Non-parametric inference of impurity transport coefficients in the ASDEX Upgrade tokamak*

 Controlling impurity ions is a critical requirement for a fusion reactor. Impurity accumulation in the core region dilutes fuel and radiates away power. However, a proper amount of impurity is likely to be required in the edge region in order to mitigate the heat load on the plasma facing components.

   The transport properties of impurity ions are often experimentally characterized by using a diffusion coefficient D and a convection velocity v. Since the realistic experimental setups usually necessitate the determination of D and v under ill-posed conditions, rigorous comparisons between simulations and experiments cannot be made without addressing the model selection and uncertainty estimation problems. To this end, the Bayesian formalism has been applied to the impurity transport studies in recent years.

   In this work, transport coefficients of light impurities (Ne, O, and C) in a Type-III ELMy H-mode plasma are inferred by using a non-parametric approach. Unlike spline functions, the non-parametric inference does not reduce the degrees of freedom for representing spatial profiles. Thus, all solutions that are consistent with the measurements and the prescribed smoothing conditions can be calculated. We report the D and v profile measurements from rho_pol=0.6-1.0.