Electronegative Gases Research Articles

Normalized Electronegative Species Effects in the Dielectric-Barrier-Discharge Plasma Actuator

Time-resolved and time-averaged net force (electrohydrodynamic force plus viscous drag) are presented for a dielectric-barrier-discharge-based aerodynamic plasma actuator operated in artificial atmospheres composed of nitrogen () with various concentrations of electronegative gasses: 1–20% oxygen () and 0.01–0.34% sulfur hexafluoride (). It is possible to deconvolve and isolate the role of oxygen to the negative-going or forward stroke of the discharge and to the region of parameter space with high : a change in oxygen content by a factor of 20 yields only a 20% increase in force produced when 12 kV AC is applied to the system but yields a 170% increase when the voltage amplitude is increased by less than a factor of 2, to 20 kV applied. It is shown that substituting for produces a mild net increase in force production that saturates at very low concentrations (0.1%) and that the increase is limited because of substantial (50%) losses of the momentum to drag even though the momentum input is increased by a factor of 2.

Physical and chemical properties of low-pressure argon-chlorine dielectric barrier discharge

The influence of adding chlorine on the characteristics of a dielectric barrier discharge in Argon is investigated on the basis of a one-dimensional fluid model. The spatio-temporal characteristics of the discharge are obtained by applying a sinusoidal voltage with a frequency and amplitude of 7 kHz and 350 V, respectively. The study shows that the discharge has a homogeneous feature across the electrodes and has only one current pulse per half cycle of the applied voltage. The calculated electric field and electric potential as well as species number densities indicated that the discharge is in glow mode, and adding chlorine as electronegative gas up to 50% does not change its mode. It is observed that the most abundant negative species are Cl− ions even in low additive chlorine. As a result, the maximum of plasma electronegativity takes place at 30% amounts of chlorine additive. The study of plasma radiations on the discharge gap shows that the main spontaneous emissions are observed at the wavelengths of 128.5 nm and 258 nm due to de-excitation of ArCl* and Cl2* molecules, respectively. Between different Ar/Cl2 mixtures, 0.99Ar−0.01Cl2 has the nearly uniform radiation in the positive column region.

Appearance of a conductive carbonaceous coating in a CO2 dielectric barrier discharge and its influence on the electrical properties and the conversion efficiency

This work examines the properties of a dielectric barrier discharge (DBD) reactor, built for CO2 decomposition, by means of electrical characterization, optical emission spectroscopy and gas chromatography. The discharge, formed in an electronegative gas (such as CO2, but also O2), exhibits clearly different electrical characteristics, depending on the surface conductivity of the reactor walls. An asymmetric current waveform is observed in the metal-dielectric (MD) configuration, with sparse high-current pulses in the positive half-cycle (HC) and a more uniform regime in the negative HC. This indicates that the discharge is operating in two alternating regimes with rather different properties. At high CO2 conversion regimes, a conductive coating is deposited on the dielectric. This so-called coated MD configuration yields a symmetric current waveform, with current peaks in both the positive and negative HCs. In a double-dielectric (DD) configuration, the current waveform is also symmetric, but without current peaks in both the positive and negative HC. Finally, the DD configuration with conductive coating on the inner surface of the outer dielectric, i.e. so-called coated DD, yields again an asymmetric current waveform, with current peaks in the negative HC. These different electrical characteristics are related to the presence of the conductive coating on the dielectric wall of the reactor and can be explained by an increase of the local barrier capacitance available for charge transfer. The different discharge regimes affect the CO2 conversion, more specifically, the CO2 conversion is lowest in the clean DD configuration. It is somewhat higher in the coated DD configuration, and still higher in the MD configuration. The clean and coated MD configuration, however, gave similar CO2 conversion. These results indicate that the conductivity of the dielectric reactor walls can highly promote the development of the high-amplitude discharge current pulses and subsequently the CO2 conversion.

E × B probe measurements in molecular and electronegative plasmas.

This paper reports on the design, the building, the calibration, and the use of a compact E × B probe that acts as a velocity filter or a mass filter for ion species. A series of measurements has been performed in the discharge and in the beam of the PEGASES (Plasma Propulsion with Electronegative GASES) ion source. PEGASES is a unique inductively coupled radio-frequency source able to generate a beam of positive and negative ions when operated with an electronegative gas. In this study, experiments have been carried out with SF6. Calibrated E × B probe spectra indicate that the diagnostic tool can be used to determine the ion velocity and the plasma composition even when many molecular fragments are present. In addition, the probe is able to detect both positive and negative ions. Measurements show a large variety of positively charged ions coming from SF6. Conversely, the beam is solely composed of F(-) and SF6(-) negative ions in compliance with computer simulations.

