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Storing excess renewable energy in hydrocarbons produced from CO2 potentially solves the intermittency issue of renewable energy sources in a green manner. The required reduction of CO2 to CO can be efficiently accomplished with non-equilibrium plasma conversion. On an industrial scale, effects of impurities on the reduction must be taken into account. During this study, the effects of oxygen are considered, as the impurity O2 is both a product of the reduction reaction and abundant in air. In this paper, the influence of O2 addition on the ro-vibrational kinetics of a pulsed DC CO2 glow discharge at 2.5–6.0 Torr—serving as a model non-equilibrium system—is studied in situ with quantum cascade laser infrared absorption spectroscopy. The temporal evolution of the ro-vibrational temperatures is measured, as well as the conversion of CO2 to CO. Trends in the temperature evolutions when increasing the flow rate from 7.4 to 30.0 sccm, varying the pressure, and increasing the O2 admixture up to 90% in increments of 10% are utilised to determine the underlying kinetic processes. Our results show that any decrease in the conversion of CO2 to CO caused by increasing O2 addition cannot be attributed to an induced change in the vibrational kinetics, since the asymmetric stretch mode of CO2—which is associated with dissociation via vibrational excitation—is not quenched. Measured changes in the temporal temperature trends are explained by species-dependent intra- and intermolecular collisional energy transfer processes.

High-quality dielectric films could enable GaN normally off high-electron-mobility transistors (HEMTs). Plasma atomic layer deposition (ALD) is known to allow for controlled high-quality thin-film deposition, and in order to not exceed energy and flux levels leading to device damage, the plasma used should preferably be remote for many applications. This article outlines ion energy flux distribution functions and flux levels for a new remote plasma ALD system, Oxford Instruments Atomfab™, which includes an innovative, RF-driven, remote plasma source. The source design is optimized for ALD for GaN HEMTs for substrates up to 200 mm in diameter and allows for Al2O3 ALD cycles of less than 1 s. Modest ion energies of

Numerous applications have required the study of $$\hbox {CO}_2$$ plasmas since the 1960s, from $$\hbox {CO}_2$$ lasers to spacecraft heat shields. However, in recent years, intense research activities on the subject have restarted because of environmental problems associated with $$\hbox {CO}_2$$ emissions. The present review provides a synthesis of the current state of knowledge on the physical chemistry of cold $$\hbox {CO}_2$$ plasmas. In particular, the different modeling approaches implemented to address specific aspects of $$\hbox {CO}_2$$ plasmas are presented. Throughout the paper, the importance of conducting joint experimental, theoretical and modeling studies to elucidate the complex couplings at play in $$\hbox {CO}_2$$ plasmas is emphasized. Therefore, the experimental data that are likely to bring relevant constraints to the different modeling approaches are first reviewed. Second, the calculation of some key elementary processes obtained with semi-empirical, classical and quantum methods is presented. In order to describe the electron kinetics, the latest coherent sets of cross section satisfying the constraints of “electron swarm” analyses are introduced, and the need for self-consistent calculations for determining accurate electron energy distribution function (EEDF) is evidenced. The main findings of the latest zero-dimensional (0D) global models about the complex chemistry of $$\hbox {CO}_2$$ and its dissociation products in different plasma discharges are then given, and full state-to-state (STS) models of only the vibrational-dissociation kinetics developed for studies of spacecraft shields are described. Finally, two important points for all applications using $$\hbox {CO}_2$$ containing plasma are discussed: the role of surfaces in contact with the plasma, and the need for 2D/3D models to capture the main features of complex reactor geometries including effects induced by fluid dynamics on the plasma properties. In addition to bringing together the latest advances in the description of $$\hbox {CO}_2$$ non-equilibrium plasmas, the results presented here also highlight the fundamental data that are still missing and the possible routes that still need to be investigated.

In situ quantum cascade laser (QCL) absorption spectroscopy is used to investigate the effect of admixed water in a pulsed CO2 glow discharge on the vibrational excitation of CO2 and CO and the conversion of CO2. Time-resolved transmittance spectra of the non-equilibrium CO2 plasma are measured with a 100 μs time resolution. A custom fitting routine is used to extract the time evolution of the gas temperature, rotational temperature and vibrational temperatures of CO2 and CO, while the CO2 conversion is determined from measured CO2 and CO number densities. Rotational Raman scattering is additionally performed in the centre of the reactor to verify measured rotational and vibrational temperatures from line-of-sight absorption spectroscopy. The plasma is operated at 6.7 mbar, with up to 10% water admixed, and is pulsed with a 5–10 ms on-off cycle, with a current of 50 mA supplied during the plasma on-time. Vibrational temperatures and CO2 conversion are not significantly affected by water admixtures below 0.5%. However, the asymmetric stretch temperature of CO2 (T 3) shows considerable quenching upon admixing 10% water vapour, with the maximum elevation above the rotational temperature (T rot) decreasing from 580 ± 86 K to 230 ± 63 K. For the vibrational temperature of CO (T CO), a similar trend is measured. However, the slopes of T 3 and T CO within the first few hundred μs after the start of the plasma remain unchanged, even when admixing 10% water vapour, suggesting equal excitation of the vibrational modes through e–V and V–V interactions. The conversion decreases by almost a factor of 4 when admixing 10% water. We argue that vibrational quenching of CO2 by water can explain part of the decrease. Changes in electron density and temperature and reactions between CO and OH can also play a role.

Over the past few decades many diagnostics have been developed to study the non-equilibrium nature of plasma. These developments have given experimentalists the possibility to measure in situ molecular and atomic densities, electron and ion densities, temperatures and velocities of species in the plasma, to just name a few. Many of the diagnostic techniques are based on the ‘photon-in, photon-out’ principle and were at first developed to perform spectroscopy on atoms and molecules. Much later they were introduced in the research of plasmas. In this foundation paper we will focus on optical-based diagnostics that are now for quite some time common use in the field of low-temperature plasma physics research. The basic principles of the diagnostics will be outlined and references will be given to papers where these techniques were successfully applied. For a more comprehensive understanding of the techniques the reader will be referred to textbooks.

