Kenneth H. Wright

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.

Charging of the International Space Station as Observed by the Floating Potential Measurement Unit: Initial Results

The floating potential measurement unit (FPMU) is a multiprobe package designed to measure the floating potential of the International Space Station (ISS) as well as the density and temperature of the local ionospheric plasma environment. The purpose of the FPMU is to provide direct measurements of ISS spacecraft charging as continuing construction leads to dramatic changes in ISS size and configuration. FPMU data are used for refinement and validation of the ISS spacecraft charging models used to evaluate the severity and frequency of occurrence of ISS charging hazards. The FPMU data and the models are also used to evaluate the effectiveness of proposed hazard controls. The FPMU consists of four probes: a floating potential probe, two Langmuir probes, and a plasma impedance probe. These probes measure the floating potential of the ISS, plasma density, and electron temperature. Redundant measurements using different probes support data validation by interprobe comparisons. The FPMU was installed by ISS crew members during an extra-vehicular activity on the starboard (S1) truss of the ISS in early August 2006 when the ISS configuration included only one 160-V U.S. photovoltaic (PV) array module. The first data campaign began a few hours after installation and continued for over five days. Additional data campaigns were completed in 2007 after a second 160-V U.S. PV array module was added to the ISS. This paper discusses the general operational characteristics of the FPMU as integrated on ISS, the functional performance of each probe, the charging behavior of the ISS before and after the addition of a second 160-V U.S. PV array module, and initial results from model comparisons.

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.

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.

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.

The emissions of broadband electrostatic noise in the near vicinity of the Shuttle Orbiter

Abstract Measurements of the Space Shuttle environment from the STS -3 and Spacelab 2 missions indicate the presence of oblique ion streams and broadband electrostatic noise. A two-dimensional theoretical model is applied to study a possible causal relationship between the ion streams and the broadband noise; especially in terms of the ion acoustic wave and ion-ion wave modes. This model predicts the generation of waves with frequencies ranging from the ion cyclotron frequency up to values greater than the ion plasma frequency, with the maximum growth rate occurring in the 10 kHz range. These results are consistent with the observational data from the STS -3 mission. The model also shows that these two wave modes can co-exist only when the wave vectors of the two wave modes are nearly perpendicular. The parametric dependence of the wave instabilities on the plasma parameters, e.g. N b / N e , T e / T i and the inclination of the wave propagation vector, is also studied.

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.

Validation of the Plasma Densities and Temperatures From the ISS Floating Potential Measurement Unit

The validation of the floating potential measurement unit (FPMU) plasma density and temperature measurements is an important step in the process of evaluating International Space Station (ISS) spacecraft charging issues including vehicle arcing and hazards to crew during extravehicular activities. The highest potentials observed on the Space Station are due to the combined Vsp times B effects on a large spacecraft and the collection of ionospheric electron and ion currents by the 160-V U.S. solar array modules. The ionospheric plasma environment is needed for input to the ISS spacecraft charging models used to predict the severity and frequency of occurrence of ISS charging hazards. The validation of these charging models requires the comparison of their predictions with measured FPMU values. The FPMU measurements themselves must also be validated for use in manned flight safety work. This paper presents preliminary results from a comparison of densities and temperatures derived from the FPMU Langmuir probes and plasma impedance probe with the independent density and temperature measurements from a spaceborne ultraviolet imager, a ground-based incoherent scatter radar, and ionosonde sites.

Intercomparison among plasma wake models for plasmaspheric and ionospheric conditions

Intercomparisons are made among the angular distributions of ions in the wake of a body moving through a space plasma as computed from three different expressions (models). Both subsonic and supersonic relative flows are considered in order to examine the wake current depletion ratios under conditions realistic for the topside ionosphere and plasmasphere. Results of these comparisons demonstrate the importance of including the thermal flux at low Mach numbers and of taking into account the angular acceptance of ion detectors in making theory-experiment comparisons. Gradients in the angular variations of the fluxes are found to be steeper near the wake-ambient interface than closer to the maximum rarefaction region of all models, although quantitatively there is considerable variation among the models. From examining the variations of the wake depletion ratio parametrically with Mach number and nor- malized potential over ranges characteristic of the plasmasphere and topside ionosphere, we find con- siderable variation with both parameters, with sensitivity to normalized potential increasing dramatically with Mach number. Overall. however, the Mach number variation appears to be the more significant over this range of parameters.

Technology development for the solar probe plus faraday cup

The upcoming Solar Probe Plus (SPP) mission requires novel approaches for in-situ plasma instrument design. SPP’s Solar Probe Cup (SPC) instrument will, as part of the Solar Wind Electrons, Alphas, and Protons (SWEAP) instrument suite, operate over an enormous range of temperatures, yet must still accurately measure currents below 1 pico-amp, and with modest power requirements. This paper discusses some of the key technology development aspects of the SPC, a Faraday Cup and one of the few instruments on SPP that is directly exposed to the solar disk, where at closest approach to the Sun (less than 10 solar radii (Rs) from the center of the Sun) the intensity is greater than 475 earth-suns. These challenges range from materials characterization at temperatures in excess of 1400°C to thermal modeling of the behavior of the materials and their interactions at these temperatures. We discuss the trades that have resulted in the material selection for the current design of the Faraday Cup. Specific challenges include the material selection and mechanical design of insulators, particularly for the high-voltage (up to 8 kV) grid and coaxial supply line, and thermo-optical techniques to minimize temperatures in the SPC, with the specific intent of demonstrating Technology Readiness Level 6 by the end of 2013.