Three-body Attachment Research Articles

Generation of seed electrons in guided ionization waves in He–O2 mixtures: The effect of negative ion cluster formation

The kinetics of electrons and negative ions was numerically studied under the conditions, which are typical for multi-pulsed guided ionization waves in He flows ejected into ambient air. It was shown that, in He with a small admixture of O2 or air, O2− ions formed due to three-body electron attachment in the discharge afterglow are rapidly converted to O4− cluster ions in pulse off time. We suggested that seed electrons are generated at the leading edge of a pulse in two steps, and these steps are dissociation of O4− ions to form O2− ions followed by electron detachment from the O2− ions. The rate of O4− dissociation was calculated in He–O2 mixtures for various reduced electric fields on the basis of a Monte Carlo simulation. This rate, as well as the rate of electron detachment from O2− ions calculated previously, was used to simulate seed electron generation in a high-voltage nanosecond pulse in He–O2 mixtures for different gas pressures. It was shown that the formation of O4− ions drastically hinders the generation of seed electrons in multi-pulsed guided ionization waves in He–O2 and He–air mixtures.

Two- and three-body attachment, electron transport and ionisation in water-air mixtures

Three-body electron attachment in the mixtures of H2O and dry air have been measured over a wide range of the density-reduced electric field, E/N, from 3–130 Td and gas pressures, for mixture combinations ranging from 1% to 50% of H2O. We have measured the regions of three-body attachment (3–30 Td) and two-body dissociative attachment (40–130 Td). Besides, the increasing amount of H2O in the mixture causes an increase in the three-body reaction rates of up to two orders of magnitude in comparison with that measured for dry air. On the other hand, the three-body attachment coefficients exceed the two-body ones (dissociative attachment) at high pressures. Good agreement has been found with previous measurements of H2O-dry air mixtures with H2O concentrations of up to 2%. We know of no previous work for higher H2O concentrations. Values of the effective ionisation coefficients and longitudinal diffusion coefficients derived from the same measurements are also presented.

The influence of humidity on positive streamer propagation in long air gap

A 2D numerical simulation of the positive streamer properties was performed in 9–12 cm plane-to-plane air gaps for various pressures and water vapor contents. It was shown that an increase in air humidity leads to hampering the streamer development and to increasing the average critical electric field required for bridging the discharge gap. The effect of humidity was most profound at atmospheric pressure and decreased with decreasing pressure. The influence of water content on the streamer properties was explained by a decrease in the streamer channel conductivity due to dissociative recombination of electrons with positive hydrated ions and enhanced three-body electron attachment to O2 molecules. The calculated critical electric field and streamer velocity in humid air gaps were compared with available experimental data.

Discussion on the mechanism leading to positive synergism in SF6 mixtures with HFO1234ze(E)

The electric strength in HFO1234ze(E)/SF6 mixtures is investigated with swarm and breakdown experiments. The density-reduced critical electric field as well as the breakdown voltage measured with both techniques, are found to be higher than that of the pure gases in mixtures with more than 10% SF6. The underlying mechanism for the observed positive synergy is investigated and the explanation proposed by Hunter and Christophorou (1985 J. Appl. Phys. 57 4377–85) is discussed for this mixture. The pressure-dependent attachment rate is found to increase with SF6 ratio thus satisfying the main requirement of the proposed mechanism in Hunter and Christophorou’s study. It appears nevertheless that due to the fast saturation with pressure and low rates in the mixtures, the three-body attachment processes account only for a small increase in the electric strength. An alternative hypothesis is proposed which considers the strong reduction of electron energies via inelastic processes in HFO1234ze(E), and is qualitatively demonstrated based on measurements and simulations.

