Abstract
Controllability of plasmas with a large bootstrap current fraction (fBS) has been investigated in JT-60U. Real time control logic for avoidance of collapses by pressure profile control through the toroidal rotation has been newly installed for the long sustainment of reversed magnetic shear plasmas. The new real time control logic can control the injection timing of the neutral beam based on the real time detection of the minimum value of safety factor (qmin) using the motional Stark effect diagnostic. Using the real time control logic, the weak reversed magnetic shear plasma with fBS ∼ 70% is sustained for ∼8 s. In such a plasma, dynamic change in the current profile, especially qmin, which was induced by the change in the pressure profile at the internal transport barrier (ITB) through the rotation control, was observed. The response of qmin to the change in the ion temperature gradient at the ITB for large fBS plasmas is stronger than that for smaller fBS plasmas.
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