In the past decade, atmospheric-pressure spatial atomic layer deposition (AP-SALD) has gained momentum as a fast deposition technique in thin-film manufacturing [1]. The key benefits of conventional ALD, such as superior control of layer thickness, conformality and homogeneity, in addition to the high throughput enabled by the spatial separation of the half-reactions and the more cost-effective processing at atmospheric pressure, make AP-SALD an interesting option for industrial large-area applications. Finally, the use of atmospheric pressure plasmas as co-reactants enables to rapidly deposit high-quality, dense films at relatively low temperatures [2]. Despite its industrial potential, the further advancement of atmospheric-pressure plasma-enhanced spatial ALD processing still lacks a detailed understanding of the underlying plasma physics and chemistry. Here, effective ALD metrology techniques are of prime importance to obtain more detailed insights into the nature of the process with the aim to improve process performance and thus to open up new industrial application fields. However, due to the very nature of the spatial concept, monitoring AP-SALD processes is rather challenging: especially in the case of close-proximity systems the substrate is typically at ~100 μm distances from the gas injection head. In this work, we employed optical emission spectroscopy (OES) and infrared spectroscopy on effluent plasma gases as diagnostics tools to study the basic chemistry of the AP-SALD process of Al2O3 films grown from Al(CH3)3 and Ar-O2 plasma in a rotary lab-scale reactor (see Fig. 1 and ref. 1). We identified the main reaction products and studied their behavior as a function of the exposure time to the precursor to verify the ALD layer-by-layer growth characteristics. The results show that using spatial separation of the ALD half-reactions and atmospheric pressure plasma as the reactant gives rise to a complex underlying chemistry (see Fig. 2). Infrared absorbance spectra show CO, CO2, H2O and CH4 as the main ALD reaction by-products originating from 1) combustion-like reactions of the methylated surface with oxygen plasma radicals and ozone, and 2) a concurrent latent thermal component related to the substrate. Moreover, CH2O and CH3OH are identified as ALD reaction by-products either formed at the surface or in the plasma by electron-induced dissociation. The investigated trends in CO2, CO and CH4 formed as a function of the exposure time confirm a self-limiting ALD behavior. Finally, the OES results corroborate that, as soon as the plasma-enhanced SALD process takes place, emission from OH and CH arises while excited oxygen species are being consumed. -------------------------------------------------------------------------------------------------------------------------------------------------------------------- References P. Poodt, A. Lankhorst, F. Roozeboom, C. Spee, D. Maas and A. Vermeer, ‘High-speed atomic layer deposition of aluminum oxide layers for solar cell passivation’, Adv. Mater., 22, 3564 - 3567 (2010). M.A. Mione, I. Katsouras, Y. Creyghton, W. van Boekel, J. Maas, G. Gelinck, F. Roozeboom, and A. Illiberi, ‘Atmospheric Pressure Plasma Enhanced Spatial ALD of ZrO2 for Low-Temperature, Large-Area Applications’, ECS Journal of Solid State Science and Technology, 6 (12) N243-N249 (2017), and refererences therein. -------------------------------------------------------------------------------------------------------------------------------------------------------------------- Figure 1

Time-resolved absolute OH density measurements in a millisecond-pulsed DC glow discharge from carbon dioxide with water admixture, by laser-induced fluorescence (LIF) spectroscopy, are presented. For this purpose, a novel technique is suggested to calibrate the OH LIF setup by means of two-photon absorption LIF spectroscopy measurements on a CO-filled gas cell. OH densities in the order of 1 × 1018 m−3 are reported at a pressure of 6.67 mbar with a water admixture of 20% and a discharge current of 50 mA. Furthermore, the time evolution of the CO2 conversion is determined from collision energy transfer LIF and validated against the literature. The time-dependent rotational temperature of OH is obtained with LIF thermometry.

We report first measurements of time-resolved absolute CO number densities and rotational temperatures in a non-thermal CO 2 plasma environment using TALIF with a nanosecond pulsed laser. Two-photon excitation spectra from the B 1Σ+(v′ = 0) ← X 1 Σ+ (v″ = 0) Q-branch are recorded and fitted to extract rotational temperatures. Absolute number densities are determined from the frequency-integrated excitation spectrum. The plasma under investigation is a pulsed glow discharge operated at a frequency of 60 Hz with an plasma-on time of 5 ms per plasma cycle, 50 mA plasma current and a pressure of 6.67 mbar. CO number densities range from (2.6 ± 0.6) × 10 22 m -3 to (1.2 ± 0.3) × 10 22 m -3, while rotational temperatures range from 370 ± 40 K to 700 ± 70 K at the beginning and end of the plasma-on phase, respectively. Our results show fair agreement with literature data.

Atmospheric-pressure Plasma-Enhanced spatial Atomic Layer Deposition (PE-s-ALD) is a high-throughput technique for synthesizing thin films at low temperatures for large area applications. The spatial separation of the ALD half-reactions and the use of an atmospheric pressure plasma as the reactant give rise to complex surface chemistry which is not yet well understood. Here, we employed gas-phase infrared spectroscopy and optical emission spectroscopy (OES) to study the underlying chemistry of the PE-s-ALD process of Al2O3 films grown at 80 C using Al(CH3)3 and an Ar-O2 plasma. We identified the reaction products and investigated their dependence on the exposure time of the substrate to the precursor. Infrared absorbance spectra show CO, CO2, H2O, and CH4 as the main ALD reaction byproducts originating from (i) combustion-like reactions of the methylated surface with O plasma radicals and O3 and (ii) a concurrent latent thermal ALD component due to produced and/or residual H2O molecules. In addition, CH2O and CH3OH were identified as reaction by-products either originating at the surface or formed in the plasma. The OES spectra provide a corroborative proof of the combustive nature of the PE-s-ALDreactions showing OH and CH emissions arising during the spatial ALD process while excited O species are being consumed.