Formation of H¯+ via electron-assisted three-body attachment of e+ to H¯

The formation of positive ions of antihydrogen ${\overline{\mathrm{H}}}^{+}$ via the three-body reaction (i) ${e}^{+}+{e}^{\ensuremath{-}}+\overline{\mathrm{H}}\ensuremath{\rightarrow}{e}^{\ensuremath{-}}+{\overline{\mathrm{H}}}^{+}$ is considered. In reaction (i), free positrons ${e}^{+}$ are incident on antihydrogen $\overline{\mathrm{H}}$ embedded in a gas of low-energy ($\ensuremath{\approx}$ meV) electrons and, due to the positron-electron interaction, a positron is attached to $\overline{\mathrm{H}}$ whereas an electron carries away the energy excess. We compare reaction (i) with two radiative attachment mechanisms. One of them is (ii) spontaneous radiative attachment, in which the ion is formed due to spontaneous emission of a photon by a positron incident on $\overline{\mathrm{H}}$. The other is (iii) two-center dileptonic attachment which takes place in the presence of a neighboring atom B and in which an incident positron is attached to $\overline{\mathrm{H}}$ via resonant transfer of energy to B with its subsequent relaxation through spontaneous radiative decay. It is shown that reaction (i) can strongly dominate mechanisms (ii) and (iii) for positron energies below 0.1 eV. In addition, it is shown that, at the energies considered, reaction (i) will not be influenced by annihilation and that the reaction ${e}^{+}+{e}^{+}+\overline{\mathrm{H}}\ensuremath{\rightarrow}{e}^{+}+{\overline{\mathrm{H}}}^{+}$ has a vanishingly small rate compared with reaction (i). We also compare the reaction (i) with the formation of ${\overline{\mathrm{H}}}^{+}$ in collisions between an antihydrogen atom and a positronium.

Research on the electron attachment of oxygen using a Frisch-grid ionization chamber

The electron attachment is studied using a Frisch-grid ionization chamber (GIC) with different number densities of O2 in the working gas and different reduced electric fields. The influence of the electron attachment on the cathode–anode two-dimensional amplitude spectrum of the GIC is measured and evaluated. The method of measuring the attachment probability per unit time using the GIC is established. The quantitative and systematic relationships between the attachment probability per unit time and the number density of O2 and the reduced electric field are obtained from experiments, where the number densities of O2 are from 0 to 2.29 × 1017 cm −3 and the reduced electric fields are from 22 to 577 V/cm/bar. The three-body attachment coefficients of O2 at different mean electron energies (< 1 eV) are measured, which are consistent with those from previous works. For the first time, the three-body attachment coefficients of CO2 at different mean electron energies (<1 eV) are measured. It is found that the three-body attachment coefficients of O2 and CO2 are nearly the same at the same mean electron energy.

Direct electron attachment to fast hydrogen in 10−9 contrast 1018 W cm−2 intense laser solid target interaction

The interaction of an ultra-short (<30 fs), high-contrast (<10−9), high-intensity (>1018 W cm−2) laser pulse with a solid target is not generally known to produce and accelerate negative ions. The transient accelerating electrostatic-fields are so strong that they ionize any atom or negative ion at the target surface. In spite of what may appear to be unfavourable conditions, here it is reported that H− ions extending up to 80 keV are measured from such an interaction. The H− ion flux is about 0.1% that of the H+ ions at 20 keV. These measurements employ a recently developed temporally-gated Thomson parabola ion spectrometry diagnostic which significantly improves signal-to-noise ratios. Electrons that co-propagate with the fast protons cause a two-step charge-reduction reaction. The gas phase three-body attachment of electrons to fast neutral hydrogen atoms accounts for the measured H− yield. It is intriguing that such a fundamental gas-phase reaction, involving the attachment of an electron to a hydrogen atom, has not been observed in laboratory experiments previously. Laser-produced plasma offers an alternative environment to the conventional charged particle beam experiments, in which such atomic physics processes can be investigated.

Analytical Results for the Three-Body Radiative Attachment Rate Coefficient, with Application to the Positive Antihydrogen Ion H+

To overcome the numerical difficulties inherent in the Maxwell–Boltzmann integral of the velocity-weighted cross section that gives the radiative attachment rate coefficient α R A for producing the negative hydrogen ion H − or its antimatter equivalent, the positive antihydrogen ion H ¯ + , we found the analytic form for this integral. This procedure is useful for temperatures below 700 K, the region for which the production of H ¯ + has potential use as an intermediate stage in the cooling of antihydrogen to ultra-cold (sub-mK) temperatures for spectroscopic studies and probing the gravitational interaction of the anti-atom. Our results, utilizing a 50-term explicitly correlated exponential wave function, confirm our prior numerical results.

