Secondary Ionization Coefficient Research Articles

Effect of Secondary Ionization Coefficient on the Breakdown Voltage in Nitrogen Gas

A theoretical investigation of the secondary ionization coefficients that responsible for the ionization current growth in uniform electric field studied. Nitrogen gas was from the range of E/p various 60 to 392 V/cm Torr, which corresponding to low values of pressures (0.05 < p < 0.684 Torr). The secondary ionization coefficients have been calculated using the breakdown criterion condition for a discharge gap in a uniform electric field. Using theoretical values of the primary ionization coefficient determined from numerically solving Boltzmann equation together with the values of the sparking distance (ds) obtained from present work. The program provided all cross-sections data from nitrogen. Experimentally, the measurement of breakdown voltage was taken after fixing sparking distance at ds= 69 cm. The results of the present work showed that, the value of breakdown voltage decreased during the increase of the secondary ionization coefficient value. Furthermore, under very low-pressure range, and within the studied sparking distance ds, the value of secondary ionization coefficient was negligible. During pressure increase, the value of secondary ionization coefficient becomes a saturation value.

Uncertainty Sources in the Estimation of the Partial Discharge Inception Voltage in Turn-to-Turn Insulation Systems

Partial discharges (PD) are one of the main causes of premature failure in low-voltage motors driven by variable-speed drives. The use of these control systems are being quite extended due to new applications, such as the more electric aircraft (MEA) or hybrid and electric vehicles, and this has pushed research towards appropriate designs of electric motors to avoid, as much as possible, the presence of PD within their windings. This article presents a model to predict the partial discharge inception voltage (PDIV) in the insulation of low-voltage machines. A value for the secondary ionization coefficient based on a statistical study is also proposed. The deviations of the model are also studied by obtaining the uncertainty of the value of that coeficient and the predicted values of the PDIV for a set of wires. This uncertainty will be compared with other error sources such as generator harmonics and humidity. Finally, the tests are done for different temperatures extend the model applicability.

Verification of Paschen Law using a Mixed Geometry Disc-Conical Electrodes

This work studies the Paschen law in electrical breakdown of a mixed geometry disc - conical electrodes from aluminum material at constant 10 cm distance using nitrogen gas. The discharge characteristices including discharge voltage, discharge current and breakdown voltage against the pressure-distance multiplied were measured at different pressures. The secondary electron emission coefficient was calculated using Townsend’s secondary ionization coefficient equation. A comparison between two similar disc and conical structures with a mixed disc - conical structure was made for the breakdown voltage, discharge current and secondary electron emission coefficient. It is concluded that the values of secondary electron emission coefficient of conical cathode are higher than of disc cathode due to higher ions density on conical cathode than that disc one.

Study of the secondary electron emission coefficient using disc and conical electrodes

In this work, glow discharge is formed by similar disc and conical electrode shapes from aluminum material at constant anode–cathode gap distance equal to 10 cm. The breakdown voltage and discharge current against the pressure × distance are measured at different pressures. The secondary electron emission coefficient is calculated using Townsend’s secondary ionization coefficient equation. A comparison is made between the breakdown voltage, discharge current, and secondary electron emission coefficient using disc and conical anode–cathode electrodes. Hence with a product of pressure and distance equal to 6 Torr cm, the secondary electron emission coefficient value of conical shape is higher than for the disc shape.

Gradual increase in secondary ionization coefficient γ and charge accumulation on a dielectric electrode during DBD with repeated breakdown

We have investigated the effect of the surface roughness of a dielectric electrode for dielectric barrier discharge (DBD). We prepared three alumina plates made from the same material with different surface roughnesses for use as electrodes. During the repeated breakdown from the first breakdown to the stationary state, we observed gradual increases in the secondary ionization coefficient and the charge accumulated on the alumina electrodes under the application of a sinusoidal voltage in argon up to a pressure of 105 Torr. A strong correlation was also observed between them, similarly to in a previous paper on CaO film electrodes. The results suggest that increasing the surface roughness of an alumina electrode results in a larger secondary ionization coefficient and greater charge accumulation on the electrode. Moreover, it is concluded that the charges that accumulate on the alumina can be liberated from the surface as initial electrons when it acts as a cathode, depending on the polarity of the alternating gap voltage. These electrons induce the formation of an atmospheric-pressure Townsend discharge. Finally, we discuss the effect of the secondary ionization coefficient in DBD.

