Sherwood 2015

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approvedsherwood_2015_poster.pdf2015-04-01 15:45:10Carson Cook

Analytical theory and numerical investigation of the shear Alfvén continuum in the presence of an island

Author: Carson R. Cook
Requested Type: Poster Only
Submitted: 2015-01-16 13:18:32

Co-authors: C.C. Hegna, D.A. Spong, S.P. Hirshman

Contact Info:
University of Wisconsin
1150 University Ave
Madison, WI   53706

Abstract Text:
In this work, we investigate the effects of an equilibrium magnetic island on the Alfvén continuum. Of particular interest is the Beta-induced Alfvén Eigenmode (BAE) gap, which is a break in the frequencies of the shear Alfvén spectrum created by finite beta and toroidal effects. This gap is important because a discrete Alfvén eigenmode can exist within the gap frequency range and will not be affected by continuum damping. Under these conditions, there is a coupling between the equation for shear Alfvén waves and the sound wave equation. Prior numerical work has shown the presence of a magnetic island causes an upshift in the BAE gap frequency.[1] The effect of a magnetic island on the BAE gap frequency is calculated analytically for a toroidal equilibrium. Using a WKB approximation of the ideal MHD equations, an island-induced BAE frequency upshift is predicted. In the presence of the island, the minimum of the continuum frequencies is shifted from the rational surface to the island separatrix. Properties of the eigenmode structure will be detailed. Numerical results of the spectrum for MST will also be presented and compared to theory. These results come from 3D MHD equilibria with islands obtained using the SIESTA code. The continuum is computed by solving the generalized eigenvalue problem obtained from the Hessian matrix of the potential energy along with the inertia matrix of the SIESTA equilibrium. The theory may help to explain some observed Alfvénic activity in MST discharges.[2]
[1] A. Biancalani, L. Chen, F. Pegoraro, and F. Zonca, Plasma Phys. Control. Fusion 53, 025009 (2011).
[2] J. Koliner, C. Forest, J. Sarff, and J. Anderson, Physical Review Letters 109, 115003 (2012).
Research supported by the U.S. DOE under grants DE-FG02-99ER54546 and DE-SC0006103.


March 16-18, 2015
The Courant Institute, New York University