Air In Uniform Fields Research Articles

Effect of humidity on the breakdown characteristics of air in uniform field for the very low frequency (VLF) band

The paper reports about the role of humidity and frequency on the electrical breakdown of air in uniform field gaps. Experiments were conducted on Rogowski-profile electrodes with gap lengths ranging from 5 to 53 mm at power frequency (60 Hz) and frequencies in the range of 18-50 kHz, corresponding to the VLF/LF bands used for long-range communication. The results show that breakdown voltage at VLF decreases with humidity, opposite to that observed at 60 Hz. Breakdown mechanisms for explaining this important phenomenon are proposed. A correction factor to calculate breakdown voltage as a function of humidity for VLF is presented.

The mechanism of the electrical breakdown of air in uniform fields at voltages up to 400 kv

Measurements of pre-breakdown ionization currents in uniform fields in dry air at pressures of about 1600 torr were carried out over a range of voltages from 165 kv to over 400 kv, corresponding to a range of values of the parameter p20d (p20 is the pressure reduced to 20°c and d is the electrode separation) from 500 to 12 600 torr cm. Over the whole range investigated the spatial growth of ionization was found to be in accordance with the well-known generalized Townsend equation. Moreover, at a value of p20ds of 12 200 torr cm (ds is the sparking distance), the sparking potential of 408 ± 9 kv predicted by using the Townsend breakdown criterion was in good agreement with the experimentally measured value of 4034 ± 05 kv. In these experiments the gas pressure and electrode separation were chosen so that the maximum value of the field used was less than about 60 kv cm-1. At this value field emission was negligible compared with the externally generated initial current I0 (similar 10-12 A), and values of I0 of this magnitude were shown, experimentally, not to give rise to significant space-charge effects in the measurement of pre-breakdown ionization. The values of the secondary ionization coefficient ω/ά were found to be markedly dependent on the state of the cathode surface showing that, just as at lower pressures and voltages, the cathode plays an important role in the breakdown process. At higher pressures (similar 5000 torr) and higher pre-breakdown fields (similar 100 kv cm-1) field emission currents of about 10-10 A were observed, which can be accounted for on the basis of the Fowler-Nordheim equation modified to take account of the presence of a thin insulating surface film: this work is currently being extended to the investigation of breakdown in these conditions, when space-charge distortion of the applied field due to high field emission from small cathode areas may become important.

Breakdown voltage calculation for air in uniform fields

Expressions are derived for the determination of the space-charge field in front of an avalanche, for the widening of the avalanche's head due to electrostatic repulsion and for the minimum breakdown voltage of uniform gaps. In principle, this last condition is based on a modification of the criterion for streamer advance developed by Raether, Meek et. al. An attempt is made to consider the avalanche as an electric dipole oriented in a direction parallel to the applied field and use is made of the radial instead of the axial field of the avalanche's head.

Measurement of Ionization and Attachment Coefficients in Humid Air in Uniform Fields and the Mechanism of Breakdown

Measurement of pre-breakdown currents and breakdown potentials in humid air in the E/p range of 30 to 40 v cm-1 (mm Hg)-1 at total pressures of 150 and 300 mm Hg with partial pressures of water vapour in the range 2.5 to 15 mm Hg indicate a pronounced increase in attachment compared with conditions in dry air. From the semi-logarithmic plots of current against electrode separation, Townsend's ionization coefficient α and an attachment coefficient η have been obtained for humid air employing the modified Townsend equations for the growth of current. From the measured breakdown potentials, values of Townsend's secondary coefficient γ have been calculated using the modified Townsend breakdown criterion. Further the percentage increase in breakdown potential in humid air has been plotted as a function of the partial pressure of water vapour. From similar measurements in pure water vapour at pressures of 10 and 20 mm Hg in the E/p range of 30 to 50 v cm-1 (mm Hg)-1, values of α/p and η/p have been obtained for water vapour. A mean cross section for attachment has been computed for various values of mean electron energies assuming a Maxwellian distribution and employing the earlier measurements of drift and agitation velocities

Measurement of Ionization and Attachment Coefficients in Dry Air in Uniform Fields and the Mechanism of Breakdown

Measurement of Ionization and Attachment Coefficients in Dry Air in Uniform Fields and the Mechanism of Breakdown

Time-lag data for spark discharges in uniform field gaps

Formative time-lags of spark breakdown have been measured for air in uniform fields with static applied voltages up to 63 kV; and the very long time lags (greater than 10-4 s) predicted by theory for such conditions have been recorded. Time-lag data have also been obtained for air in uniform fields with impulse spark voltages up to 150 kV for standard impulse waveforms, and also for lower voltages with non-standard impulse waveforms. For the latter condition the data indicate an impulse ratio of the order of 1.01. Comparative data for sphere gaps are included.

Formative Time Lags of Spark Breakdown in Air in Uniform Fields at Low Overvoltages

An attempt has been made to establish the region of validity of the streamer and Townsend mechanisms of spark breakdown in air by measurements of the formative time lag of the breakdown process.Formative time lags were measured in a uniform field for overvoltages of a few percent down to as close to threshold as possible. Such measurements have been carried out as a function of pressure (atmospheric to a few cm of Hg) and plate separation (0.3 to 1.4 cm). For pressures greater than 200 mm Hg, the formative time lags very close to threshold are of the order of 100 \ensuremath{\mu}sec and longer. These times are orders of magnitude lnger than those previously reported. For all pressures studied, the time lags decrease as the percent overvoltage (percent o.v.) increases, until at about two percent above threshold, the formative times are of the order of 1 \ensuremath{\mu}sec. The time lag vs percent o.v. curve is independent of pressure from atmospheric down to 200 mm Hg. At a given percent o.v., the time lags increase linearly with plate separation. Changing the approach voltage from two to four kv below breakdown does not affect the results appreciably. The number of initiating electrons at the cathode has been varied by a factor of about seven, and this again does not materially alter the results.The long time lags and their dependence on pressure and percent o.v. cannot be explained by secondary emission of electrons from the cathode by positive ion bombardment. The proper explanation is the enhancement of field intensified ionization due to field distortion acting in conjunction with a photoelectric secondary process. The experiment demands an extension of the streamer concept of breakdown, and makes very questionable the role of positive ion bombardment of the cathode in spark breakdown for the pressures and plate separations studied. No transition region for change of streamer to Townsend breakdown was found.