International audience; This work employs in situ rotational Raman spectroscopy to study the effect of N2 and O2 addition to CO2 in pulsed glow discharges in the mbar range. The spatiotemporally resolved measurements are performed in CO2 and 25%, 50% and 75% of N2 or O2 admixture, in a 5–10 ms on-off cycle, 50 mA plasma current and 6.7 mbar total pressure. The rotational temperature profile is not affected by adding N2, ranging from 400 to 850 K from start to end of the discharge pulse, while the addition of O2 decreases the temperature at corresponding time points. Molecular number densities of CO2, CO, O2 and N2 are determined, showing the spatial homogeneity along the axis of the reactor and uniformity during the cycle. The measurements in the N2 containing mixtures show that CO2 conversion factor α increases from 0.15 to 0.33 when the content of N2 is increased from 0% to 75%, demonstrating the potential of N2 addition to enhance the vibrational pumping of CO2 and its beneficial effect on CO2 dissociation. Furthermore, the influence of admixtures on CO2 vibrations is examined by analysing the vibrationally averaged nuclear spin degeneracy. The difference between the fitted odd averaged degeneracy and the calculated odd degeneracy assuming thermal conditions increases with the addition of N2, demonstrating the growth of vibrational temperatures in CO2. On the other hand, the addition of O2 leads to a decrease of α, which might be attributed to quenched vibrations of CO2, and/or to the influence of the back reaction in the presence of O2.

This work uses in situ narrowband quantum cascade laser (QCL) absorption spectroscopy to study the effect of N2 on the time evolution of gas temperature, rotational temperature and the vibrational temperatures of CO2 and CO in a pulsed glow discharge. Three colinear QCLs are used to scan three regions of about 1 cm-1 between 2179.20 and 2253.51 cm-1, including (v 1,v2-l 2,v 3)\to (v 1,v 2-l2,v3+1) C O 2 transitions up to the asymmetric stretch level v 3 = 6, as well as (v CO) → (v CO + 1) CO transitions up to v CO = 1. A fitting routine is used to extract temperatures from the measured absorption spectra. The time resolved measurements are performed in CO2, admixed with up to 90% N2, with the plasma operated with a 5-10 ms on-off cycle, a discharge current of 50 mA and a pressure of around 6.7 mbar. The time evolution of the gas temperature has been measured and agrees well with the time evolution of the rotational temperature. The asymmetric stretch vibrational temperature T 3 of CO2 reaches a maximum of 1060 K at 0.7 ms for pure CO2, while T 3 goes up to 2250 K for a N2 content of 90% and stays constant until the plasma is switched off. Both T 3 and the vibrational temperature of CO T CO show a clear non-equilibrium with respect to the rotational temperature T rot. Both do not equilibrate with the rotational temperature T rot between consecutive plasma cycles for a N2 content above 70%, although T 3 and T CO always equilibrate with each other in the afterglow. The symmetric stretch and bending mode temperature T 12 is elevated more with respect to the rotational temperature for increasing N2 content, while the maximum of the rotational temperature decreases for increasing N2 admixtures, which might be attributed to the energy stored in the vibrational modes of N2, CO2 and CO. Additionally, an indication of an increase in the total pressure during the plasma on-time due to heating and a subsequent decrease in the afterglow due to cooling was found for a pure CO2 plasma.

International audience; The excitation and relaxation of the vibrations of CO2 as well as the reduction of CO2 to CO are studied in a pulsed glow discharge. Two diagnostics are employed, being (1) time-resolved in situ Fourier transform infrared (FTIR) spectroscopy and (2) spatiotemporally resolved in situ rotational Raman spectroscopy. Experiments are conducted within a pressure range of 1.3-6.7 mbar and a current range of 10-50 mA. In the afterglow, the rate of exponential decay from the asymmetric stretch temperature (T3) to the rotational temperature (Trot) is found to be only dependent on Trot, in the conditions under study. The decay rate ρT3-Trot follows the relation ρT3-Trot = 388 s-1 exp((Trot - 273 K)/(154 K)). Pressure and varying concentrations of CO and (presumably) atomic oxygen did not show to be of significant influence. In the active part of the discharge the excitation of T3 showed to be positively related to current and negatively to pressure. However, the contribution of current to vibrational excitation is ambiguous: the conversion of CO2 and therefore the fraction of CO in the discharge, is found to be strongly dependent on the current, with a conversion factor of 0.05 to 0.18 for 10 mA to 50 mA, while CO can contribute to the excitation through near-resonant collisions. A clear relation between the elevation of T3 and the dissociation of CO2 could not be confirmed, though conversion peaks are observed in the near afterglow, which motivate future experiments on vibrational ladder-climbing directly after termination of the discharge.

This is the second of two papers presenting the study of vibrational energy exchanges in non-equilibrium CO2 plasmas in low-excitation conditions. The companion paper addresses a theoretical and experimental investigation of the time relaxation of ∼70 individual vibrational levels of ground-state CO molecules during the afterglow of a pulsed DC glow discharge, operating at pressures of a few Torr and discharge currents around 50 mA, where the rate coefficients for vibration-translation (V-T) and vibration-vibration (V-V) energy transfers among these levels are validated (Silva et al 2018 Plasma Sources Sci. Technol. 27 015019). Herein, the investigation is focused on the active discharge, by extending the model with the inclusion of electron impact processes for vibrational excitation and de-excitation (e-V). The time-dependent calculated densities of the different vibrational levels are compared with experimental data obtained from time-resolved in situ Fourier transform infrared spectroscopy. It is shown that the vibrational temperature of the asymmetric stretching mode is always larger than the vibrational temperatures of the bending and symmetric stretching modes along the discharge pulse - the latter two remaining very nearly the same and close to the gas temperature. The general good agreement between the model predictions and the experimental results validates the e-V rate coefficients used and provides assurance that the proposed kinetic scheme provides a solid basis to understand the vibrational energy exchanges occurring in CO2 plasmas.

The implementation of in situ rotational Raman spectroscopy is realized for a pulsed glow discharge in CO2 in the mbar range and is used to study the rotational temperature and molecular number densities of CO2, CO, and O2. The polarizability anisotropy of these molecules is required for extracting number densities from the recorded spectra and is determined for incident photons of 532 nm. The spatiotemporally-resolved measurements are performed in the same reactor and at equal discharge conditions (5-10 ms on-off cycle, 50 mA plasma current, 6.7 mbar pressure) as in recently published work employing in situ Fourier transform infrared (FTIR) spectroscopy. The rotational temperature ranges from 394 to 809 K from start to end of the discharge pulse and is constant over the length of the reactor. The discharge is demonstrated to be spatially uniform in gas composition, with a CO2 conversion factor of 0.15 ± 0.02. Rotational temperatures and molecular composition agree well with the FTIR results, while the spatial uniformity confirms the assumption made for the FTIR analysis of a homogeneous medium over the line-of-sight of absorption. Furthermore, the rotational Raman spectra of CO2 are related to vibrational temperatures through the vibrationally averaged nuclear spin degeneracy, which is expressed in the intensity ratio between even and odd numbered Raman peaks. The elevation of the odd averaged degeneracy above thermal conditions agrees well with the elevation of vibrational temperatures of CO2, acquired in the FTIR study.

