Description
Disruption mitigation remains a critical challenge for future tokamaks, where runaway electron formation during the disruption may lead to severe wall damage (Ratynskaia, 2025). Studies have shown that minimizing the hot-tail seed is crucial for achieving runaway electron mitigation in large tokamaks (Vallhagen, 2024). In this work, we investigate the impact of neutral beam injection (NBI) on the hot-tail runaway electrons during the thermal quench of disruptions in the Tokamak à Configuration Variable (TCV) tokamak.
A neutral beam deposition model has been implemented in the DREAM framework (Hoppe, 2021), accounting for beam geometry, charge-exchange, ionization processes, and energy partition between electrons and ions. The deposition model is benchmarked against analytical calculations (Rome, 1974) and applied to TCV discharges, with free parameters inferred using Bayesian techniques (Järvinen, 2022).
To isolate the effect of NBI, additional power scans are performed in which free parameters are held constant. These scans demonstrate a clear increase in the electron temperature with increasing NBI power, confirming that NBI influences the thermal quench time. The impact on runaway electron generation is studied using both fluid- and kinetic models of the hot-tail mechanism. While fluid simulations show that increasing NBI initially suppresses hot-tail formation, kinetic simulations reveal a later phase characterized by a larger electric field that instead increases runaway electron production. Overall, the net effect of NBI on hot-tail generation is found to be modest, indicating that NBI has little influence on runaway electron generation through the hot-tail mechanism. However, the results show the importance of competing thermal and electric field dynamics in determining runaway production during the thermal quench.