Reduction (or enhancement) of stellarator turbulence by impurities

The deliberate injection of impurities into the plasma core has been proven to increase the energy confinement time, and to reduce the turbulent fluctuations and the turbulent ion heat diffusivity in the Large Helical Device (LHD) [1] and in Wendelstein 7-X (W7-X) [2]. However, in general, impurities are neglected in the context of gyrokinetic modeling of bulk plasma turbulence in stellarators. In this talk, we present a systematic study of the impact of impurities on stellarator bulk turbulence, and in particular on the turbulent heat fluxes in W7-X. By means of nonlinear multispecies gyrokinetic simulations with the code stella [3], we demonstrate that if the impurity density gradient and the bulk ion density gradient are parallel (resp. antiparallel), impurities can significantly reduce (resp. enhance) turbulent heat losses. We will explain that this is caused by the intrinsic capacity of impurities to modify bulk plasma turbulence, and not by trivial effects such as dilution or changes in the bulk plasma profiles. For the relevant situation where the impurity and bulk ion density gradients are parallel, the heat flux exhibits a minimum at a certain impurity concentration. These results show the potential of impurities for enabling access to enhanced confinement regimes in stellarators.

References:

[1] Nespoli, F. et al. (2021) Nature Physics 18, 350.

[2] Lunsford, R. et al. (2021) Physics of Plasmas 28 082506.

[3] Barnes, M. et al. (2019) Journal of Computational Physics 391, 365–380.