LOW-FREQUENCY INSTABILITY EXCITED BY A MESH GRID IN A DOUBLE-PLASMA DEVICE

Coherent low-frequency oscillations whose frequency is lower than the ion plasma frequency are generated by a mesh grid with negative d.c. bias in a double-plasma device. The frequency is inversely proportional to the one-quarter power of the grid voltage under a fixed plasma density, and is proportional to the ion plasma frequency when the grid bias is kept constant. With potential profiles near the grid, which are measured by an emissive probe, the transit time of ions is numerically calculated. The inverse of the transit time agrees well with the oscillation frequency. The oscillation is considered to be excited by a klystron bunching effect of incident ions towards the ion-rich sheaths.