Jaroslav P. Novak

Early stages of channel formation in a short-gap breakdown

A description and analysis of the solution of a two‐dimensional model for a HV breakdown of a short gap is presented. The model consists of the electron, ion, and excited‐atom conservation and Poisson equations and is applied to a plane‐parallel gap with an electrode separation of 0.48 mm in helium gas at atmospheric pressure and a temperature of 293 K subjected to an electrical field of 10 kV cm−1. Two‐dimensional plots of the charged and excited‐particle densities and electric field components are presented and discussed. It is shown that in the first, diffusion‐controlled, stage density profiles are close to a Gaussian distribution with an effective radius increasing in time. The subsequent stage is controlled by the space‐charge field, causing prominent constriction of the electron density channel. In consequence, a high ionization near the discharge axis results in a virtual narrowing of the ion and excited‐atom profiles as well, and the forming conductive chanel exhibits a tendency towards constrict...

Theoretical and experimental investigations on the process of high-pressure discharge development

The voltage breakdown behavior of a plane-parallel gap of 0.48-mm length filled with helium was examined at atmospheric pressure with admixtures of dry air at relative pressures of 0, 10/sup -4/, 3*10/sup -4/, and 10/sup -3/. The initial stages of the breakdown were investigated by means of a quantitative model consisting of the electron, ion, and excited-particle conservation equations and the Poisson equation. The system of equations was solved for an applied voltage of 180 V, at one single partial pressure of the impurities. Two numerical routines were used for the solution: a commercial IMSL subroutine TWODEPEP, and a newly developed method of solution in several fractional steps. The results were compared and found to be in reasonable agreement although the new method indicated a somewhat slower rate of rise, particularly concerning electron density. The new method permits extension of the calculation up to electron densities equal to almost two orders of magnitude above the earlier limit. >

Collisional phenomena and current buildup in the electrical breakdown of hydrogen

The electrical breakdown of hydrogen is examined by means of a two-dimensional mathematical model applied to the case of a 0.50-mm gap at atmospheric pressure. The calculations indicate a very rapid discharge development, characterized by a marked deviation of the axial density profiles from an exponential dependence, but with an approximately exponential variation of the discharge current. The axial-electron and ion-density profiles are similar in form and nearly balanced in magnitude. Notwithstanding the small magnitude of the space-charge field, the radial profiles of the electron and ion density displayed a definite tendency towards constriction, i.e., the incipient formation of a spark-type channel. >

Effect of air admixture in helium on the rate of the breakdown current rise

The influence of dry air admixtures on the breakdown mechanism of helium in small gap separations was investigated using plane parallel stainless-steel electrodes enclosed within a Pyrex chamber. The experiments, carried out using different admixtures of air, shown that the time constant of the rate of breakdown current rise is a linear function of the product of the overvoltage across the gap and the partial pressure of the admixture. An empirical formula is derived to express this linear relationship. The rate of the breakdown current rise is calculated, using a quantitative model of the breakdown process; although these values somewhat exceed the experimental ones, the theoretical and experimental values are found to converge when measurements are performed at a slightly elevated relative partial air pressure of 3*10/sup -4/. >

Transient response of a free‐burning discharge: Quantitative model

The work presented here deals with the application of the theoretical model of the nonequilibrium discharge developed by one of us to a free‐burning discharge in argon subjected to a current impulse. The paper includes a description of the system of transport equations and a discussion of the boundary conditions and numerical solutions. The response to a 0.5‐μs current ramp from 0.4 to 1.2 A is studied in detail. The variation of the basic parameters, such as the electron density, temperature, heat flux and diffusion velocity, the heavy‐particle temperature, and heat flux and the average mass velocity is shown as a function of time and space. Radial profiles of the initial and final states are shown to agree well with the results of the earlier steady‐state model.

