Allen Andersen

Measurements of Endurance Time for Electrostatic Discharge of Spacecraft Materials: A Defect-Driven Dynamic Model

Electrostatic breakdown leads to the majority of anomalies and failures attributed to spacecraft interactions with the plasma space environment. It is therefore critical to understand how the electrostatic field strength (FESD) of spacecraft materials varies due to environmental conditions, such as duration of applied electric field, rate of field change, history of exposure to high fields, and temperature. We have developed a dual-defect, thermodynamic, mean-field trapping model in terms of recoverable and irrecoverable defect modes to predict probabilities of breakdown. Fits to a variety of measurements of the dependence of FESD of insulating polymers on endurance time, voltage ramp rate, and temperature based on this model yield consistent results. Our experimental results for the prototypical materials, low-density polyethylene and polymer (PI or Kapton HN), suggest that the values of FESD from standard handbooks, or cursory measurements that have been used routinely in the past, substantially overestimate the field required for breakdown in common spacecraft applications, which often apply subcritical fields for very long time periods as charge accumulates.

Pre-breakdown arcing as proxy for DC dielectric breakdown testing of polymeric insulators

Pre-breakdown arcing is proposed as a key indicator of DC breakdown properties of polymeric dielectric materials. Voltage step-up tests were performed for films of low-density polyethylene (LDPE) at ramp rates significantly lower than is common in most studies, allowing for the observation of both breakdown and transient pre-breakdown current spikes. The distributions of the breakdowns versus applied field were compared with the more frequent pre-breakdown arcs. A strong correlation was observed for LDPE between the distribution of breakdowns in step-up tests and the distribution of pre-breakdown arcing. Pre-breakdown arcing distributions are much easier to obtain than breakdown distributions and may be an efficient indicator of the minimum field at which breakdown can occur, leading to accelerated test methods. The possible physical origins underlying pre-arcing and its relation to breakdown distributions are discussed.

Voltage Ramp-rate Dependence of DC Breakdown in Polymeric Insulators: Physical Models versus Data

The standard handbook values for dielectric breakdown strength of necessity come from accelerated test methods. In some cases, the breakdown voltage may vary significantly with voltage ramp rates; therefore, a theoretical model for the ramp-rate dependence of breakdown is needed to extrapolate from realistic tests to long-duration material service lifetimes. Series of step-up to breakdown tests were performed for ramp rates from 0.5 to 500 V/s for biaxially-oriented polypropylene (BOPP), low density polyethylene (LDPE), and polyimide (PI) films. The data were fit with standard empirical methods, as well as two physics-based defect-driven models. Empirical models can be fit to a given data set; however, they offer little-if any-physical insight. Voltage ramp rate can, in some materials, significantly affect the breakdown field. However, the physical models, one of which was successful in describing preliminary data, were shown to be inadequate for the other materials tested.