Characterization of Turbulence in Negative Triangularity DIII-D Plasmas using Beam Emission Spectroscopy

Negative Triangularity (NT) shaped plasmas at DIII-D demonstrate high performance, ELM-free operation (H98 > 1, βN ≥ 2.5, fGW > 1) with improved turbulent characteristics which are not fully understood [1]. Turbulence is measured with localized (kyρs < 1), high speed (1 MS/s), multichannel (64 channels), 2D density fluctuation measurements using Beam Emission Spectroscopy (BES) [2]. In strong NT plasmas (δ∼-0.5), low amplitude fluctuations (n ̃/n<0.5%) consistent with Ion Temperature Gradient (ITG) turbulence are observed propagating in the ion-diamagnetic direction for ρ∼0.65-0.85. In the edge (ρ>0.85), modes consistent with Trapped Electron Mode (TEM) turbulence are observed propagating in the electron-diamagnetic direction with reduced fluctuation amplitude at the separatrix. Upcoming experiments are planned to sweep triangularity at constant power from δ=+0.2→-0.2. First, low beam power below the L-H power threshold P_aux<P_LH will keep the plasma in L-mode in Positive Triangularity (PT). Then, high beam power P_aux>P_LH will be used to capture the transitions from H-mode in PT and weak NT to the ELM-free NT-edge. Finally, a mix of NBI and ECH power will be used to alter the balance between ion and electron turbulent modes. An overview of the experimental discharges and the preliminary turbulent results will be presented. BES fluctuation analysis can shed light on how negative triangularity influences turbulent transport, improving our understanding of NT as a core-edge integration solution.

[1] A. Marinoni et al 2021 Nucl. Fusion 61 116010

[2] GR. McKee et al 2010 Rev. Sci. Instrum. 81, 10D741