Impact of E × B shear flow on low-n MHD instabilities

Abstract

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.

E × Bflow shear drive of the linear low-nmodes of EHO in the QH-mode regime

“…These results are consistent with the observation in recent DIII-D experiments [13] of coherent EHOs at high rotation (and hence high E × B flow shear at the plasma edge) and a transition to broadband electro magnetic turbulence at low rotation. MHD simulations indicate that modes with a broad wavenumber spectrum in the linear simulations usually will evolve into broadband turbulence in the nonlinear simulations due to significantly enhanced modemode couplings compared to a narrow wavenumber spectrum [17,19,27]. Our results are further consistent with the kink/ peeling mode properties revealed in simulations using the ideal-MHD codes ELITE [4] and MINERVA [14], and more recently using MHD codes M3D-C1 [16] and BOUT++ [17,19].…”

“…Previously, the K-H instability was shown to be stabilized in the tokamak plasma edge by strong magnetic shear [18]. However, a most recent BOUT++ simulation [19] indicated that the mode remained unstable even after the K-H drive was turned off artificially. These developments therefore beg the question: how could the E × B flow shear destabilize the kink/peeling mode in the absence of the K-H drive?…”

Curvature of Radial Electric Field Aggravates Edge Magnetohydrodynamics Mode in Toroidally Confined Plasmas

“…Overall, R impacts the growth rate mainly through modulating the cross phase. In summary, R is the proper quantity to quantify the E r profile effect, in contrast to the familiar E r shear or shearing paradigm [19].…”

“…Vortex-kink-ballooning mode.-There is accumulating experimental and simulation evidence indicating that a deeper E r well may facilitate the excitation of a low-n edge MHD mode, e.g., the EHO [3,[19][20][21], other than having a stabilizing effect. These facts point to a reconsideration of the E r and MHD interaction in the edge region, i.e., incorporating the driving effect by E r curvature.…”

E × B flow shear mitigates ballooning-driven edge-localized modes at high collisionality: experiment and simulation

“…The theoretical works predict that E × B shear can affect the magnitude and evolution of the cross phase of the velocityṽ and pressureP fluctuations in the peeling-ballooningmode-driven heat flux. In one instance, this causes the evolution from ELMing H-mode to quiescent (Q) H mode [3][4][5][6]. Experimental results on DIII-D report that the increased E × B velocity shear can also help to excite the edge harmonic oscillations (EHO), which replaces ELMs and provides reduced continuous heat flux to divertor plates.…”