Radial impurity transport in the pedestal of ASDEX Upgrade plasmas in H-mode, L-mode, and the QCE regime

Abstract We present an experimental study of radial impurity transport in the pedestal, comparing ASDEX Upgrade discharges across different operation regimes. The analyses employ our recently developed framework that extracts flux surface-averaged diffusion and convection profiles from the stationary balance of multiple impurity charge state densities. High radial resolution and rigorous uncertainty quantification are achieved through a customized diagnostic setup of the charge-exchange spectroscopy and with Bayesian sampling to solve the complex inverse problem. For optimal diagnostic feasibility the presented study focuses on neon transport. Dedicated discharges were performed with neon puffing in high confinement mode (H-mode) with large, type-I, edge localized modes (ELMs), in low confinement mode (L-mode), and in the quasi-continuous exhaust (QCE) regime. The inferred transport coefficients are compared with neoclassical modeling to disentangle collisional and anomalous contributions. Our results validate previous findings of predominantly neoclassical impurity transport between type-I ELMs, caused by the turbulence suppressing edge transport barrier in the H-mode pedestal. We also recover the expected strong turbulent impurity diffusion in the L-mode edge, and qualitatively confirm that large ELMs act on impurities as additional diffusion. The QCE regime is a promising confinement scenario for future reactors due to the absence of large ELMs in combination with high separatrix densities. Its pedestal dynamics are modified by the presence of small, type-II, ELMs together with a quasi-coherent mode. Their impact on impurity transport is quantified as significant anomalous diffusion in the pedestal, leading to weaker impurity density gradients.