Effect of gas properties on the dynamics of the electrical slope asymmetry effect in capacitive plasmas: comparison of Ar, H2 and CF4

Tailored voltage excitation waveforms provide an efficient control of the ion energy (through the electrical asymmetry effect) in capacitive plasmas by varying the ‘amplitude’ asymmetry of the waveform. In this work, the effect of a ‘slope’ asymmetry of the waveform is investigated by using sawtooth-like waveforms, through which the sheath dynamic can be manipulated. A remarkably different discharge dynamic is found for Ar, H2, and CF4 gases, which is explained by the different dominant electron heating mechanisms and plasma chemistries. In comparison to Argon we find that the electrical asymmetry can even be reversed by using an electronegative gas such as CF4. Phase resolved optical emission spectroscopy measurements, probing the spatiotemporal distribution of the excitation rate show excellent agreement with the results of particle-in-cell simulations, confirming the high degree of correlation between the excitation rates with the dominant heating mechanisms in the various gases. It is shown that, depending on the gas used, sawtooth-like voltage waveforms may cause a strong asymmetry.

Ignition delay of a pulsed inductively coupled plasma (ICP) in tandem with an auxiliary ICP

Plasma ignition delays were observed in a ‘main’ inductively coupled plasma (ICP), in tandem with an ‘auxiliary’ ICP. The Faraday-shielded ICPs were separated by a grounded metal grid. Power (13.56 MHz) to the main ICP was pulsed with a frequency of 1 kHz, while the auxiliary ICP was operated in continuous wave (cw) mode. In chlorine plasmas, ignition delay was observed for duty cycles greater than 60% and, in contrast to expectation, the delay was longer with increasing duty cycle up to ~99.5%. The ignition delay could be varied by changing the auxiliary and/or main ICP power. Langmuir probe measurements provided the temporal evolution of electron temperature, and electron and positive ion densities. These measurements revealed that the plasma was ignited shortly after the decaying positive ion density (n+), in the afterglow of the main ICP, reached the density () prevailing when only the auxiliary ICP was powered. At that time, production of electrons began to dominate their loss in the main ICP, due to hot electron injection from the auxiliary ICP. As a result, increased from a value below , improving inductive power coupling efficiency, further increasing plasma density leading to plasma ignition. Plasma ignition delay occurred when the afterglow of the pulsed plasma was not long enough for the ion density to reach during the afterglow. Besides Cl2, plasma ignition delays were also observed in other electronegative gases (SF6, CF4/O2 and O2) but not in an electropositive gas (Ar).

Dielectric barrier discharge-based plasma actuator operation in artificial atmospheres for validation of modeling and simulation

We present an experimental case study of time-resolved force production by an aerodynamic plasma actuator immersed in various mixtures of electropositive (N2) and electronegative gases (O2 and SF6) at atmospheric pressure using a fixed AC high-voltage input of 16 kV peak amplitude at 200 Hz frequency. We have observed distinct changes in the discharge structures during both negative- and positive-going voltage half-cycles, with corresponding variations in the actuator's force production: a ratio of 4:1 in the impulse produced by the negative-going half-cycle of the discharge among the various gas mixtures we explored, 2:1 in the impulse produced by the positive-going half-cycle, and cases in which the negative-going half-cycle dominates force production (by a ratio of 1.5:1), where the half-cycles produce identical force levels, and where the positive-going half cycle dominates (by a ratio of 1:5). We also present time-resolved experimental evidence for the first time that shows electrons do play a significant role in the momentum coupling to surrounding neutrals during the negative going voltage half-cycle of the N2 discharge. We show that there is sufficient macroscopic variation in the plasma that the predictions of numerical models at the microscopic level can be validated even though the plasma itself cannot be measured directly on those spatial and temporal scales.

