Two-step ionization in non-equilibrium SF₆discharges at high current density

Abstract

In the pressure range 10-100 mbar, the discharge development in SF 6 has been investigated in the photo-triggered excitation scheme for current density in the range 10-1000 A cm −2 and pulse duration of about 60 ns. Thanks to the homogeneity of the photo-triggered discharge, a self-consistent zero-dimensional model can be used to predict the temporal evolution of the electrical parameters. From a detailed comparison between the experimental results and the modelling predictions, evidence for the occurrence of a two-step ionization process in high current density discharges in SF 6 is reported and the corresponding collision cross-section is estimated. Moreover, it is shown that the amount of the two-step ionization is directly correlated to the density of the electrical charge transferred to the plasma per unit surface. This two-step ionization becomes the main source of electron multiplication whenever the transferred charge per unit surface is higher than 15 µC cm −2 .