Effect of Temperature on Insulation Properties of SF₆/N₂ Mixed Gas Under AC Voltage

The insulation properties of SF6/N2 mixed gas are important for the insulation design of high-voltage equipment such as gas-insulated switchgear (GIS) and gas-insulated transmission line (GIL). However, the influence of temperature on insulation performance of SF6/N2 under alternating current (ac) voltage is rarely studied. In this article, we did gas breakdown experiments with SF6/N2 mixed gas at temperatures changing from −50 °C to 100 °C. The experimental results indicate that the breakdown voltages of SF6/N2 mixed gas increase with the increase in temperature. In order to explain the phenomenon, the three-body attachment process in compressed SF6 is introduced to analyze the electron avalanche process affected by temperature. The streamer and leader discharge criteria are introduced to qualitatively explain the influence of temperature on the breakdown voltages. It is found that the streamer channel radius becomes larger at elevated temperatures, which causes the increase in breakdown voltage.

Measurements of the electron swarm parameters of R1225ye(Z) (C3HF5) and its mixtures with N2 and CO2

The electron swarm parameters of R1225ye(Z) (cis--pentafluoroprop-1-ene) and its mixtures with the buffer gases N2 and CO2 (content of R1225ye(Z) from 1% to 50%) are experimentally investigated using a pulsed Townsend experiment. The analysis of the electron avalanche displacement current yields the effective ionization rate coefficient, the electron drift velocity and the longitudinal electron diffusion coefficient mainly around the density-reduced critical electric field , which is of interest for application in medium or high voltage insulation. In pure R1225ye(Z), increases with increasing pressure from around 280 Td at 2 kPa to 300 Td at 14 kPa. We assume that this is due to a three-body attachment rate coefficient of approximately . Mixtures of R1225ye(Z) with N2 and CO2 are discussed and a comparison to the structurally similar gases HFO1234ze(E) and l-C3F6 is given, which also exhibit three-body attachment.

Three-dimensional simulation of ionic wind in a laminar premixed Bunsen flame subjected to a transverse DC electric field

The role of the ionic wind effects on modifying flame dynamics was demonstrated by detailed computational models. Full three-dimensional simulations were conducted to reproduce and describe the response of a laminar premixed methane–air Bunsen flame subjected to a transverse DC electric field at saturation condition. The chemical kinetic mechanism employed a methane–air skeletal mechanism with an optimized ionization model to predict the positive and negative ions that are important for generating the electric currents. Given the strong dependence of the ionic wind on the amount of charged species created by chemi-ionization, the ion production rate was optimized to match the measured saturation current. The simulation successfully reproduced the flame tilt toward the cathode. The ionic winds blowing from the flame toward the electrodes in both rightward and leftward directions were also captured. The calculated flow field is qualitatively consistent with the PIV experimental data. Accurate description of the three-body electron attachment to oxygen and the charge transfer reactions generating heavy anions was found to be critical in simulating the flame–electric field interaction. This is a first demonstration of the ionic wind effect by full three-dimensional simulations. Further investigation by both experiment and modeling are required in future work to address quantitative differences.

Simulation of pulsed positive streamer discharges in air at high temperatures

Two-dimensional simulations of an atmospheric-pressure streamer discharge at high gas temperatures were performed in humid air at initial gas temperatures, T0, in the range 300 K–600 K with the same electrode and applied voltage conditions as those used in Ono and Kamakura (2016 Plasma Sources Sci. Technol. 25 044007). The simulation was validated by comparing its results to experimentally obtained discharge currents, primary streamer velocities, and secondary streamer diameters and lengths. This paper discusses the mechanisms underlying the temperature effects in terms of the behaviour of the charged particles in the primary and secondary streamers. At T0 = 300 K, the main decay processes in the primary streamer are the electron recombination reactions with cluster ions and electron three-body electron attachment with O2 and H2O, while the main decay process in the secondary streamer is the two-body electron attachment to O2. Although, at T0 = 600 K, the main decay processes in both streamers are still recombination and two- and three-body electron attachment reactions, the rates of these reactions decrease owing to the increase in the gas temperature, which leads to the increased conductivity of streamer discharge channels at high gas temperatures.

