Abstract
It is shown that the drift waves can become unstable due to the shear flow produced by externally applied electric field. The modified Rayleigh instability condition is obtained which is applicable to both electron-ion and electron-positron-ion plasmas. It is proposed that the shear flow driven drift waves can be responsible for large amplitude electrostatic fluctuations in tokamak edges. In the nonlinear regime the stationary structures may appear in electron-positron-ion plasmas as well as electron-ion plasmas. A particular form of the shear flow can give rise to counter-rotating dipole vortices and vortex chains. The speed and amplitude of the structures are affected by the presence of positrons in the electron ion plasma. The relevance of this investigation to laboratory and astrophysical plasmas is pointed out.