Secondary ionization coefficient γ of MgO, SrO and CaO and the correlation between γ and charge accumulated on CaO in argon

An experimental investigation of Townsend's secondary ionization coefficient γ is carried out for MgO, SrO and CaO film electrodes. These oxides are utilized, or are to be utilized, in next-generation plasma display panels. All of the experiments are performed in argon. In particular, in the case of CaO film, our attention is attracted to the effect of charge accumulated on CaO film. For this purpose, the breakdown voltage Vs is observed from waveforms of the gap voltage and charge accumulated on the dielectric film electrodes with the repeated breakdown. Townsend's criterion is utilized to estimate γ from the breakdown voltage. In the case of the CaO film, γ increases gradually with repeated breakdowns and reaches a stationary value. The increase of γ is considered to be due to the charge that accumulates on the dielectric electrode triggering the next breakdown. Therefore, it is concluded that the increased γ is sustained by a combined process consisting of restarted electrons that have accumulated on the CaO film and the original γ action, as observed in metallic electrodes in a low-pressure gas discharge.

Measurement of secondary ionization coefficient of CaO film electrode

The secondary ionization coefficient γ of a CaO film electrode is investigated taking into account the difference in breakdown voltage obtained by repeated voltage applications. Such measurement is performed under a sinusoidal voltage of 0.5 Hz. If the CaO film electrode acts as the cathode, breakdown voltage gradually decreases and converges to an almost constant value after several breakdowns. From the obtained results, the γ of the CaO film electrode is determined for each breakdown using Townsend’s criterion. The γ in the first breakdown is lower than those in subsequent breakdowns, particularly in the steady state. The difference in γ is considered to originate from accumulated charges on the CaO film electrode.

Influence of the hot filament on the electrical breakdown characteristics in the presence of Ar/N2

The influence of a hot filament on the electrical breakdown characteristics is studied for different ratios of argon and nitrogen gases for a wide range of pressure. The vacuum tube consists of two parallel plane stainless steels used as cathode and anode accompanied with a tungsten filament located behind the cathode. Paschen’s curves are obtained for different ratios of argon and nitrogen as a function of pressure for various electric currents of the hot filament. The first and second Townsend coefficients as well as the ionization efficiency and secondary ionization coefficient are obtained for different filament currents. In addition, the influences of the nitrogen partial pressure on the forgoing parameters are obtained. It is shown that, increase of the filament current causes the decrease of the electrical breakdown voltage which is more pronounced in low pressures. Furthermore, introducing the nitrogen gas leads to the increase of the breakdown voltage and decrease of the ionization efficiency as well as the first and second Townsend coefficients. Moreover, it is concluded that, in the middle range of pressure, the presence of the hot filament results to the electrical breakdown which reveals the linear features.

Secondary Ionization Coefficient of MgO and Accumulated Charge

An experimental study on Townsend's secondary ionization coefficient γ of MgO is carried out in accordance with a previously reported sequential procedure. A sinusoidal voltage is applied between the MgO film electrode and a stainless-steel electrode in the frequency range of 0.1 Hz–2 kHz. The breakdown voltage is determined from the observed waveforms of applied voltage and accumulated charge on the MgO film electrode. The influence on the breakdown voltage of the voltage induced by the accumulated charge is investigated. We found that the accumulated charge does not affect the breakdown voltage at low frequency or the DC voltage, but it affects the breakdown voltage at high frequency. Using the breakdown voltage, we determine Townsend's secondary ionization coefficient γ of MgO. The obtained γ for MgO in the study is compared with other reported values. It is found that γ for MgO is larger than those of metallic electrodes.

Secondary Ionization Coefficient of MgO and Accumulated Charge

An experimental study on Townsend's secondary ionization coefficient γ of MgO is carried out in accordance with a previously reported sequential procedure. A sinusoidal voltage is applied between the MgO film electrode and a stainless-steel electrode in the frequency range of 0.1 Hz–2 kHz. The breakdown voltage is determined from the observed waveforms of applied voltage and accumulated charge on the MgO film electrode. The influence on the breakdown voltage of the voltage induced by the accumulated charge is investigated. We found that the accumulated charge does not affect the breakdown voltage at low frequency or the DC voltage, but it affects the breakdown voltage at high frequency. Using the breakdown voltage, we determine Townsend's secondary ionization coefficient γ of MgO. The obtained γ for MgO in the study is compared with other reported values. It is found that γ for MgO is larger than those of metallic electrodes.

