DECAF mode locking forecaster based on MHD mode torque balance and island size threshold*

Tokamak experimental fusion reactors need to avoid major disruptions to be commercially viable. One phenomenon that can lead to plasma disruptions is called a neoclassical tearing mode (NTM). NTMs are rotating disturbances in the plasma, but they can slow down and lock to the lab frame due to fluid and electromagnetic forces that drag their rotation to that reference frame. When these modes lock, they can cause excessive device metal fatigue due to impulsive electromagnetic forces and material damage to the first wall or other device components due to non-axisymmetric heat loads and/or runaway electrons, so predicting and avoiding locked modes is crucial for successful tokamak operation. A forecasting model has been developed that calculates the balance of torques that result in maintaining steady-state mode rotation. If the input torque is inadequate, the model forecasts a frequency threshold below which a rotating mode is expected to lock. Performance of this forecaster has been tested across devices of varying aspect ratio and error field conditions. Analysis has been carried out across the KSTAR, NSTX, and MAST-U databases to see how well the model predictions match the observation of disruptions across the machines. Furthermore, a threshold criteria was developed to track the changing volume of the magnetic island that contributes to the warning level in forecasting a mode lock. The size of the island increases until saturation, with the mode field perturbation interacting with the device error field and conducting structure. Therefore, it can be used to validate the loss of torque balance measured from the mode rotation dynamics.