Description
Plasma-wall interaction, or more generally the scrape-off layer (SOL, region of the plasma with open magnetic surfaces), plays a key role in controlled magnetic fusion, both in terms of machine integrity and fusion performance. In these regions we observe a complex interaction between the plasma and the neutrals, whose presence can be due to recycling, gas injection, outgassing or simply low-temperature plasma recombining. Standard fluid models simplify these processes (namely charge exchange, ionisation and recombination) using Maxwellian assumptions and linearised source terms. In this contribution, we revisit the impact of these assumptions in the ideal case of an hydrogen plasma interacting with hydrogen atoms.
We first present the comprehensive derivation of plasma-neutral interaction terms in the kinetic framework. Unlike fluid approaches [1], the model naturally retains non-Maxwellian features of both plasma and neutral distributions. Since explicitly accounting for velocity-dependent differential cross-sections, it furthermore captures the full kinetic impact of such inelastic collisions on both ions and electrons, and not only through channels like energy or momentum exchange, [2] [3]. We moreover include energy losses due to excitation, multi-step ionisation and radiative recombination, and demonstrate that the kinetic model reduces to standard fluid source terms under the appropriate assumptions of Maxwellianity and linearity.
Secondly, we compare the newly derived kinetic model to existing ones based on fluid hypotheses for the plasma such as [1], [2] or [3], across tokamak-relevant conditions, spanning ion temperatures from 5 to 200 eV, as well as a range of neutral temperatures and fluid velocities. Substantial discrepancies are observed, even for Maxwellian distributions. For charge exchange, notably, the kinetic model yields momentum and energy exchange rates that are 2 to 3 times higher than those predicted by fluid descriptions. Further deviations are observed in higher-order fluid moments. We will present a discussion of some of the expected implications in the SOL and outer core.
[1] N. Horsten et al 2017 Nucl. Fusion 57
[2] D. Reiter et al 2005 Fusion Sci. Technol. 47
[3] T. N. Bernard et al 2022 Phys. Plasmas 29