Progress in Simulating Turbulent Electron Thermal Transport in NSTX
Authors: Walter Guttenfelder, et al.
Abstract:
Nonlinear simulations based on multiple NSTX discharge scenarios have progressed to help
differentiate unique instability mechanisms and to validate with experimental turbulence and transport data. First
nonlinear gyrokinetic simulations of microtearing (MT) turbulence in a high-beta NSTX H-mode discharge
predict experimental levels of electron thermal transport that are dominated by magnetic flutter and increase with
collisionality, roughly consistent with energy confinement times in dimensionless collisionality scaling
experiments. Electron temperature gradient (ETG) simulations predict significant electron thermal transport in
some low and high beta discharges when ion scales are suppressed by E x B shear. Although the predicted
transport in H-modes is insensitive to variation in collisionality (inconsistent with confinement scaling), it is
sensitive to variations in other parameters, particularly density gradient stabilization. In reversed shear (RS) Lmode
discharges that exhibit electron internal transport barriers, ETG transport has also been shown to be
suppressed nonlinearly by strong negative magnetic shear, s<<0. In many high beta plasmas, instabilities which
exhibit a stiff beta dependence characteristic of kinetic ballooning modes (KBM) are sometimes found in the core
region. However, they do not have a distinct finite beta threshold, instead transitioning gradually to a trapped
electron mode (TEM) as beta is reduced to zero. Nonlinear simulations of this "hybrid" TEM/KBM predict
significant transport in all channels, with substantial contributions from compressional magnetic perturbations.
As multiple instabilities are often unstable simultaneously in the same plasma discharge, even on the same flux
surface, unique parametric dependencies are discussed which may be useful for distinguishing the different
mechanisms experimentally.
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2013 Nucl. Fusion 53 093022 doi:10.1088/0029-5515/53/9/093022
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Download PPPL-4933 (pdf 280 KB 23pp)
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