Two-dimensional density evolution local to the inversion layer during sawtooth crash using Beam Emission Spectroscopy at DIII-D tokamak

We present methods for analyzing light fluctuations measured by Beam Emission Spectroscopy (BES) data to obtain the plasma density evolution in a 2-D plane associated with rapid sawtooth crash events at the DIII-D tokamak. A method is developed to remove sawtooth induced edge light pulse from the BES data, as intense D_α  emission produced by edge recycling during sawtooth events makes traditional spectroscopic filtering and data analysis techniques insufficient. The cross-calibration of 64 BES channels is checked using a novel technique to ensure accurate measurements. The relationship between large-amplitude light fluctuation, δI/I₀, and density variation, δn_i/n_i0, is discussed for these intense sawtooth oscillations. 2-D images of δn_i/n_i0 show a significant spatial variation across an 8 cm (radial) × 20 cm (poloidal) area spanning the sawtooth inversion layer. Multiple density oscillations at ~13 kHz are observed at each sawtooth event that are synchronized with magnetic field fluctuations observed with edge magnetic probes. Density has been observed to spatially vary from ∼ 3.6 to 6.3 × 10¹³ cm^-3 about the sawtooth inversion region at the latter end of the crash [Bose et. al. to be submitted to Rev. Sci. Instrum. (2021)].