Recently, the stationary high confinement operations with improved pedestal conditions have been achieved in DIII-D [K. H. Burrell , Phys. Plasmas, 056103 (2016)], accompanying the spontaneous transition from the coherent edge harmonic oscillation (EHO) to the broadband MHD turbulence state by reducing the neutral beam injection torque to zero. It is highly significant for the burning plasma devices such as ITER. Simulations about the effects of × shear flow on the quiescent H-mode (QH-mode) are carried out using the three-field two-fluid model in the field-aligned coordinate under the BOUT++ framework. Using the shifted circular cross-section equilibriums including bootstrap current, the results demonstrate that the × shear flow strongly destabilizes low-n peeling modes, which are mainly driven by the gradient of parallel current in peeling-dominant cases and are sensitive to the shear. Adopting the much more general shape of × shear ([Formula: see text]) profiles, the linear and nonlinear BOUT++ simulations show qualitative consistence with the experiments. The stronger shear flow shifts the most unstable mode to lower-n and narrows the mode spectrum. At the meantime, the nonlinear simulations of the QH-mode indicate that the shear flow in both co- and counter directions of diamagnetic flow has some similar effects. The nonlinear mode interaction is enhanced during the mode amplitude saturation phase. These results reveal that the fundamental physics mechanism of the QH-mode may be shear flow and are significant for understanding the mechanism of EHO and QH-mode.

We investigate how the E × B shearing profile impacts the energy transport induced by the MHD mode in the edge of H-mode plasmas. By enhancing the outer layer (ψnor ≈ 0.9–1) shear, the growth rate spectrum γ(n) gets narrowed with the fastest growth rate having a lower toroidal mode number. In the nonlinear stage, the MHD fluctuations tend to reach a coherent, oscillating state, which is analogous to a quiescent(Q) H mode state with edge-harmonic-oscillations. By enhancing the inner layer (ψnor ≈ 0.7–0.8) shear, γ(n) shifts to lower n, and its width does not change much. In the nonlinear stage, the edge fluctuations tend to stay in an incoherent, oscillating state, which is analogous to a turbulent QH-mode state. We also explore the underlying physics of the results and show that they are closely related to the different cross-phase dynamics of the edge MHD fluctuations induced by the variation of the edge E × B shearing profile.

Results presented here are from 6-field Landau-Fluid simulations using shifted circular cross-section tokamak equilibria on BOUT++ framework. Linear benchmark results imply that the collisional and collisionless Landau resonance closures make a little difference on linear growth rate spectra which are quite close to the results with the flux limited Spitzer-Härm parallel flux. Both linear and nonlinear simulations show that the plasma current profile plays dual roles on the peeling-ballooning modes that it can drive the low-n peeling modes and stabilize the high-n ballooning modes. For fixed total pressure and current, as the pedestal current decreases due to the bootstrap current which becomes smaller when the density (collisionality) increases, the operational point is shifted downwards vertically in the Jped – α diagram, resulting in threshold changes of different modes. The bootstrap current can slightly increase radial turbulence spreading range and enhance the energy and particle transports by increasing the perturbed amplitude and broadening cross-phase frequency distribution.

The corner roll (CR) in the Rayleigh-Bénard convection accounts for the behaviors of convection flow and heat transport in the corner. The three-dimensional direct numerical simulations for 108 < Ra < 5 × 109 show that CR presents a well-defined similarity and multi-layer structure. The CR can be described by a stream function derived from the homotopy and the structure ensemble dynamics. The scale of CR has a negative scaling r = 0.770Ra−0.085. A new scaling theory for CR suggests the scalings for Recr ∼ Ra0.248 and Nucr ∼ Ra1/3 very close to the simulated ones.