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 radial potential profile is inhomogeneous, to the nonsegmented source side, where the radial potential profile is homogeneous, the ions experience an axial acceleration that has an inhomogeneous radial profile. This configuration with the mesh is referred to as the parallel configuration. Laser-induced-fluorescence is used to measure the ion-velocity-distribution function on the nonsegmented-source side with and without the mesh inserted. Measurements show that the above configurations result in ion temperature anisotropy, 1<(T⊥∕T‖)<5, and radially nonuniform ion flows with a maximum in the shear profile (dU⊥∕dx)max as large as 0.6Ωi and (dU‖∕dx)max as large as 0.17Ωi where Ωi is the ion gyrofrequency.