Frankie Wong

Electrostatic Discharge Testing of Multijunction Solar Array Coupons After Combined Space Environmental Exposures

A set of multijunction GaAs/Ge solar array test coupons provided by Space Systems/Loral was subjected to a sequence of five-year increments of combined space environmental exposure tests. The test coupons capture an integrated design intended for use in a geosynchronous (GEO) space environment. A key component of this test campaign is performing electrostatic discharge (ESD) tests in the inverted gradient mode. The protocol of the ESD tests is based on the ISO standard for ESD testing on solar array panels [ISO-11221]. The test schematic in the ISO reference has been modified with Space System/Loral designed circuitry to better simulate the on-orbit operational conditions of its solar array design. Part of the modified circuitry is to simulate a solar array panel coverglass flashover discharge. All solar array coupons used in the test campaign consist of four cells constructed to form two strings. The ESD tests are performed at the beginning-of-life (BOL) and at each five-year environmental exposure point. The space environmental exposure sequence consists of ultraviolet radiation, electron/proton particle radiation, thermal cycling, and xenon ion thruster plume erosion. This paper discusses the coverglass flashover simulation, the ESD test setup, and the importance of the electrical test design in simulating the on-orbit operational conditions. Results from fifth-year testing are compared to the baseline ESD characteristics determined at the BOL condition.

Combined Space Environmental Exposure Test of Multijunction GaAs/Ge Solar Array Coupons

The purpose of this test program is to understand the changes and degradation of the space solar array panel components, including its electrostatic discharge (ESD) mitigation design features in their integrated form, after multiple years (up to 15) of simulated Earth geosynchronous (GEO) space environment. A set of multijunction GaAs/Ge solar array test coupons was subjected to five-year increments of combined environmental exposure tests. These tests consisted of the following: simulated ultraviolet (UV) radiation, ESD, electron/proton particle radiation, thermal cycling, and simulated ion thruster exposures. The solar radiation simulation was produced using a mercury-xenon lamp with wavelengths in the UV spectrum ranging from 230 to 400 nm. The ESD test was performed in the inverted-gradient mode using a low-energy electron (3-6 keV) beam exposure. The ESD test also included a simulated panel coverglass flashover for the primary arc event. The electron/proton radiation exposure included 1.0-MeV electrons, 100-keV electrons, and 40-keV protons. Thermal cycling included simulated transient Earth eclipse for satellites in geosynchronous orbit. With the increasing use of ion thruster engines on many satellites, the combined environmental exposure test also included ion thruster exposure to determine the impact on solar array performance, as well as the ion thruster interaction to ESD events. Before and after each increment of combined environmental exposures, the coupons underwent visual inspection using high power magnification and electrical tests that included characterization by large-area pulse solar simulator, dark I -V, insulation resistance, and electroluminescence. This paper discusses the test objective, test methodologies, and preliminary results after five and ten years of simulated combined environmental exposure tests.

Electrostatic discharge test with simulated coverglass flashover for multi-junction GaAs/Ge solar array design

Space Systems/Loral (SS/L) successfully completed electrostatic discharge (ESD) tests of Multi-junction (MJ) GaAs/Ge solar array design in geosynchronous space environment. This ESD test was based on ISO-11221, Space systems - Space solar panels -Spacecraft Charging Induced Electrostatic Discharge Test Methods. In addition to the ISO reference for the test schematic, SS/L implemented modified test circuitry to better simulate the on-orbit operational conditions of our solar array design. The ESD test circuit also included simulated solar array panel coverglass flashover. The ESD test program utilized a 25-cell coupon that had been subjected to 2,000 thermal cycles caused by earth eclipses in GEO orbit and >12,000 thermal cycles caused by the shadow of the spacecraft antennas. Other ESD test coupons are 4-cell coupons that, after baseline ESD experiments, can later be subjected to combined space environmental exposures tests. To demonstrate design robustness, we performed ESD tests to voltages and currents that are higher than that of on-orbit solar array operational voltages and currents. This paper discusses the coverglass flashover simulation, ESD test setup, the importance of the electrical test design in simulating the on-orbit operational conditions, and the test results.

Electrostatic Discharge Tests on Solar Array Wire Coupons Subjected to Simulated Space Environment Aging

Solar array wire coupons have successfully completed an environmental life test that simulates 15 years at geosynchronous orbit. The environments included: ultraviolet (UV), electron, and proton irradiation; thermal cycling; electrostatic discharge (ESD); and ion thruster plume exposure on an electrostatically charged coupon. The test articles consisted of two wire coupons: one simulated the sun-facing side and the other simulated the shade side. UV irradiation was performed only on the sun-facing side coupon. Tests were performed at beginning-of-life, 7.5 years, and end-of-life. At each age point, ESD tests were performed using an electron beam at a worst case geosynchronous space environment flux of 1 nA/cm2. The test setup included capacitance that simulated a whole panel array harness, and a solar array simulator that generated in-flight array current profiles. The test coupon configuration contained aspects of the full panel wiring topology, which included potential fault conditions on wires. Visual inspections, documented with photographs, and isolation resistance tests were performed after each environment exposure to ensure the integrity of the wire insulation. This paper/presentation discusses each environment test level, test condition, and results from the various environmental age points.

Spacecraft Charging Analysis of Large GEO Satellites Using MUSCAT

A series of spacecraft charging analyses was carried out for large GEO satellites using Multi-Utility Spacecraft Charging Analysis Tool. The purpose was to derive the number of electrostatic discharges expected for 15 years in orbit, which would be used as the basis of the number of primary electrostatic discharges (ESDs) to be used in future solar cell coupon ESD tests. The combinations of GEO plasma parameters (electron density, proton density, electron energy, and proton energy) that have a high probability of occurrence have been identified first. For each of those plasma parameters, a charging analysis was done. In the simulation, the time for the differential voltage between solar array cover glass and the spacecraft chassis to reach the ESD inception threshold was calculated. The results indicate that, for a typical GEO satellite, the expected number of ESDs in 15 years is on the order of 10 000, agreeing well with the previous work where another satellite was simulated by NASCAP/GEO.

