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

Long-wavelength density fluctuations ( k ρ i <1) are studied using beam emission spectroscopy (BES) at the edge of DIII-D L-mode plasmas ( ρ = 0.88–1.1) in scenarios with electron cyclotron heating (ECH) power ramp ( P ECH up to 1.5 MW), neutral beam injection (NBI) power ramp ( P NBI up to 2.5 MW), and injected torque scan (−1 < T inj <0.6 Nm). We find that broadband turbulent density fluctuations ( f ∼ 20–120 kHz) have a non-Gaussian distribution. The skewness of δ n / n changes sign from negative at ρ <0.95–0.97 to positive at ρ > 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 ⟨ v ~ r n ~ 2 ⟩ is calculated to characterize turbulence spreading at the plasma edge. During ECH/NBI power ramps and at counter- I p injected torque, ⟨ v ~ r n ~ 2 ⟩ 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 ⟨ v ~ r n ~ 2 ⟩ is observed with co- I p torque. A correlation between co- I p torque, turbulence intensity δ n / n at ρ = 0.97, and increased srape-off layer (SOL) heat flux decay length λ q is found in the torque scan scenario, showing that edge turbulence plays a material role in determining the SOL conditions and heat flux width.