Elizabeth Reynolds

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 et al., 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.

Laser-induced-fluorescence characterization of velocity shear in a magnetized plasma column produced by a segmented, Q-machine source

Control over the radial profile of one component of ion drift speed U, either perpendicular or parallel to the magnetic field, is demonstrated in the magnetized plasma column for two configurations of a double-ended Q machine in which one of the hot plates is segmented into a central disk and two concentric annuli. Each hot plate can be heated to ∼1800°C, and the segmented hot plate is bombarded with barium vapor, resulting in quasineutral, magnetized, barium-ion plasma. Since the electric potential in the plasma magnetically maps to the segments, the radial profile of plasma potential, and hence the E×B flow, can be made inhomogeneous. This configuration is referred to as the perpendicular configuration. A negatively biased, cross-sectional mesh, inserted between the two ends of the plasma column, reflects electrons from each source causing the radial profile of potential on the nonsegmented source side of the mesh to become uniform. In traversing the mesh sheath from the segmented source side, where the...

Inhomogeneity scale lengths in a magnetized, low-temperature, collisionless, Q-machine plasma column containing perpendicular-velocity shear

Radial inhomogeneity scale lengths for radial electric field, ion density, and magnetic-field-aligned (parallel) electron-drift velocity have been measured and interpreted in magnetized, low-temperature, collisionless plasma. The effect of a narrow layer of inhomogeneity in these parameters on the excitation of electrostatic ion-cyclotron waves is investigated. When the ion Larmor radius ρi is on the order of, or larger than, the half-width at half-maximum σr{Er} of the Gaussian-like, radially localized, radial electric-field profile Er(r), the radial profile of the azimuthal ion rotation velocity, measured using laser-induced fluorescence (LIF), has a peak that, because of finite-Larmor-radius effects, is significantly lower than the peak of the combined radial profile of the E×B and diamagnetic drift velocities. Results of an experimentally validated test-particle simulation are presented and applied using experimentally relevant electric-field profiles. Two experimental configurations are explored for ...