Avoidance of Disruptions due to Vertical Displacement Events via Novel Real-Time Stability Assessment

Disruption avoidance via the DECAF approach has been achieved on KSTAR using a novel real-time vertical stability assessment and a multiactuator feedback control strategy. Development of disruption avoidance strategies with reactor-relevant reliability is an urgent activity, enabling future fusion power plants. The stability metric employed is based on a new formulation of a vertical force gradient balance metric evaluated across the poloidal cross section of the plasma, with parameters tuned using historical data. Evaluation of this metric on a validation set of 400 recent KSTAR shots indicates >82% of VDEs can be avoided via feedback control. Essential to its calculation is the toroidal current density distribution in the plasma. Measurement of this profile faster than fully-converged equilibrium reconstructions can deliver is found to improve forecaster performance and is achieved with a surrogate model that takes as input magnetic diagnostic measurements and outputs the current profile on a basis comprising the top principal components of historical current profiles (from past equilibrium reconstructions). This method solves the non-uniqueness problem typically faced when reconstructing current profiles directly from diagnostics while improving computation time and accuracy. On average, profiles produced by this model reach coefficients of determination of > 0.99 with respect to those from equilibrium reconstructions. The avoidance actuators employed include poloidal field coils and an electron cyclotron heating system. The multiactuator approach is shown in this first demonstration to allow disruption avoidance while minimizing impact to operational performance. This ability, along with its flexibility and speed, make this new approach an attractive option for avoidance of these types of disruptions in reactors. Supported by US DOE Grants DE-SC0020415, DE-SC0021311, and DE-SC0018623