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 of free electrons, provides a new explanation for the increase of the breakdown voltage of air with humidity. The measurements also yield values of the true (α−η), 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 (α−η) values included the effect of detachment and gave no agreement with the streamer criterion.