Chapter V the breakdown of dielectrics

A number of studies of the statistics of Townsend avalanche production were made in 1958. Frommhold (1), bas examined the conditions under which the theory of Wijsman, relating the number of avalanches to their size, is valid. Using counting techniques, he measured avalanche size distributions in oxygen, air and methane, and found the Wijsman statistics valid providing the field was not too close to that necessary for breakdown, in which case secondary γ processes produced “satellite” avalanches. The mean avalanche sizes corresponded to those predicted from known values of the ionization coefficient α when allowance was made for attachment processes in oxygen and air. Schlumbohm (2) has extended this work to include ethyl alcohol, methane, acetone and methylal, and, in addition to the deviations observed by Frommhold, has found that in many cases there was a marked paucity of small avalanches in the distribution. This has been correlated with the parameter κ = E/αUi, where E is the field strength and Ui is the ionization potential. Unless κ ≫ 1 the Wijsman distribution cannot be valid. Schmidt-Tiedemann (3), using a fast oscilloscopic technique to examine currents due to individual avalanches at lower values of pd (pressure x gap) than were used by Vogel and Raether (1957), have obtained values for the second Townsend coefficient, γ, for clean copper, oxidized copper and oxidized tungsten in nitrogen, hydrogen and oxygen. Their technique permits separate evaluations of that part of γ due to positive ions, γi, and that due to photons, γp. Depending upon the gas or the cathode material, one or the other of these effects may predominate. Of particular interest is the overall value of γ obtained with oxidized tungsten, 10⁻⁹, a value low enough to permit streamer breakdown to occur even at low pd. Kessemeier (4) has constructed an interesting device in which positive ions are held back by a double grid system placed between the anode and cathode, while electrons are transmitted. By thus eliminating positive-ion γ effects at the cathode, avalanches of very large size can be obtained.