Comparing local plasma density and temperature evolution during sawtooth crash events in DIII-D

The sawtooth crash in fusion plasmas leads to a fast drop in core electron temperature, and several models have been proposed for the fast temperature relaxation, including fast magnetic reconnection provided by two-fluid effects or the plasmoid instability, or growth of higher-mode-number pressure driven instabilities. Experiments were conducted at DIII-D through the Frontier Science program where the Beam Emission Spectroscopy (BES) system was used for 2-D localized density measurements near the inversion layer (q=1 surface) during sawtooth. We analyzed the BES data by doing channel cross-calibration and subtraction of edge light [Bose et. al. to be submitted to Rev. Sci. Instrum. (2021)] to obtain time-domain movies of the plasma density evolution. The observations show that a (1,1) structure with high plasma density grows just before the temperature crashes and persists for a few plasma rotations afterward. We correlated the evolution of localized plasma density (BES), temperature (ECEI), and magnetic field (from external magnetics) in time to understand the physics of sawtooth crash. Initial MHD simulation results to complement the experimental finding are presented.