Dale C. Ferguson

Interactions of High-Voltage Solar Arrays with Their Plasma Environment: Ground Tests

Five different types of solar arrays have been tested in a large vacuum chamber. Arc inception voltages, arc rates, and current collections were measured for samples with different coverglass materials and thickness, interconnect designs, and cell sizes. It is shown that the array with wrapthrough interconnects has the highest arc threshold and the lowest current collection. Coverglass designs with overhang result in a decrease of current collection and increase of arc threshold. Doubling coverglass thickness does not improve measured array parameters. Both arc inception voltage and current collection increase significantly with increasing the sample temperature to 80◦C. The sample with exposed interconnects demonstrated a significant (30 times) drop in arc rate after it underwent five thermal cycles in a clean chamber to outgas its surfaces.

DIRECT DRIVE HALL THRUSTER SYSTEM DEVELOPMENT

The status of development of a Direct Drive Hall Thruster System is presented. In the first part, a study of the impacts to spacecraft systems and mass benefits of a direct-drive architecture is reviewed. The study initially examines four cases of SPT-100 and BPT-4000 Hall thrusters used for north-south station keeping on an EXPRESS-like geosynchronous spacecraft and for primary propulsion for a Deep Space-1 based science spacecraft. The study has also been extended to include the impact of direct drive on orbit raising for higher power geosynchronous spacecraft and on other deep space missions as a function of power and velocity change. The major system considerations for accommodating a direct drive Hall thruster are discussed, including array regulation, system grounding, distribution of power to the spacecraft bus, and interactions between current-voltage characteristics for the arrays and thrusters. The mass benefit analysis shows that, for the initial cases, up to 42 kg of mass savings is attributable directly to changes in the propulsion hardware. When projected mass impacts of operating the arrays and the electric power system at 300V are included, up to 63 kg is saved for the four initial cases. Adoption of high voltage lithium ion battery technology is projected to further improve these savings. Orbit raising of higher powered geosynchronous spacecraft is the mission for which direct drive provides the most benefit, allowing higher efficiency electric orbit raising to be accomplished in a limited period of time, as well as nearly eliminating significant power processing heat rejection mass. The total increase in useful payload to orbit ranges up to 278 kg (11%) for a 25 kW spacecraft, launched from an Atlas IIA. For deep space missions, direct drive is found to be most applicable to higher power missions with a velocity change up to several km/s, typical of several Discovery-class missions. In the second part, the status of development of direct drive propulsion power electronics is presented. The core of this hardware is the heater-keeper-magnet supply being qualified for the BPT- 4000 by Aerojet. A breadboard propulsion power unit is in fabrication and is scheduled for delivery late in 2003.

Interactions of High-Voltage Solar Arrays with Their Plasma Environment: Physical Processes

The results of an experimental study and theoretical analysis are presented to reveal some important physical mechanisms of electrostatic discharge inception due to charging of conductor‐dielectric junctions immersed in low-density plasmas. Two samples of conventional solar arrays and four quartz‐metal junctions were used to investigate the effects of arcing within a wide range of neutral gas pressures, ion currents, and electron number densities. Collected data allowed the study the correlation between external parameters (plasma density, additional capacitance, bias voltage, and metal work function) and arc characteristics (arc rate, arc current pulse width and amplitude, plasma contamination, and intensities of spectral lines). The locations of arc sites on solar array samples were determined by video camera, and it is shown that the most probable sites for arc inception are interconnect‐ coverglass junctions, even though some arcs were initiated in gaps between cells. The effect of surface conditioning (a decrease of the arc rate due to outgassing) was clearly demonstrated. Moreover, a considerable increase in arc rate due to the absorption of molecules from the air has been confirmed. The analysis of optical spectra (220‐680 nm) reveals intense narrow atomic lines (Ag, Cu, and H) and wide molecular bands (OH, CH, SiH, and SiN) that imply a complicated mechanism of arc plasma generation. The rate of plasma contamination due to arcing was measured by employing a mass spectrometer. The arc threshold was increased to above 350 V (from 190 V) by keeping a sample in vacuum for seven days. The results obtained are important for the understanding of the arc inception mechanism, which is essential for progress toward the design of high-voltage solar arrays for space applications.

