Joel T. Galofaro

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.

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.

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.

Inception and Prevention of Sustained Discharges on Solar Arrays

Sustained arc between adjacent cells is certainly a catastrophic event that results in significant loss in the power delivered to spacecraft systems. In order to prevent this kind of discharges, the threshold magnitudes of voltage and current should be determined in ground tests and compared with respective operational parameters. It is necessary to demonstrate that the results of ground tests depend on solar array designs but do not depend on the simulated environment and the electrical circuitry arrangement. A thorough analysis of about 20 tests performed in different laboratories has been conducted in this work. Sustained arc current thresholds were established for a variety of solar array designs and confronted with well-known magnitudes for vacuum arcs. If both voltage and string current magnitudes exceed the threshold values, the gaps between adjacent strings can be filled in with insulating material (RTV). Comprehensive ground tests demonstrated a high efficiency of this method. However, the lifetime of modern spacecraft spans for 10-15 years, and aging of RTV due to space radiation and temperature variations may cause critical changes in insulator properties. Thus, it seems reasonable to prepare a sample with RTV-grouted gaps, to undergo it proton fluence and thermal cycling equivalent to a few years in the geosynchronous orbit, and to test the sample against sustained arc inception. This program was also realized in the test described in this paper.

Interaction of Charged Spacecraft with Electric Propulsion Plume: On Orbit Data and Ground Test Results

On-orbit observations and ground tests demonstrate interaction between charged spacecraft and electrothermal thruster-generated plasma. On-orbit measurements and test results are presented for plasma diagnostics and solar array performance during long-term exposure of flight solar panel. The long-term performance of a flight 70 V 2mtimes4 m GEO solar array exposed to a 2 kW arcjet plasma environment was studied

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.

Electrical breakdown of anodized coatings in low-density plasmas

A comprehensive set of investigations has been performed involving arcing on a negatively biased anodized aluminum plate immersed in a low-density argon plasma at low pressures (Po w 7.5X 10~ torr). These arcing experiments were designed to simulate electrical breakdown of anodized coatings in a low-Earth orbital environment. When electrical breakdown of an anodized layer occurs, an arc strikes, and there is a sudden flux of electrons accelerated into the ambient plasma. This event is directly followed by ejection of a quasineutral plasma cloud consisting of ejected material blown out of the anodized layer. Statistical analysis of plasma cloud expansion velocities has yielded a mean propagation velocity, v 19.4 ± 3.5 km/s. As the plasma cloud expands into the ambient plasma, energy in the form of electrical noise is generated. The radiated electromagnetic noise is detected by means of an insulated antenna immersed in the ambient plasma. The purpose of the investigations is 1) to observe and record the electromagnetic radiation spectrum resulting from the arcing process, 2) to make estimates of the travel time of the quasineutral plasma cloud based on fluctuations to several Langmuir probes mounted in the ambient plasma, and 3) to study induced arcing between two anodized aluminum structures in close proximity.

Effect of Solar Array Size on Sustained Arc Inception

The probability of sustained arc inception depends on many factors including primary arc peak current and pulse width. Multiple tests in simulated plasma environments were performed to extract the most important factors influencing primary arc pulse characteristics. It is clearly demonstrated that the solar array size has very little impact on sustained arc inception. Solar array operational voltage and string current, together with the array design features, are the critical issues in the prevention of catastrophic discharges. In this paper we show the experimental and theoretical justifications for making this statement. Nomenclature C = capacitance, F Cs = surface capacitance, F*m Esc = second crossover, V I = arc current, A Im = peak current, A Mi = ion mass, kg R = resistance, Ohm Te = electron temperature, eV U = voltage, V Uc = semiconductor potential, V Ug = coverglass potential, V Us = SAS voltage, V V = plasma expansion speed, m*s d = semiconductor thickness, m dc = coverglass thickness, m e = electron charge, C j = current density, A*m jb = beam current density, A*m jf = field emission current density, A*m ji = ion current density, A*m l = plasma cloud radius, m n = electron number density, m r0 = cathode spot radius, m t = time, s s = gap size, m w0 = specific energy, J*kg E = electric field, V*m ΛD = Debye length, m Σ = surface charge density, C*m β = field enhancement factor γ = adiabatic exponent δ = secondary emission yield e = dielectric permittivity ρ = charge density, C*m ρm = metal density, kg*m σ = coverglass conductivity, S*m τL = charging time in LEO, s τc = capacitor charging time, s τe = explosion time, s τf = front time, s τp = pulse width, s ωi = ion plasma frequency, Hz

Arcing in LEO: Does the Whole Array Discharge?

The conventional wisdom about solar array arcing in LEO is that only the parts of the solar array that are swept over by the arc-generated plasma front are discharged in the initial arc. This limits the amount of energy that can be discharged. Recent work done at the NASA Glenn Research Center has shown that this idea is mistaken. In fact, the capacitance of the entire solar array may be discharged, which for large arrays leads to very large and possibly debilitating arcs, even if no sustained arc occurs. We present the laboratory work that conclusively demonstrates this fact by using a grounded plate that prevents the arc-plasma front from reaching certain array strings. Finally, we discuss the dependence of arc strength and arc pulse width on the capacitance that is discharged, and provide a physical mechanism for discharge of the entire array, even when parts of the array are not accessible to the arc-plasma front. Mitigation techniques are also presented.