Dynamics of zonal shear collapse with hydrodynamic electrons

Journal Article · Fri Jun 08 00:00:00 EDT 2018 · Physics of Plasmas

DOI:https://doi.org/10.1063/1.5030345· OSTI ID:1540209

Hajjar, R. J. ^[1]; Diamond, P. H. ^[2]; Malkov, M. A.  ^[3]

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1. Univ. of California, San Diego, CA (United States)
2. Univ. of California, San Diego, CA (United States); Southwestern Inst. of Physics, Chengdu (China)
3. Center for Astrophysics and Space Sciences, University of California San Diego, La Jolla, California 92093, USA; Department of Physics, University of California, San Diego, California 92093, USA

This paper introduces a theory for the collapse of the edge zonal shear layer, as observed at the density limit at low β. We investigate the scaling of the transport and mean profiles with the adiabaticity parameter α, with special emphasizes on fluxes relevant to zonal flow (ZF) generation. We show that the adiabaticity parameter characterizes the strength of production of zonal flows and so determines the state of turbulence. A 1D reduced model that self-consistently describes the spatiotemporal evolution of the mean density $$\overline{n}$$, the azimuthal flow $$\overline{v}_{y}$$, and the turbulent potential enstrophy ε=$$\langle$$$$\tilde{n}$$–∇²$$\tilde{\phi}$$)²/2$$\rangle$$—related to fluctuation intensity—is presented. Quasi-linear analysis determines how the particle flux Γ_n and vorticity flux Π=–χ_y∇²v_y+Π^res scale with α, in both hydrodynamic and adiabatic regimes. As the plasma response passes from adiabatic (α>1) to hydrodynamic (α<1), the particle flux Γ_n is enhanced and the turbulent viscosity χ_y increases. However, the residual flux Π^res—which drives the flow—drops with α. As a result, the mean vorticity gradient ∇²$$\overline{v}$$_y=Π^res/χ_y—representative of the strength of the shear—also drops. The shear layer then collapses and turbulence is enhanced. The collapse is due to a decrease in ZF production, not an increase in damping. A physical picture for the onset of collapse is presented. The findings of this paper are used to motivate an explanation of the phenomenology of low β density limit evolution. A change from adiabatic (α=k$$^{2}_{z}$$v$$^{2}_{th}$$/(||ω||ν_ei)>1) to hydrodynamic (α<1) electron dynamics is associated with the density limit.

• Research Organization: Univ. of California, San Diego, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES)
• Grant/Contract Number: FG02-04ER54738
• OSTI ID: 1540209
• Alternate ID(s): OSTI ID: 1441086
• Journal Information: Physics of Plasmas, Vol. 25, Issue 6; ISSN 1070-664X
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY