Ionisation waves in low pressure SF6 gas

Two modes of ionisation waves were observed in the pressure range 0.5-1 Torr and current range 0.1-0.3 mA. With Mode I waves the frequency changed from 20 to 80 kHz with the discharge current. The wave showed a B- type property with both phase and group velocities above 106 cm s-1. The other mode (Mode II) was a kHz wave with a group velocity less than 2*103 cm s-1, which changed from the forward wave to the backward wave with the discharge current. The forward wave showed an exponential increase of both phase velocity and wavelength. Numerical analysis taking into account four species of charged particles agrees qualitatively with the experimental results, and suggests that Mode I was induced by positive ions and Mode II by heavy negative ions.