Density-gradient-driven collisional drift waves are identified by the dependences of ω and k on density, temperature, magnetic field, and ion mass, and by comparisons with a linear theory which includes resistivity and viscosity. Abrupt stabilization of azimuthal modes is observed when the stabilizing ion diffusion over the transverse wavelength due to the combined effects of ion Larmor radius and ion-ion collisions (viscosity) balances the destabilizing electron-fluid expansion over the parallel wavelength, determined by electron-ion collisions (resistivity). The finite-amplitude (ũ/n0 ≃ 10%) coherent oscillation, involving the entire plasma body, shows a phase difference between density and potential waves (which is predicted by linear theory for growing perturbations). The wave-induced radial transport exceeds classical diffusion, but is below the Bohm value by an order of magnitude. Although observations have been extended to magnetic fields three times those for drift-wave onset, turbulence has not been encountered.
Read full abstract