Stellarator microinstabilities and turbulence at low magnetic shear

Abstract

Gyrokinetic simulations of drift waves in low-magnetic-shear stellarators reveal that simulation domains comprised of multiple turns can be required to properly resolve critical mode structures important in saturation dynamics. Marginally stable eigenmodes important in saturation of ion temperature gradient modes and trapped electron modes in the Helically Symmetric Experiment (HSX) stellarator are observed to have two scales, with the envelope scale determined by the properties of the local magnetic shear and an inner scale determined by the interplay between the local shear and magnetic field-line curvature. Properly resolving these modes removes spurious growth rates that arise for extended modes in zero-magnetic-shear approximations, enabling use of a zero-magnetic-shear technique with smaller simulation domains and attendant cost savings. Analysis of subdominant modes in trapped electron mode (TEM)-driven turbulence reveals that the extended marginally stable modes play an important role in the nonlinear dynamics, and suggests that the properties induced by low magnetic shear may be exploited to provide another route for turbulence saturation.

Theory of ITG turbulent saturation in stellarators: Identifying mechanisms to reduce turbulent transport

“…However, it might be possible to overcome the unfavorable linear stability properties by appealing to a strong nonlinear saturation mechanism. Indeed, nonlinear gyrokinetic simulations of trapped electron turbulence in HSX geometry exhibit significant signatures of damped eigenmodes as a prominent feature of saturated turbulence [50].…”

Advancing the physics basis for quasi-helically symmetric stellarators

“…Nonlinear flux-tube gyrokinetic calculations were performed to describe ion temperature gradient (ITG) turbulence at the s = 0.5 surface using the GENE code (Jenko et al 2000) for various values of the ion temperature scale length a/L Ti assuming adiabatic electrons. In figure 6, the heat flux in dimensionless gyro-Bohm units for WISTELL-A is compared to two other configurations, the HSX configuration, which has been well analysed for turbulent transport (Faber et al 2015;Pueschel et al 2016;Faber et al 2018;McKinney et al 2019), and a third 'turbulent reduced' configuration, which was the result of a separate optimization calculation. The turbulence-reduced configuration is identical to the configuration in Bader et al (2019) labelled 'Opt.…”

A comparison of turbulent transport in a quasi-helical and a quasi-axisymmetric stellarator

“…The black lines indicate the α = θ − ι -ξ = 0 flux-tube domains of each configuration, where θ and ξ are the poloidal and toroidal angle in Boozer coordinates, respectively. accurately described (Faber et al 2018). In Figure 1, examples of n pol = 2 flux-tube domains of each configuration are shown, where n pol indicates the number of poloidal transits of the flux tube.…”

“…Prior gyrokinetic studies of HSX flux tubes indicate is required for proper convergence (Faber et al. 2018).…”