Breakdown potential of water vapour between plane parallel electrodes

Breakdown potentials of water vapour are measured at room and elevated temperatures between plane parallel electrodes made of stainless steel. The water vapour pressure is varied in the range 22·5-573 Torr and the temperature of both the vapour and the metal electrodes from 298-374 K. A minimum breakdown potential of 2000 V is observed in the p0d range (normalized vapour pressure times electrode gap length) 18-23 Torr cm. The generalized secondary ionization coefficient γ in water vapour is calculated, as a function of the ratio of electric field and gas pressure, from the measured breakdown potentials and using the recently published values of primary ionization and attachment coefficients.