Using geometry to optimize turbulence in quasi-helically symmetric stellarators*

Reducing turbulent energy and particle losses represents one of the major challenges facing magnetically confined fusion and is a major design goal of a future stellarator.  Observations from fluid modeling and gyrokinetic simulations indicates saturation of microturbulence in the quasi-helically symmetric stellarator HSX is strongly influenced by three-wave, nonlinear energy transfer between stable and unstable eigenmodes mediated by a third, non-zonal mode[1,2].  The nonlinear coupling strength is sensitive to the local magnetic geometry on the flux surface and suggests that geometry may be manipulated to optimize energy transfer between unstable and stable modes, reducing microturbulence levels and fluxes.  To help design a turbulence-optimized stellarator, an analytic, three-field fluid model for ITG turbulence saturation in general 3D geometry has been implemented within an optimization framework.  The first turbulence optimization results will be presented and compared against nonlinear gyrokinetic simulations, focusing on identifying geometric mechanisms responsible for increased confinement.

[1] Hegna et al. PoP, 25, 022511 (2018)
[2] Faber et al. To be submitted to JPP, 2018