Suppressing Trapped-Electron-Mode-Driven Turbulence in Quasisymmetric Equilibria via 3D Shape Optimization

A goal of stellarator optimization is to find configurations that reduce turbulent transport using three-dimensional (3D) shaping. Trapped-electron-mode (TEM) turbulence can play a significant role in quasi-symmetric stellarators [1]. Gyrokinetic simulations suggest that the heat flux of TEM turbulence correlates with the free energy in background temperature and density gradients, as quantified by an available energy (AE) metric [2]. Towards this end, a new optimization framework is developed using local 3D MHD equilibrium solutions [3]. This approach has been successfully employed to improve the quasi-symmetry properties while simultaneously reducing AE in helically rotating negative and positive triangularity stellarators. The gyrokinetic code GENE is used to assess the local TEM linear characteristics and nonlinear behavior. Linear analysis shows this optimization approach can strongly suppress TEMs. However, universal instabilities (UI) and electron-temperature-gradient modes can be unstable. Nonlinear simulations show that UIs drive large heat fluxes at finite density gradients, while there are no significant fluctuations on ion scales with only electron temperature gradients. Also, an increase in plasma β to 4x10^-3 reduces UI growth rates and substantially decreases the heat flux. These insights suggest AE may be used as an optimization target to successfully reduce TEM turbulence while raising questions on the role of UIs in turbulence-driven transport in stellarators.

References

[1] B. Faber et al., Phys. Plasmas 22 (2015)

[2] R. J. J . Mackenbach et al., Phys. Rev. Lett. 128 (2022)

[3] J. M. Duff et al., Phys. Plasmas 29 (2022)