Dynamics and Stability of Resistive Wall Mode in the JT-60U High-β Plasmas

The dynamics and stability of the resistive wall mode (RWM) are investigated in the JT-60U high-β plasmas, and the sustainment of high-β plasmas above the ideal no-wall β-limit βno-wall N has been demonstrated using a plasma rotation. To suppress an n = 1 RWM that limits the achievable βN in the regime βN > βno-wall N , the plasma rotation is kept larger than the critical rotation. Therefore, we have successfully sustained βN ' 3.0 for ∼ 1 s or βN ' 2.8 for ∼ 5 s. In this high-βN regime, the MHD instabilities relating the RWM, that is, an n = 1 bursting mode and a slowly growing mode, have been observed. These modes sometimes induce the plasmas rotation damping and triggering the RWM. Especially, the slowly growing mode can reduce the rotational shear around the q = 2 surface and finally trigger the RWM. These results shows the rotational shear at the rational surface is also important to determine the RWM stability as well as the plasma rotation speed. Moreover, the plasma rotation braking owing to the RWM have been observed. It is found that the RWM distortion can clearly induce the plasma rotation braking inside the q = 2 surface.