The plasma wake of mesosonic conducting bodies. Part 1. An experimental parametric study of ion focusing by the plasma sheath

Abstract

An experimental, parametric investigation has been made of the disturbed ion flow field immediately downstream from conducting bodies of spherical and long cylindrical geometries in a collisionless, mesosonic plasma stream. The ion-acoustic Mach number, S, and the normalized test body potential, Φb, were varied, while the test body radii were of the order of a Debye length for all cases. In addition to standard diagnostics, the ion flux intensity, energy and direction were measured with the recently developed differential ion flux probe. These vector measurements provide direct observations of ion streamlines deflected across the wake axis at an angle proportional to sin-1 (|Φb|½/S) and show that some deflected streamlines are bunched together to form converging ion beams which can exist in the near wakes of both test body configurations. The angle of inclination of the converging ion beams depends linerarly on Φb, while the position of their intercept on the wake axis varies with S|Φb|½. The vector flux measurements also provide direct observations of secondary ion trajectory deflexions in the mid-wake region. These features exhibit parametric dependences that are in good agreement with theoretical calculations that include the effects of the test body and space charge potentials.