Plasma flows generated by an annular thermionic cathode in a magnetized plasma*

A LaB₆ thermionic emitter of annular shape is used in the LAPD device to create off-axis heating conditions for various transport studies. Since the emitter is biased relative to a distant anode (15 m away) the entire magnetized plasma develops a self-consistent, potential structure that simultaneously generates transverse and axial flows with shear. This study uses swept Langmuir probe techniques and Mach probes to map the three-dimensional flow patterns and their dependence on bias and plasma parameters. By implementing additional biasing configurations it is possible to control the magnitude of the flows and their shear strength. The experimental measurements are compared to predictions of a Braginskii code that incorporates the boundary conditions associated with thermionic injection.