Abstract Details
Abstracts
Author: Xiang Fan
Requested Type: Pre-Selected Invited
Submitted: 2019-02-20 18:00:47
Co-authors: P.H.Diamond, L.Chacon
Contact Info:
UCSD
9585 Genesee Ave, Apt E1
San Diego, CA 92121
USA
Abstract Text:
This study elucidates the physics of the classic problem of turbulent resistivity in 2D MHD. The nonlinear structure of 2D MHD is identical to that of reduced MHD, and so is fundamental to all problems involving anomalous dissipation in electromagnetic turbulence in a strongly magnetized plasma. This analysis is of particular importance to anomalous dissipation associated with ELMs. Previous studies found that the turbulent transport is suppressed even when a weak large scale magnetic field is present, using mean field theory. We further noticed that, absent an imposed, ordered, in-plane magnetic field, a blob-and-barrier structure forms spontaneously, with magnetic energy concentrated in thin, linear barriers, located at the interstices between blobs of magnetic potential A. One characteristic of the existence of blob-and-barrier structure is in the PDF of A: this PDF is bimodal when there are transport barriers in the suppression stage. Two peaks still arise on the PDF of A even if its initial condition is unimodal. The barriers quench the transport and decay of magnetic energy, relative to the kinematic prediction. The barriers are intermittent, so quenching cannot be addressed by mean field theory. The transport bifurcation underlying barrier formation is linked to the inverse cascade of <A^2> and negative resistivity, which together induce local bistability. Negative resistivity results from magnetic flux coalescence on larger scales. Local negative resistivity and magnetic field feedback imply a bifurcation in magnetic transport. For small scale forcing, spontaneous layering of the magnetic potential occurs, with barriers located at the interstices between layers. This may be thought of as a ‘staircase’ of magnetic fields. Coarsening of the layers occurs in time, prior to the final decay of the field.
This work is supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, under Award Number DE-FG02-04ER54738.
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