Ion Drag on a Highly Negatively Biased Solar Array DANIEL E. HASTINGS & MENGU CHO Summary Highly biased solar arrays are found to have a number of significant interactions with the space environment. The negatively biased parts of the array undergo enhanced ion drag and also suffer from destructive arcing. The enhanced drag suffered by highly biased solar arrays is studied with the PIC code. A new model of the drag is developed and the results are compared to recent experimental work. The drag calculations contain the effect of having the conductor surrounded by dielectrics as well as the charging of the dielectric by electrons. Introduction Future space systems, such as the space station, call for solar arrays to deliver anywhere from 25 kW to several megawatts of power. At these power levels, weight and efficiency constraints will demand solar arrays which operate at several hundred volts. The greatest voltage used so far has been the 100 V used on Skylab [1], and it has been seen that at voltages above this, there is significant interaction between the solar array and the ambient plasma. Solar arrays typically have the interconnectors between the solar cells exposed to the space environment. The solar array is then connected electrically in some fashion to the space system on which it resides. In equilibrium, the net current drawn from the space plasma to the entire space system including the solar arrays must be zero. This is achieved when some part of the array floats positively with respect to space and the rest of the array floats negatively. The positive part of the array attracts electrons and the negative part of the array attracts ions. Beyond a critical negative voltage, solar arrays undergo arcing on their negative ends which can rapidly destroy the solar cells. Conversely, beyond a critical positive voltage, the solar array begins to collect anomalously high currents. This jump in current collection is called ‘snapover’. The parts of the array which are negatively charged will suffer from ion drag. The ion drag for a large three dimensional solar array has been studied numerically and experimentally [2, 3]. In the experimental and numerical work it is shown that for practical high-voltage solar arrays in low earth orbit that the iron drag could be as much as a quarter of the aerodynamic drag on the solar array. This is a significant drag and is much larger than would be estimated by treating the ion flow like a neutral flow. Daniel E. Hastings (Class of 1956 Career Development Associate Professor of Aeronautics and Astronautics, Member AIAA) & Mengu Cho (Research Assistant), Department of Aeronautics and Astronautics, MIT, Cambridge, MA 02139, USA.
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