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We have investigated the radiation-induced conductivity (RIC) in Kapton-like polymers in which it increases with an accumulating dose at large dose rates and long irradiation times. Such a behavior is very useful for spacecraft applications as it allows mitigating the spacecraft charging problems. Also, we studied ordinary polymers whose RIC steadily falls after reaching an initial maximum. To interpret experimental results, we used the semi-empirical Rose–Fowler–Vaisberg model. Numerical and experimental results have been compared with published data.

Multilayer insulation (MLI) can provide large thermal benefits for the storage of cryogenic propellants on-orbit, reducing the heat loads through large areas on storage tanks by two orders of magnitude. Such MLI has flown in space either within contained environments, such as a vacuum jacketed tank (tank in a tank), or external to a spacecraft, but heavily grounded to prevent the build-up of charge on the blanket. The grounding lugs deployed on a traditional spacecraft blanket require each layer to be electrically and thermally grounded to a metallic lug that passes through all the reflective layers. Thus, the metallic lug can have as much heat load through it as several square meters of blanket, significantly reducing the benefit of using the MLI (by a factor as high as 30). As NASA explores the benefits of applying MLI to large cryogenic stages, various environmental effects may change the application of the MLI to those stages. These environmental effects must be both understood and accounted for in the development of MLI, specifically for these applications. Testing was performed to understand the charge build up that multiple different outer covers would generate in both low earth orbit (LEO) and geostationary orbit (GEO). The test set up included an aluminum 6061 baseplate, a 25-mm-thick polyurethane foam, and either 10 or 25 layers of MLI (approximately 2.5 layers per mm) with an outer cover material. Cover materials tested included indium tin oxide (ITO), germanium, and aluminum, all deposited on a 2-mil-thick Kapton substrate. Testing occurred at room temperature and at a temperature of around 170 K. Results indicated that the blankets with Aluminum or ITO outer covers did not arc at voltages less than 250-V negative in simulated LEO and electron beam irradiation of 5.8-keV energy and up to 3-nA/cm2 current density in simulated GEO conditions. These results can be used to minimize the number of grounding points for large blankets or perhaps eliminate them.

This paper is dedicated to the characterization of internal charging and discharging processes of different systems or materials in medium earth orbit (MEO) environment. Experiments for MEO were carried out at the ONERA, Toulouse, France, using the Geostationary Dielectric Unit Radiation electron beam facilities. Assuming 2.5 mm of Al shielding on the spacecraft, a worst case electron spectrum emerging from the shielding has been calculated from previous works. Different materials or systems have been tested in this configuration: printed circuit board (PCB) and insulating materials used for cables or connectors [polyetheretherketone, ethylene tetrafluoroethylene (ETFE), and Kapton]. The experiments revealed that PCB or ETFE cables might be prone to present a high risk of charging and discharging after several days under extreme environment. Electrostatic discharges have then been detected on PCBs. An experimental and numerical method has also been defined and validated at ONERA to assess the charging levels in long-term space environment through short-duration experiments performed at higher electron current. This method requires the extraction of electric parameters (especially radiation-induced conductivity) of the tested materials. From these parameters, it was demonstrated that the extrapolation of the experimental results was feasible for most space used materials.

In the past few years, the Aerospace Corporation and other labs have been exploring how electrostatic discharges propagate over the surface of a positively charged dielectric surfaces, like solar arrays, on a spacecraft. One goal of these explorations is ultimately to aid in the extrapolation of transient currents measured on coupons in the laboratory to currents expected on full-scale spacecraft assemblies. In the popular plasma bubble model, the electrostatic discharge plasma is expected to expand radially from a single arc initiation point at the ion sound velocity, which depends on the composition of the conductor at that initiation point. In this paper, we examine how changing the conductor material at the arc initiation site affects propagation on a positively charged Kapton surface. By measuring current flowing into a series of concentric electrodes beneath this Kapton surface, we can monitor surface neutralization as a function of time and radial distance from the arc site. Results will be presented for a selection of cathode materials, and implications for existing and future models will be discussed.

