Latest progress in steady state plasma research on the Japan Atomic Energy Research Institute Tokamak-60 Upgrade

The JT-60U [Y. Kusama and the JT-60 team, Phys. Plasmas 6, 1935 (1999)] high performance plasmas, such as high poloidal-beta high-confinement mode (high βp H-mode) and reversed magnetic shear (RS) plasmas, have been optimized towards a regime of steady state operation concerning high confinement, high β and high bootstrap current fraction which are requirements of a steady state fusion plasma. What is essential to keep improved confinement in these plasmas is to maintain the internal and/or the edge transport barriers (ITB and/or ETB). A key issue to achieve non-inductive current drive relevant to a steady state fusion reactor is to increase the fraction of the bootstrap current and match the spatial profile to the optimum. In JT-60U, RS plasmas have been optimized not only for the high performance but also for reactor relevant steady state research. In 1999, as the result of the optimization, the equivalent deuterium–tritium (D–T) fusion gain (QDTeq) of 0.5 was sustained for 0.8 s, which is roughly equal to the energy confinement time, by utilizing feedback control of the stored energy. Furthermore, in a RS plasma with H-mode edge, a confinement enhancement factor of 3.6 was maintained for 2.7 s with a large bootstrap current fraction. In addition to the existing radio frequency and the negative-ion based neutral beam heating systems, the newly installed electron cyclotron range of frequency system extended the improved confinement study to a regime where the electron temperature is higher than the ion temperature. The required power to obtain the internal transport barrier in a RS plasma was found not to be sensitive to the toroidal magnetic field. The regime of improved confinement was extended to higher electron density using argon puffing.