Torque balance analysis of rotating MHD for disruption prediction and avoidance in KSTAR*

Reactor scale tokamak devices require a low disruptivity rate to be a viable fusion energy producing system. An important precursor to disruptions is the presence of rotating MHD events that are often neoclassical tearing modes (NTM). Through electromagnetic and fluid drag torques, NTMs with a saturated island width can cause slowing of the mode and plasma rotation and lock it to the wall reference frame. A balance of the driving torque from the NBI, drag from perpendicular viscous diffusion drag and electromagnetic forces on the mode, and its inertia is used to model the rotation dynamics. Threshold rotation frequencies are derived from this model below which the mode rotation is expected to lead to a locking, serving as a disruption forecaster. Mode identification is computed most accurately by Fourier analysis of a toroidal array of magnetic probes or using simpler approaches more generally amenable to real-time calculation. From the rotation, the torque components are then calculated based on conditions for the expected drag torque ratios at the mode onset, changes in frequency, and Mirnov signal amplitudes. This technique will be employed for offline and real-time analysis of KSTAR plasmas with potential to signal use of active control or disruption mitigation systems