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Title: Characterizing electron temperature gradient turbulence via numerical simulation

Abstract

Numerical simulations of electron temperature gradient (ETG) turbulence are presented that characterize the ETG fluctuation spectrum, establish limits to the validity of the adiabatic ion model often employed in studying ETG turbulence, and support the tentative conclusion that plasma-operating regimes exist in which ETG turbulence produces sufficient electron heat transport to be experimentally relevant. We resolve prior controversies regarding simulation techniques and convergence by benchmarking simulations of ETG turbulence from four microturbulence codes, demonstrating agreement on the electron heat flux, correlation functions, fluctuation intensity, and rms flow shear at fixed simulation cross section and resolution in the plane perpendicular to the magnetic field. Excellent convergence of both continuum and particle-in-cell codes with time step and velocity-space resolution is demonstrated, while numerical issues relating to perpendicular (to the magnetic field) simulation dimensions and resolution are discussed. A parameter scan in the magnetic shear, s, demonstrates that the adiabatic ion model is valid at small values of s (s<0.4 for the parameters used in this scan) but breaks down at higher magnetic shear. A proper treatment employing gyrokinetic ions reveals a steady increase in the electron heat transport with increasing magnetic shear, reaching electron heat transport rates consistent with analyses of experimentalmore » tokamak discharges.« less

Authors:
; ; ; ; ; ; ; ; ; ;  [1];  [2];  [2];  [3];  [2];  [2];  [2];  [2];  [4];  [2]
  1. Lawrence Livermore National Laboratory, Livermore, California 94551 (United States)
  2. (United States)
  3. (CRPP), Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne (Switzerland)
  4. (Germany)
Publication Date:
OSTI Identifier:
20860430
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 13; Journal Issue: 12; Other Information: DOI: 10.1063/1.2402510; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; CONVERGENCE; CORRELATION FUNCTIONS; ELECTRON TEMPERATURE; ELECTRONS; FLUCTUATIONS; HEAT FLUX; HEAT TRANSFER; ION TEMPERATURE; IONS; MAGNETIC FIELDS; NUMERICAL ANALYSIS; PLASMA; PLASMA CONFINEMENT; PLASMA SIMULATION; RADIATION TRANSPORT; SHEAR; TEMPERATURE GRADIENTS; TOKAMAK DEVICES; TURBULENCE

