Jean-Michel Siguier

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.

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.

Effect of Surface Discontinuities and Electrical Network Architecture on the Flashover

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.

Study on Secondary Arcing Occurrence on Solar Panel Backside Wires With Cracks

Space environment exposure may create cracks on solar panel backside wires. In terms of the wiring design of the solar array backside, environmental constraints applied on each wire are identical. Thus, the probability of two adjacent wires having cracks facing each other is very high. This configuration presents a risk of secondary arc occurrence, which can lead to a destructive process such as arc tracking. In order to determine in which conditions electrostatic discharges (ESDs) can lead to an arc, we have carried out an experimental study on solar panel backside-like samples. We have tested different types of wires presenting artificial mechanical cracks or space simulation aging cracks, set on a solar panel backside coupon. The wires are connected to a secondary arc test setup including a solar array simulator (SAS) set to different current values. As both direct and inverted potential gradient (IPG) charging are theoretically possible on geostationary orbit and low earth orbit, we have carried out, in the JONAS vacuum chamber (ONERA facility), both charging types. Direct charging was achieved with an electron gun and IPG with a plasma source. Results are presented for the two types of charging, different types of wires, and different SAS current values. They show that, in direct charging conditions, ESD propagates along the wires but the plasma density is too weak to provoke an arc. In IPG situation, ESDs are able to trigger different arc types, such as nonsustain arc, temporary sustain arc, and permanent sustain arc, depending on SAS current values.

Influence of Different Parameters on Flashover Propagation on a Solar Panel

This paper first presents a short review of the recent experiments performed on large test fixtures, whose aim was to evaluate the flashover (FO) propagation during an electrostatic discharge. Then, the model of plasma bubble expansion is presented and compared with the results collected during the review. We will show that the model can represent qualitatively and quantitatively most of the results. The parameters of the model are mainly the ion velocity and the backscattered electron emission yield. The other experimental parameters influencing the FO propagation will be discussed.

Arcing Test on an Aged Grouted Solar Cell Coupon With a Realistic Flashover Simulator

We have performed arcing tests on an aged grouted solar cell coupon provided by Kyutu Institute of technology (Japan) under New Energy and Industrial Development Organization (Japan) grant. Aging is simulated by electrons, protons, and UV irradiations combined with thermal cycling, corresponding to 10 years in the geostationary orbit. Arcing tests are performed with a European standard setup implemented with two different flashover simulators. Instead of using a large capacitance corresponding to the missing solar panel surface, we have implemented two more realistic devices: a 2.4-m2 Kapton surface charged in an inverted potential gradient mode, which releases an average flashover current of 6-A peak at 200 μs and RLC circuit, deduced from the European Etude et Modelisation des Arcs sur Generateur Solaire study (large flashover on an 8-m2 solar array panel). This circuit is connected between the solar array simulator (SAS) line and the surrounding environment with a ringshape electrode, which releases an adaptable flashover current of 5-A peak during 600 μs (average values). The results present the occurrence of different arc types versus current/voltage SAS values. A comparison is made with other tests, on a similar coupon, performed by U.S. and Japanese laboratories and also with comparable coupons without grouting.

Analysis of Solar Array Performance Degradation During Simulated Flashover Discharge Experiments on a Full Panel and Using a Simulator Circuit

The main activity of the European Space Agency EMAGS3 study (published at 12th Spacecraft Charging Technology Conference Kitakyushu, Japan) was to simulate in-orbit electrostatic discharge, called flashover, on a flight representative panel of dimensions 2 m × 4 m in an environment to achieve inverted gradient conditions on the solar cells. Electrical performance was measured after the panel has suffered from about 1400 flashovers, some 300 have been recorded and at least 24 of them covered more than 75% of the surface. The comparison with the last measurements before testing revealed no degradation beyond 1%. Obvious differences in the shape of the obtained curves could be attributed to increased serial resistances in the test circuit. A flashover simulation circuit was designed to reproduce the pulse shape measured on the panel. Experiments on single cells using this circuit demonstrated its ability to substitute the flashover in ground tests.

Measurements of Physical Parameters Characterizing ESDs on Solar Cell and Correlation Between Spectral Signature and Discharge Position

Electrostatic discharges (ESDs) on solar cells are a possible cause of dramatic consequences such as secondary arcs responsible for definitive power losses. To cope with these significant implications, different approaches are followed such as design rules reducing voltage between the adjacent cells, conductive layers, or grouting to try to reduce discharges triggering. However, ESDs on solar cells cannot be completely avoided and having a good knowledge of their characteristics is essential for prevention, prediction, and modeling. In this paper, we describe how the plasma emitted during an ESD on a solar cell can be analyzed with dynamic tools such as triple probes and time-resolved optical spectroscopy. These techniques are used to obtain the results on plasma density and electron temperature that can be compared with outputs from ESD and flashover propagation models. While time-resolved optical spectroscopy is used on a single point (the point where optical fiber is focused on), triple probe is also used for spatial measurements. With this technique, electron density is measured at several distances from the discharge point providing both temporal and spatial information. In a second time, the optical signature measured by optical spectroscopy is correlated with scanning electron microscope observations showing the existence of two kinds of triple points at cells edge. These two kinds of discharges have different optical signatures showing either elements from the active junction or from the substrate and rear electrode. These discharges are also distinguished by microscopic observations and images of cells edges confirm the previous results. These results show the importance of the silver back electrode and also of the eventual presence of covering glue on the position of the discharge. They provide information for models but let us also imagine the possible mitigation methods.