Growth Of Ionization Currents Research Articles

Calculation of Formative Time Lags in Dry Air in Crossed Electric and Magnetic Fields

A formula has been derived for the calculation of formative time lags in dry air with high applied overvoltages on the basis of temporal and spatial growth of ionization currents. The formula is then extended to a discharge in E×B fields by considering the influence of a crossed magnetic field on each of the parameters involved in the current build up process, according to the effective reduced electric field (EREF) concept. For the first time Somerville's formula for the loss of electrons to the cathode in a magnetic field has been shown to be applicable in the calculation of formative time lags. At zero magnetic field and low magnetic field strengths, the calculated formative time lags in dry air agree very well with the measure values. Calculated formative time lags over a range of magnetic field strengths and over voltages are presented. In a strong magnetic field and high overvoltages, the time lag calculations show that the secondary ionization coefficient is entirely due to photoelectric action at the cathode.

Growth of ionization currents in nitrogen

Townsend's primary and secondary ionization coefficients α/p and γ were determined in nitrogen over a wide range of E/p (100-1000 V cm−1 Torr−1) and p (0·4 to 12 Torr at 0 °C) using the pressure variation technique. This technique, along with the Gosseries method of evaluation of ionization coefficients, seems to be more suitable at higher values of E/p, since the errors in these coefficients could be minimized by a suitable selection of p and d, thus eliminating the non-equilibrium ionization condition.

Growth of ionization currents in carbon tetrafluoride and hexafluoroethane

Values of the ionization coefficient α and attachment coefficient η have been obtained in CF4 and C2F6 at 20°C using the modified Townsend method. In CF4 the reduced field E/p was varied from 40 to 90 V cm−1 torr−1 and the reduced coefficients α/p and η/p were found to be independent of pressure from 5 to 20 torr. In C2F6 the range of E/p was from 70 to 190 V cm−1 torr−1 and the pressure was varied from 2·5 to 30 torr. Values of α/p and η/p are in good agreement at the lower pressures but at 30 torr it was necessary to assume a small detachment coefficient δ/psimilar, equals2 × 10−3 cm−1 torr−1, in order to obtain consistent results.

Electron Attachment in Oxygen

THE various collisional processes of electron attachment in electronegative gases are of interest in the examination of the properties of ionized gases not only in gas discharge tubes but also in the upper atmosphere, so that the data on attachment cross-sections are important. When the electric field E is high enough to produce ionization by electrons by collision, the attachment cross-sections cannot be measured by the direct methods possible at low electron energies. At high electron energies, however, the total cross-section can be calculated from data on the coefficient of attachment defined in a way analogous to that of α, the coefficient of primary ionization. The coefficient of attachment must be computed from measurements on the growth of ionization currents, preferably between parallel plates. In the presence of attachment the steady-state spatial growth of ionization in a uniform electric field at a constant value of E/p (p the gas pressure) can be represented by: where I is the amplified ionization current, at an electrode separation d, resulting from a small externally maintained current I0(∼10−13 A), α′ is the apparent primary ionization coefficient, ω a generalized secondary ionization coefficient and at the total attachment coefficient representing the action of several possible attachment processes.

Theory of Secondary Emission of Electrons in Hydrogen

This paper describes a theoretical investigation of the mechanism of ejection of secondary electrons at the cathode surface in electric discharges in hydrogen. Theoretical estimates of the magnitude of the coefficient δ/α (due to photons) are obtained from the energy balance equation of an electron stream moving in a gas under the influence of an electric field. The magnitudes of the coefficient γ (due to ions) are obtained from the theory of resonance neutralization and Auger neutralization and de-excitation of an ion at a metal surface. Comparison is made between such theoretical estimates and those values of γ and δ/α obtained from an analysis of the measurements of the temporal growth of ionization currents. Satisfactory agreement is obtained when the comparison is made at low values of the ratio of the electric field E to gas pressure p (i.e. in the range E/p similar 50 v cm-1 mmHg-1). At larger values of E/p (similar 200 v cm-1 mmHg-1) the agreement is satisfactory only when the mobility of hydrogen ions is taken to be about 20 cm2 sec-1 v-1 when computing γ and δ/α from the experimental data on temporal growth of ionization. This value of the ion mobility is about three times greater than the previously accepted value obtained by linear extrapolation of experimental data for values of E/p < 100 v cm-1 mmHg-1. The separate contributions to the total γ of the various processes of neutralization and subsequent electron ejection from the cathode are estimated over the full range of E/p from 50 to 250 v cm-1 mmHg-1.

Precision Measurements of Ionization Coefficients in Uniform Static Fields

Recent work has shown that the growth of ionization currents in air and nitrogen up to high values of the parameter pd (where p is the gas pressure and d the gap distance) follows the well-known generalized Townsend equation and that the observed breakdown potentials are in agreement with those calculated from the Townsend breakdown criterion. Having established the nature of the general mechanism the next stage is to determine the ionization coefficients α and ω/α as accurately as possible. The present paper gives an analysis of the errors in the coefficients arising from experimental errors in the measurement of the ionization currents, and on the basis of the analysis defines the experimental conditions necessary for the accurate determination of the coefficients at high values of pd in hydrogen. The apparatus designed to satisfy these conditions is described, and the values of the ionization coefficients, determined for a range of pd from about 200 to 900 mm Hg cm in hydrogen, are given. Investigation of the effect of the state of the cathode surface on the secondary ionization coefficient showed that even at the comparatively high value of pd similar 350 mm Hg cm in hydrogen, the cathode played a significant role in the growth of ionization currents and thus in the setting of the breakdown criterion.

Growth of Pre-Breakdown Ionization Currents in Hydrogen

THE growth of ionization currents with distance at constant low values of E/p (E is the field, p the gas pressure) at high values of p has been investigated in this laboratory primarily to obtain precise determinations of the ionization coefficients α and ω/α in conditions when special attention was paid to the state of the gas and cathode surface. Some of the results obtained preliminary to the main part of this investigation are, however, of general interest.

Ionization processes in the electrical breakdown of gases

In elucidating the phenomena of breakdown it is necessary first to find the general mechanism by which the various processes of ionization interact to produce the necessary amplification of current, and then to identify the particular ionization processes themselves. It was the exact prediction, both of the form of the growth of ionization and of the breakdown potentials, when the spark parameter pd was less than about 200 (mm of mercury × cm), that first established the validity of the Townsend theory based on the continuous development of ionization by primary and secondary ionization processes; the main characteristics of this theory are given. The problem of breakdown at higher pressures when pd is 760 (mm of mercury × cm) is next discussed, and recent experimental and theoretical work on the growth of ionization currents and breakdown is described from which it is concluded that the general Townsend mechanism can also account for breakdown in this range of the spark parameter. This mechanism is then discussed in relation to other properties of breakdown involving the spatial-temporal development of currents, breakdown under impulse voltages, under non-uniform fields, and, finally, in highly compressed gases at very high voltages.