Arifur R. Khan

Charge transport properties of dielectrics revealed by isothermal surface potential decay

Revealing the charge transport properties of space grade high insulation materials can benefit the mitigation of electrostatic discharges (ESD) on spacecraft. The charge transport properties of polyimide are investigated by an isothermal surface potential decay (ISPD) experiment under a simulated space environment chamber. After irradiated the sample by an electron gun, the 2-D surface potential distributions are measured by a non-contact potential probe. From the surface potential decay curves, we obtain current density at steady state ISPD. Analyzing the steady state current density against surface potential, we find two regimes. One is Ohmic regime and another is Space Charge Limited Current (SCLC) regime, which are separated at around -950 V with the sample thickness of 27 μm at 298 K. Ohmic resistivity and effective charge carrier mobility are calculated from these two regimes, respectively. In addition, the trap density of polyimide is derived from the SCLC theory, when the sample is charged to high initial surface potential.

Surface and volume charge transport properties of polyimide revealed by surface potential decay with genetic algorithm

It is very important to understand the surface and volume charge transportation properties of high insulating materials, such as polyimide, in order to find suitable method to mitigate the electrostatic discharge (ESD) of certain sensitive components on spacecraft. An isothermal surface potential decay (ISPD) experiment is performed inside a ground based vacuum chamber on polyimide under a simulated space environment. Immediately after low energy electron beam irradiation on polyimide, the 2D surface potential distributions are measured by a non-contact potential probe under five various temperatures from 298 to 338 K. The surface potential decay of the insulating material can be divided into two categories: transient process and steady state process. The steady state process is determined by the surface and volume charge transportation properties of dielectric. An ISPD model with genetic algorithm (GA) is developed to reveal the steady state surface potential decay experimental results. From the GA analysis, we obtain the surface resistivity, volume Ohmic resistivity, and charge carrier mobility of polyimide at various temperatures. After analyzing the surface and volume charge transportation properties of the material as a function of temperature, we find that the surface resistivity, volume Ohmic resistivity, and charge carrier mobility are well fitted with the Arrhenius law. Consequently, surface activation energy, volume activation energy, and trap energy of polyimide are found as 0.30 eV, 0.32 eV, and 0.54 eV, respectively.

Development of Mission Payloads Onboard High Voltage Technology Demonstration Satellite HORYU-II

High Voltage Technology Demonstration Satellite HORYU-II is a nanosatellite (30-cm cubic shape, 7 kg) developed by students at the Kyushu Institute of Technology. One of the objectives of this satellite is orbital demonstration of high-voltage technologies. The satellite with the highest voltage generation in low earth orbit (LEO) has been the International Space Station, generating 160 V. In orbit, especially LEO, the use of high voltage over 200 V induces arcing. HORYU-II is aimed to demonstrate new designs of solar array that can generate the power, free of arcs, by producing 300 V via a specially designed solar array itself, not via a conventional dc/dc converter. If successful, HORYU-II will become the first spacecraft in the world that achieves 300 V photovoltaic power generations in space. In this paper, we describe the detail of the mission payload development and verification. Preliminary flight results obtained since the satellites launch on May 18, 2012 to the sun-synchronous orbit at 680-km altitude are also briefly presented.

Charge transport properties of insulators revealed by surface potential decay experiment and bipolar charge transport model with genetic algorithm

To evaluate spacecraft charging level and predict surface and internal electrostatic discharging (ESD) probability, it is important to know the charge transport properties of high insulation materials, such as epoxy resin/glass composition (FR4), Teflon and polyimide. In present work, the charge transport properties of the space insulators are revealed by a bipolar charge transport (BCT) model combined with genetic algorithm (GA). It has been found that the BCT model can be used to simulate the experimental surface potential decay (SPD) results, and these two results are in good agreement with each other. The BCT model consists of charge injection, conduction, trapping, detrapping, and recombination processes. Stochastically initiating a series of charge transport parameters by GA, we can compute the SPD curves of materials by the BCT model. Used GA operates, the best fitting SPD curve of the experimental results can be obtained. From the comparison of the calculated and the experimental SPD results, we obtain the charge transport properties of FR4 and polytetrafluoroethylene (PTFE).

Effects of Ultraviolet Irradiation and Atomic Oxygen Erosion on Total Electron Emission Yield of Polyimide

Polymers used on low earth orbit (LEO) spacecraft surface suffer from an ultraviolet (UV) irradiation and orbital atomic oxygen (AO) erosion. These degradations may change the total electron emission yield (TEEY) of the materials and ultimately result in unexpected surface charging. In this paper, we chose polyimide (PI) film, a thermal control material, and carried out two types of ground-based degradation. The degradation methods were UV irradiation with five different equivalent solar hours, and AO erosion with two fluences equivalent to 0.5 and 1 year LEO flight time, respectively. Using an Auger microscope-based TEEY measurement system with a scanning measuring method, the TEEY of virgin and degraded PI films were tested and analyzed comparatively. The X-ray photoelectron spectrum, field-emission scanning electron microscope, and computational simulations were also used for element bonds analysis, roughness imaging, and electron depth calculation to discover the mechanisms of the degradation and their effects on the TEEY of the material.

