Virginie Inguimbert

Secondary Electron Emission on Space Materials: Evaluation of the Total Secondary Electron Yield From Surface Potential Measurements

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.

Parametric Study of a Physical Flashover Simulator

A physical flashover (FO) simulator has been developed by ONERA/DESP and CNES. The objective of this simulator is to represent the missing cells when testing small coupons in the laboratory. The aim of this paper is to present the results of a parametric study which has been performed on a sample constituted by a solar-array coupon made of six cells and a simulator constituted of a large surface of metallized polymeric film. Experiments were performed in the ONERA/DESP facility called JONAS which is a 9-m3 vacuum chamber equipped with a plasma source and a 10-keV electron gun. Electrostatic discharges (ESDs) occur in the inverted potential gradient (IPG) configuration obtained either by electrons or by plasma. The FO was characterized by measuring the neutralization current on the different surfaces with current probes. Therefore, we could get charge quantity, duration, and velocity. The surface potential of the coupon and the polymeric film were monitored before and after ESD by a potential probe, giving a good correlation with the amount of charges participating to the discharge. In order to determine what the limits of the FO are and what parameters can monitor it, we have studied different configurations: 1) electron or plasma IPG charging; 2) surfaces from 0.5 to 14 m2; 3) geometries-cylinder, ring, rectangular, and discontinuous surface; 4) primary arc locations-cell edge or interconnectors; and 5) absolute satellite capacitance values-between 300 pF and 300 nF. Analysis of the results is given for these configurations.

Measurements of the Flashover Expansion on a Real-Solar Panel—Preliminary Results of EMAGS3 Project

When a primary discharge occurs on a solar array, it is important to understand what would be the maximum flashover expansion. This value would then be representative of in-flight scenarios on full panel size. This paper presents the results of the experimental campaign performed in the frame of the European Space Agency EMAGS3 project “Flash-over evaluation on large solar panels” and where we measure flashovers in different conditions on a real-solar panel. This experimental campaign is conducted in the large vacuum chamber of Industrieanlagen-Betriebsgesellschaft mbH (IABG) (Germany) on a solar panel of 4 × 2 m provided by Astrium-Germany and organized in 52 linear strings of silicon cells covered by Cerium doped borosilicate glass (CMX) coverglasses (CG). During the test, several parameters are studied such as inverted potential gradient (IPG) obtained in plasma or in electrons and test of flashover expansion over a gap between panels by addition of a small panel. The main difficulty is in the evaluation of the value and homogeneity of the initial potential gradient to be able to determine the initial stored charge. The development of a model of flashover expansion has contributed significantly to the comprehension of the results and the assessment of the initial stored charge. During the first step (IPG by electrons at room temperature) ~ 200 electrostatic discharges (ESDs) are recorded of which 12 discharge of the theoretical stored charge in the CG. In IPG by plasma at room temperature, ~ 100 ESDs are recorded, 12 discharging of the theoretical stored charge including two that discharged the panel completely. With this test campaign we demonstrate that, even if the probability is not very high, an ESD on a solar panel could lead a flashover to expand and neutralize the complete surface of an 8- m2 panel. In addition, we see that the flashover can continue across a gap of 10 cm.

Electrostatic Discharge and Secondary Arcing on Solar Array—Flashover Effect on Arc Occurrence

A good representativeness of the test setup used for the qualification of solar arrays is fundamental. We know now that the flashover current involved in the electrostatic discharge is related with the total dielectric surface neutralized during the electrostatic discharge. This paper presents experiments on samples of different sizes, performed in a large vacuum chamber, JONAS, installed in ONERA/DESP, Toulouse, France. The samples are tested in the inverted voltage gradient configuration obtained with electrons. We observed that secondary arcs obtained on a large sample (i.e., with a long flashover) had much longer durations than those on a small sample, i.e., the flashover is a critical parameter for secondary arc initiation and sustainment.

ESDs on Solar Cells—Degradation, Modeling, and Importance of the Test Setup

Cumulative electrostatic discharges (ESDs) on spacecraft solar cells result in the degradation of their performances. In this paper, silicon solar cells are tested in inverted voltage gradient situation obtained in plasma. ESDs occurring on the cells are detected by both electrical and optical signatures. To be representative of flight ESDs, the test setup must avoid unwanted coupling with ground arcs. The degradation is then evaluated by measuring the current-voltage characteristic of the cell in darkness. The equivalent shunt resistance allows quantifying this degradation, which can be attributed to material deposition on the cell edge or to local cell carbonization due to arcing. Visual inspection of the cells allows us to correlate ESD location and local degradation of the cell. The important parameter for solar cell degradation is the amount of energy dissipated during the discharge. A model of the plasma expansion from the cathode spot is compared to measurement. This model explains the current rise during the first phase of the discharge, which is the same for normal ESDs and coupling with ground ESDs.

