On the interpretation of kinetic effects from ionization in fluid models and its impact on filamentary transport

Abstract

In regions where a plasma is not fully ionized, such as the edge and scrape-off layer (SOL) regions in a tokamak, the charged particles may be subject to strong sources from interactions with neutral atoms and molecules. Such sources, e.g., from electron impact ionization, can introduce kinetic effects, as the ionized particles may have flow velocity and temperature different from that of the main species. If treated in the conventional fluid picture, this kinetic effect emerges as a frictional heating term. In this paper, the physics of this term is discussed, both for un-magnetized and magnetized plasmas. The fluid source terms are mapped back to the kinetic sources to provide a consistent picture for future model comparison. In the limits of low and high ratios between the rates of thermalization and ionization, a multi-ion species drift-fluid model is applied to assess the impact of this kinetic effect on SOL drift-plane plasma transport. This is done by modeling a seeded blob where the ions follow either a single- or double-Maxwellian velocity distribution function (VDF). It is found that the robustness of the magnetized plasma VDF in the drift-plane and the limited effect on the vorticity source ensure that the impact of kinetic effects on the perpendicular blob evolution is small, even in the limit of high ionization to thermalization rate ratio, where kinetic effects to the ion VDF are significant.