Bao Hoang

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.

First Preliminary Results From U.S. Round-Robin Tests

The first preliminary results are reported from the U.S. Round-Robin Test on Plasma Expansion Speed. The tests were performed at the NASA Glenn Research Center on two coupons (six strings) of International Space Station (ISS) solar cell arrays, with a separate small array to obtain arcs (because it is so difficult to get ISS arrays to arc). ISS arrays were used because they have no exposed interconnects to act as bare current collectors and confuse the experimental results. The preconstructed ISS strings were laid out approximately parallel to the plasma expansion velocity to allow for the best test of the simple plasma expansion front current waveform model. Several Langmuir probes were arranged above and to the sides of the sample to allow for measurement of the plasma propagation speed. In the initial set of tests, primary arcs and the consequent current waveforms were measured in a Low Earth Orbit-type plasma. In a second set of tests, two electron guns with diffusers were used to provide an approximately uniform Geosynchronous Earth Orbit-type environment, and primary arcs and current waveforms were obtained. The objective of this and other round-robin tests is to characterize primary arc waveforms in terms of speed and degree of discharge of arc plasmas produced by primary arcs, and their dependences on environment, capacitance per unit area, arc voltage, temperature, and so on. The final goal is to allow engineering estimates of arc current peaks and half-widths to allow confident design and construction of space solar arrays, and to allow mitigation techniques to be evaluated. This is the first in an extended series of tests to be performed at six different U.S. facilities, and with participation from ten different U.S. organizations.

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.

Low-Thrust Geostationary Transfer Orbit (LT2GEO) Radiation Environment and Associated Solar Array Degradation Modeling and Ground Testing

Low-thrust geostationary transfer orbits (LT2GEO) are found to offer significant low cost options thus making them very attractive for GEO missions. However, LT2GEOs increase the transfer orbit time from days to months, thereby causing a significant increase in the time that satellites traverse the most intense trapped particle radiation belts. This paper describes the LT2GEO radiation environment and provides some practical implications using new solar cell technologies and materials. New solar cell and coverglass testing protocols are established using Monte Carlo transport modeling and experimental results are given for Qioptiq CMG borosilicate coverglass.

AFRL Round-Robin Test Results on Plasma Propagation Velocity

The speed plasma propagates across a charged solar panel after a primary arc is one of the most important, yet poorly known, quantities in determining Electrostatic Discharge (ESD) currents for spacecraft arcing events. A review of the literature over the last two decades reveals that measured propagation velocity varies by as much as an order of magnitude. To overcome this deficiency, a round-robin set of tests was initiated with partners from industry, academia, NASA and the U.S. Air Force. This paper will provide the most recent results from the Air Force Research Laboratory testing conducted at the Spacecraft Charging and Instrument Calibration Laboratory.

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.