Investigation of multi-ion heat and neoclassical transport using new edge main ion measurements at ASDEX Upgrade

This study provides new insights into multi-ion heat transport and the validation of neoclassical theory at the edge of H-mode plasmas. Utilizing a high-resolution main ion charge exchange recombination spectroscopy system, the first characterization of edge deuterium temperature (TD) and toroidal velocity ( v ϕ , D ) in a metal wall environment is presented. Dedicated experiments which examine the impact of the heating mix on TD and v ϕ , D in the ASDEX Upgrade tokamak are discussed. An unexpected temperature difference between main ions (TD) and impurities (Tz) was discovered when increasing P ECRH , T D > T z . The new temperature measurements have been used to solve the multi-ion heat transport equations with the astra transport code. The interpreted deuterium (χD) and impurity (χz) heat diffusivities have been compared to fluid (tglf) and gyrokinetic (gkw) models. While the dependence of a qualitatively similar χ D / χ z on the ion to electron heat flux ( Q i / Q e ) was identified in both experiment and simulation, discrepancies in the absolute value between the two are found when temperature differences between deuterium and impurities are present. The mechanism for the χ D / χ z dependence on Q i / Q e is the stronger resonant interaction of impurities with low drift frequency turbulence modes in comparison to deuterium. Importantly, it is shown that considering a single ion species (i), and assuming T D = T z gives reasonable estimates of χ D ≈ χ i for the cases studied here. On the contrary, the evaluation of χz is very sensitive to ion temperature differences, which must be considered for accurate impurity ion heat transport description. Additionally, differences between v ϕ , D and v ϕ , z were compared to neoclassical calculations. Neoclassical theory can accurately describe v ϕ , D (provided an independent measurement of v ϕ , z or Er) in the steep gradient region, but not at the pedestal top nor bottom, highlighting the complexity of edge transport phenomena.