Development of advanced operation scenarios in weak magnetic-shear regime on JT-60U

Fully non-inductive discharge having a relaxed current profile and a high bootstrap current fraction fBS = 0.5 has been realized in the high-βp ELMy H-mode discharge with weak magnetic-shear having q95 = 5.8, qmin = 2.1 and q(0) = 2.4, where qmin and q(0) are the safety factor q at the minimum and the plasma centre, respectively. The rest of the plasma current is externally driven by neutral beams and lower-hybrid waves. The safety factor profile evaluated by the motional Stark effect diagnostic is kept unchanged for 0.7 s at the end of the full current drive (CD) sustainment for 2 s (1.5 times the current relaxation time τR). The loop voltage profile is spatially uniform at 0 V at the end of the sustainment. This demonstration shows, for the first time, that the steady sustainment of full-CD plasma is possible at high fBS = 0.5 and reasonably low q95 = 5.8 regime and is stably controlled by appropriate external current drivers. On the other hand, when the combination of bootstrap current and externally driven current does not match the steady current profile, a slight change in the current profile due to current relaxation resulted in the appearance of magnetohydrodynamic instability, e.g. neo-classical tearing mode, in a high-beta plasma. This discharge clarifies the importance of the demonstration of steady current profile developed here. These discharges contribute to the ITER steady-state operation scenario development.