Low-frequency sheath instability stimulated by an energetic ion component

Spontaneous low-frequency oscillations have been observed in the circuit of a positively biased electrode immersed in a non-Maxwellian laboratory plasma containing an energetic ion component produced by the resonant absorption of a short microwave pulse in a nonuniform plasma column. The oscillations are found to be due to an instability of the electron-rich sheath. The instability with its characteristic frequency below the ion plasma frequency is driven by the energetic ion component reflected in the sheath area. A qualitative model of the instability is suggested.