Magneto-Fluid Dynamics Seminar

The large space of possible stellarator configurations[1] provides a means to optimize different physics problems by changing the magnetic geometry, which has been most readily achieved in neoclassical-transport-optimized stellarators such as the quasi-isodynamic W7-X stellarator and the quasi-helically symmetric HSX stellarator. Due to its complicated physical nature and the need to analyze complex numerical simulations, comparatively little progress has been made understanding how magnetic field geometry affects transport due to drift-wave-driven microturbulence.  However, recent theory and simulation work has indicated the importance the choice of 3D geometry plays on determining microturbulence saturation and subsequent transport levels in stellarators. For Ion-Temperature-Gradient-driven (ITG) turbulence, simulations indicate turbulence saturation in W7-X is primarily moderated by nonlinear interactions between turbulent eddies and zonal flows, while in HSX saturation occurs mostly through nonlinear eddy-eddy interactions[2].  Detailed analysis of the nonlinear three-wave turbulent interactions in an analytic fluid model of ITG turbulence further indicate that for HSX, energy transfer is strongly influenced by three-wave nonlinear interactions between unstable modes and stable modes at similar scales[3].  This nonlinear model provides a natural optimization metric, as nonlinear energy transfer is quantified by a three-wave interaction time, where the frequencies involved in the interaction time have been modified by the turbulent nonlinearity through a nonlinear closure.

 

Promisingly, HSX possess the flexibility to alter its MHD equilibrium by adding a magnetic well or hill component.  Nonlinear gyrokinetic GENE simulations show an ITG-driven turbulence minimum when the magnetic hill is increased, where larger ITG-driven transport is observed with a large magnetic well term.  These results are replicated by the nonlinear ITG fluid model, where three-wave interaction times peak when the transport is minimized, indicating a more prominent role of stable modes with increasing magnetic hill, while three-wave interaction times are smaller with increasing magnetic hill.  This picture will be compared against detailed analysis of the spectral gyrokinetic energy transfer, which are obtained with an advanced diagnostic developed to track the evolution of nonlinear energy transfer both between different wavenumbers and different eigenmode that can expose the role of stable modes in turbulence saturation[4].  These results form the motivation for optimization starting from the HSX configuration, focused on increasing three-wave interaction times to decrease turbulent transport.

 

[1] M. Landreman, W. Sengupta, and G.G. Plunk, J. Plasma Physics, 85, 90580103 (2019)

[2] G.G. Plunk, P. Xanthopoulos, and P. Helander, Phys. Rev. Lett., 119, 105002 (2017)

[3] C.C. Hegna, P.W. Terry, and B.J. Faber, Phys. Plasmas, 25, 022511 (2018)

[4] G.G. Whelan, M.J. Pueschel, P.W. Terry, J. Citrin, I.J. McKinney, W. Guttenfelder, and H. Doerk., “Saturation and Nonlinear Electromagnetic Stabilization of ITG Turbulence”, submitted to Physics of Plasmas.