Electron Kinetics in the Sheath of Atmospheric Pressure Plasmas in Helium and Helium:H2O Vapor
Abstract
The electron energy distribution function (EEDF) plays a central role in determining the rates of electron-induced reactions in the bulk plasma, the sheath, and at the plasma-surface interface. However, the electron kinetics within the sheath and near the surface are generally not well described by conventional Boltzmann solvers, which neglect the sheath electric field and electron losses to the surface. In this study, we investigate electron kinetics in atmospheric pressure plasmas in pure helium and helium-water vapor mixtures using a hybrid model that combines a fluid description of the collisional sheath with a particle kinetic Monte Carlo model for electrons. The floating wall potential is determined taking into account electron diffusion through the sheath and the non-Maxwellian EEDF. Our results demonstrate that electron kinetics within the sheath can exhibit both local and nonlocal behavior, even at atmospheric pressure. The transition between these regimes is governed primarily by the ratio of the electron energy relaxation length to the sheath thickness. At high plasma densities, where the sheath thickness becomes smaller than the electron energy relaxation length, the EEDFs approach the nonlocal limit, exhibiting significant changes in electron kinetic energy toward the wall. The addition of water vapor reduces the energy relaxation length and shifts the electron kinetics toward the local regime. In the nonlocal regime, the electron mean kinetic energy decreases substantially toward the wall, accompanied by reductions of more than an order of magnitude in electron-impact excitation and ionization rate coefficients. These findings demonstrate that modeling of electron-induced processes in the sheath and at plasma-surface interfaces requires accounting for nonlocal electron kinetics and electron losses to the surface, even at atmospheric pressure.
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BibTeX
@misc{kortshagen2026electron,
title = {{Electron Kinetics in the Sheath of Atmospheric Pressure Plasmas in Helium and Helium:H$_2$O Vapor}},
author = {Uwe Kortshagen},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.05653},
eprint = {2611.05653},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.05653},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Electron Kinetics in the Sheath of Atmospheric Pressure Plasmas in Helium and Helium:H_2O Vapor
AU - Uwe Kortshagen
PY - 2026
DA - 2026/11/03
PB - arXiv
DO - 10.48550/arxiv.2611.05653
UR - https://arxiv.org/abs/2611.05653
AB - The electron energy distribution function (EEDF) plays a central role in determining the rates of electron-induced reactions in the bulk plasma, the sheath, and at the plasma-surface interface. However, the electron kinetics within the sheath and near the surface are generally not well described by conventional Boltzmann solvers, which neglect the sheath electric field and electron losses to the surface. In this study, we investigate electron kinetics in atmospheric pressure plasmas in pure helium and helium-water vapor mixtures using a hybrid model that combines a fluid description of the collisional sheath with a particle kinetic Monte Carlo model for electrons. The floating wall potential is determined taking into account electron diffusion through the sheath and the non-Maxwellian EEDF. Our results demonstrate that electron kinetics within the sheath can exhibit both local and nonlocal behavior, even at atmospheric pressure. The transition between these regimes is governed primarily by the ratio of the electron energy relaxation length to the sheath thickness. At high plasma densities, where the sheath thickness becomes smaller than the electron energy relaxation length, the EEDFs approach the nonlocal limit, exhibiting significant changes in electron kinetic energy toward the wall. The addition of water vapor reduces the energy relaxation length and shifts the electron kinetics toward the local regime. In the nonlocal regime, the electron mean kinetic energy decreases substantially toward the wall, accompanied by reductions of more than an order of magnitude in electron-impact excitation and ionization rate coefficients. These findings demonstrate that modeling of electron-induced processes in the sheath and at plasma-surface interfaces requires accounting for nonlocal electron kinetics and electron losses to the surface, even at atmospheric pressure.
ER -