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The Floating Potential Probe (FPP) operated on the International Space Station (ISS) from December 2000 to April 2001. During that time, it took many measurements of the ISS floating potential and the low-Earth-orbit electron density and temperature. Those measurements were used as inputs to the Environment WorkBench (EWB) model of ISS potentials (originally developed by the Science Applications International Corporation for NASA, but now sometimes called the Boeing Plasma Interaction Model), which is used even today to predict charging levels for ISS. FPP is now completely defunct, having been removed and jettisoned from ISS. With the advent of the new floating potential measurement unit (FPMU) on ISS and the beginning of ISS operations with three large sets of solar array panels instead of just one, a review of FPP measurements can offer comparisons with the new FPMU data and perhaps improve the accuracy of future ISS charging predictions. In particular, FPP measurements during times of low electron temperature and high electron density (the times of worst ISS charging) will be brought forward for comparison with the newly obtained FPMU data. Index Terms--Floating Potential Probe (FPP), International Space Station (ISS), plasma interactions.

Plasma ground-testing results, obtained at the John H. Glenn Research Center National Plasma Interaction Facility, are presented for a number of thin-film photovoltaic cells. The cells represent a mix of promising new technologies identified by the Air Force Research Laboratory under the Space Science Technology Experiment (SSTE-4) Program. The current ground-testing efforts are aimed at characterizing both the performance and survivability of thin-film technologies in the harsh Low Earth Orbit space environment where they are intended to be flown. Measurements of parasitic currents and arc threshold voltages were performed in situ under strictly controlled charging conditions for both amorphous silicon (a-Si) and copper indium gallium selenide cells. Surface flashing on the large-area a-Si cells revealed that microdischarges in the dielectric surface do not appear to cause any apparent degradation to the cells. Catastrophic arc testing between adjacent cells resulted in no sustained arcs. Similar catastrophic arc tests between adjacent strings resulted in self-extinguished nonsustained arc extensions on time scales of approximately 100 [micro]s in length. All cell efficiency measurements were performed in the Solar Cell Calibration Laboratory prior to plasma testing and then once again after the completion of the plasma tests. Index Terms--Photovoltaic cells, photovoltaic space power systems, plasma measurements.

Emission spectra of arc plasmas contain valuable information regarding arc sites, plasma parameters and composition, and temporal dynamics of plasma expansion. Emission spectra within the range of 200-800 nm were measured for trigger arcs on triple junctions and sustained arcs between solar array strings. Spectra were recorded from arcs on several different brands of conventionally designed solar array samples immersed in simulated low-Earth-orbit plasma. The arc sites and cathode materials were determined for plasmas generated by primary discharges and sustained arcs. Comparative analysis of temporal dynamics of arc current pulse waveforms with spectral line intensities allowed obtaining estimates for plasma expansion rates. Statistical processing of spectral line intensities resulted in calculations of arc plasma electron temperatures and surface densities of some atomic species. The presence of a flux of energetic electrons (above 100 eV) was confirmed by analysis of the structure of the excited atom and ion energy levels. Index Terms--Arc current, arc plasma, arc spectrum, cathode material, sustained arc, temperature.

Tests of arcing and current collection in simulated-space-plasma conditions have been performed at the National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) in Cleveland, OH, for over 30 years and at the Marshall Space Flight Center (MSFC) in Huntsville, AL, for almost as long. During this period, proper test conditions for an accurate and meaningful space simulation have been worked out, comparisons with actual space performance in spaceflight tests and with real operational satellites have been made, and NASA has achieved the authors' own internal standards for test protocols. It is the purpose of this paper to communicate the test conditions, test procedures, and types of analysis used at the NASA GRC and MSFC to the space environmental testing community at large, to help with international space-plasma arcing-testing standardization. Discussed herein are the neutral gas conditions, plasma densities and uniformity, vacuum chamber sizes, sample sizes and Debye lengths, biasing samples versus self-generated voltages, floating samples versus grounded samples, test electrical conditions, arc detection, preventing sustained discharges during testing, real samples versus idealized samples, validity of low Earth orbit tests for geosynchronous Earth orbit samples, extracting arc threshold information from an arc rate versus voltage tests, snapover, current collection and glows at the positive sample bias, Kapton pyrolysis, thresholds for trigger arcs, sustained arcs, dielectric breakdown and Paschen discharge, tether arcing and testing in very dense plasmas (i.e., thruster plumes), arc mitigation strategies, charging mitigation strategies, models, and analysis of test results. Finally, the necessity of testing will be emphasized, not to the exclusion of modeling, but as part of a complete strategy for determining when and if arcs will occur, and preventing them from occurring in space. Index Terms--Aerospace simulation, arc discharges, environmental testing, plasma measurements.