Arcing on Aluminum Anodized Plates Immersed in Low-Density Plasmas

A number of experiments have been done to study characteristics of the plasma contamination and electromagneticradiation generated by arcing on anodizedaluminum platesimmersedinlow-density plasma. Thelow-Earthorbit plasma environment was simulated in a plasma vacuum chamber, where the parameters could be controlled precisely. Diagnostic equipment included two antennas, a mass spectrometer, a spherical langmuir probe, a wire probe, and a very sensitive current probe to measure arc current. All data except for mass spectrometry were obtainedindigitalformwithasamplingintervalof2.5nsthatallowedustostudytheradiationspectrumatfrequencies up to 200 MHz. We found that the level of interference considerably exceeds the limitations on the level of electromagnetic noise set by technical requirements on Space Shuttle operation. Experiments with two independently biased plateshaveshown thatthearcing onset on oneplategeneratesa pulseof currentonthesecond plateand that the secondary current pulse has a signie cant amplitude. The sampling interval for mass spectrometry was 250 ms. This allowed us to obtain the rate of plasma contamination due to arcing. A signie cant degradation of the coating layer was determined by measurement of the resistance of the plate, which had experienced a few hundred arcs.

International Round-Robin Tests on Solar Cell Degradation Due to Electrostatic Discharge

Teppei Okumura∗ Japan Aerospace Exploration Agency, Tsukuba 305-8505, Japan Mengu Cho Kyushu Institute of Technology, Kitakyushu 804-8550, Japan Virginie Inguimbert ONERA, 31055 Toulouse, France Denis Payan Centre National d’Etudes Spatiales, 31401 Toulouse, France Boris Vayner Ohio Aerospace Institute, Cleveland, Ohio 44142 and Dale C. Ferguson∗∗ U.S. Air Force Research Laboratory, Albuquerque, New Mexico

Best Geosynchronous Earth Orbit Daytime Spacecraft Charging Index

Recently, the debate on what is the best daytime Geosynchronous Earth Orbit spacecraft charging index has been reopened. In this paper, the conclusions of one of the recent papers on the subject are verified by comparing Nascap-2k results with charging and fluxes measured on the Spacecraft Charging at the High Altitudes, Intelsat, Defense Satellite Communications System, and Los Alamos National Laboratory Geosynchronous Earth Orbit satellites. In addition, a refined measure of charging is presented as the total thermal electron flux above a certain minimum energy that is well above the second crossover point in secondary electron emission. The use of this type of index is justified by correlations between Nascap-2k simulation results and total fluxes above a range of energies. The best minimum energy to use is determined for spacecraft of different design and surface materials. Finally, the optimum Geosynchronous Earth Orbit daytime spacecraft charging index is obtained, and its use for predicting and res...

Survey of International Space Station Charging Events

With the negative grounding of the 160V Photovoltaic (PV) arrays, the International Space Station (ISS) can experience varied and interesting charging events. Since August 2006, there has been a multi-probe p ackage, called the Floating Potential Measurement Unit (FPMU), availa ble to provide redundant measurements of the floating potential of th e ISS as well as the density and temperature of the local plasma environment. The FPMU has been operated during intermittent data campaigns since August 2006 and has collected over 160 days of information reg arding the charging of the ISS as it has progressed in configuration from one to three PV arrays and with various additional modules such as the European Space Agency?s Columbus laboratory and the Japan Aeros pace Exploration Agencys Kibo laboratory. This paper summarizes the charging of the ISS and the local environmental conditions that contr ibute to those charging events, both as measured by the FPMU.

The Neutral Gas Desorption and Breakdown on a Metal-Dielectric Junction Immersed in a Plasma

