Magnetic turbulence suppression by a helical mode in a cylindrical geometry

Abstract

To study processes involved in a helical structure formation in reversed field pinch devices, the scaling of a turbulent boundary layer width associated with a vortex structure having large shears of magnetic field and flow is obtained for reduced magnetohydrodynamics. The coherent vortex, with its flow and magnetic shears, interacts with Alfv en turbulence, forming a turbulent boundary layer at the edge of the vortex. The layer arises from the balance between turbulence diffusion rates and shearing rates and suppresses the turbulence in the structure. The suppression of turbulence impedes relaxation of the coherent vortex profiles, leading to long coherence times. The scaling of the boundary layer width reveals that both magnetic shear and flow shear can effectively suppress magnetic turbulence. V

Thermal transport dynamics in the quasi-single helicity state

“…This scaling is stronger that that inferred in Ref. 11. Because the two calculations represent different aspects of the QSH lifetime the differences are not significant and the two results are qualitatively consistent.…”

“…(19) and (20) on the plasma current, or equivalently the poloidal component of the equilibrium magnetic field B h . This dependence has been described previously, 11,13 but those accounts did not address all sources of B h dependence.…”

“…While this model is extracted from reduced MHD, it should be stressed that it is a heuristic model -useful for characterizing qualitative behavior -but is neither quantitatively accurate nor rigorous. Apart from the obvious shortcoming of reducing the spectrum to two modes, its mode coupling description is heuristic, it does not explicitly address any role played by the m ¼ 0 coupling intermediary, 19 it does not account for mode structure beyond the boundary layer effect of the shear suppression theory, 11 and does not account for 3D effects or toroidal geometry. Despite its shortcomings, it falls within the performance norms of predator-prey models used for characterizing H-mode transitions 20,21 and internal transport barrier dynamics.…”

“…14 In the QSH state, the long-lived structure is the innermost resonant tearing mode. 11 Both the magnetic and flow shears of this mode can suppress the coupling to stable tearing modes resonant at larger radii. This occurs when a shear-induced asymptotic boundary layer becomes smaller than nominal tearing-mode island widths.…”

“…Both are barriers to mode coupling and turbulent transport, both are caused by shearing of fluctuation structures, 10,11 and both are triggered when certain externally controlled quantities surpass a critical threshold. Both can exhibit dithering dynamics with a repeated sequence of transitions and back transitions.…”