Advanced tokamak research on long time scales in JT-60 Upgrade

The duration of advanced tokamak plasma operation in JT-60 Upgrade (JT-60U) [A. Kitsunezaki , Fusion Sci. Technol. 42, 179 (2002)] has been extended on long time scales exceeding the current diffusion time (tau(R)) and close to the wall saturation time. A very high bootstrap current fraction (f(BS)) of 75% was maintained for 7.4 s (2.7 tau(R)), while a normalized beta of 2.5 was maintained for 15.5 s (similar to 9.5 tau(R)). The current profile reaches stationary conditions in similar to tau(R) for the small f(BS) regime, while a longer time is required for the large f(BS) regime. A plasma with a weak shear q profile, similar to requirements for the ITER steady-state operational scenario, was successfully maintained. The particle inventory in the wall was observed to saturate in repeated long-pulse (similar to 30 s) H-mode discharges with edge localized modes. The analysis of neutral particles in the scrape-of-layer plasma indicates different time scales are involved for the wall saturation. Ferritic steel tiles have been installed in the vacuum vessel to reduce the toroidal field ripple towards extending the advanced tokamak regime further in the coming operational campaign.