Disruption event chain analysis of high-radiation and high-density triggered disruptions in NSTX and MAST-U

The tokamak density limit represents an upper threshold on a critical plasma parameter that must be optimized to maximize fusion energy output without disruption. Many theoretical and empirical models seek to predict density limit disruptions, but none are universally applicable across tokamak configurations. The common thread through disruptive events associated with density limit disruptions (e.g. local cooling, increased radiation or transport, H-L back transition) is loss of energy balance. We hypothesize that high density causes disruptions via multiple physical mechanisms, each exhibiting distinct event chains triggered by local or global energy balance loss. We examine entire tokamak databases using the DECAF (Disruption Event Characterization and Forecasting) code [1] to build event chains in disruptive shots characterized by high density and radiation, focusing on spherical tokamaks NSTX and MAST-U, and comparing with conventional tokamak KSTAR. This multi-machine analysis reveals various event sequences leading to disruption when triggered by elevated density and radiation, demonstrating that multi-physics prediction models are required. Results identify the most common event sequences leading to disruption and their variations across different tokamak configurations. These event chains establish the foundation for developing improved physics-based disruption prediction models through this novel event chain methodology while enhancing our fundamental understanding of the underlying physical mechanisms.

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