Theory of electrical breakdown in air-the role of metastable oxygen molecules

Abstract

A theory is given of electrical breakdown in air for non-uniform electric fields. It is proposed that space-charge effects tend to make the electric field in the streamer column uniform at a value of E/N for which effective ionization and attachment rates are equal (E is the electric field and N the gas number density). It is also proposed that a1 Delta g metastable states of molecular oxygen, produced in the pre-breakdown corona and streamer processes, have a dominant role in determining E/N in the streamer channel, because of their ability to detach electrons from O2- ions and thus reduce the effective attachment coefficient. Assuming an effective value of alpha /N=10-22 cm2 ( alpha being the ionization coefficient) at E/N=20 Td, predictions of breakdown fields in dry air and humid air are in fair agreement with experimental results for rod-rod breakdown with electrode spacings of from 0.5 m to 2.5 m. The effect of the metastables is to reduce the breakdown field applicable to plane parallel electrodes by a factor of about 6. It is predicted that 1% water vapour in air increases breakdown fields by about 11%. A breakdown criterion is derived defining the critical E/N for breakdown in terms of electron transport coefficients and metastable rate constants.