Investigation Into Surface Potential Decay of Polyimide by Unipolar Charge Transport Model

To predict spacecraft charging levels and mitigate electrostatic discharges, it is very important to understand the charge transport properties of highly insulating materials such as polyimide. The combination of surface potential decay (SPD) experiment and unipolar charge transport (UCT) model investigates the charge transport properties of polymide. In a simulated space environment chamber, the SPD experiments are performed on polymide. After irradiated by electron beams, the surface potential distributions of the sample are measured by a noncontact potential probe (Trek). The UCT model that consists of relaxation polarization, charge injection, charge migration, charge trapping, and detrapping processes simulates the SPD results. In the numerical UCT model, we solve the charge continuity equation by Runge-Kutta discontinuous Galerkin method and Poissons equation by boundary element method. The least square error between the experimental and the simulated SPD results is searched by genetic algorithm (GA). From the GA calculation, we optimize the charge transport parameters of polyimide. Then, we calculate the space charge evolution properties during the SPD process of polyimide by the UCT model. These charge transport parameters, then, can be used to study the surface charging or deep dielectric charging of the insulator.

Development of Electron-Emitting Film for Spacecraft Charging Mitigation

Prevention of spacecraft charging and discharging has become increasingly important as geostationary Earthorbit satellites employ higher bus voltages. There are numerous mitigation techniques against spacecraft charging, including electron emission from the spacecraft chassis. A new electron emission device operating in a completely passive manner has been developed, which uses the field enhancement at the triple junction where the interface of metal and insulator is exposed to space. It has been named electron-emitting film for spacecraft charging mitigation (ELF’S CHARM). Microetching was applied to polyimide-copper laminated film to manufacture a laboratory prototype. This prototype ELFmaintains the emission current at the steady state from the triple junctions instead of leading to arcing. The electric field at the triple junction is macroscopically enhanced by charging the polyimide film and microscopically by dielectric impurities on the copper surface. The laboratory experiments confirmed a stable current emission from 10 to 100 A for 4 hr from a 5-mm square sample having a 500m microetching pattern. Recently, the endurance of this ELF design has been confirmed by 100 hr of accumulated emission testing.

Environment Exposure Tests of Electron-Emitting Film for Spacecraft Charging Mitigation

A new electron-emitting device operating in completely passive manner has been developed to prevent spacecraft charging and discharging. It is named as electron-emitting film (ELF) for spacecraft charging mitigation. This emitter (ELF) utilizes the field enhancement at the triple junction formed at the interface where metal and insulator are met and exposed to vacuum. ELF emits prebreakdown emission current that might lead to arcing at the triple junction. Hence, it balances the input and output currents to the spacecraft during substorm. After ensuring the robustness of this ELF against ground handling and in-orbit contamination, laboratory experiment confirmed continuous electron emission for 100 accumulated hours to assure the endurance. In this paper, its durability against high-energy electron and proton irradiation equivalent to ten solar years in GEO is reported. Effect of heat cycling (-150 °C-100 °C) and vacuum ultraviolet irradiation in GEO on this emitter is also completed. Postemissions of this ELF confirm the durability under those harsh space environments.

In-Orbit Demonstration of Newly Developed Passive Electron-Emitting Film for Spacecraft-Charging Mitigation

Recently, a passive electron-emitting film has been developed for the purpose of neutralization of all types of spacecraft charging. Applications are inclusive of satellites with high bus voltage in geostationary Earth orbit. This passive-type component emits electrons continuously from its triple junction under an inverted potential gradient condition after being exposed to a geostationary Earth orbit substorm. In this paper, the verification test has been conducted after designing circuitry that measures the surface potential as well as the emission of the film. In the ground test, the emitter was biased to a high negative potential with respect to the vacuum chamber to simulate the spacecraft potential achieved while floating in space plasma. The substorm condition and the satellite charging are simulated by irradiating the emitter with an electron beam. Under these conditions, the performance of the emitter has been confirmed via the developed circuitry. These results allow the flight demonstration of...

Charging and Discharging Phenomena of Plastic Encapsulated Microcircuits Used in Nanosatellites

Nanosatellites are generally used in low Earth orbits for diverse applications such as remote sensing, space weather monitoring, technology demonstrations, and even system engineering education. During their orbit lifetime, they often fly through the polar region and/or the South Atlantic Anomaly zone. Electrons at these regions are sufficiently energetic enough to penetrate into the thin aluminum exterior shield of nanosatellites to reach the interior electronics. Hitting these electrons may cause differential charging on electronics that is ultimately responsible for internal electrostatic discharge. On the ground, energetic electron beam experiments are conducted on the plastic encapsulated microcircuits samples that reveal electrical charging and discharging on the electronics residing inside the nanosatellites. Conformal-coated electronics show effective charging mitigation due to high energetic electrons.