Spacecraft Dielectrics Research Articles

Influence of electron beam irradiation on DC surface flashover of polyimide in vacuum

A satellite operates in a harsh environment including high vacuum and radiations. The vacuum-dielectric interface is the most vulnerable area to flashover discharge. Moreover, the presence of electron radiation will further decrease the flashover voltage. However, the influence of electron radiation on surface flashover remains unclear. We have performed a study on vacuum DC surface flashover during electron beam irradiation as a function of electron kinetic energy and incident direction on polyimide representative of spacecraft dielectrics. The electron radiation was accompanied by electron deposition and bombardment effects on the polyimide surface. The surface flashover voltage was experimentally found to significantly depend on the kinetic energy and incident angle of the electron beam. By calculating the trajectories of incident electron beam, we further found that the surface flashover voltages are closely connected to the irradiated area. During low energy electron beam irradiation, kinetic electrons are repelled away from the dielectric surface by the applied electric field, and deposited electrons will be dominant in the flashover process, promoting the surface flashover voltage. However, when the electron beam energy is high, kinetic electrons can overcome the applied electric field and strike the dielectric surface to generate secondary electrons, which will replace the field-emission electrons to serve as seed electrons, and initiate the flashover at a much lower applied voltage. These observations and analyses are expected to benefit the research into mitigating spacecraft discharge, and promote the knowledge of surface flashover.

ESD Propagation Dynamics on a Radially Symmetric Coupon

An increasing body of research has been dedicated to determining the magnitude and scaling of spurious electrostatic discharge current transients on solar arrays and other spacecraft dielectrics under inverted gradient charging conditions. A flashover propagation velocity is typically determined, and assumed to be a single constant number that characterizes the collection of events. The complexity of flight-like test articles and diversity of methods used for deriving this velocity from laboratory data have led to apparently conflicting results. In this paper, we attempt to clarify the radial dynamics of electrostatic discharge (ESD) plasmas on a positively charged surface via the use of a simple test article, composed of eight concentric annular electrodes covered with Kapton. Time-resolved induced ESD currents, recorded as a function of electrode radial distance from the inception point, were used to derive empirical velocity distributions. These distributions lead to a number of effects, such as $\sim 1/r^{2}$ scaling with distance at the beginning of a discharge. Implications for ESD propagation and comparison with previous studies are discussed.

Monte Carlo calculations of characteristic quantities of low-energy electron irradiation to spacecraft dielectrics

The calculations of the characteristic quantities of low-energy electron (⩽20keV) irradiation to the five typical spacecraft dielectrics, i.e. epoxy, kapton, mylar, polyethylene, and teflon, have been performed by means of Monte Carlo method. These characteristic quantities include the electron backscattering coefficient, the depth distributions of both energy deposition and deposited electrons, and the maximum penetration depth of deposited electrons in the dielectrics. A Monte Carlo model has been specifically constructed for simulating the transport of low-energy electrons in spacecraft dielectrics (organic materials). In this model, the description of the inelastic scattering of energetic electron is based on the dielectric approach developed previously and the Born–Ochkur’s exchange correction is included. Especially, the optical energy loss functions of organic materials can be obtained using an empirical evaluation. In addition, the mean cross section based on the Mott model is proposed for calculating the elastic scattering of energetic electrons in organic materials for high simulation efficiency. The constructed Monte Carlo model has been examined by a series of calculations and comparisons with the reported experiments and other theoretical results. For the dielectrics under consideration and in the energy range of E0⩽20keV, the calculated electron backscattering coefficients and the extrapolated range of deposited electrons are listed at selected energies in numerical form for convenient use, and an empirical expression of estimating the extrapolated range in the energy range of 1–20keV is given. The distribution characteristics of both energy deposition and deposited electrons are presented, and it is found that kapton and mylar present the close characteristic quantities for each other, which is of significance for the choice of the dielectrics in design of spacecraft. The characteristic quantity calculations presented in this work are a first attempt and may be useful for studies of the surface charging and the internal dielectric charging on spacecraft.

Experimental Research and Numerical Simulation on Deep Charging for Spacecraft Dielectrics

空间辐射环境中高能电子诱发的介质材料深层充放电效应是威胁航天器安全的重要因素之一. 本文采用不同束流强度的电子枪电子, 研究了不同厚度的聚酰亚胺薄膜的深层充电过程; 利用Sr90放射源电子模拟GEO轨道高能电子环境, 研究了在其辐照下聚甲醛树脂和聚四氟乙烯材料的表面电位变化; 实验观测了深层放电产生的电流脉冲和电场脉冲. 提出了深层充电模型, 较好地模拟了实验测量结果, 并且分析了深层充电平衡电位和平衡时间随电子束流强度和介质电阻率的变化规律. 实验和数值模拟结果初步揭示了深层充放电效应的特征及规律, 表明深层充电现象随着电子束流强度和介质电阻率的增加而趋于明显, 介质电阻率是影响深层充电平衡电位和平衡时间的主要因素.

