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On optimizing stellarators to microinstabilities: key geometric quantities

Author: C. C. Hegna
Requested Type: Poster Only
Submitted: 2015-01-16 14:34:12

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University of Wisconsin-Madison
1500 Engineering Drive
Madison, WI   53706
USA

Abstract Text:
An emerging theme for improving the stellarator concept is the manipulation of three-dimensional shaping in order to affect microinstability induced anomalous transport. In this work, key geometric quantities are identified for three classes of linear local modes: ballooning instabilities, toroidal ion temperature gradient driven instabilities (ITG) and trapped particle modes. For each of these modes, a common crucial geometric factor appears associated with the components of the curvature vector (both normal and geodesic) and the integrated local shear. This quantity appears in the instability drive term for both ballooning and toroidal ITG modes and also appears as part of the bounce averaged precessional drift of trapped particles associated with the drive for trapped particle instabilities. At small beta, the local shear can be associated the normal torsion of a field line [1]. A common property of the normal torsion of many stellarator configurations is that it has regions with both 'tokamak-like' and 'stellarator-like' values, where the latter regions are large and localized near the 'corners' of the magnetic flux surfaces. A possible route to optimization can be found by manipulating the symmetry properties of the geodesic curvature relative to the normal torsion. Example local 3D equilibrium [1] are constructed to accommodate this potential optimization scheme.

[1] C. C. Hegna, Phys. Plasmas 7, 3921 (2000).


*Research supported by U. S. DoE under grants DE-FG02-99ER54546 and DE-SC00006103.

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