50 / 2021-06-18 14:20:28

Electrostatic gyrokinetic turbulence in stellarators and its role in the transport of impurities

fusion,turbulence,transport,stellarators,Impurities

Keeping a low content of impurities in the plasma, in order to avoid intolerable radiation and fuel dilution levels, is a necessary condition to achieve high performance conditions in present day experiments and to make thermonuclear fusion feasible at reactor scale. It is therefore of paramount importance to quantitatively understand the mechanisms that determine the impurity transport in the plasma.

On the side of gyrokinetic turbulent modelling, the computational cost of solving the gyrokinetic system of equations for the three-dimensional magnetic geometry of stellarators is particularly high, even for electrostatic turbulence in the flux tube approximation, when several species with disparate masses are accounted for. Moreover, there have historically been many fewer numerical codes capable of handling stellarator geometry compared with the numerous tools available to address the turbulent transport problem in axisymmetric tokamak configurations. Therefore, it has not been until recently that multispecies nonlinear gyrokinetic simulations have been carried out for large stellarators, like LHD or Wendelstein 7-X (W7-X), see [1] and [2] respectively.

In the present contribution, the turbulent transport of the different species in multi-species stellarator plasmas is studied with the delta-f gyrokinetic code stella [3], with emphasis on the transport of impurities. The code uses a mixed implicit-explicit algorithm that greatly reduces, compared to fully explicit schemes, the necessary computational resources. The advantages of this approach are exploited to present a comprehensive picture of the turbulent impurity transport considering several stellarator configurations. It is found that ordinary diffusion carries the largest fraction of the total radial impurity transport, comparable to the experimentally measured. Regarding the convective part, radially inwards and outwards contributions, depending on the turbulence driving gradients, are obtained. In W7-X, the density gradient of the main species, reduces the background turbulence and, with it, the convection and diffusion coefficient noticeably. Apart from the turbulent impurity transport itself, its relative weight with respect to the neoclassical transport channel and the role of non-trace impurities on the driven turbulence will be aspects discussed as well.



References:

[1] J. M. García-Regaña et al. J. Plasma Phys. 87 (1) 855870103 (2021).

[2] M. Nunami et al. Phys. Plasmas 27 (5) 052501 (2020).

[3] M. Barnes, F. I. Parra and M. Landreman. J. Comp. Phys. 391 365-380 (2019).