Extraction and neutralization of positive and negative ions from a pulsed electronegative inductively coupled plasma

Almost electron-free (ion-ion) plasmas can be transiently formed during the afterglow phase of pulsed plasmas in electronegative gases. In ion-ion plasmas, both positive and negative ions can be extracted which makes them advantageous for a number of applications. In this paper, we investigate the extraction and acceleration of positive and negative ion beams from a pulsed inductively coupled plasma in SF6. The plasma is bounded by two electrodes biased synchronously with the discharge modulation. It is shown that when a DC bias voltage is applied during the afterglow phase, positive/negative ions are accelerated in a positive/negative space charge sheath formed in front of one of the electrodes. The energy of extracted ions closely follows the amplitude of the applied bias voltage (25–150 V) and the peak beam current density reaches 2 A m−2. With a view to using the described system as a source of energetic neutral beams for low damage material processing, simultaneous extraction and surface neutralization of positive and negative ions using an extraction electrode with high aspect ratio apertures is investigated.

Mode Transition of Trichel pulses

The trichel pulse is a typical kind of negative corona current observed in electronegative gases. In this work, stochastic behavior of the trichel pulse has been investigated. The experiment is performed in a negative corona reactor consisting of a stainless steel pin and a stainless steel powered by a dc high voltage source. The characteristic parameters distributions of corona current pulses, including the amplitude, rise time, half-wave time, and repetition frequency, are analyzed statistically. The results show there is a mode transition during the period of voltage increasing. This transition process happens in a certain voltage region, and change of pulse amplitude is the main difference between the two modes.

Modelling of aged cavities for partial discharge in power cable insulation

In this study, the ageing process of cavities in solid insulation of power cables is divided into three stages, and for each stage a circuit model is proposed. Each of these circuit models preserves the basic characteristics of the cavity under the circumstances of the corresponding stage and provides the expected behaviour of generated partial discharge (PD) pulses. This set of models simulate cavity wall effects, the non-uniformity of internal electrical field and charge deployed on the wall surfaces with considering the existence of electronegative gases according to the requirements of the three defined ageing stages. Finally, the simulation results, in the forms of phase resolved PD patterns are presented showing good agreement with the published experimental measurements drawn from accelerated ageing process setups.

Dielectric properties of gas mixtures with C<sub>3</sub>F<sub>8</sub>/C<sub>2</sub>F<sub>6</sub> and N<sub>2</sub>/CO<sub>2</sub>

Thanks to its excellent insulation and cutoff performances, SF6 gas has been applied to power equipment since the 1960s and is now widely used for several kV to 1,000 kV-class GIS, GCB, and GIL. However, since 1997, when SF6 was designated at COP3 as a greenhouse gas to be reduced, there has been a wish to use an alternative insulating gas to pure SF6 gas. In the present study, alternative gases were selected from among mixtures excluding SF6 with the need to reduce GWP (global warming potential) in mind. Gas mixtures containing such substances and with a boiling point of -20°C or less, chemically stable, non-toxic, and not ozone-depleting were prioritized. Furthermore, the availability and environmental performance were taken into consideration when deciding on component gases. Consequently, to launch this series of studies, four types of gas mixtures were used combining a gas in Group A (C2F6, C3F8) – electronegative gases with relatively high dielectric strength – and a gas in Group B (N2, CO2) – gases existing in the natural world. The GWP of SF6 is 22,800 whereas that of C2F6 is 12,200 and that of C3F8 is 8,830, or several times smaller than that of SF6. In the present paper, insulation characteristics were experimentally obtained while varying the mixture ratio under a quasi-uniform electric field assuming GIS. Consequently, compared to the GWP of pure SF6, the GWP was about 12% to 38% for gas mixtures with C3F8/N2 or C3F8/CO2 and 18% to 70% for gas mixtures with C2F6/N2 or C2F6/CO2. Consequently, it emerged that, while assuming breakdown voltage proportional to gas pressure, the GWP was likely to be reduced by 30% to 90% while maintaining dielectric strength. In addition, a study was conducted on the synergism of a gas mixture through analysis using the Boltzmann equation. Consequently, the synergism was confirmed while its degree varied depending on the type of each gas mixture, and the mechanism thereof was clarified.

