Simulation Of Microtearing Turbulence In NSTX
Authors: W. Guttenfelder, J. Candy, S.M. Kaye, W.M. Nevins, E. Wanag, J. Zhang, R.E. Bell, N.A. Crocker, G.W. Hammett, B.P. LeBlanc, D.R. Mikkelsen, Y. Ren, H. Yuh
Abstract:Thermal energy confinement times in NSTX dimensionless parameter scans increase
with decreasing collisionality. While ion thermal transport is neoclassical, the source of
anomalous electron thermal transport in these discharges remains unclear, leading to
considerable uncertainty when extrapolating to future ST devices at much lower collisionality.
Linear gyrokinetic simulations find microtearing modes to be unstable in high collisionality
discharges. First non-linear gyrokinetic simulations of microtearing turbulence in NSTX show
they can yield experimental levels of transport. Magnetic flutter is responsible for almost all the
transport (~98%), perturbed field line trajectories are globally stochastic, and a test particle
stochastic transport model agrees to within 25% of the simulated transport. Most significantly,
microtearing transport is predicted to increase with electron collisionality, consistent with the
observed NSTX confinement scaling. While this suggests microtearing modes may be the
source of electron thermal transport, the predictions are also very sensitive to electron
temperature gradient, indicating the scaling of the instability threshold is important. In addition,
microtearing turbulence is susceptible to suppression via sheared E×B flows as experimental
values of E×B shear (comparable to the linear growth rates) dramatically reduce the transport
below experimental values. Refinements in numerical resolution and physics model assumptions
are expected to minimize the apparent discrepancy. In cases where the predicted transport is
strong, calculations suggest that a proposed polarimetry diagnostic may be sensitive to the
magnetic perturbations associated with the unique structure of microtearing turbulence.
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Submitted to: Physics of Plasmas (December, 2011)
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Download PPPL-4730 (pdf 275 KB 27 pp)
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