Steady-state improved confinement studies in the JT-60U tokamak

As a result of dynamic control of the heat deposition and current profile, notably a large fusion product 4×1020 m−3 s keV (equivalent QDT=0.25) was sustained for 1.5 s in the high-confinement mode (H-mode) discharge accompanied with the edge localized modes (ELM), while the highest performance achieved transiently was 1.2×1021 m−3 s keV (equivalent QDT=0.46). A doubly formed transport barrier at the q=3 surface and near the edge is essential for the improvement in confinement. It has also been found that the active shear formation in the toroidal flow velocity, as well as the magnetic shear, can increase the critical edge pressure at the onset of ELMs, which constrain the maximum fusion performance. Further extension to the high-βN (=〈βt〉apBT/Ip) regime yielded the values of βN=2.9, βp=2.6 and the H-factor (=τE/τEITER-89P) of 2.5 for 0.7 s under the full current-drive condition. In these high-βN discharges, the bootstrap component was 74% of the total plasma current before the beam injection, while the neutral beam current drive (NBCD) fraction was 37%. It is stressed that effective current drive in the central region, which can sustain the peaked current profile against the bootstrap component, is indispensable for the efficient steady-state operation. Major results from the confinement physics studies, such as the characteristics of the primary barrier and the turbulence transport across the secondary barrier, are also addressed in this paper.