Theory and 3D fluid simulations of turbulences in detached SOL plasmas

In this paper, we present theory and 3D simulations of scrape-off-layer (SOL) turbulences in detached plasmas. The detached plasmas are characterized as rapid variation of the equilibrium plasma profiles along the magnetic field line between the x-point and the divertor. Our analysis shows that the steep parallel gradients near the x-point might play the same role as surface impedance in the plasma sheath and thus drive conducting-wall-like mode or axial shear mode. The 3D Fluid Simulations of edge turbulence in the electrostatic limit is described by a four-field model of the reduced Braginskii equations in a model x-point geometry, including drives for (1) conducting-wall mode, (2) Kelvin-Holmhotz mode, (3) curvature-driven mode, (4) Δxvparallel, and (5) dissipative axial shear mode. For uniform axial equilibrium profiles, we have obtained a flute mode structure along the field line. Fluctuating amplitudes for eφ/Te ≥ n/n0 ≥ Te/Te ∼ 0.2, and diffusivities χe ∼De ∼ 2 m2/s, are comparable to 2D results. We also find Vi/Cs ∼ Te/Te and the momentum diffusivity Χm ≅ De/2; both are peaked at the sheath entrances. In a detached plasma with Te0/Ted = 50 and Ne0/Ned = 0.2, and without magnetic shear, we find that either a conducting-wall-like mode or dissipative axial shear mode is the dominant drive even if turbulence drive from the sheath is small. The axial fluctuation profiles between x-points are approximately flute. The relative fluctuating amplitudes are comparable to the case for uniform axial equilibrium profiles. But diffusivities are much higher. The magnetic shear effect and transport scaling for detached regimes are discussed