Time-resolved measurements of electron density in nanosecond pulsed plasmas using microwave scattering

In this work, microwave scattering was utilized to measure the electron number density produced in a nanosecond-long high-voltage breakdown of air between two electrodes in a pin-to-pin configuration. The approach allows tracing the absolute electron number in the plasma volume during the high-voltage pulse and the plasma decay for wide range of plasma and discharge parameters. Electron number density decayed on the timescale of about several μs governed by plasma-chemical processes including dissociative recombination, three-body attachment and formation of cluster ions.

Lower ionospheric electron density changes following lightning discharges

The effect of lightning-induced electromagnetic (EM) waves on electron density (Ne) of the lower ionosphere is calculated by using the four-component Glukhov–Pasko–Inan (GPI) model which is modified by including two-body attachment reactions and associative detachment with negative ions of active species (namely, O and N atoms). As a result of the calculations, it is seen that three-body attachment reactions take place at about 50–73 km altitudes, associative detachment takes place at approximately 73–85 km altitudes and two-body attachment reactions take place at approximately 85–95 km altitudes on the electron density. While the effect on electron density of electric field of lightning-induced EM wave is weak at altitudes where associative detachment is effective, it is strong at altitudes where two-body attachment reactions are effective. Also, it is shown that the electron density increases up to approximately 5.5 times when compared with the background density values due to electrons occurring as a result of associative detachment with negative ions of active species.

Comparison of gases for electrical insulation: Fundamental concepts

Several fluorinated compounds have been introduced by chemical and equipment manufacturers to replace the widely used sulfur hexafluoride (SF6) as electrical insulation gas. Unlike SF6, these compounds are admixed to the atmospheric gases nitrogen, oxygen or carbon dioxide to avoid liquefaction at typical operating temperatures. The aim of this paper is to give a direct and clear comparison of the two key parameters, namely the breakdown voltage and the minimum operating temperature, of recently proposed insulation gases as well as to support the selection process of novel compounds. This article covers the basic physics and essential models necessary to characterize the electric strength and the liquefaction temperature of gas mixtures. This includes the description of electron avalanches, negative ion formation by electron attachment and the phenomenon of three-body attachment.

Monte Carlo simulations of electron transport coefficients in low temperature streamer discharge plasmas

Streamer is usually present at the initial stage of atmospheric pressure air discharge, which occurs in nature as a precursor to lightning, transient luminous events in upper atmosphere and has much potential applications in industry, such as the treatment of polluted gases/liquids, assisted combustion, plasma enhanced deposition etc. Streamer is a multi-scale problem both in time and in space, which brings much difficulty to the conventional diagnostic approaches. In past decades, fluid or particle-fluid hybrid models have been frequently used for understanding the mechanisms of streamer discharges because of their high efficiencies of calculations. Accuracies of the electron transport coefficients (including drift/diffusion coefficient, ionization/attachment coefficient, electron mean energy and extra) play a key role in ensuring the correctness of the fluid or hybrid simulations. As far as we know, BOLSIG+ and MAGBOLTZ are two typical tools for obtaining the electron transport coefficients and have been widely utilized in previous models. BOLSIG+ uses two-term approximation which is not sufficient for some molecular gases, MAGBOLTZ cannot calculate the bulk transport coefficients:these data are required for some models. METHES is an additional tool for computing electron transport coefficients, however, specific platform is required which is not very user-friendly. As sorts of drawbacks exist in currently available calculating tools, in the paper, a Monte Carlo model is developed for computing the electron transport coefficients in gases, the model is flexible to choose any type of gas mixture and its accuracy has been validated by comparing with BOLSIG+ and METHES. Furthermore, the influences of N2-O2 mixture and three-body attachment process in high gas pressures on the transport coefficient are investigated. It is worth mentioning that three-body attachment process can significantly change the electron transport properties at a relatively low reduced electric field. Thus, specific attention must be paid to the transport coefficients if simulation is performed at a high pressure. In addition, differences between the bulk and flux coefficients are analyzed which are not distinguished in some previous models. Finally, we further validate the present Monte Carlo model by performing simulation of streamer discharge in atmospheric N2, which shows that the improved electron transport coefficient from our Monte Carlo model can improve the simulated plasma properties, in particular at the interior of the streamer channel. The existence of divergence at the tip of the streamer channel might be due to our local field approximation; if a density gradient term is included in the impact ionization term and local electron energy approximation of the electron transport coefficients is used, the accuracy of the fluid can be improved further.

