Operational Status of the International Space Station Plasma Contactor Hollow Cathode Assemblies

Abstract

Abstract The International Space Station has onboard two Aerojet Rocketdyne developed plasma contactor units that perform the function of charge control. The plasma contactor units contain NASA Glenn Research Center developed hollow cathode assemblies. NASA Glenn Research Center monitors the on-orbit operation of the flight hollow cathode assemblies. As of May 31, 2013, HCA.001-F has been ignited and operated 123 times and has accumulated 8072 hours of operation, whereas, HCA.003-F has been ignited and operated 112 times and has accumulated 9664 hours of operation. Monitored hollow cathode ignition times and anode voltage magnitudes indicate that they continue to operate nominally. Introduction The International Space Station (ISS) power system is designed with high voltage solar arrays operating at output voltages of typically 140 to 160 V (Ref. 1). The power system is configured with a “negative ground” that electrically ties the habitat modules, structure, and radiators to the negative tap of the solar arrays. As a result of the electrical configuration and the plasma current balance, early analyses indicated that the habitat modules, structure, and radiators may float to voltages as high as 120 V with respect to the ambient space plasma (Ref. 1). Furthermore, given the large negative floating potentials, there exist the probability for deleterious interactions of the ISS and the space plasma. In 1992, a decision was made to baseline a plasma contactor on the ISS, from a variety of solutions, as a means to mitigate these interactions (Ref. 2). A plasma contactor establishes a low impedance plasma bridge to control the spacecraft potential relative to the local space plasma. Figure 1 illustrates the current path enabled through a plasma contactor. The plasma contactor unit (PCU) consists of a hollow cathode assembly (HCA), power electronics unit (PEU), and gas feed system (GFS). The entire unit is contained within an avionics box referenced as the orbit replaceable unit (ORU) (Ref. 1). The HCA within each PCU generates the electrons required for ISS charge control, and is electrically tied to the negative single point ground of the ISS main truss segment. The HCA consists of a low-work function insert for electron emission, a body tube with a heater, and an electrical isolator. During HCA operation, a continuous xenon flow of 6.0 to 7.5 sccm is maintained, a 3.0 A current to the anode is emitted, and the HCA emits to the space plasma for space charge control. NASA Glenn Research Center (GRC) developed and delivered the flight HCAs, shown in Figure 2, for integration into the PCUs. The NASA GRC HCA development program included manufacture, test, and installation of engineering, qualification, and flight model HCAs according to the program requirements (Refs. 3 and 4). This included long duration testing of the devices to meet the 18,000 operational hours requirement. A 28,000+ hours operation test (Ref. 5) and a 19,000+ hours operation test (Ref. 6) were among the tests performed in support of this goal. Table 1 presents the final status of the life tested hollow cathode assemblies where all life tests were terminated voluntarily. In addition, a high fidelity development model HCA attained 32,000 successful ignition cycles, demonstrating the ISS requirement of 6,000 ignition cycles with at least 99 percent reliability (Ref. 7). HCA heater testing was also conducted to determine the cyclic life capability of the heater, which was found to be around