Investigation of Turbulence Properties in Negative Triangularity Plasmas on DIII-D using Beam Emission Spectroscopy

Negative triangularity (NT) shaped plasmas demonstrate ELM-free plasmas with H₉₈ > 1, β_N ≥ 2.5, and f_GW >1. Turbulence characteristics are measured with localized (k_yρ_s <1), high speed (1 MS/s), 2D (64 channels) density fluctuation measurements using Beam Emission Spectroscopy (BES). Turbulence amplitudes in NT plasmas peak just inside the separatrix, similar to positive triangularity L-mode, but with low core turbulence amplitudes of ñ/n~0.5%, similar to positive triangularity H-modes. Scanning triangularity from -0.01 to -0.12 at fixed power input induces an H-mode to ELM-free NT-edge transition, however turbulence amplitudes decrease from ñ/n of 13% to 7% in the edge and from 0.8% to <0.2% inside ρ≈0.85. Increasing co-beam injected power tends to induce a radially broadened turbulence amplitude profile with lower peak values. Poloidal correlation lengths, decorrelation times and poloidal velocities indicate an electron diamagnetic directed mode with exceptionally broad wavenumber spectrum from k_yρ_s<0.05 to k_yρ_s>0.5 peaked near ρ≈0.95. 2D BES velocimetry is used to assess poloidal flow spectrum and particle flux <Ṽ_rñ>. H-mode like confinement in negative triangularity is achieved by effective turbulence reduction in the core while facilitating an ELM-free edge.