Radial characteristics of nonequilibrium discharges

A two‐temperature model of a free‐burning discharge at atmospheric pressure is derived from Grad’s 13‐moment approximation of the Boltzmann equation and applied to a low‐current discharge in argon. The collision terms are calculated on the basis of available cross sections using expressions given by Alievskii and Zhdanov. The paper also presents a discussion of the problem of boundary conditions. Finally, radial profiles of the discharge parameters are calculated in the current range 0.5–14 A and the results, when compared to available experimental data, show satisfactory agreement.

Spectroscopic measurements in nitrogen arcs with respect to local thermodynamic equilibrium

A wall-stabilized nitrogen arc at atmospheric pressure was studied spectroscopically in the current range from 20 to 70 A with the aim of detecting departures from LTE. Measurements of the relative intensity of several N(I) lines showed that the populations of the upper excited states are in equilibrium for currents greater than approx. 25 A. Comparisons of the excitation temperatures obtained from relative intensities with the electron temperature and with the temperature curves calculated for different departures from LTE show that deviations from equilibrium are small at the arc center for electron densities above approx. 4 × 1016 cm-3. Deviations between the radial profiles of the electron and excitation temperatures were observed for currents between 35 and 55 A.

Electric field and electrode potential drops of arcs and glow discharges in air

A simple experimental arrangement consisting of two arcs burning in series has been devised for measurement of the voltage gradient in the discharge column. The method is based on reciprocal variation of the length of the discharges so that the total length during the measurement is constant. Experiments have been performed in the current range between 20 mA and 4 A in dry air at pressures between 20 and 150 kPa. In the arc mode the current dependence of the field can be approximated by the power function E∝Iα, where the exponent α for pressures between 60 and 150 kPa is −0.54 and slightly higher at lower pressures. The pressure dependence (at constant current) can be expressed in similar form E∝pm with the exponent m equal to 0.15 and 0.24 for currents of 1 and 4 A, respectively, and for pressures above 40 kPa. Average values of the zero‐length voltage were obtained in the arc and glow discharge modes, namely, 14.5 V in the case of the arc and 367, 346, 331, and 331 V for the glow discharge at pressures ...

Electron swarm properties of CCl2F2/SF6 mixtures

The ionization and attachment coefficients for CCl2F2/SF6 mixtures were determined from measurements of prebreakdown ionization currents in a steady‐state Townsend discharge at a pressure p25=4 Torr and temperature of 25 °C. The curves representing the ionization coefficient (α) for reduced fields in the range from 110 to 180 V cm−1 Torr−1 exhibit a small maximum at about 15% of CCl2F2, while those representing the attachment coefficient (η) flatten out between 0% and 50% CCl2F2 and decrease at higher partial densities. The limit field obtained using the condition α=η shows two extrema, i.e., a minimum at approximately 20% and a maximum at 75% of CCl2F2, while the curves obtained from the breakdown‐potential measurements available in the literature show only a maximum, at roughly the same position. Theoretical values of the transport coefficients were obtained by numerically solving the Boltzmann equation using previously determined effective sets of electron collision cross sections [J. P. Novak and M. F...

Quantitative model of a short-gap breakdown in air

A two-dimensional model of a short-gap breakdown in air is presented consisting of the Poisson equation and the set of conservation equations for electrons, positive ions and stable and unstable negative ions. Recent data on collisional interactions in air are reviewed to determine the transport coefficients, ionization and attachment coefficients and excitation and detachment rates; these are expressed as analytical functions of the local electric field. The conservation equations are formulated in a two-dimensional approximation assuming rotational symmetry. The model is used to study electrical breakdown and discharge formation in a short gap bounded by parallel metallic electrodes at pressures of 1.01, 10.1 and 101 kPa. The results show that at low currents the discharge at all pressures exhibits similar characteristics, typical for a low pressure glow discharges. The calculations were carried out up to the currents of 14.4, 14.3 and 1.35 mA at 1.01, 10.1 and 101 kPa.