Density Fluctuation Statistics and Turbulence Spreading Dynamics at the Edge-SOL Interface on DIII-D

Turbulence properties of long-wavelength ion gyro-scale ( kρ_i< 1) density fluctuations are investigated at the edge of DIII-D L-mode plasmas (ρ=0.88-1.1) using 2D Beam Emission Spectroscopy (BES) and advanced imaging analyses. Turbulence intensity flux〈~V_R~n〉was calculated using BES velocimetry in the region 0.9<ρ<1.1 for the first time at DIII-D to characterize turbulence spreading. Radial profiles of turbulence intensity flux 〈~V_R~n〉show that it is directed inward inside the separatrix, which demonstrates inward spreading of turbulence from the edge plasma region towards the core. The observation of positive and negative skewness of δn/n supports a theory of ‘blob-void’ pair formation as the origin of turbulence spreading: inward spreading is associated with the inward movement of negative density fluctuations, or ‘voids’.

Turbulence spreading from the plasma edge to the SOL is of great interest because this can increase the SOL width, hence lowering the heat flux to the divertor. Handling divertor heat loads is crucial for the safe operation of plasma-facing components in future devices like ITER.

In L-mode plasmas significantly weaker inward spreading is observed with co-I_p neutral beam torque. During the torque scan, a strong correlation is observed between the power scrape-off length λ_q, injected torque T_inj and turbulence amplitude δn/n(ρ= 0.97), showing that edge turbulence plays a material role in determining the SOL conditions and heat flux width.