Theoretical Research On Spacecraft Charging Caused By Electron Radiation In Space

Various spacecraft operating in the space radiation environment will suffer from the irradation of electrons with different energies and flux,forming deep dielectric charging or surface charging,called the spacecraft charging.When reaching its threshold,the electric field within the dielectric induced by the electron irradation can lead to electrostatic discharge.The discharge will not only induce direct damage to spacecraft,but also produce the electromagnetic interference to the normal operation of space equipment.Due to these,a considerable attention has been paid to the spacecraft charging.By means of the ground simulation test,in-orbit flight test,and numerical simulation,a lot of studies on the mechanism governing the dielectric charging and on the anti-radiation of material have been carried out,giving valuable research results.In this dissertation,the spacecraft charging induced by the electron irradiation has investigated.Firstly,the origin of the spacecraft charging as well as its evolution,the space radiation environment,and spacecraft dielectric material are introduced,and the interactions of electrons with spacecraft dielectric materials as well as the corresponding numerical simulation method are analyzed.On this basis,the electric field of deep dielectric charging,surface potential of surface charging,and the improvement of the dielectric material have been investigated numerically based on Geant4 software packages.This dissertation includes the following contents and results.1.The electric field distribution within the dielectric due to deep dielectric charging has been investigated.A one-dimensional model of describing the electric field in the flat dielectric charging has been derived,and using this model the effects of injection current density and radiation-induced conductivity on the electric field have been explored.The main conclusions are obtained as follows:(1)For a fixed spacecraft dielectric material,the electron energy dependence of the maximum range of electrons penetrating into the material has been obtained by numerical simulation.The dependence is in better agreement with experiment than that given by the empirical formula,examining effectiveness and accuracy of the present numerical simulation.(2)Injection current density is closely related to the electron flux and it decreases with increasing electron penetration depth.With the increase of the electron penetration depth,the radiation-induced conductivity increases,decrease after reaching its maximum,and finally presents a stability,showing an obvious nonlinear dependence on electron penetration depth.In this dissertation,the injected electron flux and energy deposition are obtained by means of numerical simulation.Based on these two quantities,the resultant injected current density and radiation-induced conductivity are of accuracy better than those evaluated by empirical formula.(3)A method based on an experimental data fitting is suggested to quantitatively analyze the strong field conductivity.The influence of the strong field conductivity on the electric field distribution due to deep dielectric charging in the composite material has been simulated and analyzed on the basis of example of the polyethylene filled with zinc oxide.The results show that the higher the proportion of zinc oxide is,the effect of decreasing the electric field intensity is more obvious.When the proportion of zinc oxide added reaches 50%.the electric field decreases down to about 106Vm-1.This indicates that the addition of inorganic fillers can effectively improve the charging electric field of polymer materials.2.The surface charging potential of spacecraft has been investigated by using a one-dimension model of describing the electric field in the flat dielectric charging.In the energy range below 1 00 keV,the influences of the irradiation time,current density,and electron energy on the surface charging potential have been systematically analyzed,suggesting the mechanism of "shallow layer charging".Both the secondary electron emission coefficient and back scattering coefficient of electrons have been obtained by numerical simulation.The main conclusions are obtained as follows.(1)Whether for metal materials or for polymer materials,the energy deposition distribution of the penetrating electrons in these materials presents the following behaviors,namely,a high electron energy leads to a large penetration depth,a deeper peak position,and a smaller peak value.(2)The effects of the irradiation time,current density,and electron energy on the electric field distribution and surface potential of the spacecraft dielectric in the shallow layer charging have been revealed.The extension of the irradiation time can give rise to the increase of the maximum electric field Emax and the surface potential when the charging is in equilibrium.Both the surface potential and Emax increase with the increase in the current density of the irradiation electrons.When the current density and irradiation time remain unchanged,Emax is not changed by electron energy,and the surface potential is inversely proportional to electron energy.(3)The dependences of the surface potential on both electron energy and the secondary electron emission coefficient have been revealed.When the energy of incident electrons is low,the secondary electron emission coefficient is high,and thus the surface potential is also high.However,as incident electron energy increases,the secondary electron emission coefficient and the surface potential decrease.The normalized surface injection current density is in complementary to the backscatter electron coefficient.3.Nonlinear effects of dielectric conductivity has been investigated.Regarding the radiation protection technology with an addition of fillers to macromolecular materials,in the case of low-density polyethylene,the effect of change of filler property on improving the electric field within the composite material due to deep dielectric charging has been systematically investigated,providing a guidance on the selection and modification of spacecraft dielectric materials.The main conclusions are obtained as follows.(1)As the proportion of the filler increases,the depth at which the electric field begins to increase and Emax gradually decreases.When the proportion of black a silicon carbide is more than 37.5wt%,if the particle size of silicon carbide is larger,then Emax is lower.On the contrary,the variation of the particle size of silicon carbide has little effect on the electric field.The effect of black a silicon carbide on reducing the electric field within composite material is superior to green a silicon carbide.The silicon carbide with βstructure has a better effect on reducing electric field within the composite materials than with a structure.(2)A set of numerical simulation system based on Geant4 is constructed to study the influence of nonlinear conductivity on spacecraft charging.The results show that the nonlinearity of dielectric conductivity is determined by a variety of factors and can effectively reduce the dielectric charging field strength in most cases.In the study of surface charging,dielectric conductivity is inversely related to surface potential and secondary electron emission coefficient.