Velocity-shear-driven drift waves with simultaneous azimuthal modes in a barium-ion Q-machine plasma

Low-frequency instabilities modified by a field-aligned flow velocity shear are investigated, where the shear is controlled using a concentrically three-segmented ion emitter in a modified double-ended Q-machine [T. Kaneko, Rev. Sci. Instrum. 73, 4218 (2002)]. When the barium-ion flow velocity shear is produced by applying different bias voltages to two of the three segments of the ion source, drift waves having azimuthal mode number m=1, 2, and 3 are observed to be excited simultaneously. It is found that the excitation-threshold value of the shear strength depends on the azimuthal mode number, as expected from a model based on kinetic theory, suggesting that the observations are explained by the competition between inverse electron Landau damping and shear-modified ion Landau damping.