fusion output power is much greater than external input power. A fusion reactor must satisfy this condition to produce net power.

Fig. 2-5 Modes of plasma confinement and ion temperature distribution Standard tokamak discharge; Ordinary operational mode of tokamak H mode (not shown); Confinement is improved compared with standard discharges. A thermal barrier formed at the plasma edge (edge barrier) inhibits heat from escaping. high plasma pressure mode; A thermal barrier formed in the plasma (internal barrier) improves confinement in the central plasma. high plasma pressure H mode; When both the edge and the internal thermal barriers are formed simultaneously, plasma confinement is improved remarkably overall (high confinement operation attained on JT-60).

The confinement of fusion plasma is evaluated in terms of the fusion product defined by; central ion temperature x confinement time x central ion density of confined plasma (see Fig. 2-4). We have attained the world's highest value of the fusion product in JT-60 as shown in Fig. 2-4. The break-even condition has almost been reached in present experiments and the self-ignition condition is targeted by the next step experiment, ITER. Almost equivalent plasma performance was obtained under some different modes of plasma operation having different distributions of ion temperature as shown in Fig. 2-5. A common feature of high plasma pressure observed in these specific confinement modes augurs well for future development, because the property of high plasma pressure is an indispensable feature required both to realize steady-state operation of the tokamak plasma and to build a compact tokamak fusion reactor. The plasma temperature of 480 million degrees centigrade we have attained in JT-60 is almost thirty times higher than the temperature at the center of the sun.

Reference The JT-60 Team, Recent JT-60U Results towards Steady State Operation of Tokamaks, 15th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville, Spain, IAEA-CN-60/A-1-I-2(1994).

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