Turbulence Spreading and Density Fluctuation Statistics at the Edge of DIII-D Positive and Negative Triangularity Plasmas

The dynamics of turbulence intensity flux〈V_Rñ〉at the edge of DIII-D positive triangularity (δ>0) L-mode and negative triangularity (NT, δ<0) plasmas, measured using 2D Beam Emission Spectroscopy (BES), demonstrates an inward turbulence spreading inside the separatrix while outside, the turbulence spreading is outward towards the scrape-off layer (SOL). Turbulence spreading from the plasma edge to the SOL is of great interest because this can increase the SOL width, hence distributing the thermal load over a larger area in the divertor. Power spectra of low-wavenumber (k_⊥ρ_i<1) plasma density fluctuations measured with BES show strong broadband fluctuations (f=20-120 kHz for δ>0 and f=20-300 kHz for δ<0) near the edge (ρ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>ρ~0.95) of both ECH+NBI and NBI-only heated plasmas. Radial velocity of turbulent eddy structures, determined using time-resolved velocimetry, shows inward movement of voids and outward movement of blobs. These observations support a theory of blob-void pair formation as the origin of turbulence spreading. Experimental data will be compared to BOUT++ simulations. The correlation between the turbulence intensity flux and the divertor heat flux width λ_q at different levels of heating power is examined.