Description
Control of impurity transport in tokamaks is crucial for achieving and sustaining good confinement in fusion reactor plasmas. Impurities can have both beneficial and detrimental effects depending on their concentration, species, and spatial distribution. For example, impurity seeding in the edge region mitigates divertor heat loads, whereas contamination by high-Z impurities such as tungsten from plasma-facing components degrades core confinement.
While bulk plasma transport is typically dominated by turbulence, neoclassical transport driven by Coulomb collisions often plays an important role for impurities because of their high charge numbers and collisionalities. The local neoclassical transport model based on the moment approach has been widely used to evaluate impurity transport. However, recent global gyrokinetic studies have reported deviations from local predictions, particularly for high-Z impurities. These discrepancies are significant for neoclassical thermal screening driven by bulk ion temperature gradients, which is essential for preventing core accumulation of heavy impurities. Although the poloidal asymmetry has been suggested as a possible mechanism, the underlying physics of the disagreement remains incompletely understood.
In this study, we perform global gyrokinetic simulations of trace impurities in an axisymmetric tokamak configuration, covering a wide range of charge numbers and masses. The simulations include effects not captured in the conventional local model, such as the finite banana
width and the compressibility of the particle and heat flows. The role of the poloidal asymmetry is also examined. Systematic comparisons between global gyrokinetic and local neoclassical calculations are conducted to clarify the parameter dependence on quantities such as normalized collisionality and orbit width. The impact of the global effects is evaluated across impurity species with different charge numbers and masses.