Time-resolved in situ FTIR spectroscopy and spatiotemporally resolved in situ Raman spectroscopy are used to study the excitation and relaxation of the vibrations of CO 2 and the reduction of CO 2 to CO in a pulsed glow discharge.

A kinetic model describing the time evolution of ∼70 individual CO2(X1Σ+) vibrational levels during the afterglow of a pulsed DC glow discharge is developed in order to contribute to the understanding of vibrational energy transfer in CO2 plasmas. The results of the simulations are compared against in situ Fourier transform infrared spectroscopy data obtained in a pulsed DC glow discharge and its afterglow at pressures of a few Torr and discharge currents of around 50 mA. The very good agreement between the model predictions and the experimental results validates the kinetic scheme considered here and the corresponding vibration-vibration and vibration-translation rate coefficients. In this sense, it establishes a reaction mechanism for the vibrational kinetics of these CO2 energy levels and offers a firm basis to understand the vibrational relaxation in CO2 plasmas. It is shown that first-order perturbation theories, namely, the Schwartz-Slawsky-Herzfeld and Sharma-Brau methods, provide a good description of CO2 vibrations under low excitation regimes.

Rotational Raman scattering on the vibrational ground state of CO 2 was performed to determine the gas temperature in narrow-gap dielectric barrier discharges (DBDs). The Raman spectrometer was equipped with a straightforward spectral filtering to mask ca. 30 cm −1 (0.85 nm) centered around the excitation wavelength of 532 nm. Linearisation of the observed transitions ( J = 18–42) was applied to retrieve gas temperatures in discharge gaps of 1 mm. The DBD was operated in pure CO 2 at atmospheric pressure and non-negligible gas heating of about 160 K was observed at 33 W injected power. Based on a simplified energy balance the gas temperature measurements were extrapolated to a broad range of injected plasma power values (0–60 W).

Vibrational temperatures of CO2 are studied in a pulsed glow discharge by means of time-resolved in situ Fourier transform infrared spectroscopy, with a 10 μs temporal resolution. A method to analyze the infrared transmittance through vibrationally excited CO2 is presented and validated on a previously published CO2 spectrum, showing good agreement between fit and data. The discharge under study is pulsed with a typical duty cycle of 5-10 ms on-off, at 50 mA and 6.7 mbar. A rapid increase of the temperature of the asymmetric stretch vibration (T 3) is observed at the start of the pulse, reaching 1050 K, which is an elevation of 550 K above the rotational temperature () of 500 K. After the plasma pulse, the characteristic relaxation time of T 3 to strongly depends on the rotational temperature. By adjusting the duty cycle, the rotational temperature directly after the discharge is varied from 530 to 860 K, resulting in relaxation times between 0.4 and 0.1 ms. Equivalently, as the gas heats up during the plasma pulse, the elevation of T 3 above decreases strongly.

International audience; We describe an original setup named SCHEME (Source of exCited HydrogEn MolEcules) designed to study the recombinative desorption mechanisms of H atoms on a surface by means of high-brilliance monochromatic synchrotron radiation (SR). H atoms are produced on electrically heated filaments under vacuum (in the absence of any discharge) and subsequently recombine on a surface to produce excited hydrogen molecules, namely, H2(v",J″). Once these levels are produced, they are probed in the vacuum chamber with 5–40 eV (248–31 nm) SR-photons. A preliminary test with Krypton has demonstrated the feasibility of SCHEME to obtain simultaneously VUV-synchrotron radiation induced fluorescence and VUV-absorption signals with room-temperature gas. In order to evaluate the hydrogen molecule dissociation induced by the heated filaments, the absolute density of H atoms has been measured by VUV-absorption. At 1800 K and 100 mTorr, we estimated a dissociation degree of 10−4, which corresponds to an absolute density of 1.8×1017 m−3/sup>. VUV-absorption spectroscopy is shown to be a valuable tool to detect H2((v″,J″) molecules up to v″ = 3. Possible improvements of the experimental setup, to detect higher v″ levels, are suggested in the end of this paper.

We report measurements of electron densities, ${n}_{e}$, and temperatures, ${T}_{e}$, in a magnetized expanding hydrogen plasma performed using Thomson scattering. The effects of applying an axial magnetic field and changing the background pressure in the plasma vessel on ${n}_{e}$ and ${T}_{e}$ along the expansion axis are reported. Magnetic field strengths ($B$ field) up to 170 mT were applied, which are one order of magnitude larger than previously reported. The main effect of the applied $B$ field is the plasma confinement, which leads to higher ${n}_{e}$. At $B$ fields larger than 88 mT the electron density along the expansion axis does not depend strongly on the magnetic field strength. However, ${T}_{e}$ is susceptible to the $B$ field and reaches at 170 mT a maximum of 2.5 eV at a distance of 1.5 cm from the exit of the cascaded arc. To determine also the effect of the arc current through the arc, measurements were performed with arc currents of 45, 60, and 75 A at background pressures of 9.7 and 88.3 Pa. At constant magnetic field ${n}_{e}$ decreases from the exit of the arc along the expansion axis when the arc current is decreased. At 88.3 Pa ${n}_{e}$ shows a higher value close to the exit of the arc, but a faster decay along the expansion axis with respect to the 9.7 Pa case. ${T}_{e}$ is overall higher at lower pressure reaching a maximum of 3.2 eV at the lower arc current of 45 A. The results of this study complement our understanding and the characterization of expanding hydrogen plasmas.

Sustainable energy generation by means of wind or from solar radiation through photovoltaics or concentrated solar power will be a significant part of the energy mix in 2025. Intermittency (due to e.g. day/night cycle) as well as regional variation of these energy sources requires means to store and transport energy on a large scale. A promising option is creating artificial solar fuels (or CO2 neutral fuels) with sustainable energy, which can easily be deployed within the present infrastructure for conventional fossil fuels.

