Study on spots on electrodes and polarity effect inversion in a nanosecond-pulse gas breakdown

Summary form only given. Pulsed discharges, especially with fast rise time and high over-voltage, usually accompany by some specific pheromones in the nonuniform electric field. In this paper, the appearance of bright spots on the electrodes in air breakdown and the inversion of polarity effect in SF₆ breakdown are investigated. Experiments were carried out by using three experimental setups with a full width at half maximum (FWHM) at a matched load of 2, 30-40 and 130 ns. The experimental results show that in the case of air breakdown, bright spots on the flat electrode appear for both positive and negative polarity of the voltage pulse. At the negative polarity of the electrode with a small radius of curvature, these bright spots on the flat electrode arise when current through the gap changes its direction and pulse duration increases. At the positive polarity of the electrode with a small radius of curvature, these bright spots on the flat electrode appear due to the participation of the dynamic displacement current in the gap conductivity. In the case of SF₆ breakdown, with the FWHM of 2 ns, the inversion of polarity effect occurs at the pressures of 0.25 MPa and lower for SF₆ and SF₆-N₂, and with the FWHM of 30-40 ns, the inversion of polarity effect occurs at the pressure lower than 0.12 MPa for SF₆. Such inversion of the polarity effect may be related to the ionization wave front. The ionization wave velocity is higher for positive polarity of voltage pulse than that for negative polarity of voltage pulse.