Dual roles of shear flow in nonlinear multi-scale interactions

Abstract

Effect of shear flow on the multi-scale nonlinear interaction in plasmas is numerically investigated by using a self-consistent Landau-fluid model. Dual roles of shear flow in the process are discovered, significantly suppressing micro-scale fluctuations and dramatically promoting macro-scale fluctuations. Furthermore, its similar dual roles in turbulent transport are also demonstrated. The novel underlying mechanism for the nonlinear promotion is identified as the formation of a large vortex flow inside magnetic island, which as a common phenomenon have been often observed in space and magnetic fusion plasmas. The theoretical prediction on the threshold of shear flow based on an analytical modeling is verified via numerical simulations.

Multiscale interaction between a large scale magnetic island and small scale turbulence

“…The poloidal flow can be important as it will be discussed in next section. In fact, the poloidal shift of the turbulence with respect to the X-point coincides with the direction of the local poloidal flow [9,27,29]. This locality of the island-associated T e turbulence is consistently observed in other experiment [43].…”

Effects of plasma turbulence on the nonlinear evolution of magnetic island in tokamak

“…Measurements of the low- k T e turbulence and the flow shear around an MI in KSTAR experiments revealed that the turbulence is nonlinearly regulated by the flow shear development around the MI and localized to the small region near the X-point. These spatial patterns of the turbulence and the shear flow are in broad agreement with the results from many fluid and gyrokinetic simulations 15 , 16 , 19 , 20 , 22 , 23 . In particular, ref.…”

Effects of plasma turbulence on the nonlinear evolution of magnetic island in tokamak

“…Though it is not shown in this paper, the flow shearing rate across the MI is also sufficiently large (ω E×B ≥ 1.0 × 10 5 [1/s]) to prevent the increased turbulence near the X-point from spreading into the MI in this case [10]. Note that these spatial patterns of the turbulence and the shear flow are in qualitative agreements with the outcomes of the fluid and gyrokinetic simulations [17,18,21,22,24,25].…”

“…Recent tokamak experiments [5,6,7,8,9,10,11,12,13,14,15] and simulations [16,17,18,19,20,21,22,23,24,25,26] found that the MI affects the evolution of the ambient broadband fluctuations (or simply referred as 'plasma turbulence' [27]). Plasma turbulence is significantly altered by the MI itself or a modification of the equilibrium profiles due to its presence.…”