International Round-Robin Tests on Solar Cell Degradation Due to Electrostatic Discharge

Teppei Okumura∗ Japan Aerospace Exploration Agency, Tsukuba 305-8505, Japan Mengu Cho Kyushu Institute of Technology, Kitakyushu 804-8550, Japan Virginie Inguimbert ONERA, 31055 Toulouse, France Denis Payan Centre National d’Etudes Spatiales, 31401 Toulouse, France Boris Vayner Ohio Aerospace Institute, Cleveland, Ohio 44142 and Dale C. Ferguson∗∗ U.S. Air Force Research Laboratory, Albuquerque, New Mexico

Plasma Bubble Expansion Model of the Flash-Over Current Collection on a Solar Array-Comparison to EMAGS3 Results

This paper describes the results analysis obtained during the experimental campaign conducted in the frame of the ESA EMAGS3 project “Flash-over (FO) evaluation on large solar panels.” A numerical model has been developed in order to better understand the characteristics of the so-called FO phenomenon. The experimental results, presented in detail in a companion paper, are analyzed in correlation with the present FO model. A basic model based on the assumption of a plasma bubble expansion in vacuum has been developed to model the FO propagation. The model holds on a two-dimensional Cartesian mesh representing the surface of the solar panel. On this mesh, the electrical potential evolution and the current collection are computed supposing that the bubble velocity is constant, the current extracted from the plasma bubble is space-charge limited, the secondary emission is the only source of electrons from the cover-glasses and its potential evolution is only due to the current collection. The inputs of the model are the potential topology before the ESD, the ESD triggering position, and the solar array characteristics (dimensions and cover-glasses capacitance). The outputs of the model are the time evolution of the total FO current, the potential map, and the current collection map on the solar array. The results comparison between the model and the experiments shows a very good agreement in the cases where the potential topology before the ESD is well known. It is especially true when using IPG by plasma because the potential profile is relatively uniform. In that case the comparison with numerical results is concluding. In the case of IPG by electron guns, the potential map before the ESD is relatively hard to obtain. In this case, the agreement between the model and the experimental result is obtained only for a limited number of cases. The detailed comparison between model results and experimental data is shown and analyzed in this paper. It appears clearly that the plasma bubble extends at the acoustic velocity of the ions generated at the cathode spot level. As a result, ESDs generated on solar cells junction or interconnects do not have the same dynamics.

Drifting Plasma Collection by a Positive Biased Tether Wire in LEO-Like Plasma Conditions: Current Measurement and Plasma Diagnostic

BETs is a three-year project financed by the Space Program of the European Commission, aimed at developing an efficient deorbit system that could be carried on board any future satellite launched into Low Earth Orbit (LEO). The operational system involves a conductive tape-tether left bare to establish anodic contact with the ambient plasma as a giant Langmuir probe. As a part of this project, we are carrying out both numerical and experimental approaches to estimate the collected current by the positive part of the tether. This paper deals with experimental measurements performed in the IONospheric Atmosphere Simulator (JONAS) plasma chamber of the Onera-Space Environment Department. The JONAS facility is a 9- m3 vacuum chamber equipped with a plasma source providing drifting plasma simulating LEO conditions in terms of density and temperature. A thin metallic cylinder, simulating the tether, is set inside the chamber and polarized up to 1000 V. The Earths magnetic field is neutralized inside the chamber. In a first time, tether collected current versus tether polarization is measured for different plasma source energies and densities. In complement, several types of Langmuir probes are used at the same location to allow the extraction of both ion densities and electron parameters by computer modeling (classical Langmuir probe characteristics are not accurate enough in the present situation). These two measurements permit estimation of the discrepancies between the theoretical collection laws, orbital motion limited law in particular, and the experimental data in LEO-like conditions without magnetic fields. In a second time, the spatial variations and the time evolutions of the plasma properties around the tether are investigated. Spherical and emissive Langmuir probes are also used for a more extensive characterization of the plasma in space and time dependent analysis. Results show the ion depletion because of the wake effect and the accumulation of ions upstream of the tether. In some regimes (at large positive potential), oscillations are observed on the tether collected current and on Langmuir probe collected current in specific sites.

Ground Plasma Tank Modeling and Comparison to Measurements

ONERA plasma tank JONAS is populated with drifting Ar+ argon ions representative of a low Earth orbit environment, produced by a Kaufman source, and slow ions created by charge exchange with the background pressure of argon. When testing a mock-up or space equipment in this tank, it is often important to determine the drifting and slow ion densities in various locations. The orbital and radial motion models are compared to experimental current-voltage measurements and to numerical results. In the range of parameters used in this paper, the radial motion model is more adapted than the orbital-motion-limited model to determine slow ion density. Nevertheless, the number of measurements is necessarily limited and discriminating between fast and slow ions is not easy. A complementary approach consists in performing a numerical simulation of the plasma dynamics in the tank. Provided the modeled physics is validated, and the modeling is calibrated through measurements, this approach supplies fast and slow ion densities everywhere with an acceptable factor of uncertainty. This approach has been followed by characterizing in detail a given plasma configuration in JONAS and modeling it with Spacecraft Plasma Interaction Software open source code. The mock-up was a simple plate. We measured the plasma characteristics through numerous current-voltage sweeps and current space profiles at given probe potential. The modeling was based on fast ion beam measurements close to the source and the residual pressure.

SPIS and MUSCAT Software Comparison on LEO-Like Environment

Two spacecraft charging software tools, the Spacecraft Plasma Interaction Software and the Multi-Utility Spacecraft Charging Analysis Tool, have been compared in the situation of a wake generated by an object immersed in dense drifting plasma. Each model takes account of the particle dynamics, the space charge effect on the electric field, and the currents collected by the object. The cross-comparison resulted in a good agreement between the two codes and with previous experiments conducted on the same configuration. In particular, a highly negative voltage imposed to the object rear side allows the collection of drifting ions at this location. The nontrivial shape of the current density map was correctly simulated by the two codes. Future works may allow to reach a better quantitative agreement.