Absolute densities of nitrous species were studied in an atmospheric pressure RF plasma jet. The measurement of NO and N2O densities has been performed mainly by means of ex situ quantum-cascade laser absorption spectroscopy via a multi-pass cell in Herriot configuration. The dependence of the species' production on individual parameters such as power, flow and oxygen, nitrogen and water admixture is shown. NO and N2O densities are found to increase with absorbed power, while an increase in the gas flow induces a decrease of these densities due to a reduction in residence time of the gas in the plasma. Actually, a change of these two parameters, absorbed power and gas flow, induces a variation of energy density. The higher energy density, the higher NO and N2O densities. The NO and N2O densities are strongly gas mixture dependent. A change of that parameter allows to choose between a NO-rich or a N2O-rich regime. NO and N2O densities increase as a function of the N2 admixture, while increasing oxygen, above a minimum value, reduces the densities of both NO and N2O. When adding water instead of oxygen to the gas mixture the reduction in the NO density is much less. For maximal NO and N2O formation a ratio of about He/N2/O2 = 99.5/0.36/0.07 is found to be the most efficient in the μ-APPJ. However, it was found that the absorbed power in the plasma always reduces with increasing admixtures. The validation of the results obtained with quantum-cascade absorption spectroscopy with mass spectrometry shows how the two measurement techniques can complement each other. Finally a comparison of our results and others works is presented.

In previous works, surface-produced negative-ion distribution-functions have been measured in H2 and D2 plasmas using graphite surfaces (highly oriented pyrolitic graphite). In the present paper, we use the srim software to interpret the measured negative-ion distribution-functions. For this purpose, the distribution-functions of backscattered and sputtered atoms arising due to the impact of hydrogen ions on a-CH and a-CD surfaces are calculated. The srim calculations confirm the experimental deduction that backscattering and sputtering are the mechanisms of the origin of the creation of negative ions at the surface. It is shown that the srim calculations compare well with the experiments regarding the maximum energy of the negative ions and reproduce the experimentally observed isotopic effect. A discrepancy between calculations and measurements is found concerning the yields for backscattering and sputtering. An explanation is proposed based on a study of the emitted-particle angular-distributions as calculated by srim.

We present a novel approach for filtering Rayleigh scattering and stray light from Raman scattering in a gas discharge, using a volume Bragg grating as a notch filter. For low frequency rotational Raman contributions, it is essential to filter out Rayleigh scattering and stray light at the laser wavelength to be able to measure an undisturbed Raman spectrum. Using the Bragg grating, having an optical density of 3.1 at the central wavelength of 532 nm and a full width at half maximum of 7 cm-1, we were able to measure a nearly full rotational CO2 spectrum (1.56 cm-1 peak-to-peak separation). The rotational temperature in a CO2 discharge was determined with an accuracy of 2%.

International audience; Thomson scattering using a Bragg grating notch filter is used to determine the electron properties of a pulsed, kHz-driven, non-thermal atmospheric pressure plasma jet in helium expanding in air. The plasma jet is allowed to freely expand or interact with targets with different electrical properties, i.e. glass, copper and water. With the same setup, Raman scattering is used to determine spatially- and time-resolved the densities and rotational temperatures of oxygen and nitrogen molecules entrained into the jet. Fast imaging is used to determine the development of the discharge in the plasma jet as well as its behavior in the plasma-target interaction zone. As the discharge approaches the target, the rise of electron density was followed by the fall of electron temperature. The discharge is influenced only over a few millimeters before it hits the target. The electron density and temperature during the spreading of the discharge on the low-permittivity target are measured to be resp. 2 × 1019 m−3 and ≈1 eV. During the return stroke on the high-permittivity and the metallic target the densities rise with a factor 1.5 resp. 2.2, and the temperature with a factor 2.5 for both cases. The discharges on the high- and low-permittivity targets extinguished soon after the initial impact of the ionization front, while the diffuse discharge on the metallic target extinguished only after the end of the voltage pulse (with a duration of 1 μs). In the diffuse discharge the electron temperature reaches 3.4 eV, the gas temperature increases by approximately 100 K and the electron density increases by approximately a factor three with respect to before its formation.

The FOM-Institute for Plasma Physics Rijnhuizen is constructing Magnum-PSI; a magnetized (3 T), steady-state, large area (80 cm2) high-flux (up to 1 024 H+ ions m-2 s-1) plasma generator. Magnum-PSI will be a highly accessible laboratory experiment in which the interaction of magnetized plasma with different surfaces can be studied. This experiment will provide new insights in the complex physics and chemistry that will occur in the divertor region of the future experimental fusion reactor ITER. Here, extremely high power and particle flux densities are predicted at relatively low plasma temperatures. Magnum-PSI will be able to simulate these detached ITER divertor conditions in detail. In addition, conditions can be varied over a wide range, such as different target materials, plasma temperatures, beam diameters, particle fluxes, inclination angles of target, background pressures, magnetic fields, etc., making Magnum-PSI an excellent test bed for high heat flux components of future fusion reactors. The design phase of the Magnum-PSI device has been completed. The construction and assembly phase of the device is in progress. In this contribution, we will present the design and construction of the Magnum-PSI experiment. The status of the vacuum system, the 3 T superconducting magnet, the plasma source, the target plate and manipulator, and additional plasma heating will be presented. The plasma and surface diagnostics that will be used in the Magnum-PSI experiment will be introduced. © 2010.

International audience; A low pressure recombining Ar plasma to which mixtures of N 2 and O 2 were added has been studied to explore the relevance of surface related processes for the total chemistry. The abundances of the stable molecules N 2, O 2, NO, N 2 O and NO 2 have been measured by means of a combination of infrared tunable diode laser absorption spectroscopy and mass spectrometry. A gas phase chemical kinetics model was developed in chemkin to investigate the contribution of homogeneous interactions to the conversion of the feedstock gases N 2 and O 2. At a partial pressure of N 2 plus O 2 less than 8 Pa, significant discrepancies between measured and calculated concentrations of N 2 O and NO 2 were observed, indicating that heterogeneous processes are dominating the chemistry in this pressure range. At a partial pressure of N 2 plus O 2 higher than 40 Pa and a relatively high fraction of admixed O 2 we observed a fair agreement between measured and calculated concentrations of NO molecules, indicating that homogeneous processes (notably Natom induced) are more dominant than heterogeneous processes.