Citation Formats

Nevins, W. M., Candy, J., Cowley, S., Dannert, T., Dimits, A., Dorland, W., Estrada-Mila, C., Hammett, G. W., Jenko, F., Pueschel, M. J., Shumaker, D. E., General Atomics, San Diego, California 92186, Department of Physics and Astronomy, UCLA, Los Angeles, California 90095-1547, Centre de Recherches en Physique des Plasmas, Lawrence Livermore National Laboratory, Livermore, California 94551, University of Maryland, College Park, Maryland 20742, Department of Mechanical and Aerospace Engineering, UCSD, San Diego, California 92093, Princeton Plasma Physics Laboratory, Princeton, New Jersey 08536, Max-Planck Institut fuer Plasmaphysik, D-85748 Garching, and Lawrence Livermore National Laboratory, Livermore, California 94551. Characterizing electron temperature gradient turbulence via numerical simulation. United States: N. p., 2006. Web. doi:10.1063/1.2402510.
Nevins, W. M., Candy, J., Cowley, S., Dannert, T., Dimits, A., Dorland, W., Estrada-Mila, C., Hammett, G. W., Jenko, F., Pueschel, M. J., Shumaker, D. E., General Atomics, San Diego, California 92186, Department of Physics and Astronomy, UCLA, Los Angeles, California 90095-1547, Centre de Recherches en Physique des Plasmas, Lawrence Livermore National Laboratory, Livermore, California 94551, University of Maryland, College Park, Maryland 20742, Department of Mechanical and Aerospace Engineering, UCSD, San Diego, California 92093, Princeton Plasma Physics Laboratory, Princeton, New Jersey 08536, Max-Planck Institut fuer Plasmaphysik, D-85748 Garching, & Lawrence Livermore National Laboratory, Livermore, California 94551. Characterizing electron temperature gradient turbulence via numerical simulation. United States. doi:10.1063/1.2402510.
Nevins, W. M., Candy, J., Cowley, S., Dannert, T., Dimits, A., Dorland, W., Estrada-Mila, C., Hammett, G. W., Jenko, F., Pueschel, M. J., Shumaker, D. E., General Atomics, San Diego, California 92186, Department of Physics and Astronomy, UCLA, Los Angeles, California 90095-1547, Centre de Recherches en Physique des Plasmas, Lawrence Livermore National Laboratory, Livermore, California 94551, University of Maryland, College Park, Maryland 20742, Department of Mechanical and Aerospace Engineering, UCSD, San Diego, California 92093, Princeton Plasma Physics Laboratory, Princeton, New Jersey 08536, Max-Planck Institut fuer Plasmaphysik, D-85748 Garching, and Lawrence Livermore National Laboratory, Livermore, California 94551. Fri . "Characterizing electron temperature gradient turbulence via numerical simulation". United States. doi:10.1063/1.2402510.
@article{osti_20860430,
title = {Characterizing electron temperature gradient turbulence via numerical simulation},
author = {Nevins, W. M. and Candy, J. and Cowley, S. and Dannert, T. and Dimits, A. and Dorland, W. and Estrada-Mila, C. and Hammett, G. W. and Jenko, F. and Pueschel, M. J. and Shumaker, D. E. and General Atomics, San Diego, California 92186 and Department of Physics and Astronomy, UCLA, Los Angeles, California 90095-1547 and Centre de Recherches en Physique des Plasmas and Lawrence Livermore National Laboratory, Livermore, California 94551 and University of Maryland, College Park, Maryland 20742 and Department of Mechanical and Aerospace Engineering, UCSD, San Diego, California 92093 and Princeton Plasma Physics Laboratory, Princeton, New Jersey 08536 and Max-Planck Institut fuer Plasmaphysik, D-85748 Garching and Lawrence Livermore National Laboratory, Livermore, California 94551},
abstractNote = {Numerical simulations of electron temperature gradient (ETG) turbulence are presented that characterize the ETG fluctuation spectrum, establish limits to the validity of the adiabatic ion model often employed in studying ETG turbulence, and support the tentative conclusion that plasma-operating regimes exist in which ETG turbulence produces sufficient electron heat transport to be experimentally relevant. We resolve prior controversies regarding simulation techniques and convergence by benchmarking simulations of ETG turbulence from four microturbulence codes, demonstrating agreement on the electron heat flux, correlation functions, fluctuation intensity, and rms flow shear at fixed simulation cross section and resolution in the plane perpendicular to the magnetic field. Excellent convergence of both continuum and particle-in-cell codes with time step and velocity-space resolution is demonstrated, while numerical issues relating to perpendicular (to the magnetic field) simulation dimensions and resolution are discussed. A parameter scan in the magnetic shear, s, demonstrates that the adiabatic ion model is valid at small values of s (s<0.4 for the parameters used in this scan) but breaks down at higher magnetic shear. A proper treatment employing gyrokinetic ions reveals a steady increase in the electron heat transport with increasing magnetic shear, reaching electron heat transport rates consistent with analyses of experimental tokamak discharges.},
doi = {10.1063/1.2402510},
journal = {Physics of Plasmas},
number = 12,
volume = 13,
place = {United States},
year = {Fri Dec 15 00:00:00 EST 2006},
month = {Fri Dec 15 00:00:00 EST 2006}
}
  • Numerical simulations of electron temperature gradient (ETG) turbulence are presented which characterize the ETG fluctuation spectrum, establish limits to the validity of the adiabatic ion model often employed in studying ETG turbulence, and support the tentative conclusion that plasmaoperating regimes exist in which ETG turbulence produces sufficient electron heat transport to be experimentally relevant. We resolve prior controversies regarding simulation techniques and convergence by benchmarking simulations of ETG turbulence from four microturbulence codes, demonstrating agreement on the electron heat flux, correlation functions, fluctuation intensity, and rms flow shear at fixed simulation cross section and resolution in the plane perpendicular tomore » the magnetic field. Excellent convergence of both continuum and particle-in-cell codes with time step and velocity-space resolution is demonstrated, while numerical issues relating to perpendicular (to the magnetic field) simulation dimensions and resolution are discussed. A parameter scan in the magnetic shear, s, demonstrates that the adiabatic ion model is valid at small values of s (s < 0.4 for the parameters used in this scan) but breaks down at higher magnetic shear. A proper treatment employing gyrokinetic ions reveals a steady increase in the electron heat transport with increasing magnetic shear, reaching electron heat transport rates consistent with analyses of experimental tokamak discharges.« less
  • Three-dimensional global numerical simulations of ion temperature gradient mode turbulence in a sheared and curved magnetic field corresponding to tokamak geometry is obtained. The key result demonstrates that there is no significant dependence of the bulk heat transport on the edge safety factor q in agreement with analytic theories. The motivation for this question in relation to tokamak experiments is discussed.
  • Negative magnetic shear is found to suppress electron turbulence and improve electron thermal transport for plasmas in the National Spherical Torus Experiment (NSTX). Sufficiently negative magnetic shear results in a transition out of a stiff profile regime. Density fluctuation measurements from high-k microwave scattering are verified to be the electron temperature gradient (ETG) mode by matching measured rest frequency and linear growth rate to gyrokinetic calculations. Fluctuation suppression under negligible ExB shear conditions confirm that negative magnetic shear alone is sufficient for ETG suppression. Measured electron temperature gradients can significantly exceed ETG critical gradients with ETG mode activity reduced tomore » intermittent bursts, while electron thermal diffusivity improves to below 0.1 electron gyro-Bohms.« less
  • Mixing of a passive scalar in statistically homogeneous, isotropic, and stationary turbulence with a mean scalar gradient is investigated via direct numerical simulation, for Taylor-scale Reynolds numbers, {ital R}{sub {lambda}}, from 28 to 185. Multiple independent simulations are performed to get confidence intervals, and local regression smoothing is used to further reduce statistical fluctuations. The scalar fluctuation field, {phi}({bold x},{ital t}), is initially zero, and develops to a statistically stationary state after about four eddy turnover times. Quantities investigated include the dissipation of scalar flux, which is found to be significant; probability density functions (pdfs) and joint-pdfs of the scalar,more » its derivatives, scalar dissipation, and mechanical dissipation; and conditional expectations of scalar mixing, {nabla}{sup 2}{phi}. A linear model for scalar mixing jointly conditioned on the scalar and {ital v}-velocity is developed, and reproduces the data quite well. Also considered is scalar mixing jointly conditioned on the scalar and scalar dissipation. Terms appearing in the balance equation for the pdf of {phi} are examined. From a solution of the scalar pdf equation two sufficient conditions arise for the scalar pdf to be Gaussian. These are shown to be well satisfied for moderate values of the scalar, and approximately so for large fluctuations. Many correlations are also presented, including {rho}({ital v},{phi}), which changes during the evolution of the scalar from a value of unity when initialized to the stationary value of 0.5{endash}0.6. {copyright} {ital 1996 American Institute of Physics.}« less