Radiation-induced Conductivity Research Articles

Frequency Factor of the Semiempirical Model for the Radiation-Induced Conductivity in Spacecraft Polymers

We have extended the existing Rose–Fowler–Vaisberg model of the radiation-induced conductivity (RIC) in polymers by incorporating the latest ideas borrowed from theoretical and experimental results obtained in the field of electronic transport in molecularly doped polymers. These include the Poole–Frenkel-type field-dependent mobility and the arguments of the dipolar disorder formalism based on the Gaussian disorder model. Theory validation uses a new experimental technique based on the electron-gun technology allowing combined-induced conductivity and carrier mobility measurements in a typical molecularly doped polymer. Quasi-band and hopping theories of the carrier transport are critically discussed.

Functional materials for ITER diagnostic systems – Radiation aspects

ITER will be the first fusion device able to maintain fusion for long periods of time and therefore suitable to test integrated technologies, materials, and physics regimes necessary for the commercial production of fusion-based electricity. Therefore, ITER provides a harsh radiation environment that represents an important challenge for diagnostic systems located in various regions inside the tokamak, in particular the first wall, divertor areas, and ports, but as well ex-vessel areas, where radiation levels are also significantly high for some electronic components. The radiation induced effects in functional materials have been identified and studied in the past in the fusion and fission community. These effects comprise nuclear heating, changes in physical and mechanical properties due to transmutation and displacement damage, radiation induced conductivity (RIC) or radiation induced electrical degradation (RIED) effects on insulators, radiation induced absorption (RIA) for optical materials, radio luminescence observed in fibers and windows, etc. This paper summarizes the current status of the functional material selection for ITER diagnostic systems and the expected radiation conditions in the areas where they are located.Additional testing will be required for some materials and assemblies, such as bolometers and magnetic coils. Special attention is paid to the tests required for qualification of safety important components, such as diagnostic window assemblies and vacuum feedthroughs.Finally, as one of the best ways to reduce the impact of radiation in materials is to develop a proper shielding strategy, some of the shielding options envisaged for ITER are presented and discussed in this paper.

Dielectric properties of alumina ceramics for fusion applications

The insulating materials for radio frequency (RF) windows and different antenna supports for heating systems and some diagnostics are still an unresolved issue in future fusion machines such as DEMO. Alumina ceramic (Al2O3) is one of the main candidate materials for these systems. The dielectric properties, such as electrical permittivity (ε) and loss tangent (tanδ), determine its power losses. Therefore, tanδ values need to be low, between 10–6 and 10–3 depending on the application. However, due to the crucial role of the manufacturing process in determining its final dielectric properties, there is the need to undertake a joint effort with the industry in order to validate a standard manufacturing route that ensures a supply of ceramic material with homogeneous and standardized dielectric properties for future fusion machines. In DEMO, these ceramic materials will be operated under extreme conditions. They will be exposed to considerable levels of neutron irradiation. Radiation Induced Conductivity (RIC) and Radiation Induced Electrical Degradation (RIED) are phenomena that influence notoriously on the dielectric performance of these materials. Therefore, their optimum properties must be tested under relevant irradiation conditions. Hence, it is important to accurately characterize their dielectric properties before irradiation to be able to compare them with the future post neutron irradiation results. Furthermore, other types of radiation such as beta and gamma are also used in the irradiation scheme to provide more information on the variation of dielectric properties of ceramics due to radiation effects. In this work, loss tangent and permittivity values of some pre-irradiated candidate materials will be presented as well as the measuring techniques used at CIEMAT. These techniques have the advantage of covering a very broad range of frequencies required for alumina applications in fusion (from kHz to GHz) and giving the best accuracy for very low losses.

