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

Long-wavelength density fluctuations (k rho(i) <1) are studied using beam emission spectroscopy (BES) at the edge of DIII-D L-mode plasmas (rho = 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 similar to 20-120 kHz) have a non-Gaussian distribution. The skewness of delta n/n changes sign from negative at rho <0.95-0.97 to positive at rho > 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)) over tilde(n2) over tilde > is calculated to characterize turbulence spreading at the plasma edge. During ECH/NBI power ramps and at counter-Ip injected torque, <(v(r)) over tilde(n2) over tilde > 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)) over tilde(n2) over tilde > is observed with co-I-p torque. A correlation between co-Ip torque, turbulence intensity delta n/n at rho = 0.97, and increased srape-off layer (SOL) heat flux decay length lambda(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.