Downstream instabilities of electronegative plasma discharges

Relaxation oscillations are observed in low-pressure inductive plasma discharges operated with argon and ${\mathrm{SF}}_6$ gas mixtures. The data suggest that instabilities may develop periodically at some downstream location. 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.