Electrostatic Discharge Tests of Solar Array Coupons With Different String-to-String Gaps Without RTV Adhesive Grout

A series of electrostatic discharge (ESD) tests was performed on solar array test coupons consisting of triple-junction InGaP2/GaAs/Ge solar cells. The string-to-string parallel gaps were not grouted with RTV adhesives. Various combinations of the gap width, the load voltage, and the string current were tested in a vacuum chamber equipped with an electron beam gun to investigate the threshold of secondary arcs. The ESD test circuit included simulated panel coverglass flashover. The insulation resistance between strings was found to decrease as the number of secondary arcs accumulates in the gap.

Test program of multi-junction GaAs/Ge solar array coupons with combined space environmental exposures

A test program was undertaken to understand the changes and degradation of the space solar array panel components in their integrated form after multiple years (up to 15) of simulated Earth geosynchronous (GEO) space environment. The solar array coupon panel design included its electrostatic discharge (ESD) mitigation design features. A set of multi-junction GaAs/Ge solar array test coupons was subjected to 5-year increments of combined environmental exposure tests. These tests consisted of: simulated ultraviolet (UV) radiation, ESD, electron/proton particle radiation, thermal cycling, simulated ion thruster exposures, and simulated ion thruster interaction with ESD events on the solar array. Before and after each increment of combined environmental exposures, the coupons underwent visual inspection using high power magnification and electrical tests that included characterization by LAPSS, dark I-V, insulation resistance, and electroluminescence. This paper discusses the test objective, test methodologies, and preliminary results after 5 years, 10 years and 15 years of simulated combined environmental exposure tests.

Does Spacecraft Potential Depend on the Ambient Electron Density

In this paper, we address the question of whether spacecraft potential depends on the ambient electron density. In Maxwellian space plasmas, the onset of spacecraft charging does not depend on the ambient electron density. The balance of electron currents causes the incoming electrons to balance with the outgoing secondary electrons. The onset is controlled by the critical or anticritical temperature of the ambient electrons, but not the electron density. Above the critical temperature, charging to negative potential occurs. If the energy of the incoming electrons increases to well beyond the second crossing point of the secondary electron yield (SEY), the value of SEY decreases to well below unity. When the secondary electron current is negligible compared with the primary electron current, the spacecraft potential is governed solely by the balance of the incoming electrons and the sum of the currents of the repelled electrons and the attracted ions. In neutral space plasma, the electron and ion charges cancel each other. But if the space plasma deviates from being neutral, then the densities can have effect on the spacecraft potential. If the ambient plasma deviates significantly from equilibrium, a non-Maxwellian electron distribution may result. For a kappa distribution, one can show that the spacecraft charging level is independent of the ambient electron density. For a double Maxwellian distribution, the spacecraft charging level depends on the electron densities. For a conducting spacecraft charging in sunlight, the charging level is low and positive. It also depends on the ambient electron density. For a dielectric spacecraft in sunlight, the high-level negative-voltage charging on the shadowed side may extend to the sunlit side and block the photoelectrons trying to escape from the sunlit side. In this case, the charging level does not depend on ambient electron density. Using coordinated environmental and spacecraft charging data obtained from the Los Alamos National Laboratory geosynchronous satellites, we showed some results confirming that spacecraft potential is indeed often independent of the ambient electron density.

Bypass diode temperature tests of a solar array coupon under space thermal environment conditions

Tests were performed on a 56-cell Advanced Triple Junction solar array coupon whose purpose was to determine margin available for bypass diodes integrated with new, large multi-junction solar cells that are manufactured from a 4-inch wafer. The tests were performed under high vacuum with coupon back side thermal conditions of both cold and ambient. The bypass diodes were subjected to a sequence of increasing discrete current steps from 0 A to 2.0 A in steps of 0.25 A. At each current step, a temperature measurement was obtained via remote viewing by an infrared camera. This paper discusses the experimental methodology, experiment results, and the thermal model.

High-Current ESD Test of Advanced Triple Junction Solar Array Coupon

Testing was conducted on an advanced triple junction (ATJ) coupon that was part of a risk reduction effort in the development of a high-powered solar array design by Space Systems/Loral, LLC (SSL). The ATJ coupon was a small, four-cell, two-string configuration that has served as the basic test coupon design used in previous SSL environmental aging campaigns. The coupon has many attributes of the flight design; e.g., substrate structure with graphite face sheets, integrated by-pass diodes, cell interconnects, room-temperature vulcanizing grout, and wire routing. The objective of this test was to evaluate the performance of the coupon after it is subjected to secondary arc testing at two string voltages (100 and 150 V) and four array currents (1.650, 2.000, 2.475, and 3.300 A). An external test circuit, unique to SSL solar array design, was built that simulates the effect of missing cells and strings in a full solar panel with special primary arc (PA) flashover circuitry. A total of 73 PAs were obtained that included seven temporary sustained arcs (TSAs) events. The durations of the TSAs ranged from 50 μs to 2.75 ms. All TSAs occurred at a string voltage of 150 V. Post-test large area pulsed solar simulator, Dark I-V, and by-pass diode tests showed that no degradation occurred due to the TSA events. In addition, the post-test insulation resistance measured was >50 GQ between cells and substrate. These test results point toward a robust design for application to a high-current, high-power mission.