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Title: Collisionless microinstabilities in stellarators. III. The ion-temperature-gradient mode

Abstract

We investigate the linear theory of the ion-temperature-gradient (ITG) mode, with the goal of developing a general understanding that may be applied to stellarators. We highlight the Wendelstein 7X (W7-X) device. Simple fluid and kinetic models that follow closely from existing literature are reviewed and two new first-principle models are presented and compared with results from direct numerical simulation. One model investigates the effect of regions of strong localized shear, which are generic to stellarator equilibria. These “shear spikes” are found to have a potentially significant stabilizing affect on the mode; however, the effect is strongest at short wavelengths perpendicular to the magnetic field, and it is found to be significant only for the fastest growing modes in W7-X. A second model investigates the long-wavelength limit for the case of negligible global magnetic shear. The analytic calculation reveals that the effect of the curvature drive enters at second order in the drift frequency, confirming conventional wisdom that the ITG mode is slab-like at long wavelengths. Using flux tube simulations of a zero-shear W7-X configuration, we observe a close relationship to an axisymmetric configuration at a similar parameter point. It is concluded that scale lengths of the equilibrium gradients constitute amore » good parameter space to characterize the ITG mode. Thus, to optimize the magnetic geometry for ITG mode stability, it may be fruitful to focus on local parameters, such as the magnitude of bad curvature, connection length, and local shear at locations of bad curvature (where the ITG mode amplitude peaks)« less

Authors:
;  [1];  [2]
  1. Max Planck Institute for Plasma Physics, EURATOM Association, Wendelsteinstr. 1, 17491 Greifswald (Germany)
  2. Euratom/CCFE Fusion Association, Culham Science Centre, Abingdon, Oxon OX14 3DB (United Kingdom)
Publication Date:
OSTI Identifier:
22251922
Resource Type:
Journal Article
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 21; Journal Issue: 3; Other Information: (c) 2014 Euratom; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 1070-664X
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; AXIAL SYMMETRY; COMPUTERIZED SIMULATION; LENGTH; MAGNETIC FIELDS; SHEAR; STELLARATORS; TEMPERATURE GRADIENTS; WAVELENGTHS

Citation Formats

Plunk, G. G., E-mail: gplunk@ipp.mpg.de, Helander, P., Xanthopoulos, P., Max-Planck/Princeton Research Center for Plasma Physics, 17491 Greifswald, and Connor, J. W. Collisionless microinstabilities in stellarators. III. The ion-temperature-gradient mode. United States: N. p., 2014. Web. doi:10.1063/1.4868412.
Plunk, G. G., E-mail: gplunk@ipp.mpg.de, Helander, P., Xanthopoulos, P., Max-Planck/Princeton Research Center for Plasma Physics, 17491 Greifswald, & Connor, J. W. Collisionless microinstabilities in stellarators. III. The ion-temperature-gradient mode. United States. https://doi.org/10.1063/1.4868412
Plunk, G. G., E-mail: gplunk@ipp.mpg.de, Helander, P., Xanthopoulos, P., Max-Planck/Princeton Research Center for Plasma Physics, 17491 Greifswald, and Connor, J. W. 2014. "Collisionless microinstabilities in stellarators. III. The ion-temperature-gradient mode". United States. https://doi.org/10.1063/1.4868412.
@article{osti_22251922,
title = {Collisionless microinstabilities in stellarators. III. The ion-temperature-gradient mode},
author = {Plunk, G. G., E-mail: gplunk@ipp.mpg.de and Helander, P. and Xanthopoulos, P. and Max-Planck/Princeton Research Center for Plasma Physics, 17491 Greifswald and Connor, J. W.},
abstractNote = {We investigate the linear theory of the ion-temperature-gradient (ITG) mode, with the goal of developing a general understanding that may be applied to stellarators. We highlight the Wendelstein 7X (W7-X) device. Simple fluid and kinetic models that follow closely from existing literature are reviewed and two new first-principle models are presented and compared with results from direct numerical simulation. One model investigates the effect of regions of strong localized shear, which are generic to stellarator equilibria. These “shear spikes” are found to have a potentially significant stabilizing affect on the mode; however, the effect is strongest at short wavelengths perpendicular to the magnetic field, and it is found to be significant only for the fastest growing modes in W7-X. A second model investigates the long-wavelength limit for the case of negligible global magnetic shear. The analytic calculation reveals that the effect of the curvature drive enters at second order in the drift frequency, confirming conventional wisdom that the ITG mode is slab-like at long wavelengths. Using flux tube simulations of a zero-shear W7-X configuration, we observe a close relationship to an axisymmetric configuration at a similar parameter point. It is concluded that scale lengths of the equilibrium gradients constitute a good parameter space to characterize the ITG mode. Thus, to optimize the magnetic geometry for ITG mode stability, it may be fruitful to focus on local parameters, such as the magnitude of bad curvature, connection length, and local shear at locations of bad curvature (where the ITG mode amplitude peaks)},
doi = {10.1063/1.4868412},
url = {https://www.osti.gov/biblio/22251922}, journal = {Physics of Plasmas},
issn = {1070-664X},
number = 3,
volume = 21,
place = {United States},
year = {Sat Mar 15 00:00:00 EDT 2014},
month = {Sat Mar 15 00:00:00 EDT 2014}
}