Chapter V the breakdown of dielectrics

Raether¹ has written a review of the work on electron avalanche growth which has been vigorously pursued in his laboratory for several years. Unfortunately the article is not yet available to the author of this digest. Several other papers, however, by members of his group have appeared. Schmidt-Tiedemann² made a theoretical study of the fields created by both positive and negative space charge in a single electron avalanche and their influence on the avalanche growth. The growth was found to differ markedly from the simple exponential Townsend build-up. Their results are in agreement with experimental evidence reported by others in the literature. Richter³ considers the probability of streamer-initiation by a single large electron avalanche. After the charge carrier number exceeds a critical value ncrit.¹ the probability of streamer formation increases monotonously with the carrier number. The probability of streamer-formation for a constant carrier number increases with applied field. If the carrier number in one avalanche exceeds 10⁸ a process comes into force giving higher ionization and thereby higher probability for larger avalanches. This may be the first stage of streamer-initiation, since streamers are observed only at carrier numbers of this magnitude. Franke⁴ studied light emission from single electron avalanches in methane and methylal vapors with a photomultiplier. The light emitted as a function of time indicated that the breakdown in methane follows several generations of avalanches. In methylal the rapid increase of light emission indicates breakdown by streamer formation as a result of a single electron avalanche. Schlumbom⁵ studied the growth of current in a single avalanche experimentally. The avalanche was initiated by an α-particle impact on the cathode surface which released between 10³ and 10⁵ electrons within a few mμsec. and several generations of avalanches could be observed. Good agreement was obtained between measured and calculated values demonstrating that the discharge is governed by collision ionization and photoemission at the cathode. Frommhold⁶ continued his studies of the electron component of the current in single avalanches. In his experiments a spark light source of short duration was used to release ∼10⁴ electrons from the cathode. Hydrogen, nitrogen, oxygen and air were studied. Measured values of ionic drift velocities in these gases and electron drift velocities in carbon dioxide and ether are given.