Rotational state-dependent attachment of He atoms to cold molecular ions: An action spectroscopic scheme for rotational spectroscopy

We present a kinetics model description of a newly developed action spectroscopic method for rotational spectroscopy based on rotational state-dependent three-body attachment of He atoms to cold molecular ions stored in a cryogenic 22-pole ion trap. The model results from numerical simulations and an approximate analytical expression are compared to measurements of the J=1–0 rotational transition of CD+, for which we obtain a refined transition frequency of 453.5218509(7)GHz. From the analysis of the spectroscopic data recorded at varying experimental conditions, e.g. over a wide range of He number densities and excitation powers, we deduce that the ternary rate coefficient in the first excited rotational state of CD+ is reduced to (55±5)% of the rotational ground state value. This decrease in the rate coefficient can be rationalized as an increase of the redissociation probability in the ternary collision process. A summary of rotational spectroscopy measurements of other molecular ions using the new method will be given, and its general applicability is discussed.

Time-resolved electron temperature and electron density measurements in a nanosecond pulse filament discharge in H2–He and O2–He mixtures

Time evolution of electron density and electron temperature in a nanosecond pulse, diffuse filament electric discharge in H2–He and O2–He mixtures at a pressure of 100 Torr is studied by Thomson/pure rotational Raman scattering and kinetic modeling. The discharge is sustained between two spherical electrodes separated by a 1 cm gap and powered by high voltage pulses ~150 ns duration. Discharge energy coupled to the plasma filament 2–3 mm in diameter is 4–5 mJ/pulse, with specific energy loading of up to ~0.3 eV/molecule. At all experimental conditions, a rapid initial rise of electron temperature and electron density during the discharge pulse is observed, followed by the decay in the afterglow, over ~100 ns–1 µs. Electron density in the afterglow decays more rapidly as H2 or O2 fraction in the mixture is increased. In He/H2 mixtures, this is likely due to more rapid recombination of electrons in collisions with and ions, compared to recombination with ions. In O2/He mixtures, electron density decay in the afterglow is affected by recombination with and ions, while the effect of three-body attachment is relatively minor. Peak electron number densities and electron temperatures are ne = (1.7–3.1) · 1014 cm−3 and Te = 2.9–5.5 eV, depending on gas mixture composition. Electron temperature in the afterglow decays to approximately Te ≈ 0.3 eV, considerably higher compared to the gas temperature of T = 300–380 K, inferred from O2 pure rotational Raman scattering spectra, due to superelastic collisions. The experimental results in helium and O2–He mixtures are compared with kinetic modeling predictions, showing good agreement.

Electron swarm parameters of the hydrofluoroolefine HFO1234ze

In this contribution, the electron swarm parameters of the hydrofluoroolefine HFO1234ze (systematic name trans-1,3,3,3-tetrafluoro-1-propene) are experimentally investigated. The analysis of the electron avalanche current measured in a pulsed Townsend experiment yields the effective ionization rate coefficient, the electron drift velocity and the longitudinal electron diffusion coefficient. The subsequent ion current is analyzed as well, to obtain separately the ionization and attachment rate coefficients. Measurements in pure HFO1234ze at different pressures show that the effective ionization rate is strongly influenced by three-body attachment and the three-body attachment rate coefficient is derived.

Direct measurement of the characteristic three-body electron attachment time in the atmospheric air in direct current electric field

We report the results of theoretical and experimental study of the characteristic time for three-body attachment of electrons produced by 100 fs UV laser pulse in the atmosphere air in the external DC electric field ranged from 0.2 to 10 kV/cm.