Parametric instability of the sheath-plasma resonance

A large amplitude RF pump signal is applied to a wire antenna in a magnetic-field-free plasma. When the pump frequency ω0 corresponds to twice the sheath-plasma resonance frequency ωr, and the pump amplitude exceeds a threshold value, spontaneous excitation of the signal at ωr = ω0/2 is observed. The process is explained as a periodic variation of the sheath thickness causing the sheath-plasma resonance to become parametrically unstable. The RF field pressure can induce density relaxation oscillations near the ion plasma frequency which are observed to modulate the instability spectrum strongly.