Validation Through Experiments of a 3-D Time-Dependent Model of Internal Charging

A new model of dielectric conductivities has been implemented in SPIS-IC. It allows modeling the time-dependent behavior of the charge transport and the electric potential inside payload components under realistic radiation environments in space. The simulation results have been compared to experiments on four validation cases going from simple 1-D geometry to more complex 3-D components such as a d-type connector. The comparison shows a good agreement in most of the cases, and the discrepancies are explained in certain cases. Finally, the results have been analyzed in term of discharge probability and severity. This study also shows the great importance of time-dependent modeling of the radiation-induced conductivity.

Modeling of Electric Fields Inside Spacecraft Dielectrics Using In-Orbit Charging Current Data

Internal charging caused by energetic electrons is a threat to critical space infrastructure such as Europe's Galileo navigation satellites, however, the mechanism involved and its relationship to space weather is still poorly understood. While the electric field developed inside on-board dielectrics is accepted to be of prime importance, it has never been measured in orbit. On the other hand, internal charging currents have now been measured in a GPS-like medium Earth orbit (MEO) over a ten-year period, so we have used these data to model the electric fields of concern. The measured charging currents provide both charge deposition and dose rate inputs to the electric field model, the latter allowing the introduction of radiation-induced conductivity (RIC) to improve realism. As expected we find that RIC is a mitigating factor for the electric fields but they can still become very large, e.g., a 1.0-mm thickness of PEEK under 0.5 mm of Al shielding would be at risk of breakdown almost throughout the mission. We find that RIC tends to reduce sensitivity to space weather perturbations of the environment such as the April 2010 storm event, and hence longer term average fluxes seem to be more important. This seems physically reasonable for the examples studied but we also know that some satellite anomalies do show a correlation with space weather storms and short term (daily) electron fluences and so further work is needed to understand such cases: for example model parameters could be adjusted to see under what circumstances short term variations are more influential. To enable the progressive improvement of such modeling in future we recommend wider deployment of charging current sensors, especially in MEO and geostationary orbits.

Experimental Measurement of Low-Intensity and Long-Duration Internal Charging Behavior

We report initial results from the EU FP7 Spacestorm project on the experimental behavior of commonly-used space dielectric materials in an electron environment where the incident electron current is significantly below safe levels specified by design standards. The realistic electron environment facility (REEF), which uses an intense strontium-90 beta-emitting radioactive source to simulate the space environment, has been recommissioned at the University of Surrey for this purpose. Using a combination of shielding and variable source-sample separation REEF can achieve a very wide dynamic range in electron current, from the very high levels associated with an extreme space weather event, down to the levels below the European Cooperation for Space Standardization low temperature (<25 °C) safety threshold of 0.02 pA/cm2. In order to allow for the accumulation of significant levels of charge at such a low current, these experiments need to be run continuously for several weeks, far in excess of typical experimental investigations of internal charging phenomena. Here we report our early results from these experiments on samples of polyether ether ketone thermoplastic polymer material. Irradiating using a range of electron currents provides insight into the effect not just of inherent material properties but also of secondary effects such as radiation-induced conductivity.

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.

Fast Measurement of Dielectric Conductivity for Space Application by Surface Potential Decay Method**Supported by the Fundamental Research Funds for the Central Universities in Nanjing University of Aeronautics and Astronautics under Grant No NS2014089.

Surface potential decay of polymers for electrical insulation can help to determine the dark conductivity for spacecraft charging analysis. Due to the existence of radiation-induced conductivity, it decays fast in the first few hours after irradiation and exponentially slowly for the remaining time. The measurement of dark conductivity with this method usually takes the slow part and needs a couple of days. Integrating the Fowler formula into the deep dielectric charging equations, we obtain a new expression for the fast decay part. The experimental data of different materials, dose rates and temperatures are fitted by the new expression. Both the dark conductivity and the radiation-induced conductivity are derived and compared with other methods. The result shows a good estimation of dark conductivity and radiation-induced conductivity in high-resistivity polymers, which enables a fast measurement of dielectric conductivity within about 600 min after irradiation.

