High‐voltage interactions in plasma wakes: Simulation and flight measurements from the Charge Hazards and Wake Studies (CHAWS) experiment

1999 
The Charge Hazards and Wake Studies (CHAWS) flight experiment flew on the Wake Shield Facility (WSF) aboard STS-60 and STS-69. The experiment studied high-voltage current collection within the spacecraft wake. The wake-side sensor was a 45-cm-long, biasable cylindrical probe mounted on the 3.66-m-diameter WSF. Operations were performed in free flight and at various attitudes while on the shuttle orbiter remote manipulator system (RMS) arm. Preflight and postflight simulations were performed using the programs Potentials of Large Objects in the Auroral Region (POLAR) and Dynamic Plasma Analysis Code (DynaPAC) and are compared here with the flight results. Both programs perform three-dimensional, self-consistent, steady state plasma simulations. During high-voltage operations the wake-side probe collected current consistent with preflight predictions. In both the flight data and the steady state simulations the current collected has a power law dependence on the potential and has a less than linear dependence on density. Growth of the sheath beyond the WSF edge controls the high-voltage current collection, and kinematic and space charge effects both play important roles in attracting ions into the wake region. Measurements made at low voltage differ from the calculations. The preflight calculations for a pure oxygen plasma predict a collection threshold at −100 V bias. The flight data show little or no threshold, implying a source of ions not accounted for in the simulations. Possible sources for these ions are discussed.
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