This is the unspecified version of the paper.This version of the publication may differ from the final published version. Permanent repository link ABSTRACTThe PEA technique is used to measure the distribution of space charge in an epoxy resin after polarisation for one week at an applied field of 7.14kV/mm over a range of temperatures. The decay of the space charge is followed for times up to 114 hours after removal of the voltage and analysed in terms of a number of alternative decay mechanisms. It is shown that the rate-determining stage of the decay mechanism is that of a thermally activated process that has been associated with charge de-trapping. At times greater than 10 2 s the de-trapping process behaves as though the space charge field does not exist and the retention time of the space charge depends only upon the depth of the deepest occupied traps and the temperature.

This is the unspecified version of the paper.This version of the publication may differ from the final published version. Permanent repository link ABSTRACTDielectric breakdown of epoxies is preceded by light emission, or so-called electroluminescence, from the solid-state material. Very little is known about the luminescence properties of epoxies. The aim of this paper is to derive information that can be used as a basis to understand the nature of the excited states and their involvement in electrical degradation processes. Three different kinds of stimulation were used to excite the material luminescence. Photoluminescence was performed on the base resin, the hardener and the cured resin. Luminescence excited by a silent discharge has been analysed to identify which of the luminescent centres are optically active upon the recombination of electrical charges and could therefore act as charge traps. Finally, the electroluminescence spectrum has been acquired and compared with the previous ones. Although the identification of the origin of these emissions is far from being complete, it has been found that the photoluminescence from the cured resin is due to in-chain chromophores, which acts as trapping centres. The excited states involved in photoluminescence also seems to be involved in electroluminescence, but other components are detected as well, which could be due to the degradation of the resin molecule under the effect of the electric stress.2

The charge distribution in low density pyethylene and polyimide films under a quasi-mono-energetic electron beam irradiation in the range [5–400 keV] is analysed using a new experimental set-up based on the pulsed electro-acoustic (PEA) method. The device allows measurements without any physical contact between the excitation electrode used in the PEA method and the irradiated surface which remains at a floating potential during the entire duration of the experiment. This special configuration makes it possible to study bulk as well as interface phenomena. In particular, emission of charges from the irradiated surface to the vacuum is in evidence as well as charge injection from the ground electrode to the bulk. The time-dependent charge distribution during irradiation is approached by combining a Monte Carlo toolkit for modelling radiation transport in matter in conjunction with a macroscopic description of charge transport based on the concept of a radiation-induced conductivity generated by the injected electrons. Although the macroscopic description yields a fair agreement with the experimental data, it fails to depict the detailed shape of the charge distribution, thereby calling for a microscopic, bi-polar model.

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This paper proposes a numerical model for describing charge accumulation in electron-beam irradiated low density polyethylene. The model is bipolar, and based on a previous model dedicated to space charge accumulation in solid dielectrics under electrical stress. It encompasses the generation of positive and negative charges due to the electron beam and their transport in the insulation. A sensitivity analysis of the model to parameters specific to electron beam irradiation is performed in order to understand the impact of each process on the space charge distribution. At last, a direct comparison between time dependent space charge distribution issued from the model and from measurements is performed. The transport parameters used for the simulations are the same as those optimized for transportation in polyethylene under an external electric field giving a robustness in the modelling approach because of the constrains on fitting parameters that must comply to a set of experimental results.PACS Numbers: 72.20Ht, 72.20Jv 1. Introduction Solid dielectrics used as thermal blanket on geostationary satellites are submitted to the flow of several types of charged particles, and particularly to electrons. These materials can accumulate charges, building up the potential inside the dielectric, meaning a potential difference between different parts of the satellite. Electrostatic Surface Discharge (ESD) can occur, leading to possible damages of the electronics of the satellite [1]. In order to prevent such ESD to happen, it is necessary to understand the dynamics of the charge transport in solid dielectrics used in space environment. A number of studies have been carried out in this domain. Some are experimental, measuring the surface potential of the irradiated samples, the current flow during irradiation [2] and the space charge distribution after irradiation [3]. Recently, two original set-ups [4-5] to measure space charge distribution in electronbeam irradiated samples have been developed on the basis of the Pulsed Electro-Acoustic (PEA) method. With these experimental tools, it is now possible to observe the dynamic of charging and discharging in electron-beam irradiated materials. On the other hand, a number of works on theoretical background and simulation has been done in order to understand and reproduce the behaviour of charge in electron beam irradiated polymers [6][7][8][9]. Our very aim is to develop space charge modelling in electron beam irradiated materials in nonstationary conditions through an approach that closely associates experiment and numerical simulation. Within the first part of this paper, we briefly review the state-of-the-art on modelling charge transport in synthetic insulations under electron beam irradiation in order to establish the position of our approach. Then, we describe the proposed model, which has been adapted from a previous one [10] developed for space charge conduction in a Low Density Polyethylene (LDPE) under DC stress. The same set of transport parameters has b...

This is the unspecified version of the paper.This version of the publication may differ from the final published version. Permanent repository link: INTRODUCTIONMost experimental studies of electrical ageing have concentrated on semi-crystalline polymers such as those used in cable insulation and capacitors (see for example [1]). Theoretical models [2][3][4] for electrical ageing have been developed on the basis of these studies. The consensus is that ageing involves the formation of lowdensity regions, though the mechanisms responsible are disputed. For example, bond scission by high-energy electrons [2,5], and mechanical deformation have both been suggested. Both of these mechanisms are related to charge injection and the subsequent formation of high local fields. The semi-crystalline polymers studied so far have similar chemistries and almost identical morphologies. They tend, therefore, to show many similarities in, for example, the size of the energy barriers for the ageing reaction, critical ageing levels, and field dependence of ageing [4]. These similarities make it difficult to discriminate between mechanisms. Epoxy resins, however, are network polymers with a different molecular chemistry to that of the semi-crystalline polymers and are thus ideal to evaluate the proposed ageing mechanisms. We have therefore studied an epoxy resin (CY1301) under both uniform field and high divergent field conditions. Uniform field conditions were used to gain baseline characteristics for the properties of the unaged epoxy resin, and also for the effects of electrical ageing in low fields. Studies in high divergent fields were made using an electrode arrangement adapted from that of [6]. A number of wires set approximately 0.5mm apart were embedded, parallel to the flat faces, in thin (∼290 µm ) flat samples. The radius of the wires ranged from 5 µm (gold plated tungsten) to 25 µm (tungsten). Relatively small voltages applied to the wires (≤5 kV DC) therefore produced local fields up to 170 kV/mm depending upon the wire radius chosen. These field levels are high enough to inject space-charge [6] without leading to instantaneous failure. This geometry, therefore, may both inject charge and simulate local stress enhancements arising from charge accumulation. The number of wires is large (∼30) so that the volume affected is big enough to allow changes on ageing to be detectable. EXPERIMENTAL INVESTIGATIONS Dielectric ResponseThe dielectric response of the epoxy exhibited a glass transition just below 40 ˚C [7]. At this temperature and above a loss peak was observed at low frequencies that merged into a power law response at higher frequencies and eventually into a high frequency loss peak at ∼3 10 5 Hz. A dc conductivity masking the low frequency side of the alpha peak was observed at T≥70 ˚C, figure 1. An Arrhenius plot of the low frequency peak show the approach to zero frequency typical of the alpha response of a glass transition, figure 2. The power law response is less clear-cut as it is partially obscured by the two...

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