High-velocity impacts of micrometeoroids and debris are proven causes of solar arrays power loss. Many studies have been carried out in this domain with relatively large size particles, in the range of 1 mm, causing obviously large damages, but with a small probability of occurrence. This paper deals with the occurrence of secondary arcing triggered on solar panel rear face cables with defects (lack of dielectric envelope) by smaller particles, in the range of 20– $80~\mu \text{m}$ (20 to 5 km/s), that is, with a probability of impact of several hundred of impacts by $\text{m}^{2}$ and by year both in low earth and geostationary orbits. Plasmas produced by high-velocity impacts on metallic (aluminum) and dielectric (Kapton) surfaces are characterized with a Langmuir triple probe (temperature, density, floating potential, and escape velocity). Secondary arcing tests on rear-side cables with cracks under high-velocity impacts are achieved using a solar array simulator with realistic in flight electrical conditions (120 $\text{V}^{-1}$ to 3 A). Secondary arcing tests and plasma characterization are also carried out with laser impacts (0.2 J YAG laser) and compared with high-velocity impacts.

An increasing body of research has been dedicated to determining the magnitude and scaling of spurious electrostatic discharge current transients on solar arrays and other spacecraft dielectrics under inverted gradient charging conditions. A flashover propagation velocity is typically determined, and assumed to be a single constant number that characterizes the collection of events. The complexity of flight-like test articles and diversity of methods used for deriving this velocity from laboratory data have led to apparently conflicting results. In this paper, we attempt to clarify the radial dynamics of electrostatic discharge (ESD) plasmas on a positively charged surface via the use of a simple test article, composed of eight concentric annular electrodes covered with Kapton. Time-resolved induced ESD currents, recorded as a function of electrode radial distance from the inception point, were used to derive empirical velocity distributions. These distributions lead to a number of effects, such as $\sim 1/r^{2}$ scaling with distance at the beginning of a discharge. Implications for ESD propagation and comparison with previous studies are discussed.

A 1-D numerical iteration software tool (NUMIT) designed for laboratory modeling has been refined, developed, and greatly enhanced, so that it can be used to model and predict the deep charging of dielectrics on spacecraft. The new tool, called Air Force NUMIT version 2 (AF-NUMIT2), can now model isotropically incident electron fluxes from realistic energy spectra that change with time. The model has been extended to incident electron fluxes an order of magnitude lower than before (≥10 keV), and aspects of the model have been checked against Monte Carlo simulations. Electron energy flux spectra, as measured by Combined Release and Radiation Effects Satellite, have been used in dozens of simulations of Kapton charging over multiple days. Kapton has been modeled with both dark conductivities and radiation-induced conductivities extending over two orders-of-magnitude. The result has been a surprisingly consistent relationship between maximum internal electric-field strengths and conductivity, regardless of the particular incident electron spectra.

Dedicated experiments have been developed at ONERA to characterize the charging and relaxation behavior of irradiated space polymers. Postirradiation analyses have been performed through potential kelvin probe and leakage or displacement current measurements. We have been able to demonstrate that polymers are submitted, after irradiation, to electron emission from their surface during the relaxation phase. This electron emission presents very low kinetics and can be intense enough to contribute at significant level to the surface potential drop of these materials. A parametric study has been performed to confirm electron relaxation emission and get a better understanding of the underlying steering mechanisms. This emission process is strongly dependent on the energy of the incident electrons used during irradiation and nature of the irradiated material. We present here the experimental results on electron emission from Kapton and Teflon and discuss the different physical mechanisms that could account for this process. The influence of charge double layer and injected radiation dose could especially explain electron emission from the surface through bulk diffusion and energy release by electron-hole recombination process.