On-line I−/Te− separation for the AMS analysis of 125I

The isobar separator for anions (ISA) was used together with a 3MV tandem accelerator mass spectrometer (AMS) to demonstrate the real time (on-line) separation of Te− from I−. Following the ion source mass spectrometry and major retardation to tens of eV, the ISA uses a radiofrequency quadrupole (RFQ) ion guide to confine and direct I− and associated Te− isobar anions through a gas-reaction cell, where chemical reactions occur at eV energies with the electronegative gas NO2. Anions are subsequently reaccelerated out of the ISA to near original ion source extraction energies for AMS analysis. At 5mTorr NO2 in the ISA gas-reaction cell, 125Te− was observed to be attenuated by a factor of ∼107 as compared to 127I− that did not experience significant (<50%) losses. A comparative test using 37Cl− and 32S− (having similar chemical properties to iodine and tellurium) showed a 32S− attenuation of >107 relative to 37Cl− under the same ISA–AMS conditions. The preferential destruction of Te− (and S−) at eV energies in the ISA is likely due to a larger favorable destruction cross-section with NO2. This study is the first demonstration of I–Te anion separation for AMS, and makes possible the use of 125I, free of the contaminant 125Te isobar after suitable sample purification, for future 129I/125I carrier-free analyses of natural samples at ultra-low trace levels.

Obtaining electron attachment cross sections by means of linear inversion of swarm parameters

We investigate electron attachment to strongly attaching gases as a function of the electron energy. The total electron attachment cross section of this electronegative sample gas is determined from the effective ionization rate constant νeff/N measured in a swarm experiment using the buffer gases N2 and CO2 with minor proportions (≲0.8%) of the sample gas SF6, and C3F8 respectively. The measured rate constants νeff/N for varying reduced electric field strengths E/N are unfolded from the electron energy distribution of the buffer gases. Different unfolding routines for so-called ill-posed problems are tested to find robust solutions for the attachment cross section. Finally, we propose Tikhonov regularization, which is a well defined algorithm for discrete linear inversion problems, as most efficient. Our method gives rise to an approximate and clearly defined solution for the attachment cross section, and it is described in detail.

The PEGASES Gridded Ion-Ion Thruster Performance and Predictions

The plasma propulsion with electronegative gases (PEGASES) thruster is a gridded ion thruster that accelerates alternately positively and negatively charged ions to provide thrust. Over the last few years, various prototypes have been tested, adequate diagnostics have been developed, and analytical models and simulations are made to better understand and control the physics involved. Here, we present the state-of-the-art in the PEGASES development as of 2013, and discuss its possible future in space.

Experimental investigation and parameter diagnosis of air high-pressure ring-shaped inductively coupled plasma

The variable parameters like electron destiny (ne), electron collision frequency, covered-area and thickness have been regarded as the key factors for the electromagnetic scattering characteristics of the covering target. Therefore, an air inductively coupled plasma (ICP) generator of all-quartz chamber of 20 cm × 20 cm × 7 cm without magnetic confinement and grounded metal surface of substantial area is designed and conducted to study the discharge process and diagnose the parameters in this paper. The shape, E-H mode transition, and structure of inductively coupled plasma are observed, and the width and thickness change due to change of power and pressure are measured in experiments. Results show that the plasma is nearly uniformly full of the chamber in E-mode, while the shape of plasma rapidly changes to a ring in H-mode and the structure of inductively coupled plasma stratified into an electronegative core and an electropositive halo. It is observed clearly that the luminance of plasma increases slowly with the RF power in E-mode, but increases significantly in H-mode, which are proved through the relative spectral intensity variation of nitrogen 337.1 nm spectral lines due to the change of power and pressure. The width and thickness of the core region increase significantly with power, while decrease apparently with increasing pressure, which could be logically explained by the variation of RF magnetic induction amplitude distribution with power and by the theoretical diffusion analyses of electronegative gas. Since a mass of oxygen electronegative ion exists in the air inductively coupled plasma, the electron density (ne) diffusion models are different for the electronegative core and the electropositive halo. It is proved also by the theoretical drift-diffusion analyses that the electron density is distributed nearly uniformly in the electronegative core and decreased sharply in the electropositive halo. The model of electromagnetic wave propagation in the ICP generator is given and the microwave interferometry is discussed in detail. The electron density in the core region under different discharge conditions is diagnosed by microwave interferometer and the electron density of edge halo is calculated by using the high-pressure diffusion model. And the electron density increases with increasing power and pressure, which range from 0.65×1011 to 3.71×1011 cm-3. But decay rate of electron density in the halo is less affected by the power at 100 Pa, while the rate is accelerated with increasing pressure. Finally, the electronic excitation temperature of the electronegative core and the electropositive halo are diagnosed by Boltzmann graphic method using emissive spectrum of auxiliary Ar. Results show that the electronic excitation temperature of the core, which ranges from 4201 to 4390 K, increases with increasing power, but decreases with increasing pressure.

