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Title: Effect of curvature in the magnetic shear profile on micro-tearing modes in the tokamak pedestal

Abstract

The pedestal region of a tokamak plasma differs from the core in several respects. This work emphasizes the magnetic shear profile in the pedestal which is distinctive for two reasons: the pedestal is close to a magnetic-field separatrix so that the q-profile is intrinsically steep, and the steep pedestal gradients cause a peak in the bootstrap current with obvious effects on q(r). In fact, the pedestal q-profile has a complicated structure yielding a curved magnetic shear profile. Global linear simulations suggest that such a q-profile could substantially affect the nature of micro-instabilities. Here, this paper analytically investigates this special feature of the pedestal, after appropriately modifying the slab-model field equations to allow for more complicated variation of the parallel wave vector. Surprisingly, the departure from a locally linear q profile (flat magnetic shear), even when small, supports a qualitatively novel mode whose width is fixed by curvature in the profile. The mode is unstable; its substantial growth rate is robust in the sense of persisting even when the q-profile modification is assumed to be small. However, this mode, originating from details of the q-profile, will not be found in a typical local ballooning gyrokinetic simulation.

Authors:
ORCiD logo [1]; ORCiD logo [1]
  1. Univ. of Texas, Austin, TX (United States). Inst. for Fusion Studies and Department of Physics
Publication Date:
Research Org.:
Univ. of Texas, Austin, TX (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1537273
Alternate Identifier(s):
OSTI ID: 1441083
Grant/Contract Number:  
FG02-04ER54742
Resource Type:
Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 25; Journal Issue: 6; 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

Mahajan, S. M., and Hazeltine, R. D. Effect of curvature in the magnetic shear profile on micro-tearing modes in the tokamak pedestal. United States: N. p., 2018. Web. doi:10.1063/1.5035565.
Mahajan, S. M., & Hazeltine, R. D. Effect of curvature in the magnetic shear profile on micro-tearing modes in the tokamak pedestal. United States. doi:10.1063/1.5035565.
Mahajan, S. M., and Hazeltine, R. D. Fri . "Effect of curvature in the magnetic shear profile on micro-tearing modes in the tokamak pedestal". United States. doi:10.1063/1.5035565. https://www.osti.gov/servlets/purl/1537273.
@article{osti_1537273,
title = {Effect of curvature in the magnetic shear profile on micro-tearing modes in the tokamak pedestal},
author = {Mahajan, S. M. and Hazeltine, R. D.},
abstractNote = {The pedestal region of a tokamak plasma differs from the core in several respects. This work emphasizes the magnetic shear profile in the pedestal which is distinctive for two reasons: the pedestal is close to a magnetic-field separatrix so that the q-profile is intrinsically steep, and the steep pedestal gradients cause a peak in the bootstrap current with obvious effects on q(r). In fact, the pedestal q-profile has a complicated structure yielding a curved magnetic shear profile. Global linear simulations suggest that such a q-profile could substantially affect the nature of micro-instabilities. Here, this paper analytically investigates this special feature of the pedestal, after appropriately modifying the slab-model field equations to allow for more complicated variation of the parallel wave vector. Surprisingly, the departure from a locally linear q profile (flat magnetic shear), even when small, supports a qualitatively novel mode whose width is fixed by curvature in the profile. The mode is unstable; its substantial growth rate is robust in the sense of persisting even when the q-profile modification is assumed to be small. However, this mode, originating from details of the q-profile, will not be found in a typical local ballooning gyrokinetic simulation.},
doi = {10.1063/1.5035565},
journal = {Physics of Plasmas},
number = 6,
volume = 25,
place = {United States},
year = {2018},
month = {6}
}

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