Downstream instabilities of Ar/SF/sub 6/ inductive plasma discharges

Summary form only given, as follows. A new class of relaxation oscillations is observed in low-frequency inductive plasma discharges operated with low-pressure argon and SF/sub 6/ gas mixtures The oscillations occur in a certain domain of gas pressure, radio frequency power, electronegative gas chemistry, and downstream region length Langmuir probes and fast video imaging show that density perturbations grow in tens of microseconds in a quasi-spherical downstream layer. Large density variations are then seen below the layer until the initial plasma profile is recovered. The data suggest that some plasma instability develops periodically in the downstream layer. Linear fluid and kinetic calculations predict ion two-stream instabilities when the positive and negative ions develop sufficiently large drift velocities of opposite sign in the downstream region. The thresholds and the growth rates of the instabilities are calculated and are found in fair agreement with the observations. The instability models can explain several key experimental features.