Research on Simulation of Deep Charging for Spacecraft Dielectrics

针对航天器介质深层充电问题,提出了一种基于蒙特卡罗模拟和充电动力学RIC模型的介质电荷分布及电场预估新方法,利用地面试验验证了其正确性.航天器介质平板充电过程被简化为屏蔽铝板与分层介质组成的Geant 4模型,通过统计方法计算出了实际入射束流下Teflon介质内的注入电流密度和剂量率分布曲线,利用RIC模型获得了背面接地时介质中的电荷密度和电场分布,利用脉冲电声法(PEA)对不同束流密度辐照下的Teflon内部空间电荷密度进行了测量.数值模拟和地面试验结果表明,Teflon在100 keV能量电子辐照下,电荷密度和电场随着束流密度的增加而不断增大,其电荷密度峰值位置约为0.042 mm,且背面接地时接地侧电场最大.由于Geant 4粒子输运模拟和RIC模型具有通用性,因此该方法适用于各种航天器介质材料.

ARC propagation, emission and damage on spacecraft dielectrics: A review

This paper is a review of the literature on the subject of arc discharges on spacecraft dielectric materials that have become charged by energetic electrons and ions. The results discussed will be mainly from laboratory measurements of the properties of these arc discharges. Arcs resulting from the charging of spacecraft dielectrics can be extremely strong because the charge over a large area is mobilized through the phenomenon of arc propagation. The resultant damage patterns on the dielectric will be shown to be related to the arc patterns, and to the optical anisotropy and crystallinity of the material. The evidence for dielectric melting is suggestive of likely contamination of nearby surfaces. The effectiveness of arc barriers sheds light on arc propagation mechanisms. Coupling to external circuits can be deduced from blowoff calculations and from nearfield measurements of arc currents. Exposed dielectric edges contributed to charge accumulation and arc triggering. Broad-spectrum, high-energy incident electrons can cause charging and discharging internal to the spacecraft.

The Discharge of Spacecraft Dielectrics and the Theory of Lichtenberg Figure Formation

A Lichtenberg discharge model for the discharge of thin spacecraft dielectrics is considered. Conducting plasma channels are assumed to exist within the dielectric. Utilizing energy balance arguments it is concluded that it is unlikely that the model can account for observed large area discharges. It is likely, however, that Lichtenberg discharges do account for observed material damage associated with all discharges.

Electrical Conductivity and Discharge in Spacecraft Thermal Control Dielectrics

Engineering data on the radiation-induced and delayed conductivity in several common spacecraft dielectrics under conditions that approximate space exposure is presented. Direct measurements of discharge propagation velocity on Kapton, Teflon and Mylar is reported. Glass, and occasionally Mylar, exhibited discharges which propagated faster than 2 × 108 cm/s, but persist at ≈10-7 s.

Internal Breakdown of Charged Spacecraft Dielectrics

The primary purpose of this paper is to suggest that low differential voltage, small energy discharges associated with internal buried charge may be a dominant mechanism by which stored electrostatic energy is released from dielectrics on board orbiting spacecraft. The evidence from space as given by Stevens is noted. Laboratory experimental evidence of Frederickson is invoked to demonstrate that discharges occur under circumstances with no external potential drop. Previous calculations are then reviewed which indicate that significant internal electric fields can exist in dielectrics charged with multiple-kilovolt electron beams under conditions involving little or no external potential drop. The internal discharge mechanism of Meulenberg is recalled, and new calculations suggesting that the space environment is conducive to the formation of the conditions required by this mechanism are presented. Experimental procedures for checking the suggestions of the paper are developed.

Experimental Verification of an ECEMP Spacecraft Discharge Coupling Model

In the space environment, spacecraft dielectrics can be charged to potentials high enough to result in discharges. JAYCOR has performed several studies to characterize discharges on spacecraft materials and components. This characterization data was used to formulate a semi-empirical model of the coupling of discharges to spacecraft structures. This paper presents the results of a joint analytical and experimental program in which this model was tested. The results described here show that the model accurately predicts discharge coupling for simple geometries, but that additional physical processes must be considered for the model to be able to predict the response of reentrant geometries.

The Effects of High-Energy Electrons on the Charging of Spacecraft Dielectrics

Results are presented of a laboratory investigation of discharges in spacecraft dielectrics using both a low-energy electron and a combined low- and high-energy electron environment. Comparison of the charging and discharge characteristics indicates that the presence of high-energy electrons in space will result in significantly different behavior from that predicted for low-energy electron simulations alone. For Kapton, fiberglass, and second-surface mirrors, the effect of the high-energy electrons is primarily to increase the conductivity, thus often limiting surface potentials to below discharge threshold values. Variation of the relative fluxes of low- and high-energy electrons can lead to discharge; however, the magnitude of the charge blowoff is significantly smaller (~1/20) than that observed upon exposure to low-energy electrons alone. Astroquartz, which for low-energy electron exposures exhibited no measurable charge blowoff, exhibited significant charge blowoff (Ip = 2 amp) upon exposure to a combined high- and low-energy electron environment.

Spacecraft Charging Effects by the High-Energy Natural Environment

Charging of dielectrics deep within spacecraft structures by the high-energy portion of the electron environment is investigated. Order-of-magnitude estimates of charging rates and saturation fields are made for spacecraft dielectrics and compared with breakdown thresholds. Results indicate that radiation-induced and natural conductivities of several spacecraft dielectrics are sufficiently low to permit charge accumulation to the point of dielectric breakdown. It is concluded that the high-energy portion of the natural environment can produce discharges and reasonably large replacement currents within the spacecraft structure and cables. Certain of the possible mitigation techniques proposed for spacecraft charging problems arising from the lower-energy portion of the environment may not be effective in protecting the spacecraft from the high-energy electron charging.