Improved confinement and transport barrier in the JT-60U high βp H mode

The plasma performance in the JT-60U high βp regime has made significant progress since 1992 (Wuerzburg Conference) as nD(0)τETi(0) = 0.44 → 1.2 x 1021 m-3·s·keV and Sn = 2.8 → 5.6 x 1016 n/s. An energy confinement time of τE = 3.6 x τITER89-PE (τITER89-PE: ITER89-P L mode scaling) and a bootstrap current fraction of 50% were achieved simultaneously. Two transport barriers, i.e. the internal and the edge transport barrier, played an essential role in this improved confinement: auxiliary heating energy was first effectively confined inside the internal transport barrier; the effective confinement volume was expanded by subsequent formation of an edge transport barrier, while the core confinement was kept improved. This sequence is helpful in avoiding localization of the high pressure region, leading to an improved β limit. (author). 11 refs, 10 figs