Characterization of the collisional transport of a high-$Z$ impurity in a Wendelstein 7-X Electron Cyclotron Resonance Heated plasma

A concern for stellarators is the central accumulation of high-$Z$ impurities driven by the radial electric field, expected to point inwards at reactor relevant conditions. Here we study the radial transport of tracer Ar16+ impurities in an ECRH plasma from the last campaign of the W7-X stellarator [Klinger et al. Nucl. Fusion 59 (2019) 112004], and compare results from collisional transport calculations to experimental values inferred from X-ray Imaging Crystal Spectrometer measurements. The calculations are performed with three radially local drift-kinetic equation solvers, SFINCS [Landreman et al. Phys. Plasmas 21 (2014) 042503], EUTERPE [Rega\~na et al. Nucl. Fusion 57 (2017) 056004] and KNOSOS [Velasco et al. arXiv:1908.11615]. These tools include effects such as advanced collision operators and potential variations along flux-surfaces which can be important when studying high-$Z$ impurities. We find that the radial collisional transport of Ar16+ is dominated by convection, and that the collisional transport can at most account for $\sim$10\% of the observed transport.