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Title: Collisional alpha transport in a weakly non-quasisymmetric stellarator magnetic field

Abstract

Alpha particle confinement is a serious concern in stellarators and provides strong motivation for optimizing magnetic field configurations. In addition to the collisionless confinement of trapped alphas in stellarators, excessive collisional transport of the trapped alpha particles must be avoided while they tangentially drift due to the magnetic gradient (the$$\unicode[STIX]{x1D735}B$$drift). The combination of pitch angle scatter off the background ions and the$$\unicode[STIX]{x1D735}B$$drift gives rise to two narrow boundary layers in the trapped region. The first is at the trapped–passing boundary and enables the finite trapped response to be matched to the vanishing passing response of the alphas. The second layer is a region that encompasses the somewhat more deeply trapped alphas with vanishing tangential$$\unicode[STIX]{x1D735}B$$drift. Away from (and between) these boundary layers, collisions are ineffective and the alpha$$\unicode[STIX]{x1D735}B$$drift simply balances the small radial drift of the trapped alphas. As this balance does not vanish as the trapped–passing boundary is approached, the first collisional boundary layer is necessary and gives rise to$$\surd \unicode[STIX]{x1D708}$$transport, with$$\unicode[STIX]{x1D708}$$the collision frequency. The vanishing of the tangential drift results in a separate, somewhat wider boundary layer, and significantly stronger superbanana plateau transport that is independent of collisionality. The constraint imposed by the need to avoid significant energy depletionmore » loss in the slowing down tail distribution function sets the allowed departure of a stellarator from an optimal quasisymmetric configuration.« less

Authors:
Publication Date:
Research Org.:
Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1610183
DOE Contract Number:  
FG02-91ER54109
Resource Type:
Journal Article
Journal Name:
Journal of Plasma Physics
Additional Journal Information:
Journal Volume: 85; Journal Issue: 2; Journal ID: ISSN 0022-3778
Publisher:
Cambridge University Press
Country of Publication:
United States
Language:
English
Subject:
Physics

Citation Formats

Catto, Peter J. Collisional alpha transport in a weakly non-quasisymmetric stellarator magnetic field. United States: N. p., 2019. Web. doi:10.1017/s0022377819000321.
Catto, Peter J. Collisional alpha transport in a weakly non-quasisymmetric stellarator magnetic field. United States. https://doi.org/10.1017/s0022377819000321
Catto, Peter J. Mon . "Collisional alpha transport in a weakly non-quasisymmetric stellarator magnetic field". United States. https://doi.org/10.1017/s0022377819000321.
@article{osti_1610183,
title = {Collisional alpha transport in a weakly non-quasisymmetric stellarator magnetic field},
author = {Catto, Peter J.},
abstractNote = {Alpha particle confinement is a serious concern in stellarators and provides strong motivation for optimizing magnetic field configurations. In addition to the collisionless confinement of trapped alphas in stellarators, excessive collisional transport of the trapped alpha particles must be avoided while they tangentially drift due to the magnetic gradient (the$\unicode[STIX]{x1D735}B$drift). The combination of pitch angle scatter off the background ions and the$\unicode[STIX]{x1D735}B$drift gives rise to two narrow boundary layers in the trapped region. The first is at the trapped–passing boundary and enables the finite trapped response to be matched to the vanishing passing response of the alphas. The second layer is a region that encompasses the somewhat more deeply trapped alphas with vanishing tangential$\unicode[STIX]{x1D735}B$drift. Away from (and between) these boundary layers, collisions are ineffective and the alpha$\unicode[STIX]{x1D735}B$drift simply balances the small radial drift of the trapped alphas. As this balance does not vanish as the trapped–passing boundary is approached, the first collisional boundary layer is necessary and gives rise to$\surd \unicode[STIX]{x1D708}$transport, with$\unicode[STIX]{x1D708}$the collision frequency. The vanishing of the tangential drift results in a separate, somewhat wider boundary layer, and significantly stronger superbanana plateau transport that is independent of collisionality. The constraint imposed by the need to avoid significant energy depletion loss in the slowing down tail distribution function sets the allowed departure of a stellarator from an optimal quasisymmetric configuration.},
doi = {10.1017/s0022377819000321},
url = {https://www.osti.gov/biblio/1610183}, journal = {Journal of Plasma Physics},
issn = {0022-3778},
number = 2,
volume = 85,
place = {United States},
year = {2019},
month = {4}
}

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