Nicolas Balcon

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.

Radiation induced conductivity in space dielectric materials

The radiation-induced conductivity of some polymers was described mainly in literature by a competition between ionization, trapping/detrapping, and recombination processes or by radiation assisted ageing mechanisms. Our aim is to revise the effect of the aforementioned mechanisms on the complex evolution of Teflon® FEP under space representative ionizing radiation. Through the definition of a new experimental protocol, revealing the effect of radiation dose and relaxation time, we have been able to demonstrate that the trapping/recombination model devised in this study agrees correctly with the observed experimental phenomenology at qualitative level and allows describing very well the evolution of radiation induced conductivity with irradiation time (or received radiation dose). According to this model, the complex behavior observed on Teflon® FEP may be basically ascribed to the competition between electron/hole pairs generation and recombination: electrons are deeply trapped and act as recombination centers for free holes. Relaxation effects have been characterized through successive irradiations steps and have been again well described with the defined model at qualitative level: recombination centers created by the irradiation induce long term alteration on the electric properties, especially the effective bulk conductivity. One-month relaxation does not allow a complete recovery of the material initial charging behavior.

Characterization of charge carrier lateral conduction in irradiated dielectric materials

A characterization method for surface charging analysis on insulators submitted to electron irradiation has been developed. This method, based on the use of two Kelvin probes (KPs), provides details on the transit time motion for injection of both holes and electrons. It can also be used to assess the isotropy of lateral electrical conduction on the sample. The feasibility of this method was tested on fluorinated ethylene propylene (Teflon® FEP) samples. It was found that central electron injection induces rapid surface charge spreading, in contrast to injection of holes. An electrical anisotropic behaviour of the sample was also detected.

Aging Effect and Induced Electric Phenomena on Dielectric Materials Irradiated With High Energy Electrons

The high radiation dose received by space used polymers may greatly alter their electric properties. This effect could, for instance, reduce significantly radiation-induced conductivity (RIC) leading to high charging risks that were not predicted from pristine sample. For an optimized qualification and prediction, it is therefore highly important to characterize the charging properties of polymers and their evolution as a function of the received dose. This paper aimed at studying aging of electric properties of four different polymers (Teflon FEP, Kapton, polyepoxy DP 490 adhesive, and silicon QS1123 adhesive) at high dose level (105 and 106 Gy). We have been able to demonstrate that aging could lead to the reduction of RIC on some polymers (for polyepoxy and silicone adhesives, and FEP) or inversely to the increase of RIC on others (e.g., Kapton). Ionization effect must, however, be considered in the analysis of the results. Relaxation processes could drastically affect the charging profile and macroscopic electric properties.

Electrical behaviour of a silicone elastomer under simulated space environment

The electrical behavior of a space-used silicone elastomer was characterized using surface potential decay and dynamic dielectric spectroscopy techniques. In both cases, the dielectric manifestation of the glass transition (dipole orientation) and a charge transport phenomenon were observed. An unexpected linear increase of the surface potential with temperature was observed around Tg in thermally-stimulated potential decay experiments, due to molecular mobility limiting dipolar orientation in one hand, and 3D thermal expansion reducing the materials capacitance in the other hand. At higher temperatures, the charge transport process, believed to be thermally activated electron hopping with an activation energy of about 0.4 eV, was studied with and without the silica and iron oxide fillers present in the commercial material. These fillers were found to play a preponderant role in the low-frequency electrical conductivity of this silicone elastomer, probably through a Maxwell–Wagner–Sillars relaxation phenomenon.

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.

Electric properties of space used polymers under high energy electron irradiation

Polymer materials are widely used on spacecraft for optical, thermal or electrical devices. These materials are submitted in space, especially in geostationary conditions, to high energy electron radiation. These electrons could induce high charging levels, which may be hazardous for spacecraft systems, and affect at great level the electric conductivity of polymers. This study reveals that radiation induced conductivity strongly evolves with the received radiation dose yielding to complex surface potential profile that differs significantly from one material to the other. These results have been analyzed in the light of a physical model that takes into account ionization, trapping / detrapping and recombination mechanisms for negative and positive charges.

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.

Relaxation Electron Surface Emission From Space Used Polymers

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.