The influence of the plasma inhomogenity on the critical velocity phenomenon

The phenomenon of a critical ionization velocity is discussed under the aspect of experimentally measured inhomogenities. The difficulties inherent in homogeneous plasma models on the phenomenon are shown. A simple sheath model is presented which can be compared with the local plasma parameters measured in a rotating plasma device. The origin of the observed turbulent heating is attributed to a modified two-stream instability occuring in the sheath under discussion. (orig.)