This text dates from 1992, and will be updated soon...
In 1990, Space Station Freedom (SSF) baselined a negatively grounded power system with high voltage (160V) photovoltaic solar arrays. Without a method to actively control the charging of the spacecraft, critical structures on the Space Station could charge to approximately -140 volts with respect to the potential of space plasma. Under these conditions, arcing to the structural surfaces is likely to occur on-orbit. Arcing is a major concern because it would seriously degrade Space Station's thermally controlled surfaces. This would take away Station's capability to radiate energy thermally, thus causing it to heat up considerably.
To minimize the negative effects described above, engineers in OAST proposed using plasma contactors to control critical Space Station surface potentials. These devices are based on hollow cathode technology developed under OAST's ongoing program in electric propulsion. A contactor consists of a contactor unit (anode, cathode, and expellant injector), a power electronics and control unit, and an expellant management system (xenon tank, valves, etc.) In operation, the contactor acts as a source of ions and electrons by emitting a xenon plasma. This plasma, in turn, acts as a neutralizing pathway for the minimization of surface charging.
Recent tests at NASA's Lewis Research Center showed that plasma contactor systems effectively eliminate arcing under "space-like" conditions. Contactors have now been baselined for use on the Space Station for this purpose. A full scale development program is currently in progress at Lewis. For additional information, contact Michael J. Patterson, NASA Lewis Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135, Mail Stop SPTD-1. Phone 216-977-7481.