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 probe moun ted in the wake of the Wake Shield Facility (WSF) during the STS-60 an d STS-69 (CHAWS I and II) missions. A three-dimensional particle-in-ce ll code is applied in analysis of the three data categories: high-volt age wake (>/similar to 100 V/), low-voltage wake (</similar to 100 VI) , and data obtained when the shield was inverted and the probe pointin g into the plasma flow (ram-oriented data). The high-voltage analysis finds that the code underpredicts the data by about a factor of two wh en the code inputs are the detector measurement of plasma density and ion temperature, assuming that all of the plasma ions were O+ and assu ming that the electron temperature was 1.5 times the ion temperature. Analysis in the other two categories allowed identification of the inp ut error. The low voltage analysis rules out low Mach number flow whic h contributes disproportionately to the current, demonstrating that it was composed of H+ (rather than ionized contamination from the Shuttl e/WSF or turbulent O+) and not present in quantities significant enoug h to invalidate the all O+ assumption of the code. The ram-oriented da ta analysis, in conjunction with space charge limited collection theor y and International Reference on the Ionosphere, 1990 version (IRI-90) , prediction, suggests that the CHAWS detectors underestimated the amb ient plasma density and thereby reconciles the high-voltage code-data comparison. Other significant findings of this work include location o f OC collection turn-on voltages, the possibility that electron heatin g (T-e similar to 5T(i)) occurred in the ram direction, and the code d emonstration of wake-side collection insensitivity to T-e, confirming its orbit motion limited nature.