Statistics of Edge Turbulence Near the Density Limit in DIII-D Plasmas

The Greenwald limit describes an empirical cutoff for tokamak plasma density where radiative collapse and disruptions occur when exceeded [1]. Recent theoretical models propose different mechanisms for this limit [2, 3]. This work analyzes Beam Emission Spectroscopy (BES) data from near density limit L-mode discharges at DIII-D. Spectral and cross-correlation analysis are used to study turbulence characteristics. Near the Greenwald limit, plasma edge density fluctuations (f~20-130 kHz) abruptly increase and then the peak and skewness zero-crossing of the normalized fluctuation amplitude δn/n move radially inward from ρ~0.95-0.98 to ρ~0.85 [4, 5], with skewness changing from negative inwards of this radius to positive outwards, consistent with an outward turbulent radial particle flux. At higher density, increasing kurtosis at the separatrix indicates an increased number of large-scale structures. Velocimetry is used to reconstruct turbulent velocity fields, and the statistical properties (skewness, kurtosis) are calculated for comparisons with simulation and theory.

[1] M. Greenwald et al., 1988 Nucl. Fusion, 28, 2199

[2] R. Singh and P.H. Diamond, 2022 Plasma Phys. Control. Fusion, 64, 084004

[3] M. Giacomin et al., 2022 Phys. Rev. Lett., 128, 185003

[4] Z. Yan et al., P.130, 2024 US-EU Transport Task Force Workshop, Asheville, NC

[5] F. Khabanov et al., 2025 US Transport Task Force Workshop, Seattle, WA