Empirical boundary detection of tearing mode onset at DIII-D

An empirical boundary for the n=1 tearing mode (TM) is developed via data-driven method and verified on thousands of DIII-D shots. The locked n=1 TM is a key precursor leading to disruptions and its predictive ability is strongly desirable for ITER and SPARC. The fitted boundary is a linear function of equilibrium parameters like collisionality, poloidal beta and the MHD risk factor (a combination of the normalized electron temperature profile width, q₉₅ and elongation). The boundary indicates with a value related to the probability of having the TM onset within 200 ms and it achieves ~85% of shot-by-shot accuracy in offline analysis of DIII-D data. Preliminary cross-machine analysis of TM onset prediction shows potential applicability of the empirical boundary to C-Mod and EAST data as well, but the relative importance of the individual parameters is different for different devices. This suggests the existence of different trigger mechanisms for the TMs, implying that the boundary could be generalized using data from various tokamaks representing different trigger mechanisms to improve its extrapolability. Finally, this new proximity metric to the n=1 TM onset has been incorporated into the real-time in DIII-D plasma control system and results from experiments will be discussed.