Sherwood 2015

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The effect of strong radial variation of the diamagnetic frequency on two-fluid stabilization of edge localized MHD instabilities*

Author: Tyler B. Cote
Requested Type: Poster Only
Submitted: 2015-01-19 08:08:46

Co-authors: C.C.Hegna, P.Zhu

Contact Info:
University of Wisconsin - Madison
1500 Engineering Drive, ERB-51
Madison, Wisconsin   53706
United States of Ame

Abstract Text:
The conventional theory for two-fluid stabilization of ballooning instabilities in tokamaks assumes the diamagnetic frequency is constant throughout the radial structure of the ballooning mode. However, this approximation is not valid in the pedestal region where large density and temperature gradient variation is present. In this work, we apply WKB theory to solve for the radial structure of the ballooning eigenmode [1] in the presence of a radially varying diamagnetic frequency [2] for a class of MHD equilibria with edge pedestal regions. Generally, the radial variation of diamagnetic frequency reduces the stabilizing influence of two-fluid physics. Quantification of this effect is studied using a semi-classical ray tracing code. Results are presented for a number of equilibria, including a simple analytic model. Preliminary results for the varied diamagnetic frequency model indicate the region of instability—identified by the toroidal mode number, n—increases significantly in comparison to that of the constant diamagnetic frequency model. This coincides with the theory that the constant frequency model over-predicts the stabilizing contributions of the two-fluid effects [2]. Future work will include comparisons of the ballooning theory with linear two-fluid calculations using the extended MHD code NIMROD for numerous equilibria.

[1] R. L. Dewar et al, Nucl. Fusion 21, 493 (1981)
[2] R. J. Hastie et al, Phys. Plasmas 7, 4561 (2000)

* Research supported by US DoE under grant DE-FG02-86ER53218.

Please include the following author annotations:
T. B. Cote (1), C. C. Hegna (1) and P. Zhu (2,1)
(1) University of Wisconsin-Madison
(2) University of Science and Tech

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