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 (QeqDT) 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. © 2000 American Institute of Physics.