Using Rouse-Fowler model to describe radiation-induced electrical conductivity of nanocomposite materials

Using the Rouse-Fowler (RF) model this work studies the radiation-induced electrical conductivity of a polymer nanocomposite material with spherical nanoparticles against the intensity and exposure time of gamma-ray, concentration and size of nanoparticles. The research has found the energy distribution of localized statesinduced by nanoparticles. The studies were conducted on polymethylmethacrylate (PMMA) with CdS nanoparticles.

The model of radiation-induced conductivity in silicon

In this paper, we consider the conduction current excited in a silicon obstacle by the action of an external flux of penetrating radiation. We use quantum kinetic equations for distribution functions of conduction band electrons and holes of the valence band in the phase space of coordinates and quasi-momentums. Effective masses, densities of states, and group velocities of particles are determined on the base of band theory of a crystal. The approximation of the continuous momentum losses due to scattering on lattice defects is performed. The applicability of the model is validated by the comparison with the experimental data for the electron average-velocity dependence on the electric-field strength and on the velocity of the electron-energy transmission to the lattice. The silicon radiation conductivity excited by a free-electron current is calculated and the compliance of the results with the theoretical estimates is demonstrated.

Investigating the electron density of multi-MeV X-ray-induced air plasmas at low pressures based on electromagnetic resonant cavity analysis

We investigate air plasmas generated by multi-MeV pulsed X-rays at pressures ranging from 10−5 to 10−1 mbar. The experimental approach used for these studies is based on measurements of resonant frequencies damping and shift for different electromagnetic modes within a cylindrical cavity. Time-integrated electron densities in X-ray-induced air plasmas are inferred from the damping rate of the measured magnetic fields and their corresponding frequency shifts. In the present study, electron densities ranging from 108 to 109 cm−3 at pressures ranging from 10−3 to 10−1 mbar have been measured. Experimental results were confronted to 3D Maxwell-Vlasov Particle-In-Cell simulations incorporating a radiation-induced electric conductivity model. The method used in this work enables determining microscopic and macroscopic physical quantities within low pressure air plasmas generated by pulsed X-ray.

Neutron irradiation and frequency effects on the electrical conductivity of nanocrystalline silicon carbide (3C-SiC)

In this present work nanocrystalline silicon carbide (3C-SiC) has been irradiated with neutron flux (∼2×1013 ncm−2s−1) up to 20 hours at different periods. Electrical conductivity of nanocrystalline 3C-SiC particles (∼18 nm) is comparatively analyzed before and after neutron irradiation. The frequency dependencies of electrical conductivity of 3C-SiC nanoparticles is reviewed at 100 K–400 K temperature range before and after irradiation. The measurements were carried out at 0.1 Hz–2.5 MHz frequency ranges and at different temperatures. Radiation-induced conductivity (RIC) was observed in the nanocrystalline 3C-SiC particles after neutron irradiation and this conductivity study as a function of frequency are presented. The type of conductivity has been defined based on the interdependence between real and imaginary parts of electrical conductivity function. Based on the obtained results the mechanism behind the electrical conductivity of nanocrystalline 3C-SiC particles is explained in detail.

Leakage Current of Grounded Dielectrics in Electron Radiation as a Diagnostic Method to Evaluate the Deep Charging Hazards in Space

Deep dielectric charging, which is a coupling process of charge deposition and charge relief, is a significant factor in spacecraft anomalies and failures. With the aim of developing a method for evaluating the deep dielectric charging hazards, this study investigated the leakage current and charging electric field of grounded dielectrics in space usage by a method combining Monte Carlo simulation and a model of radiation-induced conductivity. The analysis adopts multiple cases of electron spectra and various thicknesses of the shielding layer and dielectric. The analysis results show that the leakage current, which can be easily measured, is strongly related to the charging electric field in various situations. Therefore, monitoring the leakage current can be a more effective approach for evaluating deep dielectric charging hazards than monitoring electron flux.

