Bernard Dirassen

Radiation Induced Conductivity in Teflon FEP Irradiated With Multienergetic Electron Beam

Teflon Fluorinated Ethylene Propylene (FEP), used as thermal blankets on satellites, has a specific electrical behavior under space ionizing environment. Charging behaviors of this dielectric material, under electron beam irradiation, is of special interest for future spacecraft needs. Hence, ground experimental parameters have been adjusted in order to distinguish between the different physical mechanisms that steers FEP charging potential. The effect of incident electron spectrum, electric field, and dose rate has been investigated through surface potential measurements during and after the irradiation process. The experimental results especially reveal that charging potential evolution as a function of time is not strongly dependent upon the electron spectrum and the electric field but varies noticeably with dose rate. 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.

Polyimide and FEP charging behavior under multienergetic electron-beam irradiation

Surface and internal charging of dielectric materials is a potential cause of surface discharges and satellite anomalies, due to the fluctuating irradiation levels induced by space environment. Understanding conduction mechanisms and reducing charging levels are therefore important industrial issues for satellite designers and manufacturers. Surface potential measurements under irradiation and after charging (potential decay) are the most significant laboratory tests to qualify and understand the charging and discharging behavior of insulating materials. We present here experimental results obtained using the SIRENE facility at ONERA. Kapton and Teflon FEP films respond differently when subjected to a 20 keV charging electron beam combined with a 400keV ionizing electron beam. The physics underlying these experimental results is discussed. A simple numerical model has been developed. It is shown that different combinations of mobility, trapping and recombination may explain the results on both materials. The complex behavior observed on Teflon FEP may be attributed to the progressive deep trapping of the negative charge, enhancing holes recombination.

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.

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.

Measurement and modeling of charge profiles in irradiated dielectrics

In order to get a better understanding on the discharge initiation principle, a charge implantation analysis in electron irradiated dielectrics is proposed. Charge depth profiles measured by using the pulse electro acoustic (PEA) method are compared with numerical simulation.

Charging Properties of Space Used Dielectric Materials

Charging effects in space are widely dominated by intrinsic and radiation-induced conductivities (RICs) of the irradiated materials. These data are needed by the community to perform charging predictions for assigned missions. This paper was dedicated to the characterization of bulk conductivity and RIC of polymers, ceramics, and glasses and their evolution in regard to sample reproducibility, composition, structure variation, and environmental conditions, such as radiation dose rate, temperature, and electric field. Three different polymers have been tested in this paper: fluorinated ethylene propylene; PolyEther Ether Ketone; and Ethylene TetraFluoroEthylene. Three nonpolymer materials have been tested as well: two borosilicate glasses and aluminum oxide Al2O3. The major objective was, for polymers, to understand physical variations of conductivity for similar materials and, for nonpolymers, to get better knowledge on the main physical contributions steering bulk conductivity and RIC at low temperature.

Charging Behavior of Polyimide/Adhesive Components During Eclipse Events in Geostationary Environment

This paper presents an experimental and numerical study carried out in the SIRENE irradiation test facility, installed at ONERA (Toulouse, France), on stacked polyimide/adhesive structures used on solar panels. This study aims at following the evolution of the charging potential built up at the surface of these structures during an eclipse event in geostationary orbit. The samples have therefore been irradiatedwith adistributed electron spectrum (from0 to 400 keV). During a first stage, the samples are submitted to light radiation at 293 K. In a second stage (eclipse phase), the electron irradiation is performed in darkness and with a decreasing temperature (down to 113 K). In a final stage (eclipse exit), the samples are submitted to light radiation with an increasing temperature (up to 293 K). We have been able to demonstrate that these solar structures are highly sensitive to temperature and light radiation since the volume conductivity of polyimide films and epoxy adhesives is submitted to noticeable variation with these operative parameters. The major conclusion is that this kind of polyimide/adhesive structure potentially used on solar panels may have to cope with elevated charging potential, which can be highly hazardous especially at the eclipse exit when the solar panel regains its power.

Analysis of Charge Transport and Ionization Effect in Space-Used Polymers Under High-Energy Electron Irradiation

Polymer materials have been tested in the dedicated experimental facility SIRENE (ONERA, Toulouse, France) designed to reproduce the electron energy spectrum met in space in the (0–400 keV) energy range and to perform electric analysis on the materials through dedicated protocols with potential, current, and contactless pulsed-electro acoustic (CNES patent) measurements. A Novel experimental approach shall as well be presented, which allowed bringing into evidence the complex response of polymers under irradiation: polarization and charge differential mobility as well as physical structural changes have been analyzed with these new techniques allowing a better understanding and prediction of charging behavior and radiation-induced conductivity evolution of these polymers under space conditions.

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.

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.