Intrinsic rotation modulation by diffusive neutral particles in tokamaks

Abstract

Abstract The modulated transport model, a model kinetic ion transport equation for the pedestal and scrape-off layer, is generalized to self-consistently include effects of a single neutral particle species. The neutrals contribute additional transport terms, modifying the v||-dependent orbit-averaged ion diffusivities of the original work and the resulting predicted intrinsic rotation of the ions. After making simplifying assumptions of the neutral transport, in particular taking the continuous transport limit via a short charge-exchange step expansion, we derive relatively simple analytic expressions that capture the diffusive neutral physics. Within the scope of the model's validity, the neutral-driven intrinsic rotation can compete with the turbulence-driven intrinsic rotation. However, for physically motivated parameters, the neutral-driven intrinsic rotation appears negligible, either on a term-by-term basis or due to a strong cancellation between the neutral-driven momentum diffusion and pinch terms. It appears that a treatment containing finite charge-exchange steps is necessary to capture neutral transport of strong flow momentum into the confined region from the scrape-off layer.