Diagnostics of Electrostatic Discharges and Postulations for ARC Mitigation Techniques on Satellite Solar Panels

Abstract

On-orbit measurements claim the presence of low density plasma in Lower Earth Orbit (LEO) and high energy electron clouds in Geo-synchronous Earth Orbit (GEO). Reportedly observed failures in various satellite components operating at higher voltage values are serious issues for Space communities. The maximum value of power generated by the solar panel is dependent on the type and configuration of solar cells laid on it. Charged species in space are responsible for building up static voltages on external satellite components at different potentials based on their dielectric properties. Proximity between two charged surfaces at different potentials results into an electrostatic discharge (ESD) or arc. An arc is defined as rapid displacement of charge either by punch-through effect, flashover propagation or release of blow-off current between surfaces and surrounding environment (nanosecond to microsecond). Arc signals are similar to transient current responses with rise time between micro to nanoseconds. According to a survey conducted by Frost and Sullivan, 33 % of satellites fail in their operations due to multiple arcs on satellite solar panels. Regions of intersection between metal (interconnect) - insulator (coverglass), metal - semiconductors (PN layers of solar cells) or insulator - semiconductor and vacuum are most probable locations for ignition of arc. These points are referred as triple junctions. A primary arc (PA) on solar cells, which dissipates the charge stored on the insulating surface in the surrounding medium affects its electrical output. If an arc gains energy from neighbouring solar cells, it is termed as sustained arc. Long duration sustained arcs termed as permanently sustained arcs (PSAs) are most destructive for the solar panels. These arcs burn entire surrounding region due to high amount of heat released during the process. Physical damages like shorting of PN junctions on the solar cell, cracks on the coverglass surface and breaking of reverse diodes are observed due to discharges. Damage to the solar cells could be most disadvantageous as it is followed by the improper functioning of other satellite components due to limited or non-availability of power. Arcs on solar panels disturb satellites’ electrical systems either by direct current injection or by induced currents due to the associated surface flashover wave. In order to protect the satellites from such situation, a pre-launch test under which solar panels are exposed to space-like environment is essential. This test would ensure the safe working ranges for satellites throughout their planned lifecycle. Thus need to develop a test facility for exposing satellite solar panels in simulated space environment, creating arcing conditions and analysing the arc signals in ground laboratories is realised for LEO and GEO environments. This thesis primarily focuses on advanced data acquisition and analysis system, proposal of arc mitigation techniques and development of an arc location predictor algorithm. Features like automated arc acquisition and categorization process, improved arc image capturing rate and algorithm for extended PSA durations are incorporated in the DAQ system. To ascertain generation of uniform differential charge on the substrate’s surface, a flood beam type electron gun is used in GEO environment. This increases the reliability of the experiments performed in the facility. A real-time controller is used to acquire electrical and optical signals during arc. This eliminates the traditional need of comparing these signals manually. Major researchers experience limitations in capturing analogous optical arc image for all the current transients observed in electrical measurements. CCD camera and the corresponding algorithm incorporated in this work ensures improved arc capturing rate. Storage memory in the computer system is allotted to meaningful arc frames thus making optimized use of it. Offline analysis is performed for identification of arc duration and amplitude on the electrical signals. An algorithm for identifying most appropriate arc frame, from the bunch of frames is designed to work offline. Provision for measurement of extended PSAs is also incorporated. Arc electrical signal having maximum duration of 1.4 seconds with the sampling rate of 20 mega-samples per second can be measured. This data is stored within the specified memory size allotted to it. Post-processing algorithm is developed within the frameworks of computation time and storage memory. V-model approach is applied to program all the DAQ modules using LabVIEW graphical programming environment. Thus an integrated facility for arc data acquisition and analysis is developed and validated. Various primary and sustained arc experiments are performed on actual and artificial triple junctions. The main purpose of these experiments is to validate the efficiency of the developed facility and propose a mitigation technique against destructive PSAs. Apart from electrical and optical signal, surface potential is also measured in GEO experiments. Based on observation obtained after analysis of the experiments, calculations for arc threshold voltage are done. This provides a primary idea about the safe voltage level for operating solar panels populated with given type of solar cell and layout configuration. Sustained arc tests, which state the probability of initial transient in current signals to continue for longer durations are also performed. As Indian satellite solar panels are fabricated in ISRO, this test is the final benchmark for defining the safe operating limits in actual space environment. Two mitigation techniques proposed in this thesis are reduced usage of grouting material and re-arrangement of solar cells on the solar panels. In the proposed configuration, enhancement of electric field at the sharp solar cell corners is regulated. This increases arc threshold voltage. Unlike other mitigation techniques which deal withmodifications in the construction of solar cells or use of new coating material, the proposed mitigation techniques are easy to incorporate in the existing facilities. Based on the vast database generated due to diversity in experiments, a model for predicting arc location on a two dimensional surface is also proposed. This goal is achieved by comparing the obtained experimental arc signal with the pre-existing library waveforms. Each waveform in the library corresponds to a specific arc location. The best match found in the comparison predicts the location of the arc signal. This model uses image processing as its backbone. Apart from arc location, arc velocity and surface propagation region can also be estimated. A decelerating velocity is considered in the simulations. Appendix A contains a library generated by the model for identification of arc initiation region.