Supershort avalanche electron beam inSF6and krypton

Abstract

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 (${\mathrm{SF}}_{6}$) in an inhomogeneous electric field were studied. One pulser operated at negative polarity with voltage pulse amplitude of $\ensuremath{\sim}130\text{ }\text{ }\mathrm{kV}$ 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 $\ensuremath{\sim}1.6\text{ }\text{ }\mathrm{ns}$. SAEB parameters in ${\mathrm{SF}}_{6}$ are compared with those obtained in krypton (Kr), nitrogen (${\mathrm{N}}_{2}$), air, and mixtures of ${\mathrm{SF}}_{6}$ 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 ${\mathrm{SF}}_{6}$ and Kr approximately ranged from several to tens of milliamps at atmospheric pressure, which were smaller than those in ${\mathrm{N}}_{2}$ and air (ranging from hundreds of milliamps to several amperes). Furthermore, the concentration of ${\mathrm{SF}}_{6}$ additive could significantly reduce the SAEB current in ${\mathrm{N}}_{2}\text{\ensuremath{-}}{\mathrm{SF}}_{6}$ mixture, but it slightly affected the SAEB current in $\mathrm{Kr}\text{\ensuremath{-}}{\mathrm{SF}}_{6}$ 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.