Abnormal polarity effect in nanosecond-pulse breakdown of SF₆ and nitrogen

The breakdown of gas gaps in an inhomogeneous electric field at subnanosecond and nanosecond voltage pulse rise times are studied, and the famous polarity effect in point-to-plane gaps is investigated. It is shown that at a voltage pulse rise time of ∼0.5 ns, the inversion of polarity effect takes place not only in electronegative gases such as SF₆, but also occurs in electropositive nitrogen. The inversion of polarity effect is related to a delay of electron emission from the plane cathode on arrival of the ionization wave front anode to the cathode. It is found that with a voltage pulse rise time of ∼0.5 ns, the inversion of polarity effect occurs at SF₆ and SF₆-N₂ pressures of 0.25 MPa and lower, and with a voltage pulse rise time of 15 ns, at a SF₆ pressure lower than 0.12 MPa.