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Title: Theory and Fluid Simulations of Boundary Plasma Fluctuations

Abstract

Theoretical and computational investigations are presented of boundary plasma microturbulence that take into account important effects of the geometry of diverted tokamaks--in particular, the effect of x-point magnetic shear and the termination of field lines on divertor plates. We first generalize our previous 'heuristic boundary condition' which describes, in a lumped model, the closure of currents in the vicinity of the x-point region to encompass three current-closure mechanisms. We then use this boundary condition to derive the dispersion relation for low-beta flute-like modes in the divertor-leg region under the combined drives of curvature, sheath impedance, and divertor tilt effects. The results indicate the possibility of strongly growing instabilities, driven by sheath boundary conditions, and localized in either the private or common flux region of the divertor leg depending on the radial tilt of divertor plates. We re-visit the issue of x-point effects on blobs, examining the transition from blobs terminated by x-point shear to blobs that extend over both the main SOL and divertor legs. We find that, for a main-SOL blob, this transition occurs without a free-acceleration period as previously thought, with x-point termination conditions applying until the blob has expanded to reach the divertor plate. We also derivemore » propagation speeds for divertor-leg blobs. Finally, we present fluid simulations of the C-Mod tokamak from the BOUT edge fluid turbulence code, which show main-SOL blob structures with similar spatial characteristics to those observed in the experiment, and also simulations which illustrate the possibility of fluctuations confined to divertor legs.« less

Authors:
; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
936982
Report Number(s):
UCRL-JRNL-227892
TRN: US0806151
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Journal Article
Resource Relation:
Journal Name: Nuclear Fusion, vol. 47, July 2007, July 1, 2007, pp. 612; Journal Volume: 47
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION; BOUNDARY CONDITIONS; CLOSURES; DISPERSION RELATIONS; DIVERTORS; FLUCTUATIONS; GEOMETRY; IMPEDANCE; PLASMA; PLATES; SHEAR; SOLS; TURBULENCE

Citation Formats

Cohen, R H, LaBombard, B, LoDestro, L L, Rognlien, T D, Ryutov, D D, Terry, J L, Umansky, M V, Xu, X Q, and Zweben, S. Theory and Fluid Simulations of Boundary Plasma Fluctuations. United States: N. p., 2007. Web. doi:10.1088/0029-5515/47/7/012.
Cohen, R H, LaBombard, B, LoDestro, L L, Rognlien, T D, Ryutov, D D, Terry, J L, Umansky, M V, Xu, X Q, & Zweben, S. Theory and Fluid Simulations of Boundary Plasma Fluctuations. United States. doi:10.1088/0029-5515/47/7/012.
Cohen, R H, LaBombard, B, LoDestro, L L, Rognlien, T D, Ryutov, D D, Terry, J L, Umansky, M V, Xu, X Q, and Zweben, S. Tue . "Theory and Fluid Simulations of Boundary Plasma Fluctuations". United States. doi:10.1088/0029-5515/47/7/012. https://www.osti.gov/servlets/purl/936982.
@article{osti_936982,
title = {Theory and Fluid Simulations of Boundary Plasma Fluctuations},
author = {Cohen, R H and LaBombard, B and LoDestro, L L and Rognlien, T D and Ryutov, D D and Terry, J L and Umansky, M V and Xu, X Q and Zweben, S},
abstractNote = {Theoretical and computational investigations are presented of boundary plasma microturbulence that take into account important effects of the geometry of diverted tokamaks--in particular, the effect of x-point magnetic shear and the termination of field lines on divertor plates. We first generalize our previous 'heuristic boundary condition' which describes, in a lumped model, the closure of currents in the vicinity of the x-point region to encompass three current-closure mechanisms. We then use this boundary condition to derive the dispersion relation for low-beta flute-like modes in the divertor-leg region under the combined drives of curvature, sheath impedance, and divertor tilt effects. The results indicate the possibility of strongly growing instabilities, driven by sheath boundary conditions, and localized in either the private or common flux region of the divertor leg depending on the radial tilt of divertor plates. We re-visit the issue of x-point effects on blobs, examining the transition from blobs terminated by x-point shear to blobs that extend over both the main SOL and divertor legs. We find that, for a main-SOL blob, this transition occurs without a free-acceleration period as previously thought, with x-point termination conditions applying until the blob has expanded to reach the divertor plate. We also derive propagation speeds for divertor-leg blobs. Finally, we present fluid simulations of the C-Mod tokamak from the BOUT edge fluid turbulence code, which show main-SOL blob structures with similar spatial characteristics to those observed in the experiment, and also simulations which illustrate the possibility of fluctuations confined to divertor legs.},
doi = {10.1088/0029-5515/47/7/012},
journal = {Nuclear Fusion, vol. 47, July 2007, July 1, 2007, pp. 612},
number = ,
volume = 47,
place = {United States},
year = {Tue Jan 09 00:00:00 EST 2007},
month = {Tue Jan 09 00:00:00 EST 2007}
}
  • Satellite observations in recent years have confirmed that the plasma sheet boundary layer is a permanent feature of the Earth's magnetotail located between the lobe and central plasma sheet during both quiet and active magnetic periods. Distinct features of the boundary layer include field aligned ion beams and intense electrostatic emissions known as broadband electrostatic noise. Since the plasma sheet boundary layer is a spatial feature of the magnetotail, within it will occur thermal mixing of the resident warm boundary layer plasma with inflowing (convecting) cold ionospheric plasma. A theoretical study involving linear theory and nonlinear numerical particle simulations ismore » presented which examine ion beam instabilities in the presence of a thermally mixed hot and cold background plasma. It is found that the free energy in the ion beams can heat the cool ionospheric plasma to ambient plasma sheet boundary layer temperatures via broadband electrostatic noise. These results, along with recent observation reports that ionospheric outflow can account for measured plasma sheet densities, suggest that the ionospheric role in plasma sheet dynamics and content may be as large as the solar wind.« less
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