Operational space assessment of vertical controllability using DECAF, and predictive capability of a vertical stability metric for tokamak plasmas*

Vertical displacement events (VDEs) in tokamaks involve large displacements of the plasma magnetic axis from the vessel midplane, often leading to disruptions. These events are of particular concern for their potential to cause damage to plasma-facing components, as well as large forces on the vessel due to halo currents generated during the disruption that run through the plasma and vessel. Detection and control of these events and mitigation or avoidance of a potential disruption is crucial. We present the results of an operational space analysis for defining regimes of vertical position controllability, compared across the NSTX, KSTAR, and MAST-Upgrade tokamaks using the DECAF approach [1]. Identification of the vertically controllable regime based on in-situ data is demonstrated to improve the accuracy of automated vertical displacement event detection to more than 99% for each of the devices studied. These findings can inform the setting of warning levels in real-time plasma control and disruption mitigation systems. Further, we present initial results of a vertical instability warning metric, indicating the potential for this metric to forecast vertical displacement events and trigger disruption avoidance procedures in a plasma control system. This research was supported by the U.S. Department of Energy under grants DE-SC0020415, DE-SC0021311, and DE-SC0018623.

[1] S.A. Sabbagh, et al., Phys. Plasmas 30, 032506 (2023). https://doi.org/10.1063/5.0133825