Abstract Details
Abstracts
Author: Benjamin J Faber
Requested Type: Poster
Submitted: 2017-03-16 11:45:00
Co-authors: C.C.Hegna, M.J.Pueschel, P.W.Terry
Contact Info:
University of Wisconsin-Madison
1150 University Ave
Madison, WI 53706
United States of Ame
Abstract Text:
Optimized stellarators, such as the Helically Symmetric eXperiment (HSX), often operate with small global magnetic shear to avoid having low-order rational surfaces and magnetic islands. Because the ballooning transformation and flux-tube concept, both integral to local gyrokinetic simulations, start to break down when the global magnetic shear becomes too small, a zero-shear approximation is attractive in this limit. In the case of HSX, local gyrokinetic simulations of drift-wave-driven microturbulence show that most features of the turbulence for $k_yrho_mathrm{s} geq 0.3$ agree quantitatively between finite- and zero-shear simulations. However, large-scale modes with $k_yrho_mathrm{s} < 0.3$ show markedly different linear characteristics.
In particular, simulations show an odd, but numerically robust, weakly unstable ion-direction-propagating mode with a two-scale nature at finite shear. The two scales are comprised of small-scale curvature-driven structure on top of an asymmetric, extended ballooning envelope, where the asymmetric envelope is linked to the small value of global magnetic shear. Despite being a subdominant mode at finite shear, it couples efficiently to zonal flows, due to the extended envelope, and through zonal flow regulation affects transport at all scales. At zero global magnetic shear, this mode dominates the long-wavelength linear and nonlinear dynamics through zonal flow coupling, making nonlinear zero-shear simulations unfeasible for HSX. In an effort to obtain an accurate zero-shear approximation beneficial for optimization studies, long-wavelength drift waves in a stellarator are examined with a fluid model and the ballooning transform, with particular attention paid to role of low magnetic shear.
This work is supported by U.S.~DoE Grant No.~DE-FG02-99ER54546.
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