Energies and probabilities of Lyman transitions of high rovibrationally excited H 2 , HD and D 2 molecules have been measured and compared with calculations. The experimental results are obtained from laser-induced fluorescence spectra that have been recorded in the spectral range from 60 500 to 83 500 cm −1 , covering 2/3 of the hydrogen Lyman band system. The necessary vacuum-UV radiation is produced by stimulated anti-Stokes Raman scattering, providing a widely tunable radiation source with narrow spectral bandwidth to resolve single Lyman transitions. The highest internal energies of detected hydrogen isotopologues are close to the dissociation limit. This extends the available data base of Lyman transitions from and to higher rotational states ( J > 10) of HD and D 2 .

The vacuum-UV laser induced fluorescence spectrum of molecular neutral species present in an H2/D2 plasma expansion has been recorded from 60 500 cm−1 to 83 500 cm−1. It will be shown that this VUV spectrum results from X 1 Σ g + ↔ B 1 Σ u + (Lyman) transitions in H2, HD and D2 molecules that are high rovibrationally excited. This is inferred from the comparison of simulated Lyman spectra based on spectroscopic data for H2, HD and D2 with the measured VUV spectra.

The dissociation of CO2 in a geometrically symmetric dielectric barrier discharge has been analysed by means of numerical modelling. A time- and space-dependent fluid model has been used, taking into account the spatial variation of the plasma between the plane-parallel dielectrics covering the electrodes. The main features of the model, including an extensive reaction kinetics for the vibrational states of CO2, are given. The modelling studies have been performed for different applied voltages, discharge frequencies, pressures, gas temperatures, and relative permittivities of the dielectrics. The model calculations show that the discharges in the positive and negative half-cycles are different for the considered standard condition, leading to a spatially asymmetric distribution of the stable neutrals like CO molecules and O atoms. The generation of CO mainly takes place during the discharge pulses, and it is dominated by electron impact dissociation. The specific energy input obtained for the broad range of parameters considered and determined for residence times reported in the literature agrees well with the corresponding experimental values. In accordance with these experiments, the calculated degree of CO2 conversion has been found to increase almost linearly with the specific energy input. Remaining discrepancies between the measured and calculated energy efficiencies are discussed.

International audience; Atomic oxygen, considered to be a determining reactant in plasma applications at ambient pressure, is routinely detected by two-photon absorption laser induced fluorescence (TALIF). Here, pressure broadening of the (2 p 4 3 P 2 ?????3 p 3 P J =0,1,2 ) two-photon transition in oxygen atoms was investigated using a high-resolution TALIF technique in normal and Doppler-free configurations. The pressure broadening coefficients determined were ##IMG## [http://ej.iop.org/images/0963-0252/25/6/06LT03/psstaa4481ieqn001.gif] γ_\textO_2 ??=??0.40??±??0.08? cm ?1 /bar for oxygen molecules and ##IMG## [http://ej.iop.org/images/0963-0252/25/6/06LT03/psstaa4481ieqn002.gif] γ_\textHe ??=??0.46??±??0.03?cm ?1 /bar for helium atoms. These correspond to pressure broadening rate constants ##IMG## [http://ej.iop.org/images/0963-0252/25/6/06LT03/psstaa4481ieqn003.gif] k_\textPB^\textO_2 ??=??9 · 10 ?9 cm 3 s ?1 and ##IMG## [http://ej.iop.org/images/0963-0252/25/6/06LT03/psstaa4481ieqn004.gif] k_\textPB^\textHe ??=??4 · 10 ?9 cm 3 s ?1 , respectively. The well-known quenching rate constants of O(3 p 3 P J ) by O 2 and He are at least one order of magnitude smaller, which signifies that non-quenching collisions constitute the main line-broadening mechanism. In addition to providing new insights into collisional processes of oxygen atoms in electronically excited 3 p 3 P J state, reported pressure broadening parameters are important for quantification of oxygen TALIF line profiles when both collisional and Doppler broadening mechanisms are important. Thus, the Doppler component (and hence the temperature of oxygen atoms) can be accurately determined from high resolution TALIF measurements in a broad range of conditions.

We measured the densities of NH and NH(2) radicals by cavity ring-down spectroscopy in N(2)-H(2) plasmas expanding from a remote thermal plasma source and in N(2) plasmas to which H(2) was added in the background. The NH radical was observed via transitions in the (0,0), (1,1), and (2,2) vibrational bands of the A(3)Pi-- X(3)Sigma- electronic transition and the NH(2) radical via transitions in the (0,9,0)-- (0,0,0) band of the A(2)A(1)-- X(2)B(1) electronic transition. The measurements revealed typical densities of 5 x 10(18) m(-3) for the NH radical in both plasmas and up to 7 x 10(18) m(-3) for the NH(2) radical when N(2) and H(2) are both fed through the plasma source. In N(2) plasma with H(2) injected in the background, no NH(2) was detected, indicating that the density is below our detection limit of 3 x 1016 m-3. The error in the measured densities is estimated to be around 20%. From the trends of the NH(x) radicals as a function of the relative H(2) flow to the total N(2) and H(2) flow at several positions in the expanding plasma beam, the key reactions for the formation of NH and NH(2) have been determined. The NH radicals are mainly produced via the reaction of N atoms emitted by the plasma source with H(2) molecules with a minor contribution from the reaction of N+ with H(2). The NH(2) radicals are formed by reactions of NH(3) molecules, produced at the walls of the plasma reactor, and H atoms emitted by the plasma source. The NH radicals can also be produced by H abstraction of NH(2) radicals. The flux densities of the NH(x) radicals with respect to the atomic radicals are appreciable in the first part of the expansion. Further downstream the NH(x) radicals are dissociated, and their densities become smaller than those of the atomic radicals. It is concluded that the NH(x) radicals play an important role as precursors for the N and H atoms, which are key to the surface production of N(2), H(2), and NH(3) molecules.

Pilot-PSI produces hydrogen plasma with a cascaded arc. It is demonstrated that the output electron density reaches 4 × 1021 m-3 by pushing the input power to 45 kW. Increasing the diameter of the discharge channel (studied from 4 to 7 mm) does not affect the plasma output but reduces the power input by up to 25%. The plasma was post-heated by feeding the plasma column with a net current. This increased the electron temperature from ∼2 to 4 eV. Calorimetric and voltage measurements on the cascade plates showed that this also influenced the discharge characteristics inside the arc.

