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Measurements of Mode Structure of Shear‐Modified Drift Wave Using Y‐ and Γ‐ Shaped Electrostatic Probes

Mode structures of drift‐wave instabilities modified by a field‐aligned flow velocity shear are investigated, where axial and azimuthal wave numbers are measured using Y‐shaped and Γ‐shaped electrostatic probes, respectively. When the ion flow velocity shear is produced by applying different bias voltages to three segments of an 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 a kinetic theory, suggesting that the observations are explained by the competition between inverse electron Landau damping and shear‐modified ion Landau damping (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)