Characterization of Rapid Charging Events due to Sheath Capacitance and Impact on the International Space Station Plasma Hazard Process

Abstract

During an Extravehicular Activity (EVA), if the Extravehicular Mobility Unit (EMU) makes galvanic contact with the International Space Station (ISS), a negative Floating Potential (FP) can lead to an arcing hazard when it exceeds -45.5 V, and a positive FP can produce a DC current high enough to stimulate the astronaut's muscles (5 mA), and also cause a hazard. The Boeing Space Environments team developed and utilizes a Plasma Interaction Model (PIM) in order to calculate the ISS FP based on the plasma environment, ISS velocity, geomagnetic field, solar array and ISS orientation, and solar array regulation to support EVA planning operations. Presently, the model excludes the sheath capacitance, resulting in the total potential drop being across the dielectric surface of the vehicle. Data from the Floating Potential Measurement Unit (FPMU) show this assumption to be generally true. However, Rapid Charging Events (RCE) are often observed in the FPMU data at eclipse exit when the electron number density, Ne, is low (<5e10 m -3). During these events, the FP can rise more than 40 V in one to five seconds. There is then a relaxation phase where the FP drops back to the normal FP values. The PIM model is not capable of producing these RCEs. It was thought that the inclusion of the sheath in PIM could improve the charging predictions, particularly as related to RCEs. A parametric study was performed to determine what portion of the measured FP is across the sheath for a range of Ne experienced by the ISS, and if the inclusion of the sheath in PIM is necessary. Results show that the potential drop across the sheath is negligible at times when the Ne > 1e11 m -3 . However, there appears to be a transitional region between 1e10 m -3 and 1e11 m -3 where the sheath capacitance becomes more significant. During those conditions the potential drop across the sheath can be larger than the potential drop across the dielectric for short periods (1-5 seconds). These results agree remarkably well with measurements made by the FPMU. The inclusion of the sheath explains why high charging measurements occur when the Ne is low at eclipse exit and even times when the solar arrays are not a significant driver (i.e., potentials often rise as the ISS flies through spread-F). Results also show that the RCEs are not a safety concern because the potential drop across the dielectric surface does not exceed -45.5 V. In that case, the EMU would not arc. This gives high confidence in the low probability of an arcing hazard occurring.Background:

Strategies for Determining Electron Yield Material Parameters for Spacecraft Charge Modeling

“…A cursory, investigation of recent studies returned a substantial list of references, which assumed that the NASCAP default values for Al were appropriate for their modeling, including Davis et al (2017), Marchand and Lira (2017), Wang et al (2017), Hughes and Schaub (2018), Schmidl et al (2018), Wolfley (2018), Bengtson et al (2019), andPandya et al (2019). It is significant to note that this ambiguity for Al has also been propagated by other international charging codes, including MUSCAT (Nakamura et al, 2018) and European Space Agency (2018).…”