Density fluctuation statistics and turbulence spreading at the edge of L–mode plasmas

Long-wavelength density fluctuations ( <1) are studied using beam emission spectroscopy (BES) at the edge of DIII-D L-mode plasmas ( = 0.88–1.1) in scenarios with electron cyclotron heating (ECH) power ramp ( up to 1.5 MW), neutral beam injection (NBI) power ramp ( up to 2.5 MW), and injected torque scan (−1 < <0.6 Nm). We find that broadband turbulent density fluctuations ( ∼ 20–120 kHz) have a non-Gaussian distribution. The skewness of changes sign from negative at <0.95–0.97 to positive at > 0.97, indicating the prevalence of density 'voids' at inner radii and density 'blobs' at outer radii and outside of the separatrix. The turbulence intensity flux is calculated to characterize turbulence spreading at the plasma edge. During ECH/NBI power ramps and at counter- injected torque, is directed inward inside the separatrix, which is evidence of inward spreading of turbulence intensity from the edge gradient region caused by the inner propagation of density 'voids'. Significantly weaker is observed with co- torque. A correlation between co- torque, turbulence intensity at = 0.97, and increased srape-off layer (SOL) heat flux decay length is found in the torque scan scenario, showing that edge turbulence plays a material role in determining the SOL conditions and heat flux width.