Effect of local E×B flow shear on the stability of magnetic islands in tokamak plasmas

Abstract

The influence of local E ϫ B flow shear on a relatively wide, constant-, magnetic island embedded in a large-aspect-ratio, low-␤, circular cross-section tokamak plasma is examined, using a slab approximation to model the magnetic geometry. It is found that there are three separate solution branches characterized by low, intermediate, and high values of the shear. Flow shear is found to have a stabilizing effect on island solutions lying on the low and high shear branches, via a nonlinear modification of the ion polarization term in the Rutherford island width evolution equation, but to have a destabilizing effect on solutions lying on the intermediate shear branch. Moreover, the effect is independent of the sign of the shear. The modification of island stability by local E ϫ B flow shear is found to peak when the magnitude of the shear is approximately v i / L s , where v i is the ion thermal velocity, and L s the magnetic shear length.

Control of neo-classical double tearing modes by differential poloidal rotation in reversed magnetic shear tokamak plasmas

“…Ofman et al and Chen et al have shown that the shear flow could effectively decrease the saturated magnetic island width when the velocity of the shear flow is below the Alfvén speed [20,21]. Fitzpatrick and Waelbroeck examined the influence of the local × E B flow shear on the magnetic island [23], in which the stabilizing effect of velocity shear is in accordance with the previous result in [20,21]. Chandra et al investigated the influence of the toroidal flow on the tearing mode instability in toroidal geometry, indicating that the toroidal flow has a stabilizing effect due to the modification in the q-profile arising from a flow induced 'Shafranov' like shift [24,25].…”

Interaction of turbulence with magnetic islands: effect on bootstrap current

“…Furthermore, it is thought that turbulence will have a significant effect on the stability of the mode [6,7,8]. The current profile can be modified by the presence of the island itself and flattening of density and temperature profiles in island vicinity.…”

“…Contributions to the current density come from various sources, including the induced current [2], bootstrap current [3,4] and polarization current [5]. Furthermore, it is thought that turbulence will have a significant effect on the stability of the mode [6][7][8]. The current profile can be modified by the presence of the island itself and flattening of density and temperature profiles in island vicinity.…”

Effect of local toroidal flow on double‐tearing modes in cylindrical geometry

“…[17] On the other hand, the toroidal plasma flow can delay the evolution of DTMs, [28] and the suppression effect depends on the magnitude of flow velocity and its shear rate on the rational surfaces. [29][30][31][32] The toroidal flows can affect the dynamics of magnetic islands through the coupling of magnetic curvature with pressure and density. [33] In order to investigate the influence of local toroidal flow (LTF) on DTMs, the reduced MHD (RMHD) simulation method is used to study the evolution of DTMs in various LTF cases in this work.…”

“…[ 17 ] On the other hand, the toroidal plasma flow can delay the evolution of DTMs, [ 28 ] and the suppression effect depends on the magnitude of flow velocity and its shear rate on the rational surfaces. [ 29–32 ] The toroidal flows can affect the dynamics of magnetic islands through the coupling of magnetic curvature with pressure and density. [ 33 ]…”