New results are presented of an experimental study and theoretical analysis of arcing on metal-dielectric junctions immersed in a low-density plasma. Two samples of conventional solar arrays have been used to investigate the effects of arcing within a wide range of neutral gas pressures, ion currents, and electron number densities. All data (except video) were obtained in digital form that allowed us to study the correlation between external parameters (plasma density, additional capacitance, bias voltage, etc) and arc characteristics (arc rate, arc current pulse width and amplitude, gas species partial pressures, intensities of spectral lines, and so on). Arc sites were determined by employing a video-camera, and it is shown that the most probable sites for arc inception are trip le-junctions, even though some arcs were initiated in gaps between cells. The effect of surface conditioning (decrease of arc rate due to outgassing) was clearly demonstrated. Moreover, a considerable increase in arc rate due to absorption of molecules from atmospheric air has been confirmed. The analysis of optical spectra (240-800 nm) reveals intense narrow atomic lines (Ag, H) and wide molecular bands (OH, CH, SiH, SiN) that confirm a complicated mechanism of arc plasma generation. The rate of plasma contamination due to arcing was measured by employing a mass-spectrometer. These measurements provided quite reliable data for the development of a theoretical model of plasma contamination, In conclusion, the arc threshold was increased to above 350 V (from 190 V) by keeping a sample in vacuum (20 micronTorr) for seven days. The results obtained are important for the understanding of the arc inception mechanism, which is absolutely essential for progress toward the design of high voltage solar arrays for space applications.

A Theory for Rapid Charging Events on the International Space Station

The Floating Potential Measurement Unit (FPMU) has detected high negative amplitude rapid charging events (RCEs) on the International Space Station (ISS) at the morning terminator. These events are larger and more rapid than the ISS morning charging events first seen by the Floating Potential Probe (FPP) on ISS in 2001. In this paper, we describe a theory for the RCEs that further elucidates the nature of spacecraft charging in low Earth orbit (LEO) in a non-equilibrium situation. The model accounts for all essential aspects of the newly discovered phenomenon, and is amenable to testing on-orbit. Predictions of the model for the amplitude of the ISS RCEs for the full set of ISS solar arrays and for the coming solar cycle are given, and the results of modeling by the Environments WorkBench (EWB) are compared to the observed events to show that the phenomenon can be explained by solar array driven charging. The situation is unique because the coverglasses have not yet reached equilibrium with the surrounding plasma during the RCEs. Finally, a prescription for further use of the ISS for investigating fundamental plasma physics in LEO is given. Already, plasma and charging monitoring instruments on ISS have taught us much about spacecraft interactions with the dense LEO plasma, and we expect they will continue to yield more valuable science when the Japanese Experiment Module (JEM) is in place.

A Desorbed Gas Molecular Ionization Mechanism for Arcing Onset in Solar Arrays Immersed in a Low-Density Plasma

Previous experimental studies have hypothesized that the onset of Solar Array Arc (SAA) initiation in low-density space plasmas is caused by a desorbed gas molecular ionization mechanism. Indeed past investigations performed at the NASA Glenn Plasma Interaction Facility tend to not only support the desorbed gas molecular ionization mechanism, but have gone as far as identifying the crucial molecular species that must be present for molecular ion dominated process to occur. When electrical breakdown occurs at a triple junction site on a solar array panel, a quasi-neutral plasma cloud is ejected. Assuming the main component of the expelled plasma cloud by weight is due to water vapor, the fastest process available is due to HO molecules and OH(+) ions, or more succinctly, dissociative molecular-ion dominated recombination processes: H2O(+) + e(-) yields H* + OH*. Recently published spectroscopic observations of solar array arc spectra in ground tests have revealed the well-known molecular OH band (302 to 309nm), as well as the molecular SiH band (387nm peak), and the molecular CH band (432nm peak). Note that the OH band is observed in emission arcs where water vapor is present. Strong atomic lines were also observed for H(sub beta) at 486nm and H(sub alpha) at 656.3nm in prior ground testing. Independent supporting evidence of desorbed gas molecular ionization mechanisms also come from measurements of arc current pulse widths at different capacitances. We will revisit an earlier first order approximation demonstrating the dependence of arc current pulse widths on the square root of the capacitance. The simple arc current pulse width model will be then be used to estimate the temperature of the arc plasma (currently believed to be somewhere in the range of 3 to 5 eV). The current paper then seeks to extend the outlined work by including numerous vacuum chamber measurements obtained with a quadrupole mass spectrometer. A small solar array was mounted inside the vacuum chamber. A plasma source, also mounted inside the vacuum chamber, is used to simulate a low-density plasma environment. The solar array is then biased to a high negative potential and allowed to arc while a mass spectrometer is used to record the partial pressure of H2O and to track other significant changes in mass (1 to 150) AMU.