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Title: Coupled ion temperature gradient and trapped electron mode to electron temperature gradient mode gyrokinetic simulations

Abstract

Electron temperature gradient (ETG) transport is conventionally defined as the electron energy transport at high wave number (high-k) where ions are adiabatic and there can be no ion energy or plasma transport. Previous gyrokinetic simulations have assumed adiabatic ions (ETG-ai) and work on the small electron gyroradius scale. However such ETG-ai simulations with trapped electrons often do not have well behaved nonlinear saturation unless fully kinetic ions (ki) and proper ion scale zonal flow modes are included. Electron energy transport is separated into ETG-ki at high-k and ion temperature gradient-trapped electron mode (ITG/TEM) at low-k. Expensive (more computer-intensive), high-resolution, large-ion-scale flux-tube simulations coupling ITG/TEM and ETG-ki turbulence are presented. These require a high effective Reynolds number R ≡ [k(max)/k(min)]2 = μ2, where μ=[ρsi/ρsi] is the ratio of ion to electron gyroradii. Compute times scale faster than μ3. By comparing the coupled expensive simulations with (1) much cheaper (less compute-intensive), uncoupled, high-resolution, small, flux-tube ETG-ki and with (2) uncoupled low-resolution, large, flux-tube ITG/TEM simulations, and also by artificially turning “off” the low-k or high-k drives, it appears that ITG/TEM and ETG-ki transport are not strongly coupled so long as ETG-ki can access some nonadiabatic ion scale zonal flows and both high-kmore » and low-k are linearly unstable. However expensive coupled simulations are required for physically accurate k-spectra of the transport and turbulence. Simulations with μ ≥ 30 appear to represent the physical range μ > 40. ETG-ki transport measured in ion gyro-Bohm units is weakly dependent on μ. For the mid-radius core tokamak plasma parameters studied, ETG-ki is about 10% of the electron energy transport, which in turn is about 30% of the total energy transport (with negligible E × B shear). However at large E × B shear sufficient to quench the low-k ITG/TEM transport, the high-k tail of the ETG-ki transport survives. In conclusion, decreasing the trapping to minimize the TEM opens a stability gap between ITG and ETG. High-k ETG transport driven by low-k ITG instability in an ETG linearly stable plasma is demonstrated.« less

Authors:
 [1];  [1];  [2]
  1. General Atomics, San Diego, CA (United States)
  2. General Atomics, San Diego, CA (United States); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). National Center for Computer Science
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Oak Ridge Leadership Computing Facility (OLCF)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1564633
Grant/Contract Number:  
AC05-00OR22725
Resource Type:
Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 14; Journal Issue: 5; Journal ID: ISSN 1070-664X
Publisher:
American Institute of Physics (AIP)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Physics

Citation Formats

Waltz, R. E., Candy, J., and Fahey, M. Coupled ion temperature gradient and trapped electron mode to electron temperature gradient mode gyrokinetic simulations. United States: N. p., 2007. Web. doi:10.1063/1.2436851.
Waltz, R. E., Candy, J., & Fahey, M. Coupled ion temperature gradient and trapped electron mode to electron temperature gradient mode gyrokinetic simulations. United States. https://doi.org/10.1063/1.2436851
Waltz, R. E., Candy, J., and Fahey, M. Fri . "Coupled ion temperature gradient and trapped electron mode to electron temperature gradient mode gyrokinetic simulations". United States. https://doi.org/10.1063/1.2436851. https://www.osti.gov/servlets/purl/1564633.
@article{osti_1564633,
title = {Coupled ion temperature gradient and trapped electron mode to electron temperature gradient mode gyrokinetic simulations},
author = {Waltz, R. E. and Candy, J. and Fahey, M.},
abstractNote = {Electron temperature gradient (ETG) transport is conventionally defined as the electron energy transport at high wave number (high-k) where ions are adiabatic and there can be no ion energy or plasma transport. Previous gyrokinetic simulations have assumed adiabatic ions (ETG-ai) and work on the small electron gyroradius scale. However such ETG-ai simulations with trapped electrons often do not have well behaved nonlinear saturation unless fully kinetic ions (ki) and proper ion scale zonal flow modes are included. Electron energy transport is separated into ETG-ki at high-k and ion temperature gradient-trapped electron mode (ITG/TEM) at low-k. Expensive (more computer-intensive), high-resolution, large-ion-scale flux-tube simulations coupling ITG/TEM and ETG-ki turbulence are presented. These require a high effective Reynolds number R ≡ [k(max)/k(min)]2 = μ2, where μ=[ρsi/ρsi] is the ratio of ion to electron gyroradii. Compute times scale faster than μ3. By comparing the coupled expensive simulations with (1) much cheaper (less compute-intensive), uncoupled, high-resolution, small, flux-tube ETG-ki and with (2) uncoupled low-resolution, large, flux-tube ITG/TEM simulations, and also by artificially turning “off” the low-k or high-k drives, it appears that ITG/TEM and ETG-ki transport are not strongly coupled so long as ETG-ki can access some nonadiabatic ion scale zonal flows and both high-k and low-k are linearly unstable. However expensive coupled simulations are required for physically accurate k-spectra of the transport and turbulence. Simulations with μ ≥ 30 appear to represent the physical range μ > 40. ETG-ki transport measured in ion gyro-Bohm units is weakly dependent on μ. For the mid-radius core tokamak plasma parameters studied, ETG-ki is about 10% of the electron energy transport, which in turn is about 30% of the total energy transport (with negligible E × B shear). However at large E × B shear sufficient to quench the low-k ITG/TEM transport, the high-k tail of the ETG-ki transport survives. In conclusion, decreasing the trapping to minimize the TEM opens a stability gap between ITG and ETG. High-k ETG transport driven by low-k ITG instability in an ETG linearly stable plasma is demonstrated.},
doi = {10.1063/1.2436851},
journal = {Physics of Plasmas},
number = 5,
volume = 14,
place = {United States},
year = {Fri May 11 00:00:00 EDT 2007},
month = {Fri May 11 00:00:00 EDT 2007}
}

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