INSTABILITIES ASSOCIATED WITH A NEGATIVE RF RESISTANCE IN CURRENT-CARRYING ION SHEATHS

Coherent instabilities on the order of the ion plasma frequency are generated by applying a dc voltage between a meshed grid and a disk target in an unmagnetized plasma. The instability occurs in the ion-rich current-carrying sheath on the negatively charged grid when some ions are reflected from the grid to the sheath edge by the potential difference between two plasmas divided by the grid. The exciting mechanism of the instability has been identified as a negative rf resistance associated with the ion inertia in the ion-rich sheath, coupled to an ion resonance caused by a positive feedback due to the reflection of ions in the sheath region. The frequency of the instability, which is proportional to the plasma density, is basically determined by the ion transit time through the sheath, and thus, is proportional to the product of inverse sheath thickness and average ion velocity related to the applied voltage.