April 15-17

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Abstracts

Author: U. Gupta
Requested Type: Poster
Submitted: 2019-02-22 16:19:15

Co-authors: C. R. Sovinec, J. Boguski, M. D. Nornberg, J. S. Sarff, K. J. McCollam

Contact Info:
University of Wisconsin-Madison
1500 Engineering Dr.
Madison, WI   53706
USA

Abstract Text:
The Quasi-Single Helical (QSH) state of Reversed Field Pinches (RFPs) is dominated by one of the m=1 helical modes with non-dominant modes also existing at finite amplitudes. It has been investigated by the RFX and MST RFP groups as a means to improve confinement. Our numerical computations with the NIMROD code seek to understand how the state is maintained in the presence of thermal transport and pressure-driven dynamics. We apply two approaches. The first generates axisymmetric pinch states through time-dependent evolution with sourcing. Restarting in 3D, albeit with restricted five-fold toroidal periodicity, leads to a 3D QSH state with a dominant m=1, n=5 helical mode at saturation. Toroidal computations produce the characteristic single-helical axis (SHAx) behavior of strong QSH, including a region of closed flux surfaces with elevated temperatures. Poloidal flows have a more complicated structure than what would be expected from the magnetic spectrum. Complex flow patterns are also observed in recent core-localized 3D ion flow measurements in the SHAx state of MST, and they indicate the significant multi-mode activity that remains in the state. Comparison with results from cylindrical computations is used to distinguish the influence of toroidal geometry. The second approach evolves just the perturbations from a symmetric Ohmic steady state with pressure. As a first step, the 3D evolution of a Z-Pinch equilibrium in cylindrical geometry with a strong guide field and pressure gradient is being investigated. Future work includes performing simulations with complete unrestricted toroidal representation that can provide insights into the spontaneous formation and sustainment of QSH conditions in RFPs.
*Supported by the U.S. DOE through grant DE-FG02-85ER53212.

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