INSTABILITY DUE TO AXIAL SHEAR AND SURFACE IMPEDANCE

The stability of plasma flow in the scrape-off layer of a tokamak, tak ing into account the surface sheath impedance and the axial shear in t he EXB flow is analyzed. An interesting stability problem arises in th e limit that end plates are sufficiently far apart, so that stability can be analyzed when the plasma is taken to interact with a single end plate. As parameters are varied, windows of instability are found, an d it is shown that growth rates are maximized for an insulating end pl ate and are also quite sensitive to the ratio of the ion diamagnetic a nd EXB drift frequencies. Mixing-length estimates of the diffusivity a re comparable to experimentally observed values.