JT-60U experimental results aimed at steady-state operation of a tokamak reactor

Recent experiments on JT-60U have been focused on the establishment of the scientific basis of advanced steady-state operation, utilizing the bootstrap current, and accordingly strove for the achievement and sustainment of improved confinement discharges. As a result, a equivalent fusion multiplication factor of QDTeq=1.25 was achieved in a transient state and QDTeq=0.5 was sustained for ∼0.8 s in the reversed shear discharges. It was then shown that active control schemes of stored energy and neutron emission rate were effective for the production of these high performances. The internal transport barrier was sustained for ∼5 s in reversed shear plasma with off-axis LHCD. In the high-βp mode of operation, simultaneous achievement of high confinement, high beta, and full non-inductive current drive with a large fraction of bootstrap current was demonstrated. For the stabilization of the neoclassical tearing mode, the ECRF system was introduced and the injection power into the plasma of 680 kW for 2 s and 250 kW for 5 s was achieved. With the negative-ion based neutral beam injection, the current drive efficiency increased to ∼1.3×1019A/W per m2. The W-shaped divertor with pumping was effective for reducing deuterium recycling, carbon impurity influx, heat load on the target tiles and the H-mode threshold power as well. Effectiveness of impurity gas puffing for the reduction of halo current was also demonstrated, which can mitigate the mechanical fatigue of the tokamak at disruptions.