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Author: Zachary R Williams
Requested Type: Poster
Submitted: 2017-03-15 11:17:08

Co-authors: M. J. Pueschel, P. W. Terry

Contact Info:
University of Wisconsin-Madison
1150 University Avenue
Madison, WI   53715

Abstract Text:
Gyrokinetic simulations of improved-confinement discharges, where large-scale instabilities are reduced, of the Madison Symmetric Torus Reversed-Field Pinch (RFP) exhibit very strong zonal flow formation when tearing mode effects are neglected. A computational study of Trapped Electron Mode (TEM) and Ion Temperature Gradient (ITG) turbulence shows that zonal flows play a much more significant role in the regulation of transport for TEM than in ITG. This trait is apparent in the nonlinear upshift of critical gradient when comparing TEM and ITG: in RFPs, the nonlinear critical gradient is a factor of four larger than the linear for TEM turbulence, while nonlinear and linear critical gradients nearly coincide in the ITG case. Additionally, the degradation of zonal flow activity due to tearing mode effects plays a significant role in determining TEM transport, but leaves ITG transport largely unaffected. This work characterizes RFP zonal flows to understand these observations.

Three specific aspects of zonal flows are presented here: Rosenbluth-Hinton zonal flow residual behavior in an RFP geometry, the effect of collisionality on the zonal-flow suppression of turbulence, and the role of secondary instability driving zonal flows. Zonal flow residuals are significantly larger in the RFP than in tokamak geometry; this is a general consequence of the low-q regime of operation in RFPs. In comparing TEM and ITG, collisionality is seen to play a significant role in TEM physics, while contributing very little to ITG, reflecting stronger linear TEM stabilization than zonal flow erosion. Finally, secondary instability analysis reveals zonal flows in the TEM regime to have twice the growth rate of the ITG case. Cumulatively, these effects are responsible for the extremely strong zonal flow activity described above, and offer insight into some important differences between ITG and TEM. This work was supported by the U.S. Department of Energy, Grant No. DE-FG02-85ER-53212.