Secondary Ionization Coefficient of Dielectric Electrode -I. Observation and Stabilization of Discharge Paths for Measurement of .GAMMA. of MgO Electrode-

Experiments for observations and stabilization of discharge paths in several electrode systems are carried out aiming at precise measurement of the secondary ionization coefficient γ of MgO film electrode. The discharge chamber is filled with Ar gas. The waveforms of the applied voltage between the electrodes and the discharge current are measured with visual observation of the discharge light. Two MgO coated electrodes are placed so that they are facing each other. For these MgO electrodes, the discharge paths take a detour, not the shortest distance. Smaller prebreakdown current pulses are observed before the breakdown. After breakdown, discontinuous discharge current is observed. Therefore, it is prepared a glass tube surrounding the discharge area. As the result, the discharge paths take a straight perpendicular for the electrode surface, and the discharge is stabilized.

キシレンによるN2(A3&Sigma;u+)の失活とキシレン分解生成物の陰極表面への付着による電流阻止作用

The collisional quenching rate coefficient of metastable nitrogen molecules N2(A3Σu+) by m-xylene (C8H10) is determined experimentally in the Townsend discharge. The diffusion coefficient of N2(A3Σu+), and the reflection coefficient of N2(A3Σu+) at the electrode surface are also determined simultaneously. During the experiment, we find that it is difficult to continue the current measurement necessary to determine the fundamental constants of N2(A3Σu+) for long period. It is considered that any by-product of xylene would be decomposed by N2(A3Σu+) is deposited on the cathode, through repeated experiments, then the current-voltage curves consistently shift to the higher-E/p0 region. For the purpose of clarifying the reason behind this behavior, we confirm that these changes are caused by the current prevention by the decreases of initial photoemission current from the cathode and the decrease in the secondary ionization coefficient γ, because the cathode surface is covered by deposition film of a by-product of decomposed xylene.

Measurement of Townsend's Secondary Ionization Coefficient of MgO Film in Argon

Townsend's secondary ionization coefficients γ of MgO in Ar are determined from the breakdown voltages and the Townsend's criterion. Breakdown voltages on MgO film are measured by the both wave forms, discharge current and applied voltage between the electrodes on the oscilloscope. According to the experimental procedure, γ of gold film and stainless steel electrodes are determined and compared with other experimental values. After the confirmation of the validity of those results, we have carried out the determination for γ of MgO. Furthermore, the influence of heating effect of MgO film to the value of γ is also discussed.

Townsend's Secondary Ionization Coefficient of MgO Film in Ar and Electron Reflection at the Surface

Previously, a Monte Carlo simulation (MCS) was carried out to determine the Townsend's secondary ionization coefficient γ of a MgO film using the experimental values of starting voltage between the electrodes covered by the MgO film. The electrodes were mounted at the center of a chamber as a modeled plasma display panel (PDP) cell filled with Ar. The purpose of the study was to examine the effect of the energy in a nonequilibrium state of electrons on the Townsend's secondary ionization coefficient. It was confirmed that the energy nonequilibrium effect appeared and the effect strongly depended on the initial energy distribution of electrons emitted from the cathode. The values of γ considered in the nonequilibrium of electron energy were approximately 20% larger than those considered in energy equilibrium under the operating conditions of the PDP. In this study, we investigated the effect of electron reflection on both electrode surfaces on the secondary ionization coefficient and the electron transmission coefficient. The electric power injection into the gap space based on the electron transmission coefficient and the dynamic characteristics of electron flux at the anode are explained using the electron reflection coefficient obtained in our previous experiments.

Stochastic and Relaxation Processes in Argon by Measurements of Dynamic Breakdown Voltages

AbstractStatistically based measurements of breakdown voltages U b and breakdown delay times td and their variations in transient regimes of establishment and relaxation of discharges are a convenient method to study stochastic processes of electrical breakdown of gases, as well as relaxation kinetics in afterglow. In this paper the measurements and statistical analysis of the dynamic breakdown voltages U b for linearly rising (ramp) pulses in argon at 1.33 mbar and the rates of voltage rise k up to 800 V s –1 are presented. It was found that electrical breakdowns by linearly rising (ramp) pulses is an inhomogeneous Poisson process caused by primary and secondary ionization coefficients α , γ and electron yield Y variations on the voltage (time). The experimental breakdown voltage distributions were fitted by theoretical distributions by applying approximate analytical and numerical models. The afterglow kinetics in argon was studied based on the dependence of the initial electron yield on the relaxation time Y 0 (τ ) derived from fitting of distributions. The space charge decay was explained by the surface recombination of nitrogen atoms present as impurities. The afterglow kinetics and the surface recombination coefficients on the gas tube and cathode were determined from a gas‐phase model. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Secondary Ionization Coefficient of MgO Film in Argon

The secondary ionization coefficient of MgO in Ar is estimated from the measured starting voltage of self-sustaining discharge between MgO electrodes in a plasma display panel. Monte Carlo simulation and Townsend's criterion of self-sustaining discharge are employed to discuss the electron transport characteristics of the electron swarm in the cell. This technique allows for precise calculations of the behavior of the electron swarm under a nonequilibrium electron energy distribution.

