The Role of Electrical Potential Barriers in LEO Spacecraft Charging

Abstract

The “rule of thumb” for LEO spacecraft charging has always been that the vehicle will charge negative, perhaps as much as 90% of its power system voltage negative of the ambient plasma. However, historically, electrical potential barriers have modified spacecraft charging in LEO a great deal. For instance, a potential barrier that forms at the edge of ISS solar cells was successfully predicted to prevent it from charging to -140 V as a result of its 160 V solar array strings. And in fact, aside from brief excursions within a minute or so after eclipse exit, it has not been seen to charge more than about -30 V (leaving the most positive part of the solar arrays at +130 V). Charging on the body of SPEAR-I, which was initially predicted to proceed to about-40 kV, was seen to only proceed to about -8 kV because of potential barriers and the Earth’s magnetic field, leaving some biased parts of the spacecraft at +38 kV relative to the plasma. And, finally, Nascap-2k studies of a hypothetical high power, high voltage spacecraft in LEO have shown that some dielectric surfaces may charge to thousands of volts positive due to potential barriers around electron collecting surfaces. Thus, potential barriers may modify LEO charging a great deal. In this paper, we build on the success of Nascap2k in modeling the ISS and SPEAR-I charging and potential barriers by performing time-lapse and particle tracking charging calculations of a hypothetical but typical high voltage spacecraft in LEO. Here, one can see the cause, formation, buildup and decay of potentials on spacecraft surfaces and in the surrounding space, and see potential barriers as they occur. The role of magnetic field strength and direction in the production of these potential barriers will be investigated. Finally, some general “rules of thumb” for LEO potential barriers will be formulated, to help allow LEO spacecraft designers and engineers to confidently build LEO spacecraft that will not arc due to differential potentials brought on by potential barriers.