Particle-in-Cell Code Analysis of Charging Hazards and Wake Studies Experiment

The Charging Hazards and Wake Studies (CHAWS) experiment characterized the ion current collection to a negatively biased Langmuir probemounted in the wake of the Wake Shield Facility (WSF) during the STS-60 and STS-69 (CHAWS I and II) missions. A three-dimensional particle-in-cell code is applied in analysis of the three data categories: high-voltagewake (> 100V ), low-voltagewake (< 100V ), and data obtainedwhen the shield was inverted and the probe pointing into the plasma  ow (ram-oriented data). The high-voltageanalysis Ž nds that the code underpredicts the data by about a factor of two when the code inputs are the detector measurement of plasma density and ion temperature, assuming that all of the plasma ions were O+ and assuming that the electron temperature was 1.5 times the ion temperature. Analysis in the other two categories allowed identiŽ cation of the input error. The low-voltage analysis rules out lowMach number  ow, which contributes disproportionately to the current, demonstrating that it was composed of H+ (rather than ionized contamination from the Shuttle/WSF or turbulent O+ ) and not present in quantities signiŽ cant enough to invalidate the all O+ assumption of the code. The ram-oriented data analysis, in conjunctionwith space charge limited collection theory and InternationalReference on the Ionosphere, 1990 version (IRI-90), prediction, suggests that the CHAWS detectors underestimated the ambient plasma density and thereby reconciles the high-voltagecode–data comparison. Other signiŽ cant Ž ndings of this work include location of O+ collection turn-on voltages, the possibility that electron heating (Te 5Ti ) occurred in the ram direction, and the code demonstration of wake-side collection insensitivity to Te, conŽ rming its orbit motion limited nature.