The FOM-Institute for Plasma Physics Rijnhuizen is preparing the construction of Magnum-PSI, a magnetized (3 T), steady state, large area (80 cm2) high-flux (up to 1024 H+ ions m−2 s−1) plasma generator. The aim of the linear plasma device Magnum-PSI is to provide a controlled, highly accessible laboratory experiment in which the interaction of a magnetized plasma with different surfaces can be studied in detail. Plasma parameters can be varied over a wide range, in particular covering the high-density, low-temperature conditions expected for the detached divertor plasma of ITER. The target set-up will be extremely flexible allowing the investigation of different materials under a large variety of conditions (temperatures, inclination, biasing, coatings, etc.). A range of target materials will be used, including carbon, tungsten and other metals, and mixed materials. Because of the large plasma beam of 10 cm diameter and spacious vacuum tank, even the test of whole plasma-facing component mock-ups will be possible. In this article, we will present the pre-design of the Magnum-PSI experiment. We will focus on the requirements of the vacuum system and the 3 T superconducting magnet. We briefly introduce some of the other sub-systems on Magnum-PSI such as the target and target manipulator.

Profiling the arc channel of a cascaded arc into an hourglass-like shape results in an extremely high light output intensity (3.5 × 1012 Wm−2 m−1 sr−1 in the near-infrared) at an input power of 2.2 kW, a good stability (better than 5 × 10−4) and a small solid angle of the emitted light beam (Ω = 0.015 sr). These optical properties are advantageous for spectroscopic applications that require a high light intensity over a broad wavelength range in a small solid angle. The absolute light output is measured and is compared with a fast and flexible modelling scheme. These simulations are used to verify the arc performance and to guide arc design. The calculated emitted radiance at moderate arc input power agrees with the experiments within 30% for moderate input power.

FOM-Rijnhuizen is building, in cooperation with its Trilateral Euregio Cluster (TEC) partners, a PSI-laboratory to study plasma surface interaction (PSI) under extreme, ITER relevant plasma conditions. The largest linear plasma generator of PSI-lab is Magnum-PSI, and is designed to deliver up to 10 MW m-2 power over a 10 cm diameter hydrogen plasma beam with an electron density (ne) up to 1021 m-3 and electron-temperature (Te) between 1–5 eV. Magnum-PSI is presently under construction and its predesign is presented. Its forerunner is Pilot-PSI, in which record plasma parameters of ne=4×1021 m-3 at Te=2 eV in a ~1 cm wide hydrogen beam confined by a magnetic field (B) =1.6 T were measured at 40 mm downstream the source nozzle. At 17 mm in front of a target (located at 0.56 m distance from the source nozzle), ne>1021 m-3 and Te= 4 eV have been demonstrated. Initial experiments on exposing fine-grain carbon samples are presented that showed up to 20 µm s-1 erosion as a demonstration of the extreme plasma conditions. Spectroscopy was applied to compare chemical erosion yields for flux densities up to 5.0×1024 m-2 s-1.

We present a time-resolved imaging and electrical study of an atmospheric pressure spark discharge. The conditions of the present study are those used for nanoparticle generation in spark discharge generator setups. The oscillatory bipolar spark discharge was generated between two identical Cu electrodes in different configurations (cylindrical flat-end or tipped-end geometries, electrode gap from 0.5 to 4 mm), in a controlled co-axial N2 flow, and was supplied by a high voltage capacitor. Imaging data with nanosecond time resolution were collected using an intensified CCD camera. This data were used to study the time evolution of plasma morphology, total light emission intensity, and the rate of plasma expansion. High voltage and high current probes were employed to collect electrical data about the discharge. The electrical data recorded allowed, among others, the calculation of the equivalent resistance and inductance of the circuit, estimations for the energy dissipated in the spark gap. By combining imaging and electrical data, observations could be made about the correlation of the evolution of total emitted light and the dissipated power. It was also observed that the distribution of light emission of the plasma in the spark gap is uneven, as it exhibits a “hot spot” with an oscillating position in the axial direction, in correlation with the high voltage waveform. The initial expansion rate of the cylindrical plasma front was found to be supersonic; thus, the discharge releases a strong shockwave. Finally, the results on equivalent resistance and channel expansion are comparable to those of unipolar arcs. This shows the spark discharge has a similar behavior to the arc regime during the conductive phase and until the current oscillations stop.

A full characterization of the Ar–NH3 expanding thermal plasma source applied in industrial processes for high-rate silicon nitride deposition is presented in terms of absolute densities of reactive species such as ions and radicals. The ion composition of the plasma was identified by mass spectrometry, while absolute ion density information was obtained by Langmuir probe measurements. N radicals were detected by threshold ionization mass spectrometry, whereas NH and NH2 radicals were measured by cavity ringdown spectroscopy. It was found that the ion density decreases from 1013 cm−3 in a pure Ar plasma to 1010–1011 cm−3 when NH3 is injected, with ArH+, , , and being the most abundant ions. The densities of N and NH both saturate at a level of 3 × 1012 cm−3 at NH3 flows above 3 sccs while the density of NH2 increases linearly with the NH3 flow and reaches a level of 4 × 1012 cm−3. When the plasma source current is increased, the densities of N and NH increase linearly, while the NH2 density remains approximately constant. Furthermore, it is revealed that most of the consumed NH3 is converted into N2 and H2 in the plasma.

The formation of new molecules in a microwave plasma, created from a mixture of Ar, CH4, N2 and O2, is investigated by means of an in-depth study of the molecular abundance in the plasma. The molecules are detected by means of tunable diode laser absorption spectroscopy and by absolute mass spectrometry. Three groups of molecules can be discerned in terms of molecular abundance: CO is predominantly formed, together with H2O, N2 and H2. The molecules CH4 and O2 are significantly depleted, but still abundant in a finite quantity. The third group is formed by several other species like NH3, NO, HCN etc. This tendency is expected to occur in every low temperature plasma containing C, O, H and N atoms. Furthermore, the combination of both techniques also allows us to make a clear distinction between the etching mode and deposition mode of the microwave reactor. Etching mainly occurs when the ratio of admixed gas flows Φ(O2)/Φ(CH4) > 0.5.

