Experimental Turbulence and Zonal Flow Characteristics in DIII-D Negative Triangularity Plasmas

Negative Triangularity (NT) shaped plasmas at DIII-D demonstrate high performance (H₉₈ > 1, β_N ≥ 2.5, f_GW > 1), ELM-free operation for P_aux>P_LH with an NT-edge when δ_avg<-0.2. The mechanisms of the improved global confinement and unique edge transport properties are not well understood [1]. Dedicated triangularity scans (δ_up=+0.41→-0.43;δ_bot=0.1-0.0) with P_aux>P_LH are performed to investigate the NT turbulence and confinement characteristics. Ion scale (k_θρ_s < 1, f<500 kHz) turbulence is measured using the Beam Emission Spectroscopy fluctuation diagnostic [2]. TEM-like turbulence is observed outside of ρ>0.9, while ITG-like turbulence is observed in the core region ρ<0.9 [3]. Core turbulence amplitudes are comparable in H-mode and NT-edge phases, while measured turbulent correlation lengths and times vary. When δ_up is more negative, the core normalized T_i gradients are increased concomitant with the observed reduction in random-walk turbulent diffusivity (L_corr²/τ_corr). Velocimetry analysis of NT-edge phases with P_aux>P_LH shows no zonal flow structures and indications that edge turbulence (ρ>0.9) is saturated via mean shear flow. In contrast, in L-mode phases (P_aux<P_LH ) zonal flows are observed and mean shearing rates are insufficient for saturation. Unlike in L-mode, NT-edge turbulence at P_aux>P_LH may remain shear flow saturated after the H-mode/NT-edge transition.