Fig. 2-1 Progress of plasma performance Typical experimental data obtained were; ion temperature of 190 million degrees, ion density of 48 trillion ions/cm3, confinement time of 1.1 seconds, plasma current of 2.6 million amperes, and plasma volume of 53 m3. The plasma current and the volume were about 80% and 50% respectively compared with those values of JET of equivalent plasma performance, indicating that the size of the reactor core plasma can be made smaller.

Fig. 2-2 Plasma performance expressed in terms of QDT and impurity content in the plasma Plasma performance has a marked dependence on the impurity content in the plasma. Effective charge number is a figure-of-merit of the impurity content in the plasma, namely the effective charge number is unity if the hydrogen plasma contains no impurities.

To further the development of a steady-state tokamak fusion reactor, confinement experiments on JT-60 have been conducted with a new W-shaped divertor installed in 1997, which has the same concept as the divertor design of ITER, a next generation experimental reactor. A divertor is required to remove impurities and extra-heat from the reactor core, and is composed of a diverted confinement (resulting from the form of the magnetic field at the plasma boundary) and some hardware components such as divertor plates. The major objective of the experiments was to demonstrate further enhancement of plasma performance using a confinement scheme termed the "negative magnetic shear mode" which was developed successfully in JT-60. In a series of experiments in June 1998, we accomplished a very high plasma performance in deuterium plasma in JT-60 that had an equivalent fusion energy multiplication factor of QDT=1.25, the world highest value exceeding the previous record of JET (EU), QDT=1.14 (Fig. 2-1). The fusion energy multiplication factor, QDT is defined as the ratio of fusion output power to input power for plasma heating, and is evaluated here for a plasma composed of a 50-50 mixture of deuterium and tritium equivalent to the actual deuterium plasma of the experiment. This result was obtained by deliberate experiments combining the new W-shaped pumped divertor which was designed to further reduce impurities in the main plasma, and the negative shear confinement scheme that has demonstrated excellent characteristics to confine high temperature plasmas, and it will surely step up the R&D for a steady-state tokamak fusion reactor at JAERI. Figure 2-2 summarizes the experimental results showing clearly that the fusion energy multiplication increases as the plasma impurities decrease.

Reference S. Ishida et al., JT-60U High Performance Regimes, Nucl. Fusion, 39, 1211 (1999).

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