Plasma-neutral collision operators for hydrogen isotopes: kinetic and gyrokinetic descriptions, comparison to fluid
Abstract
Plasma-neutral interactions play a critical role in the dynamics of the edge of magnetically confined fusion devices. Neutral atoms, originating from recycling at the wall, gas injection, outgassing, or volumetric recombination, affect the velocity distribution of both ions and electrons, potentially changing the local instability drives. Standard fluid models simplify these processes (namely charge exchange, ionization and recombination) using Maxwellian assumptions and linearized source terms. In this paper, we revisit the impact of these assumptions in the case of a hydrogen plasma interacting with hydrogen atoms. We first present a comprehensive derivation of plasma-neutral interaction terms in the kinetic and gyrokinetic frameworks. Unlike fluid approaches, the model naturally retains non-Maxwellian features of both the plasma and neutral distribution functions. By explicitly accounting for velocity-dependent differential cross-sections, the model 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. We moreover include energy losses due to excitation, multi-step ionization and radiative recombination, and demonstrate that the fluid moments of the kinetic model reduce 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 assumptions for the plasma, over a range of tokamak-relevant conditions, both in terms of their effects on the distribution functions and their fluid moments. Substantial discrepancies are observed, even for Maxwellian distributions. For charge exchange, the kinetic model yields momentum and energy exchange rates up to twice those predicted by fluid descriptions. We furthermore show that a fluid treatment of ionization greatly overestimates the mean kinetic energy of the produced electrons. These results highlight the limitations of fluid descriptions of plasma-neutral interactions and the importance of kinetic effects for accurately modelling momentum and energy exchange in the edge plasma.
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BibTeX
@misc{matthieu2026plasmaneutral,
title = {{Plasma-neutral collision operators for hydrogen isotopes: kinetic and gyrokinetic descriptions, comparison to fluid}},
author = {Protais Matthieu and Guilhem Dif-Pradalier and Yanick Sarazin and Virginie Grandgirard and Yannick Marandet and Srikanth Sureshkumar and Naren Varadarajan},
howpublished = {HAL},
year = {2026},
month = oct,
url = {https://cea.hal.science/cea-05795126v1},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - Plasma-neutral collision operators for hydrogen isotopes: kinetic and gyrokinetic descriptions, comparison to fluid
AU - Protais Matthieu
AU - Guilhem Dif-Pradalier
AU - Yanick Sarazin
AU - Virginie Grandgirard
AU - Yannick Marandet
AU - Srikanth Sureshkumar
AU - Naren Varadarajan
PY - 2026
DA - 2026/10/12
PB - HAL
UR - https://cea.hal.science/cea-05795126v1
AB - Plasma-neutral interactions play a critical role in the dynamics of the edge of magnetically confined fusion devices. Neutral atoms, originating from recycling at the wall, gas injection, outgassing, or volumetric recombination, affect the velocity distribution of both ions and electrons, potentially changing the local instability drives. Standard fluid models simplify these processes (namely charge exchange, ionization and recombination) using Maxwellian assumptions and linearized source terms. In this paper, we revisit the impact of these assumptions in the case of a hydrogen plasma interacting with hydrogen atoms. We first present a comprehensive derivation of plasma-neutral interaction terms in the kinetic and gyrokinetic frameworks. Unlike fluid approaches, the model naturally retains non-Maxwellian features of both the plasma and neutral distribution functions. By explicitly accounting for velocity-dependent differential cross-sections, the model 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. We moreover include energy losses due to excitation, multi-step ionization and radiative recombination, and demonstrate that the fluid moments of the kinetic model reduce 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 assumptions for the plasma, over a range of tokamak-relevant conditions, both in terms of their effects on the distribution functions and their fluid moments. Substantial discrepancies are observed, even for Maxwellian distributions. For charge exchange, the kinetic model yields momentum and energy exchange rates up to twice those predicted by fluid descriptions. We furthermore show that a fluid treatment of ionization greatly overestimates the mean kinetic energy of the produced electrons. These results highlight the limitations of fluid descriptions of plasma-neutral interactions and the importance of kinetic effects for accurately modelling momentum and energy exchange in the edge plasma.
ER -