Understanding Cold-Pulse Dynamics in Tokamak Plasmas Using Local Turbulent Transport Models*

A long-standing enigma in plasma transport has been resolved by the modeling of cold-pulse experiments conducted on the Alcator C-Mod and DIII-D tokamaks with a local turbulent transport model, TGLF-SAT1 [1]. The model is able to capture the full dynamics of cold-pulse experiments, demonstrating that the existence of nonlocal transport phenomena is not necessary for explaining the behavior and time scales of cold-pulse experiments in tokamak plasmas [2]. Observed for more than twenty years, controlled edge cooling of low-density plasmas triggers a core electron temperature increase on time-scales faster than an energy confinement time, which appear to challenge the local transport paradigm encapsulated in electromagnetic drift-wave turbulent transport models. The quasilinear model TGLF-SAT1 includes a new saturation rule, motivated by recently uncovered cross-scale turbulence coupling, that captures the nonlinear upshift (Dimits shift) of the critical gradient, higher stiffness, and enhanced importance of TEM transport, which are important for reproducing the cold pulse dynamics. For Alcator C-Mod laser blow-off (LBO) cold-pulse experiments, TGLF-SAT1 is able to quantitatively capture the prompt onset of the core electron temperature inversion, with a magnitude that is qualitatively consistent with experimental trends, as well as the disappearance of cold pulse dynamics at high-density. New experiments conducted at DIII-D, guided by predictive analysis to identify plasma conditions that should exhibit temperature inversions, and actuated by a new LBO system, confirm the predicted cold pulse regime and provides evidence of fast density perturbation dynamics using a high-resolution profile reflectometer.
[1] Staebler 2017 Nucl. Fusion 57 066046
[2] Rodriguez-Fernandez 2018 Phys. Rev. Lett. 120 075001