Towards increasingly local electron kinetics: neutral pressure effects on remote plasma production in a convergent magnetic field
Abstract
Remote plasma production in magnetic nozzles (i.e., convergent-divergent magnetic fields) - where the density peak occurs far from the region of primary RF excitation - is currently attributed to energetic electrons formed near the antenna which transport their energy along the magnetic field lines to drive non-local ionisation downstream. While this phenomenon has been extensively studied at low neutral pressures where the electron relaxation mean free path λ rel is greater than the plasma length scale L , the effect on remote plasma production of increased collisionality in high-pressure regimes where λ rel ≤ L has not yet been directly characterised. This study presents axial profiles of the main plasma parameters and electron energy probability functions (EEPFs) in the ADR-T linear device, operated at argon pressures of 0.25 Pa and 0.85 Pa, spanning conditions from λ rel > L to λ rel ≤ L . At lower pressure, the ion saturation current peaks near the magnetic throat, the EEPF retains a high-energy tail with a bi-Maxwellian structure at the location of maximum field strength and the ionisation rate coefficient does not peak under the antenna - consistent with non-local electron kinetics. At 0.85 Pa, the density peak is observed to shift upstream towards the plasma source, the high-energy tail is progressively depleted, and the EEPF evolves into a Druyvesteyn-like distribution, while the ionisation rate coefficient decreases monotonically downstream, suggesting increasingly local electron kinetics. Across the pressure range analysed, both the axial location of the density peak and the electron relaxation mean free path vary monotonically with pressure, providing experimental observation that electron collisions affect the maximum axial distance over which non-local remote ionisation could be sustained, and proposing neutral pressure as a controllable parameter for tailoring plasma production in convergent-divergent magnetic field configurations.
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BibTeX
@article{caldarelli2026towards,
title = {{Towards increasingly local electron kinetics: neutral pressure effects on remote plasma production in a convergent magnetic field}},
author = {Antonella Caldarelli and Kazunori Takahashi and Rod W Boswell and Christine Charles and Stephane Mazouffre},
journal = {Journal of Physics D: Applied Physics},
year = {2026},
month = oct,
doi = {10.1088/1361-6463/aeb13c},
url = {https://hal.science/hal-05785318v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Towards increasingly local electron kinetics: neutral pressure effects on remote plasma production in a convergent magnetic field
AU - Antonella Caldarelli
AU - Kazunori Takahashi
AU - Rod W Boswell
AU - Christine Charles
AU - Stephane Mazouffre
PY - 2026
DA - 2026/10/07
JO - Journal of Physics D: Applied Physics
DO - 10.1088/1361-6463/aeb13c
UR - https://hal.science/hal-05785318v1
AB - Remote plasma production in magnetic nozzles (i.e., convergent-divergent magnetic fields) - where the density peak occurs far from the region of primary RF excitation - is currently attributed to energetic electrons formed near the antenna which transport their energy along the magnetic field lines to drive non-local ionisation downstream. While this phenomenon has been extensively studied at low neutral pressures where the electron relaxation mean free path λ rel is greater than the plasma length scale L , the effect on remote plasma production of increased collisionality in high-pressure regimes where λ rel ≤ L has not yet been directly characterised. This study presents axial profiles of the main plasma parameters and electron energy probability functions (EEPFs) in the ADR-T linear device, operated at argon pressures of 0.25 Pa and 0.85 Pa, spanning conditions from λ rel > L to λ rel ≤ L . At lower pressure, the ion saturation current peaks near the magnetic throat, the EEPF retains a high-energy tail with a bi-Maxwellian structure at the location of maximum field strength and the ionisation rate coefficient does not peak under the antenna - consistent with non-local electron kinetics. At 0.85 Pa, the density peak is observed to shift upstream towards the plasma source, the high-energy tail is progressively depleted, and the EEPF evolves into a Druyvesteyn-like distribution, while the ionisation rate coefficient decreases monotonically downstream, suggesting increasingly local electron kinetics. Across the pressure range analysed, both the axial location of the density peak and the electron relaxation mean free path vary monotonically with pressure, providing experimental observation that electron collisions affect the maximum axial distance over which non-local remote ionisation could be sustained, and proposing neutral pressure as a controllable parameter for tailoring plasma production in convergent-divergent magnetic field configurations.
SN - 0022-3727
SN - 1361-6463
ER -