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12th Spacecraft Charging Technology Conference 14-18 May, 2012 Kitakyushu Japan

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Measurements of the electrostatic field strength of thin film insulating materials due to interactions with the space plasma environment are one of the most important concepts to understand for the effective design of spacecraft. It is therefore critical to understand how electrostatic field strength (FESD) of spacecraft materials varies due to environmental conditions such as temperature, duration of applied electric field, rate of field changes, and history of exposure to high fields. This research STUDY emphasizes experimental and theoretical investigations on the FESD of polymeric insulators as a function of temperature, applied field, and time to breakdown. It suggests that values of FESD from standard handbooks or cursory measurements that have been used routinely in the past by the spacecraft charging community substantially overestimate the field required for breakdown in common spacecraft situations.

Electrostatic discharge tests for two prototypical materials with different types of trap state densities—low density polyethylene (LDPE) and polyimide (PI or Kapton HNTM)—were conducted by applying a high voltage across the material in parallel plate geometry using a modified ASTM D149-99 method. FESD was determined as a sustained, rapid rise in I-V curves measured in a custom, high vacuum chamber (50% below the nominal breakdown voltage. Ramp rate dependence was found by varying the rate and magnitude of the incremental voltage step used to reach electrostatic breakdown. Slower ramp rates (as low as 20 V steps at 4 s intervals) resulted in substantially lower FESD values than tests conducted the maximum ramp rate of 500 V/s recommended in ASTM 3755 standards. Very short, unsustained arcing was also observed beyond a material dependant threshold electric field; the frequency of these arcs was field dependant. For LDPE, a small linear temperature dependence of FESD for LDPE was observed in the range of 150 K to 240 K. Above 240 K there was a shift in FESD near a structural phase transition in LDPE, at which other electrostatic properties have also exhibited discontinuities.

These experimental results are compared with thermodynamic mean field multiple trapping models of the electric field induced aging process and available measurements. Numerous studies have shown that electrical aging can be characterized by defect creation within the material from bond stress due to local and applied electric fields. We introduce a modified dynamic temperature-dependent electrostatic discharge model which accurately predicts observations of two distinct regions of negative logarithmic decay of the endurance time as a function of applied field, which are attributed to separate recoverable and irrecoverable defect mechanisms. Relevant examples of both recoverable and irrecoverable defect mechanisms are discussed; these are characterized by two parameters, a mean separation of sites and an activation energy. The interdependence of these mechanisms may explain the unusual transition observed in the crossover field region between the mechanisms. The observed frequency of the short unsustained arcs is consistent with the recoverable defect part of the model. We further discuss these results in terms of a more comprehensive unified theory for electron transport in highly disordered insulating materials, which allows a correlation between fitting parameters and more fundamental materials properties such as atomic scale structure and bonding, mobility, transition probabilities, and spatial and energetic distributions of trap states.


Presented at the 12th Spacecraft Charging Technology Conference 14-18 May, 2012 Kitakyushu Japan

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