Controllability of large bootstrap current fraction plasmas in JT-60U

摘要

Controllability of plasmas with a large bootstrap current fraction (f_(bs)) 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 (q_(min)) using the motional Stark effect diagnostic. Using the real time control logic, the weak reversed magnetic shear plasma with f_(BS) ～ 70% is sustained for ～8 s. In such a plasma, dynamic change in the current profile, especially q_(min), 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 q_(min) to the change in the ion temperature gradient at the ITB for large f_(BS) plasmas is stronger than that for smaller f_(BS) plasmas.