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.

Ion acceleration and anomalous transport in the near wake of a plasma limiter

“…As a rule of thumb, electrons in NIPs become unimportant when the electron-ion density ratio falls significantly below the ion-electron thermal velocity ratio. This condition is not met in the present plasma so electrons should be taken into account.͒ Ion flux peaks 45,46,52 were observed intermittently in the midwake region of this system several gyrolengths ͑10.5-11.1 cm͒ downstream in the Ba ϩ /e Ϫ plasma at magnetic-field strengths of 4 and 6 kG ͑but not at 2 kG͒. Peaks were noisy and not reproducible in shape or magnitude.…”

“…͑Their fickleness is believed due, in part, to the perturbations by the Langmuir probe diagnostic.͒ Ion flux peaks have been observed in unmagnetized and partially magnetized plasmas. 45,46,52 They are believed to result from electrostatic ion focusing by fields associated with the obstacle. As described by Taylor, 57 two types of electric fields may arise around electrically floating obstacles in an unmagnetized plasma: ͑1͒ a negative Debye sheath of scale length D , and ͑2͒ a negative potential gradient due to the difference in thermal velocities between electrons and ions.…”

“…[40][41][42][43][44][45][46] The near wake, immediately downstream of the obstacle, consists of a void and a thin region of disturbed plasma at the interface of the void and ambient plasma. For an unmagnetized plasma, this boundary makes an angle ϭtan Ϫ1 (1/M ) with the plasma streaming vector, where M is the plasma Mach number, the ratio of plasma drift to thermal speeds.…”

“…Thus, the plasma flow is supersonic with respect to the obstacle. The middle wake, characterized by an ion flux peak 45,46 bridges the near and far wakes. In the far wake, the plasma makes its transition back to ambient plasma conditions.…”

Ion temperature enhancement in the wakes of ionospheric spacecraft

“…It has been well established that ions are accelerated into the void region behind a spacecraft from the ambient plasma at the sides of the wake [Stone, 1981b …”