Kelvin-Helmholtz instabilities in tokamak edge plasmas

The parallel Kelvin-Helmholtz instability is investigated as a possible exp lanation for poloidal asymmetries of density fluctuations which reverse wit h the plasma current direction. It is shown that these modes are localized around the position where the radial gradient of parallel velocity is maxim um. Two mechanisms lead to unstable Kelvin-Helmholtz modes; the acceleratio n of ions in a presheath and the anomalous Stringer spin-up due to asymmetr ies of the particle flux. Up-down asymmetries are explained by combining th ese two effects. Depending on the limiter configuration, the Stringer effec t amplifies or weakens the flow due to presheath acceleration. This type of asymmetry reverses with the plasma current direction. (C) 1999 American In stitute of Physics. [S1070-664X(99)03510-7].