Benjamin Faber, University of Wisconsin-Madison

"Advances in gyrokinetic simulation of trapped electron mode turbulence in HSX"

Abstract: The ability to manipulate three-dimensional magnetic field structure to modify confinement properties has enabled neoclassical-transport-optimized stellarators, like the Helically Symmetric eXperiment (HSX), to be attractive candidates for future fusion experiments. An exciting research prospect is to further manipulate the magnetic geometry to optimize for turbulent transport; however, this requires at least a basic understanding of turbulence in stellarators to proceed. Nonlinear, flux-tube gyrokinetic simulations of density-gradient-driven Trapped Electron Mode (TEM) turbulence in HSX show several unique features. Two distinct spectral fluctuation regions are observed: long-wavelength slab-like TEMs localized by global magnetic shear that extend along field lines and shortwavelength TEMs localized by local magnetic shear to a single helical bad curvature region. The slab-like TEMs require computational domains significantly larger than one poloidal turn and are computationally expensive, making turbulent optimization studies challenging. A computationally more efficient, zero-average-magnetic-shear approximation is shown to sufficiently describe the relevant nonlinear physics and replicate finite-shear computations, and can be exploited in quasilinear models based on linear gyrokinetics as a feasible optimization tool. TEM quasilinear heat fluxes are computed with the zero-shear approximation and compared to experimentally-relevant nonlinear gyrokinetic TEM heat fluxes for HSX.