Flashover corresponds to the differential charging neutralization of dielectric parts on satellite surfaces when an electrostatic discharge (ESD) is triggered. In solar panels, it is supposed to have effects on solar cells aging and secondary arc occurrence. Laboratory experiments show that flashover propagation sometimes involves the whole charged surface, but is generally partial in terms of charge amount and covered surface. An assumption is made that these charged dielectric surfaces are never plain due to many discontinuities as for solar panels, intercell gaps, inserts, and so on. To establish which parameters monitor or stop flashover propagation, we study neutralization ratio of the flashover on 1- m2 charged surfaces. We test and modified Kapton surfaces with different types of discontinuities added on the surface as dielectric sheathed and unsheathed wires. We also test a 1- m2 solar panel coupon and compare results with Kapton surface. Based on this coupon, different electrical network architectures for solar cell strings are tested (parallel, series, and interlaced circuit) to verify if they have an effect on flashover characteristics and to determine the way the replacement current circulates in the solar cell strings. These tests are performed in the JONAS facility, which is a 9- m3 vacuum chamber equipped with two electron guns and several measurement devices as surface potential probe and transient current probes. The sample under test is biased at a negative high voltage and charged with electrons to be in an inverted potential gradient situation to trigger ESDs. In Kapton film tests, ESDs are triggered on a six solar cells coupon set in the center of the Kapton surface. Results show 2-D surface voltage before and after flashover correlated to neutralized charges and solar cell strings replacement current. Effect or noneffect of surface topology on neutralization ratio and flashover propagation is shown for the Kapton film and the solar panel coupon.

Secondary electron emission (SEE) is one of the main parameters controlling spacecraft potential. It also plays an important role in the triggering of the multipactor phenomenon occurring in waveguides (electron avalanche in microwave electric fields). In this paper, we propose an original method adapted to low-energy SEE measurements on dielectrics and conductors (incident electron energy below 20 eV). It is based on Kelvin probe (KP) surface potential measurements after electron irradiation. It is particularly well suited to insulating materials but can also be used on metals by letting the sample potential float. We present results of SEE measurements performed on metals used in waveguides, Kapton, Teflon, and CMX cover glass. In order to avoid any experimental artifact due to the earth magnetic field and conduct accurate low-energy measurements with the KP method, the distance between the electron gun and the sample is chosen to be negligible compared to the Larmor radius.

Spacecraft charging codes require accurate models of electron yields as a function of accumulated charge to correctly predict the charge buildup on spacecraft. The accumulated charge creates equilibrium surface potentials on spacecraft resulting from interactions with the space plasma environment. There is, however, a complex relation between these emission properties and the charge built up in spacecraft insulators. This paper focuses on different methods appropriate to determine the fundamental electronic material property of the total electron yield as the materials accumulate charge. Three methods for determining the uncharged total yield are presented: 1) The dc continuous beam method is a relatively easy and accurate method appropriate for conductors and semiconductors with maximum total electron yield σmax ; 1024 Ω·cm; and 3) the yield decay method is a very difficult and time-consuming technique that uses a combination of measurement and modeling to investigate the most difficult materials with σmax >; 4 and ρ up to

Charging of materials by incident radiation is affected by both environmental and physical conditions. Modifying a material's physical surface will change its reflection, transmission, and absorption of the incident radiation which are integrally related to the accumulation of charge and energy deposition in the material. General arguments for incident and emitted photons, electrons, and ions are considered. An optical analysis of the effects of surface modification on spacecraft charging parameters on prototypical polyimide Kapton HN and Cu samples is presented. Samples were roughened with abrasive compounds ranging from 0.5 to 10 μm in size, comparable to the range of incident wavelengths. They were also contaminated with thin layers of DC 704 diffusion pump oil. Using a UV/VIS/NIR light source and a diffraction grating spectrometer, measurements were performed on pristine and modified materials. The measured spectra confirmed that surface modifications induce expected changes in optical reflection, transmission, and absorption. The generally increased absorption observed results in increased photon energy deposited in the material, leading to increased charge emission through the photoelectric effect.

Electron-induced electron yields of high-resistivity high-yield materials - ceramic polycrystalline aluminum oxide and polymer polyimide (Kapton HN) - were made by using a low-fluence pulsed incident electron beam and charge neutralization electron source to minimize charge accumulation. Large changes in the energy-dependent total yield curves and yield decay curves were observed, even for incident electron fluences of < 3 fC/mm2. The evolution of the electron yield as charge accumulates in the material is modeled in terms of electron recapture based on an extended Chung-Everhart model of the electron emission spectrum. This model is used to explain the anomalies measured in highly insulating high-yield materials and to provide a method for determining the limiting yield spectra of uncharged dielectrics. The relevance of these results to spacecraft charging is also discussed.