Turbulence driven magnetic reconnection causing long-wavelength magnetic islands

Abstract

Magnetic reconnection caused by turbulence in a current sheet is studied by means of numerical simulations of fluid equations. It is found that turbulence produces long-wavelength magnetic islands even if the current sheet is so thick that spontaneous magnetic reconnection does not occur. Thus, turbulence modifies the threshold of magnetic island formation predicted by the conventional theory of spontaneous magnetic reconnection in a current sheet. In spite of the fact that the turbulence is driven by a short-wavelength instability due to a pressure gradient, the length of the magnetic island is the same order as the system size. The width of the island is several times the ion Larmor radius, and stronger turbulence causes wider magnetic islands. This suggests that the turbulence can trigger neoclassical tearing modes, which are the main nonlinear instability that limits the plasma pressure in magnetically confined plasmas. The long-wavelength magnetic island is formed by merging of small-scale magnetic islands.

Nonlinear stability of magnetic islands in a rotating helical plasma

“…The detailed modeling of the anomalous effects is still an open issue. 43 Therefore, we treat these coefficients as phenomenological parameters. In addition, the neoclassical thermal transport is not considered, since that is typically smaller than the anomalous perpendicular thermal transport.…”

Magnetic island evolution in hot ion plasmas

“…The model used here is a reduced set of two-fluid equations. 18,35 The model consists a set of equations that describes temporal evolutions of the electrostatic potential U, the density n, the parallel ion velocity v k , and the ion temperature T i . In keeping with our electrostatic approach, we neglect the rate of growth of the island in the electron parallel momentum equation (Ohm's law).…”

“…16,17 More recently, two of us have demonstrated a third mechanism whereby ion temperaturegradient turbulence can robustly give rise to magnetic islands of size several times the ion Larmor radius, which may be sufficient to initiate NTM growth. 18 These islands are created by a process of merging, through inverse cascade, of the much smaller-scale magnetic islands that are formed as a by-product of ion temperature gradient driven (ITG) turbulence in finitebeta plasma. The possible existence of additional phenomena related to the interaction of turbulence and magnetic islands, 19 and the relative importance of the three proposed mechanisms are the subject of ongoing research.…”

Nonlinear dynamics of turbulence driven magnetic islands. I. Theoretical aspects

“…Both the 3/2 and 2/1 NTM 21,22 have been seen to grow without the existence of an MHD triggering event. The possibility of a turbulence triggered NTM onset has been suggested in the past 14,15,23,24 but remains to some extent an open question to date. It has been shown, in numerical simulations, that small scale interchange turbulence can generate a turbulence driven magnetic island (TDMI), either by a simple mode beating mechanism in the vicinity of a low order rational surface 15 or remotely by spatial transfer of energy of turbulent modes to the lowest available low order rational surface.…”

“…4 Previous research has identified several different causes for the formation of seed islands such as an unstable tearing mode, [5][6][7] MHD events such as sawtooth 8 and fishbones, 9 error fields, 10,11 edge localized modes (ELMS), 12 nonlinear mode coupling, 13 and turbulence. 14,15 This diversity of origins poses serious challenges for experimentally anticipating their appearance and choosing appropriate control parameters for their avoidance and suppression. 4 Present experimental efforts towards the control of NTMs rely primarily on tracking the appearance of seed islands at various mode rational surfaces and preventing their growth by replacing the missing bootstrap current inside them through externally driven helical currents.…”