Supershort avalanche electron beam in SF₆ and krypton

Runaway electrons play an important role in the avalanche formation in nanosecond- and subnanosecond- pulse discharges. In this paper, characteristics of a supershort avalanche electron beam (SAEB) generated at the subnanosecond and nanosecond breakdown in sulfur hexafluoride (SF₆) in an inhomogeneous electric field were studied. One pulser operated at negative polarity with voltage pulse amplitude of ∼30 kV and rise time of 0.3 ns. The other pulser operated at negative polarity with voltage pulse amplitude of 70 kV and rise time of ∼1.6 ns. SAEB parameters in SF₆ are compared with those obtained in krypton (Kr), nitrogen (N₂), air, and mixtures of SF₆ with krypton or nitrogen. Experimental results showed that SAEB currents appeared during the rise-time of the voltage pulse for both pulsers. Moreover, amplitudes of the SAEB current in SF₆ and Kr approximately ranged from several to tens of milliamps at atmospheric pressure, which were smaller than those in N₂ and air (ranging from hundreds of milliamps to several amperes). Furthermore, the concentration of SF₆ additive could significantly reduce the SAEB current in N₂-SF₆ mixture, but it slightly affected the SAEB current in Kr-SF₆ mixture because of the atomic/molecular ionization cross section of the gas had a much greater impact on the SAEB current rather than the electronegativity.