**1. Introduction**

The charging and discharging of dielectric materials under space radiation environment are the main factors that cause anomalies in a spacecraft. Koons et al. counted the abnormal failures of the spacecraft, suggesting that 54.2% from the total 299 cases were caused by the charging and discharging of dielectric materials [1]. A spacecraft is inevitably exposed to space plasma, energetic particles radiation, extreme temperature, cosmic rays, etc. [2]. A situation has to be taken into consideration that partial accumulation of space charges and high electric field occur when energetic electrons penetrate through the aluminum shield and deposit in the surface or deep layer of insulating materials. When the maximum electric field of

insulating material exceeds a certain threshold, electrostatic discharge (ESD) will occur. Consequently, it will lead to the deterioration of insulating materials and even the failure of the whole electronic equipment. With the rapid increasing interest on space exploration, several countries are making efforts to build a Space Solar Power Station (SSPS) with megawatts or even gigawatts [3]. The reliability of the spacecraft becomes a very important problem. Polyimide is widely used in spacecraft system because of its good insulating, mechanical and antiaging properties [4]. Therefore, the charging and discharging mechanism of polyimide under electron irradiation and high voltage is a research focus in the field of spacecraft reliability.

Surface dielectric charging and deep dielectric charging are two kinds of dielectric charging, which are divided by the incident electron energy range and discharge position [2]. Surface dielectric charging refers to the deposition of low-energy electrons (e.g., 1–50 keV) on the dielectric surface and the induction of surface potential, while the deep dielectric charging refers to the penetration of highenergy electrons (e.g., 0.1–10 MeV) from the dielectric surface, deposition within the insulating materials and establishment of internal electric field [5]. Modeling the dielectric charging based on secondary electron yield, surface potential decay processes and characteristic parameters is the research focus in surface dielectric charging [2, 6–10]. While for deep dielectric charging, the charge transport properties of insulating materials irradiated by energetic electrons are key issues, and several models have been proposed to investigate it [11–14]. There are two types of typical models: the radiation-induced conductivity (RIC) model and the charge generation-recombination (GR) model. RIC model describes the transport processes of electrons in insulating materials under the irradiation of electron beam. It is a macroscopic model in which the parameters are given by the measurement of radiation-induced conductivity [14]. GR model describes the generation and recombination processes of electron-hole pairs in insulating materials. It is a microscopic model in which some specific parameters are difficult to be determined.

Charge behavior on the dielectric surface layer or the deep layer under electron irradiation has an important influence on discharging properties. As to DC surface flashover, it implies that the essence of surface flashover is the charge transport behavior across gas-solid interface under high electric field, which involves charge trapping and de-trapping properties in dielectric surface layer, secondary electron emission properties, impact ionization of gas molecules and electron multiplication properties in gaseous phase (or desorbed gas). The development process and formation of surface flashover is a coupling effect of the above factors. The vacuum surface flashover voltage of dielectric material irradiated by electrons is much lower than that in vacuum or gaseous atmosphere. At present, several theories have been postulated to explain the surface flashover phenomenon in vacuum, among which the theory of secondary electron emission avalanche (SEEA) is dominant [15]. The flashover of insulating material in vacuum under electron beam irradiation is also closely related to the field-emission electrons emitted from the cathode-dielectricvacuum triple junction (CTJ) and secondary electrons (SE) [16]. A large number of experimental studies emphasize the effects of deposited charges in the dielectric surface layer, while few data can be obtained about the effect of kinetic electron from the electron beam on surface flashover [17]. On the aspect of dc electrical breakdown mechanism of polyimide, it has been proven that under the action of a high electric field, charges are injected into the insulating materials, and space charges are accumulated [18–20]. The electric field distortion appears inside the insulating materials caused by the accumulated space charges. When the maximum local electric field exceeds a threshold value, the electrical breakdown will occur [19, 21].

In this chapter, the charging and discharging phenomena of dielectric materials under electron irradiation environment were introduced. The electrical properties

*Charging and Discharging Mechanism of Polyimide under Electron Irradiation and High Voltage DOI: http://dx.doi.org/10.5772/intechopen.92251*

of polyimide were investigated. The surface and deep charging process and model of polyimide radiated by electrons were analyzed. Then, the experimental results of DC surface flashover during electron irradiation with different energies, fluxes and incident angles were investigated.
