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Description
Mean-field transport coefficients are the main work-horse for the modelling of the edge plasma of magnetic fusion devices and are heavily used in the frame of predictive studies for the design and preparation of operation of future devices. These codes offer a high-fidelity description of plasma-neutrals and multi-species interactions at the cost of a simplified description of transverse turbulent transport in the form of a gradient-diffusion model. In practice, transverse transport is prescribed by 3 ad-hoc diffusion coefficients (for particles, parallel momentum and energy) for every single particle species in the plasma. While experimental Scrape-Off-Layer (SOL) width scaling laws can serve as a guide-line to tune the transport coefficients of the dominant species (in general hydrogenic ions), very little information is available to guide the choice of sensible transport coefficients for impurities.
In this contribution, we report on a first evaluation of the turbulent transport of light impurities from 3D turbulence simulations performed with the SOLEDGE3X code. Of particular interest for this study is the capability of SOLEDGE3X to model plasmas of arbitrary composition thanks to its implementation of the multi-fluid Zhdanov closure. We consider a TCV attached deuterium plasma based on the TCV-X21 reference scenario. Carbon is chosen here as the impurity of interest. Two simulations are considered: one in which carbon is self-consistently generated by erosion processes at the divertor target plates and one in which it is injected in the plasma from the outboard mid-plane. By doing so we can compare whether the resulting transport differs whether the carbon source is downstream or upstream of the main species flow. In both simulations, turbulence is found to be the dominant contributor to the transverse transport of carbon as it is for the deuterium. A key difference with the main species however is that in the concentrations considered here (a few percents) carbon mostly behaves like a passive tracer. This results in a diffusive behavior for carbon independently of it being injected from the mid-plane or at the targets, allowing carbon to radially penetrate inwards against the deuterium flux. Like for the main species, the transport coefficients of carbon are found significantly inhomogeneous in space, with enhanced transport in the outboard mid-plane and along the outer divertor leg. We finally compare the amplitudes of transport coefficients for carbon and deuterium and come up with 0th order recommendations for mean-field models.