Measurements of Corona Discharge in Non-Uniform Field Freon GAP

Corona Discharge have been recorded in pin-to pin electrode at gas pressure gaps with high degree of non-uniformity electric field. For a wide range of experimental applicability the pin to pin electrode was used over pressure range from 1bar to 3bar and gap spacing from 0 to 5mm.High –pressure dc corona discharge experiments have been carried out in a 16cm long glass chamber with an inner diameter and outer diameter are 16.8cm, and 19cm respectively. Corona onset voltage and breakdown voltages were reached. The design considerations of the setup allow to use pin to pin electrode type geometries. Electronegative gases such as air, Freon (R22), were separately used for low temperature corona generation. The experiments have been carried out under ambient Laboratory conditions of temperature (19-24 oC), and humidity. The present experiment were studies the relation between the (V-P) and (V-d), on other hand, we study the relation between the current and applied voltage.

Influence of oxygen gas on characteristics of self-organized luminous pattern formation observed in an atmospheric dc glow discharge using a liquid electrode

Self-organized luminous pattern formation is observed in the liquid surface of an atmospheric dc glow discharge using a liquid electrode with a miniature helium flow. The factors affecting pattern formation are the gap length, discharge current, helium mass flow rate and polarity. The pattern shape depends on the conductivity and temperature of the liquid electrode. A variety of patterns were observed by changing the conductivity and temperature of the liquid. We clarified that the self-organized pattern formation depends on the amount of electronegative gas, such as oxygen, in the gas in the electrode gap. When an oxygen gas flow was fed to the liquid surface from the outside in an obliquely downward direction, namely, the amount of oxygen gas on the liquid surface was increased locally, self-organized pattern formation was observed in the region with the increased amount of oxygen gas. When the amount of oxygen in the gas in the gap was changed by using a sheath flow system, the appearance of the pattern changed. The presence of oxygen gas strongly affected the self-organized pattern formation of the atmospheric dc discharge using a liquid anode.

Prediction of the dielectric strength for c-C4F8 mixtures with CF4, CO2, N2, O2 and air by Boltzmann equation analysis

The dielectric strength of c-C4F8, and mixtures of c-C4F8 with CF4, CO2, N2, O2 and air, is studied through solution of the Boltzmann equation. The reduced ionization coefficient α/N and reduced attachment coefficient η/N are calculated, allowing the reduced effective ionization coefficient (α–η)/N and the critical reduced electric field strength (E/N)cr (the reduced electric field for which (α–η)/N = 0), to be determined. A high value of (E/N)cr for an electronegative gas, such as those considered here, indicates good insulating properties. It is found that c-C4F8–N2 and c-C4F8–air have very similar (E/N)cr values, higher than those of the other three mixtures, and superior even to that of pure SF6 for c-C4F8 concentrations above 80%. Comparison of the results obtained for c-C4F8 and c-C4F8–N2 with experimental values from the literature supports the validity of the approach taken here and the parameters used.

Partial discharges and breakdown in C3F8

Traditional search processes of gases or gas mixtures for replacing SF6 involve time consuming measurements of partial discharges and breakdown behaviour for several voltage waveforms and different field configurations. Recently a model for prediction of this behaviour for SF6 was described in literature. The model only requires basic properties of the gas such as the critical field strength and the effective ionization coefficient, which can be obtained by swarm parameter measurements, and thermodynamic properties, which can be calculated. In this paper, we show for the well-known and electronegative gas octafluoropropane (C3F8) that it is possible to transfer the model developed for SF6 to this gas to describe the breakdown behaviour of C3F8. Thus the model can be beneficial in the screening process of new insulation gases.

Abnormal polarity effect in nanosecond-pulse breakdown of SF6 and nitrogen

Abstract The breakdown of gas gaps in an inhomogeneous electric field at subnanosecond and nanosecond voltage pulse rise times are studied, and the famous polarity effect in point-to-plane gaps is investigated. It is shown that at a voltage pulse rise time of ∼0.5 ns, the inversion of polarity effect takes place not only in electronegative gases such as SF6, but also occurs in electropositive nitrogen. The inversion of polarity effect is related to a delay of electron emission from the plane cathode on arrival of the ionization wave front anode to the cathode. It is found that with a voltage pulse rise time of ∼0.5 ns, the inversion of polarity effect occurs at SF6 and SF6–N2 pressures of 0.25 MPa and lower, and with a voltage pulse rise time of 15 ns, at a SF6 pressure lower than 0.12 MPa.