The densities of NH and N H2 radicals in an Ar-N H3 plasma jet created by the expanding thermal plasma source were investigated for various source-operating conditions such as plasma current and N H3 flow. The radicals were measured by cavity ringdown absorption spectroscopy using the (0,0) band of the A ¿3 ¿ X ¿-3 transition for NH and the (0,9,0)-(0,0,0) band of the A~ A12 ¿ X~ B12 transition for N H2. For NH, a kinetic gas temperature and rotational temperature of 1750±100 and 1920±100 K were found, respectively. The measurements revealed typical densities of 2.5× 1012 cm-3 for the NH radical and 3.5× 1012 cm-3 for the N H2 radical. From the combination of the data with ion density and N H3 consumption measurements in the plasma as well as from a simple one-dimensional plug down model, the key production reactions for NH and N H2 are discussed. © 2005 American Institute of Physics.

We treat the kinetics of the three dominant species in a hydrogen plasma expansion, which was studied using various laser spectroscopic techniques. Whereas the ground-state H/sub 2/ molecules expand like a normal gas, the behaviour of H atoms and ro-vibrationally excited molecules is considerably altered as a consequence of their reactivity. The hydrogen atoms are primarily lost via surface association. In the source, H atoms are lost on the surface of the nozzle, reducing the atomic flux, and in the expansion, the H atoms diffuse out due to large H atom concentration gradients between the plasma and the background that exists as a result of surface association on the vessel walls. The loss process of H atoms in the nozzle, surface association, is the main production process of H/sub 2/ molecules with high rotational and vibrational excitation. The behaviour of these ro-vibrationally excited H/sub 2/ molecules in the expansion shows the relaxation process from the high temperature in the upstream region to the low temperature of the downstream region. The lower rotational levels (J 7) do not because of the large energy spacing between the hydrogen rotational levels. The consequence of this high rotational excitation is demonstrated in the dissociative attachment of electrons to H/sub 2/

It has been conjectured that atomic oxygen and ozone can have a great influence on the plasma oxidation of ultrathin aluminum for magnetic tunnel junctions. In order to measure the density of O and ozone, two-photon absorption laser induced fluorescence measurements are performed in the dc glow plasma that is used for the oxidation process. It was found that ozone is much more abundantly present compared to atomic oxygen. Using in situ, real-time ellipsometry measurements, we prove that ozone is not directly involved in the oxidation process.

Cavity ring-down detection techniques can sensitively determine frequency-dependent absorption cross-sections of gasses. However, so-called line-width problems and amplified spontaneous emission of the laser light source lowers the technique’s quantitative accuracy. Using phase-shift cavity ring-down spectroscopy (PSCRD), we measured absolute line intensities of the spin-forbidden transitions in the b 1 Σ g + ( ν ′ = 0 ) ← X 3 Σ g - ( ν ″ = 0 ) band of molecular oxygen. Our results were within 4% of values obtained from the HITRAN database, demonstrating the accuracy of PSCRD, when corrected for amplified spontaneous emission. Its high sensitivity (2 × 10−8 cm−1), simplicity and high duty cycle make PSCRD a powerful diagnostic technique.

An experimental setup is built to detect spatially resolved rovibrationally excited hydrogen molecules via laser-induced fluorescence. To excite the hydrogen molecules, laser radiation is produced in the vacuum UV part of the spectrum. The laser radiation is tunable between 120 nm and 230 nm and has a bandwith of 0.15 cm–1. The wavelength of the laser radiation is calibrated by simultaneous recording of the two-photon laser induced fluorescence spectrum of nitric oxide. The excited hydrogen populations are calibrated on the basis of coherent anti-Stokes Raman scattering measurements. A population distribution is measured in the shock region of a pure hydrogen plasma expansion. The higher rotational levels (J>5) show overpopulation compared to a Boltzmann distribution determined from the lower rotational levels (J5).

The aim of this article is to determine the role of carbon atoms in the growth of hydrogenated amorphous carbon (a-C:H) films by means of an argon/acetylene expanding thermal plasma. Cavity ring down absorption spectroscopy is used to detect metastable carbon atoms by probing the 1s2 2s2 2p 3s 1P11s2 2s2 2p2 1S0 electronic transition. In addition to absorption measurements, the emission of the same transition is monitored by means of optical emission spectroscopy. These two measurements provide information about the local production of the C atoms and about their reactivity in the gas phase. It will be shown that under growth conditions in an Ar/C2H2 expanding thermal plasma, the metastable carbon density is also representative for the ground state carbon density. From obtained results it is concluded that the carbon atoms react rapidly with acetylene in the gas phase and therefore their contribution to the growth of hard diamond-like a-C:H films can be neglected. Only at low acetylene flows, the condition when soft polymer-like films are deposited, carbon atoms are detected close to the substrate and can possibly contribute to the film growth.

The strong non-equilibrium conditions provided by the plasma phase offer the opportunity to beat traditional thermal process energy efficiencies via preferential excitation of molecular vibrations. Simple molecular physics considerations are presented to explain potential dissociation pathways in plasma and their effect on energy efficiency. A common microwave reactor approach is evaluated experimentally with Rayleigh scattering and Fourier transform infrared spectroscopy to assess gas temperatures (exceeding 104 K) and conversion degrees (up to 30%), respectively. The results are interpreted on a basis of estimates of the plasma dynamics obtained with electron energy distribution functions calculated with a Boltzmann solver. It indicates that the intrinsic electron energies are higher than is favorable for preferential vibrational excitation due to dissociative excitation, which causes thermodynamic equilibrium chemistry to dominate. The highest observed energy efficiencies of 45% indicate that non-equilibrium dynamics had been at play. A novel approach involving additives of low ionization potential to tailor the electron energies to the vibrational excitation regime is proposed.

Supersonically expanding stationary thermal plasma, formed by a thermal cascaded arc is studied. Due to the low chamber pressure (20-100 Pa) the results of continuum mechanics model can be doubtful. This is why these results are validated against kinetic Monte Carlo simulation and experimental data obtained by means of laser induced fluorescence. The analysis proves that continuum mechanics is still applicable for the velocity and temperature field predictions downstream of the shock region. However, the shock formation and some non-equilibrium effects typical for supersonic flow can be correctly studied only with the help of kinetic simulations. We show that the errors in the results using continuum mechanics can be attributed to the presence of flow gradients. These errors diminish when the shock regions are thickened due to rarefaction, viscosity and heat conductivity. Besides, both methods show that the effect of the chamber geometry on the plasma flow field is important.