Rotational and vibrational temperatures in a hydrogen discharge with a magnetic X-point

A novel plasma source with a magnetic X-point has been developed to probe an alternative for cesium-free negative hydrogen ion production. This study presents first results for the gas and vibrational temperatures in the source at 1 Pa and various RF powers. The temperatures are obtained from analysis of the intensity distribution of the molecular Fulcher-α bands. The gas temperature increases with the RF power, while the vibrational temperature remains constant in the studied range of RF powers. Both quantities show no appreciable spatial dependence. The obtained high values of the vibrational temperatures indicate a high population of the vibrational levels, favourable for the volume negative ion production. A theoretical concept indicates the presence of an optimum value for the vibrational temperature at which the negative hydrogen ion yield by volume processes has a maximum. Coincidently, the measured value is close to this optimum. This indicates that the novel concept can provide certain advantages compared to other sources based on volume production

Comment on: Measurement of the force exerted on the surface of an object immersed in a plasma

Surfaces exposed to a plasma experience a certain pressure that pushes them away from the volume. This effect has been investigated experimentally in a recent article by Thomas Trottenberg, Thomas Richter, and Holger Kersten from Kiel University/Germany [Eur. Phys. J. D 69, 91 (2015)]. The experimental results are impressive and have actually drawn the attention of the community to an interesting question which so far has been largely ignored. In addition to their experimental results the Kiel group proposes also a rough concept in order to explain their findings which provides certainly a basic qualitative understanding of the physical processes involved. However, on a closer inspection the picture developed so far is not entirely satisfying and the problem seems to require a more fundamental approach. This comment shows that the effect of the wall pressure can be described exactly using only analytical methods. The physical situation is analyzed by three different approaches. First, the simple case of only one spatial dimension is presented in detail. Second, the case of spherical symmetry is analyzed by some simplifying assumptions in order to investigate the effect of higher dimensionality. Third, a formal derivation for arbitrary geometry is given. This general result includes the one-dimensional case but does not allow a convenient connection between the pressures at the wall and in the center. Finally, the results are summarized and some conclusions are drawn

Temporally resolved optical emission spectroscopic investigations on a nanosecond self-pulsing micro-thin-cathode discharge

International audienceAt atmospheric pressure in Ar, a micro-thin-cathode discharge operates in a self-pulsing mode due to periodic ignition of a nanosecond spark discharge with a long living afterglow (several hundred nanoseconds). In this mode, optical emission spectra of the nanosecond spark and the afterglow are investigated. The electron density and temperature in the pure Ar discharge are measured by the Stark broadening and shift of the Ar 3p6 → 1s5 transition (415.859 nm). The nanosecond spark has an electron density of the order of 1017 cm-3 and an electron temperature of 5 eV. The gas temperature is obtained by analyzing the emission spectra of the N2 second positive system with an admixture of 0.5% N2. The measured gas temperature agrees very well with the result of a zero-dimensional kinetic simulation. The temporal development of the spatial distribution of separate emission lines shows that not only the nanosecond spark but the afterglow is also strongly localized. The temporal development of the emission spectrum provides powerful proof that the nanosecond spark discharge, due to thermionic emission, occurs in the self-pulsing mode with nanosecond current peaks

Foundations of magnetized radio-frequency discharges

This is the second part of a set of two papers on radio-frequency (RF) discharges, part of a larger series on the foundations of plasma and discharge physics. In the first paper (Chabert et al 2021 Plasma Sources Sci. Technol. 30 024001) the two basic configurations of RF discharges commonly used in industrial applications, the capacitive and the inductive discharges, are presented. The introduction of an external magnetic field to these discharges results in not only a quantitative enhancement of their capabilities but also leads to qualitatively different interaction mechanisms between the RF field and the plasma. This provides rich opportunities for sustaining dense plasmas with high degrees of ionization. On one hand, the magnetic field influences significantly the particle and energy transport, thus providing new possibilities for control and adjustment of the plasma parameters and opening even lower operation pressure windows. On the other hand, when the magnetic field is introduced also in the region where the plasma interacts with the RF field, qualitatively new phenomena arise, that fundamentally change the mechanisms of power coupling to the plasma—the electromagnetic energy can be transported as waves deeper into the plasma volume and/or collisionlessly absorbed there by wave resonances. The characteristics of these discharges are then substantially different from the ones of the standard non-magnetized RF discharges. This paper introduces the physical phenomena needed for understanding these plasmas, as well as presents the discharge configurations most commonly used in applications and research. Firstly, the transport of particles and energy as well as the theory of waves in magnetized plasmas are briefly presented together with some applications for diagnostic purposes. Based on that the leading principles of RF heating in a magnetic field are introduced. The operation and the applications of various discharges using these principles (RF magnetron, helicon, electron cyclotron resonance and neutral loop discharges) are presented. The influence of a static magnetic field on standard capacitive and inductive discharges is also briefly presented and discussed

Machine learning-based prediction of the electron energy distribution function and electron density of argon plasma from the optical emission spectra

Arellano F.J., Kusaba M., Wu S., et al. Journal of Vacuum Science and Technology A 42, 053001 (2024) https://doi.org/10.1116/6.0003731.Optical emission spectroscopy (OES) is a highly valuable tool for plasma characterization due to its nonintrusive and versatile nature. The intensities of the emission lines contain information about the parameters of the underlying plasma-electron density n e and temperature or, more generally, the electron energy distribution function (EEDF). This study aims to obtain the EEDF and n e from the OES data of argon plasma with machine learning (ML) techniques. Two different models, i.e., the Kernel Regression for Functional Data (KRFD) and an artificial neural network (ANN), are used to predict the normalized EEDF and Random Forest (RF) regression is used to predict n e . The ML models are trained with computed plasma data obtained from Particle-in-Cell/Monte Carlo Collision simulations coupled with a collisional-radiative model. All three ML models developed in this study are found to predict with high accuracy what they are trained to predict when the simulated test OES data are used as the input data. When the experimentally measured OES data are used as the input data, the ANN-based model predicts the normalized EEDF with reasonable accuracy under the discharge conditions where the simulation data are known to agree well with the corresponding experimental data. However, the capabilities of the KRFD and RF models to predict the EEDF and n e from experimental OES data are found to be rather limited, reflecting the need for further improvement of the robustness of these models

The influence of the spatial nonuniformity on the measurement of Ar*(1s 5

International audienceThe self-absorption technique is a simple method to determine the line integrated density of metastable atoms in low-pressure plasmas. In order to employ this technique, it is necessary to make an assumption on the spatial profiles of both the emission intensity and the absorbing species, which usually are unknown and can be highly nonuniform. Taking Ar*(1s5) atoms in a capacitively coupled plasma as an example, the influence of nonuniformity on the measurement of the line integrated density of the absorbing species is investigated analytically. It is proved that when the two spatial profiles are the same, the obtained line integrated density is independent of the functional form of this profile. This is also true if the density of the absorbing species is small (weak absorption), even though the emission profile is different from that of the absorbing species. However, if the density is high (strong absorption), the choice of the profiles has a significant influence on the deduced line integrated density. In the experiment, it is found that in argon-oxygen mixture discharges, in which Ar*(1s5) density is low (weak absorption), the measured densities by self-absorption are in very good agreement with the results obtained from laser absorption. However, in pure argon discharge, when the density is high (strong absorption), the measured densities by self-absorption are significantly smaller than that by laser absorption. Both phenomena have been predicted by the model results. The smaller densities obtained by self-absorption in the pure argon discharge are attributed to the assumption of the same spatial profile used in the model