Real-time locking dynamics analysis of rotating MHD for disruption prediction and avoidance in KSTAR

Tokamak reactors require low disruptivity to support commercial viability. An important precursor to disruptions is the locking dynamic of rotating MHD events that are often neoclassical tearing modes (NTM). The drag of electromagnetic and fluid viscosity torques can cause the slowing down of NTM's and lock them to device conducting structure. A balance of the driving torque from the NBI, and drag from perpendicular viscous diffusion drag and electromagnetic forces on the mode, as well as its inertia, are used to model the mode rotation dynamics. Rotation frequencies below which the mode rotation is expected to lead to locking serve 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 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 amplitude. The technique is employed for database and real-time analysis of KSTAR plasmas. Database analysis is carried out to validate the model. In real-time, the forecaster was used to trigger controlled shut-down or disruption mitigation systems.