THE INFLUENCE OF IMPURITIES ON THE DIELECTRIC STRENGTH OF SF6 FOR POSITIVE POLARITY; THE NATURE OF INITIATORY ELECTRONS

ABSTRACT Two kinds of sources of seed electrons are analysed through the time lag distributions, namely the collisional detachment from negative ions preexisting in the gap and from negative ions formed by attachment of an electron released from the cathode and drifting towards the critical volume. In both cases, the role of impurities, especially water vapour and oxygen, are emphasized. The application of a bias voltage shows that the second mechanism is a mixed effect of absorbed and adsorbed water vapour molecules on the cathode surface.