Study On Surface Charge Accumulation Characteristics Of Polyimide Under Electron Irradiation

The spacecraft is irradiated by charged particles in space during normal operation.The dielectric material on its surface will cause electrostatic discharge problems,although it has good insulation properties.Because the material has a large resistivity,it is difficult for electrons to escape after being injected into the material and eventually stay inside it,which causes the surface of the material to be charged.The spacecraft will also encounter different temperature environments during normal operation,which makes its surface potential uneven.It will also distort the local electric field on the surface of the material and this will increase the risk of electrostatic discharge.Therefore,it is of great significance to study the surface charge accumulation and trap characteristics of dielectric materials under electron irradiation.According to the operating environment and experimental requirements of the spacecraft in low earth orbit,this paper designs and manufactures an electron gun and a surface potential measurement module,which are based on the principle of thermionic emission and the principle of the probe method.The vacuum and temperature control system has been improved and the surface potential measurement system of the dielectric material has also been constructed under electron irradiation.Finally,it was debugged and verified.The system can achieve continuous adjustment of electron energy(0～7.4 keV)and current density(0～1.40 nA/cm2),control of sample temperature,and measurement of surface potential in one-dimensional space.As a commonly used dielectric material for spacecraft,polyimide is used as a research object.The distribution of surface potential was tested when the charge is accumulating at different electron energies(1.6 keV,3.3 keV,4.9 keV,6.6 keV and 7.4 keV),current density(0.07 nA/cm2 and 1.40 nA/cm2)and temperature(243 K and 343 K).The test results show that the saturation time of the surface potential is positively correlated with electron energy and temperature and negatively correlated with current density.The saturation potential is positively correlated with electron energy and current density and negatively correlated with temperature.The electron energy,current density,and temperature all make the surface potential present a low potential at the center of the irradiation and a high potential at the edge of the irradiation.The trend becomes more obvious as their value increases.By calculating the electron incidence depth,injected charge density and energy loss,the accumulation characteristics of surface charges under electron beam irradiation are analyzed.The surface potential distribution was tested when the charge dissipated under different electron energy(1.6 keV and 7.4 keV),current density(0.07 nA/cm2 and 1.40 nA/cm2)and temperature(243 K and 343 K).And the trap parameter distribution is calculated by the Surface potential Decay Method under different experimental conditions.The electron energy,current density,and temperature affect the charge accumulation characteristics mainly by affecting the electron incidence depth,the number of injected electrons,and the trapping,detrapping,and migration of charges.The higher the electron energy,the greater the electron incidence depth and distribution range.Then the injected charge is more easily captured by the deep trap,which increases the density of the charge.The greater the current density,the more charge is captured by the trap.The increase in temperature increases both the intrinsic conductance and the radiation-induced conductance of the material.The trapped charges are more likely to detrape after being thermally excited and migrate to the ground electrode under the action of the self-built electric field.