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Title: Study of the parametric dependence of linear and nonlinear microtearing modes in conventional tokamak discharges

 [1];  [1];  [2];  [1];  [3]
  1. Department of Physics, Lehigh University, Bethlehem, Pennsylvania 18015, USA
  2. Chalmers University of Technology and EURATOM-VR Association, Gothenburg, Sweden
  3. Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, Pennsylvania 18015, USA
Publication Date:
Sponsoring Org.:
OSTI Identifier:
Grant/Contract Number:
FG02-92ER54141; SC0013977
Resource Type:
Journal Article: Publisher's Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 25; Journal Issue: 1; Related Information: CHORUS Timestamp: 2018-01-08 13:25:33; Journal ID: ISSN 1070-664X
American Institute of Physics
Country of Publication:
United States

Citation Formats

Rafiq, T., Kritz, A. H., Weiland, J., Luo, L., and Schuster, E. Study of the parametric dependence of linear and nonlinear microtearing modes in conventional tokamak discharges. United States: N. p., 2018. Web. doi:10.1063/1.5009105.
Rafiq, T., Kritz, A. H., Weiland, J., Luo, L., & Schuster, E. Study of the parametric dependence of linear and nonlinear microtearing modes in conventional tokamak discharges. United States. doi:10.1063/1.5009105.
Rafiq, T., Kritz, A. H., Weiland, J., Luo, L., and Schuster, E. 2018. "Study of the parametric dependence of linear and nonlinear microtearing modes in conventional tokamak discharges". United States. doi:10.1063/1.5009105.
title = {Study of the parametric dependence of linear and nonlinear microtearing modes in conventional tokamak discharges},
author = {Rafiq, T. and Kritz, A. H. and Weiland, J. and Luo, L. and Schuster, E.},
abstractNote = {},
doi = {10.1063/1.5009105},
journal = {Physics of Plasmas},
number = 1,
volume = 25,
place = {United States},
year = 2018,
month = 1

Journal Article:
Free Publicly Available Full Text
This content will become publicly available on January 8, 2019
Publisher's Accepted Manuscript

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  • Microtearing modes (MTMs) have been identified as a source of significant electron thermal transport in tokamak discharges. In order to describe the evolution of these discharges, it is necessary to improve the prediction of electron thermal transport. This can be accomplished by utilizing a model for transport driven by MTMs in whole device predictive modeling codes. The objective of this paper is to develop the dispersion relation that governs the MTM driven transport. A unified fluid/kinetic approach is used in the development of a nonlinear dispersion relation for MTMs. The derivation includes the effects of electrostatic and magnetic fluctuations, arbitrarymore » electron-ion collisionality, electron temperature and density gradients, magnetic curvature, and the effects associated with the parallel propagation vector. An iterative nonlinear approach is used to calculate the distribution function employed in obtaining the nonlinear parallel current and the nonlinear dispersion relation. The third order nonlinear effects in magnetic fluctuations are included, and the influence of third order effects on a multi-wave system is considered. An envelope equation for the nonlinear microtearing modes in the collision dominant limit is introduced in order to obtain the saturation level. In the limit that the mode amplitude does not vary along the field line, slab geometry, and strong collisionality, the fluid dispersion relation for nonlinear microtearing modes is found to agree with the kinetic dispersion relation.« less
  • Linear and nonlinear magnetohydrodynamic (MHD) calculations have been performed to study the stability of tokamak discharges with reversed or negative central magnetic shear using the finite aspect ratio (FAR) suite of MHD codes [Charlton , J. Comput. Phys. 86, 270 (1990)]. The linear calculations confirm anew that radially localized resistive interchange modes can be unstable in the reversed or negative central magnetic shear region. The calculations further show that these resistive interchange modes are more unstable for toroidal mode numbers n higher than n=1. The nonlinear calculations do however demonstrate that toroidal mode number n=1, enhanced by the nonlinear couplingsmore » of the linearly more unstable higher n toroidal harmonics, dominates the nonlinearly saturated steady state. While the resistive interchange modes may account for the precursors detected experimentally, the saturated levels of magnetic fluctuations obtained in the nonlinear calculations do not appear large enough to cause the experimentally observed global collapse of such discharges.« less
  • Microtearing modes are an important drive of turbulent heat transport in present-day fusion plasmas. We investigate their linear stability under very-low collisionality regimes, expected for the next generations of devices, using gyrokinetic and drift-kinetic approaches. At odds with current opinion, we show that collisionless microtearing instabilities may occur in certain experimental conditions, particularly relevant for such devices as reversed field pinches and spherical tokamaks.
  • Electron-scale turbulence is predicted to drive anomalous electron thermal transport. However, experimental study of its relation with transport is still in its early stage. On the National Spherical Tokamak Experiment (NSTX), electron-scale density fluctuations are studied with a novel tangential microwave scattering system with high radial resolution of {+-}2 cm. Here, we report a study of parametric dependence of electron-scale turbulence in NSTX H-mode plasmas. The dependence on density gradient is studied through the observation of a large density gradient variation in the core induced by an edge localized mode (ELM) event, where we found the first clear experimental evidencemore » of density gradient stabilization of electron-gyro scale turbulence in a fusion plasma. This observation, coupled with linear gyro-kinetic calculations, leads to the identification of the observed instability as toroidal electron temperature gradient (ETG) modes. It is observed that longer wavelength ETG modes, k{sub Up-Tack }{rho}{sub s} Less-Than-Or-Equivalent-To 10 ({rho}{sub s} is the ion gyroradius at electron temperature and k{sub Up-Tack} is the wavenumber perpendicular to local equilibrium magnetic field), are most stabilized by density gradient, and the stabilization is accompanied by about a factor of two decrease in electron thermal diffusivity. Comparisons with nonlinear ETG gyrokinetic simulations show ETG turbulence may be able to explain the experimental electron heat flux observed before the ELM event. The collisionality dependence of electron-scale turbulence is also studied by systematically varying plasma current and toroidal field, so that electron gyroradius ({rho}{sub e}), electron beta ({beta}{sub e}), and safety factor (q{sub 95}) are kept approximately constant. More than a factor of two change in electron collisionality, {nu}{sub e}{sup *}, was achieved, and we found that the spectral power of electron-scale turbulence appears to increase as {nu}{sub e}{sup *} is decreased in this collisonality scan. However, both linear and nonlinear simulations show no or weak dependence with the electron-ion collision frequency, {nu}{sup e/i}. Instead, other equilibrium parameters (safety factor, electron density gradient, for example) affect ETG linear growth rate and electron thermal transport more than {nu}{sup e/i} does. Furthermore, electron heat flux predicted by the simulations is found to have an order-of-magnitude spatial variation in the experimental measurement region and is also found to be much smaller than experimental levels except at one radial location we evaluated. The predicted electron heat flux is shown to be strongly anti-correlated with density gradient, which varies for a factor of three in the measurement region, which is in agreement with the density gradient dependence study reported in this paper.« less
  • Geodesic Acoustic Modes (GAMs) have been predicted and subsequently observed in many toroidal plasma devices. Bicoherence studies on various devices have suggested three-wave mode coupling processes between GAMs and high frequency turbulence. Thus the parametric coupling of GAMS to drift waves and/or ion temperature gradient(ITG{r_brace} modes is a potential candidate for excitation of these modes. In this paper we discuss the resonant three-wave coupling mechanism for the excitation of GAMs by ITG and finite beta drift waves in homogeneous and inhomogeneous plasmas and compare theoretical predictions with observed characteristics of the GAMs.