The influence of water vapor on avalanches in air

Abstract

In the early stages of breakdown of a gas individual avalanches of charged particles are formed. Measurements on avalanche currents with the electrical method can yield new information on discharge processes if the measuring system has a high time resolution (1.3 ns in our case) and if the avalanche is initiated at the cathode by means of a very short (0.6 ns) light pulse. Avalanches in atmospheric air show an aftercurrent caused by delayed electrons which result from detachment. Measurements at various humidities, an extensive analysis and comparison with computer calculations give, for E /p = 35 V /cm Torr, the following result. The negative oxygen ions formed lose their electrons gradually with a timeconstant of 83 ns. Collisions however, cause conversion to a nondetaching type of ion. This conversion process has a characteristic time of 32 ns in dry air and is more rapid when water molecules are present, down to 3 ns at a water vapor pressure of 11.25 Torr. The occurrence of these processes, where water molecules cause a reduction of the number offree electrons, provides a new explanation for the increase of the breakdown voltage of air with humidity. The measurements also yield values of the true (a -17), the ionization minus the attachment coefficient of air. This parameter leads to the number of electrons in the head of the avalanche which, used in the streamer criterion, gives good agreement with experimental breakdown voltages. Older (a -17) values included the effect of detachment and gave no agreement with the streamer criterion.PACS numbers: 52.80.Dy, 51.50. + v 3286

VLF/LF High-Voltage Design and Testing

“…Verhaart and van der Laan (1984) indicate that this asym atmospheric air (containing normal amounts of humidity) is expected to be 26.2 kVp/cm at STP as opposed to 24.4 kVp/cm for air, this provides a possible explanation as to why his 60-Hz corona onset data are asy instead of 24.4 kVp/cm. Dc measurements of corona onset on wires above a ground plane have been undertaken by Gallo et al (1969) as a function of temperature and humidity.…”

“…The temperature us 3 . Kuffel concludes that at 60 Hz the humidity effect is unlikely to exceed 2 or 3% over the range of humidity normally encountered in the laborato (1 with the findings of Davies e pulse breakdown and the findings of Verhaart and van der Laan (1984) that the breakdown strength of air containing water vapor is approximately independent of the humidity. However, it does not seem to be consistent with their finding that the fundamental breakdown strength of air is approximately 8% greater than the breakdown strength of dry air when humidity is present.…”

“…The amount of the increase is a function of the electric field, increasing rapidly with field strength. A partial explanation for this is contained in Verhaart an van der Laan (1984). They used dc measurements to determine the value of the effective ionization ptote for dry air.…”

An Efficient Procedure to Identify and Quantify New Molecules for Insulating Gas Mixtures

“…Usually, photocathodes are operated in vacuum [33,34,35]. However, in PT experiments [36,37,38,39], the photocathode necessarily operates in the presence of gases at 100 Pa to 100 kPa total pressure. Under these conditions, one must consider the adsorption of gas molecules on the photocathode surface [40], and the interaction of photoelectrons with the gas particles in the immediate vicinity of the cathode.…”

“…In many cases a stainless steel cathode was used in conjunction with a 266 nm laser system [41,36,37]. In other cases an aluminum cathode was used with a 355 nm laser [38] or a 337 nm laser [39]. In these cases the laser pulse energy used to be between 120 µJ and 20 mJ.…”