Ion collection by Mach probes in flowing unmagnetized plasmas.

摘要

Plasma behavior in flowing unmagnetized plasmas was investigated using ion collection with Mach probes. The experiments were performed with planar and spherical Mach probes in two different devices, one a DC Multi-Dipole plasma device for subsonic flow within a presheath region and the other a double plasma device for supersonic flow. In the presheath region, the Mach probe measurement using the expression R = exp(KM), where R is the ratio of upstream to downstream current, K is a calibration factor equal to 1.34, and M is Mach number, agreed with the velocity profile derived from ion flux conservation using the potential profile and showed that the ions accelerated mostly near the wall, which was different than LIF and Wave measurements. Due to the presence of background plasma, supersonic flows were determined from the ion beam detection by the upstream side of the planar Mach probe. The very negatively biased grid did not fully repel primary electrons even when the Debye length was comparable to the grid opening. Based on the experimental data, the ineffectiveness of grid biasing suggests that the operation of gridded energy analyzers should be reexamined. Asymmetry reversal in ion collection was found in supersonic and subsonic ion flows. These data are in qualitative agreement with a recent numerical calculation by Hutchinson. Higher ion flux on the downstream side of the probe compared to the upstream side flux occurred for low ion temperature and high probe bias compared to the electron temperature, relatively low ion drift velocity, and Debye length comparable to probe radius. As suggested by Hutchinson, it was found that K depended on the plasma parameters, especially for finite Debye length and high probe bias. Asymmetry reversal leads to negative K, and emphasizes the lack of validity of using a constant K except for narrow ranges of the parameters. This study is the first experiment to demonstrate the non-intuitive phenomenon predicted by Hutchinson's numerical calculation.