Nonlinear effect caused by radiation-induced conductivity of cable irradiated by X-ray

Nonlinear effect caused by radiation-induced conductivity of cable irradiated by X-ray

Internal Dielectric Charging Simulation of a Complex Structure With Different Shielding Thicknesses

To find the real risk points of internal dielectric charging (IDC), 3-D IDC simulation is performed to a typical dielectric structure considering GEO severe radiation environment (FLUMIC3). Charge transportation simulation is carried out by Geant4 and electric field is numerically computed according to the current conservation law. The aluminum shielding thickness is varied to investigate the shielding dependence. By adjusting the lower energy limit of the incident electron spectrum according to the shielding thickness, a technique is proposed to improve the simulation efficiency. The statistic condition in Geant4 is checked and attention is paid to the influence of mesh on the peak electric field. The simulation data show that the peak electric field occurs on the grounding edge and local mesh refinement on the critical charging point is of significance to make sure the real charging level. Increasing the shielding thickness could mitigate IDC. However, this mitigation effect is weak in the case where radiation-induced conductivity dominates the total conductivity (at low temperature). In this case, even using a 3-mm aluminum shielding, the peak electric field can reach the level of $10^{7}$ V/m.

Effects of proton pre-irradiation on radiation induced conductivity of polyimide

Abstract As an important parameter to evaluate the charge/discharge behaviors, radiation induced conductive behaviors of proton pre-irradiated polyimide films were investigated. The results indicate that the proton pre-irradiation results in decrease down to three orders in radiation induced conductivity (RIC) of polyimide as the proton (60–110 keV) fluence is just 1×10 14 p/cm 2 . Further analysis implies that, the dominate effect on the RIC degradation is proton pre-irradiation induced displacement damage, which results in the decomposition of pyromellitimide in polyimide.

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: 1) fluorinated ethylene propylene; 2) PolyEther Ether Ketone; and 3) 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.

Experimental and Theoretical Studies of Radiation-Induced Conductivity in Spacecraft Polymers

We have reviewed the basic results in the radiation-induced conductivity and the bulk charging of polymers obtained by our group during the past 30 years, which are mostly unknown in the West. Special attention is given to a new experimental technique extensively used in our investigations based on an electron-gun technology allowing combined induced conductivity and carrier mobility measurements. Quasi-band and hopping theories of the carrier transport are critically discussed.

Analysis of Charging Kinetics on Space Dielectrics Under Representative Worst Case Geostationary Conditions

Prediction of the surface charging behavior of the materials used in space often raises concerns about the definition of the applied electron energy spectrum used for irradiation during testing. Radiation-induced conductivity (RIC) cannot be discarded for a proper qualification of these materials. But RIC level is significantly dependent upon radiation dose and dose rate. We have demonstrated that for some energy spectra, severe potentially hazardous charging levels were predicted by models when radiation history was ignored. However, the charging effect is strongly smoothed by the radiation history of the material. We have been able therefore to reveal in this paper that a good electric qualification of space materials under electron irradiation requires the definition of realistic electron energy spectrum and implies to take into account radiation history in space.

Radiation-Induced Conductivity of Space Used Polymers Under High Energy Electron Irradiation

Polymers are widely used on spacecraft for different specific functions such as thermal and electrical insulation, mechanical support, and adhesion. These polymers are highly sensitive to radiation encountered in space. Their electric properties are especially significantly altered leading to a very specific behavior in terms of charge transport and discharge processes. Different dedicated facilities have been developed at ONERA, with Centre National d’Etudes Spatiales support, for the characterization of space used materials in representative conditions. Thanks to the use of these different facilities, it has indeed been demonstrated that radiation-induced conductivity of space polymers strongly affects the charging surface potential and depends on several parameters (radiation dose rate, total radiation dose, temperature, and on the induced electric field) through complex physical mechanisms that are described in this paper. The sensitivity of polymers on these different parameters strongly depends on the polymer trap distribution and molecular configuration. Experimental and numerical results are presented in this paper, coupled with the different experimental techniques developed and applied to this work.