The action of hydrogen in low-pressure r.f.-plasma nitriding

Samples of AISI 316 austenitic stainless steel were nitrided in a low-pressure r.f. plasma using various mixtures of nitrogen and hydrogen with the aim of elucidating the action of hydrogen in plasma nitriding. Exposure of samples to a pure hydrogen discharge prior to treatment in a pure nitrogen discharge increases both the thickness of and the nitrogen concentration in the treated layer compared to an unexposed sample. However, treatment in an N 2–25% H 2 mixture gives even greater layer thickness and nitrogen content. Separate in-situ measurements of the secondary ionisation coefficient, using a pre-breakdown discharge with an AISI 316 sample as the central part of an AISI 316 cathode surface, show a marked increase in secondary electron emission after hydrogen plasma exposure, presumably due to oxide removal. This is direct evidence in support of previous conclusions that hydrogen acts to increase ionisation. The fact that nitriding with an N 2–H 2 mixture produces the best result indicates the action of a second effect in addition to the effects of increased ionisation. Strong evidence for this was provided by a treatment in which hydrogen was present for part of the nitriding step only. Finally, it is suggested that the increase in the thickness of the treated layer caused by the presence of hydrogen points to an action deep (∼μm) beneath the surface of the sample.

The relationship between electron—molecule collision cross‐sections, experimental Townsend primary and secondary ionization coefficients and constants, electric strength and molecular structure of gaseous hydrocarbons

Experimental Townsend primary ionization coefficients are concisely presented for 24 gaseous hydrocarbons starting with methane. Applying the traditional Townsend formula for the primary ionization coefficient enables the saturation ionization constant, A , and the inelastic barrier constant, B , to be obtained for each hydrocarbon. Making use of the Lewis formula for B allows this constant to be further analysed in terms of available excitation and ionization potentials and total electron‐molecule collision cross‐sections, and enables the as‐yet unknown‐except for methaneinelastic collision cross‐sections to be found for each hydrocarbon. Using the inelastic collision cross‐section data for methane, the only hydrocarbon for which this is available, vindicates for the first time the Lewis formula and shows that the vibrational cross‐sections have to be included in the inelastic one for these molecular gases. Inserting the ionization coefficients and/or constants into Townsend's sparking criterion allows Paschen characteristics to be presented up to 500 kV. Each of these sets of data is linked in turn to the number of hydrogen atoms/carbon‐hydrogen bonds rather than the carbon number and thus this electrical classification differs from the chemical one. This is further corroborated by the hydrogen molecule being the natural precursor to methane in its electrical characteristics and by the total collision cross‐section in the relevant electron energy range being determined by the hydrogen atoms in the hydrocarbon molecule. For the alkanes tested, propane is the first member of the series; for the 1‐alkenes, propene‐1 is, while for the methyl‐alkenes, ethene‐1 is the first.

Measurement of the effective ionisation coefficient in SF6 and air mixtures at high E/p20 values

The effective ionisation coefficient alpha /p20(= alpha /p20- eta /p20, alpha and eta being respectively the Townsend first ionisation coefficient and the electron attachment coefficient) and the total secondary ionisation coefficient gamma T for SF6-air mixtures have been measured by the steady-state Townsend method for 150 V cm-1 Torr-1 <or=E/p20<or=2000 V cm-1 Torr-1. The results show that alpha /p20 decreases with decreasing SF6 percentage partial pressure k for E/p20<or=220 V cm-1 Torr-1 and increases with increasing k above this E/p20 value. The alpha /p20-values of the present mixtures are summarised in a form of formula for the prediction and simulation of the ionisation current development and breakdown in the mixtures. The inverse current plots according to Gosseries are studied for the present measurements, but they do not give any evidence of pronounced non-equilibrium which could cast doubt on the validity of the measurement at high E/p20.

Experimental determination of the primary and secondary ionisation coefficients in krypton and xenon

The primary ionisation coefficient alpha of Kr and Xe has been evaluated from measurements of the spatial growth of ionisation currents between plane-parallel gold electrodes for the respective ranges 40<E/n<250 Td and 60<E/n<400 Td. Deviations from Kruithof's values are found, especially at the lower of range E/n. In the same gases the secondary ionisation coefficient gamma has been determined from measurements of the breakdown voltage. It has been clearly established that gamma depends on the electrode separation and a qualitative explanation is proposed.