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Title: Magnetic island evolution in the presence of ion-temperature gradient-driven turbulence

Turbulence is known to drive and sustain magnetic islands of width equal to multiples of the Larmor radius. The nature of the drive is studied here by means of numerical simulations of a fluid electrostatic model in 2D (single helicity) sheared-slab geometry. The electrostatic model eliminates the coalescence of short wavelength islands as a mechanism for sustaining longer wavelength islands. In quiescent islands, the polarization current, which depends on the propagation velocity of the island through the plasma, plays a critical role in determining the growth or decay of island chains. For turbulent islands, the unforced propagation velocity is significantly changed by strong zonal flow. The simulations show, however, that the turbulent fluctuations in the current density are much larger and faster than those in the zonal flow, and that they dominate the steady-state perturbed current density. In order to distinguish the roles of the zonal flow from the direct action of the fluctuations on the islands, a new diagnostic is implemented. This new diagnostic separates the effects of all the sources of parallel current. These are the curvature (which drives Pfirsch-Schlüter currents) and the divergences of the viscous and Reynolds stresses (the latter driving polarization currents). The new diagnosticmore » also enables the contributions from short and long wavelengths to be separated for each term. It shows that in the absence of curvature, the drive is dominated by the contributions to the polarization current from the short wavelength fluctuations, while the long-wavelength fluctuations play a stabilizing role. In the presence of unfavorable curvature, by contrast, the effects of the short- and long-wavelength contributions of the polarization current reverse roles but nearly cancel, leaving the Pfirsch-Schlüter current as the dominant drive.« less
Authors:
 [1] ;  [2]
  1. National Institute for Fusion Science, Toki 509-5292 (Japan)
  2. Institute for Fusion Studies, University of Texas at Austin, Austin, Texas 78712 (United States)
Publication Date:
OSTI Identifier:
22218329
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 20; Journal Issue: 12; Other Information: (c) 2013 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; COMPUTERIZED SIMULATION; CURRENT DENSITY; FLUCTUATIONS; HELICITY; LARMOR RADIUS; MAGNETIC ISLANDS; MAGNETOHYDRODYNAMICS; PLASMA DRIFT; PLASMA FLUID EQUATIONS; PLASMA SIMULATION; PLASMA WAVES; POLARIZATION; REYNOLDS NUMBER; SHEAR; STEADY-STATE CONDITIONS; STRESSES; TEMPERATURE GRADIENTS; TURBULENCE