14th Spacecraft Charging Technology Conference
This study investigates very short duration (<1ms) flashes caused by rapid discharge arcs from charged insulating epoxy “glue dots” to an underlying grounded substrate. There were 36 separate ~3 mm diameter hemispherical bisphenol/amine epoxy “glue dots” positioned around the conducting polymeric/carbon nanocomposite substrate edge exposed simultaneously to nearly identical electron fluxes. Most arcs are found to be random events which occur when built up charge produces an electric field large enough for electrostatic breakdown to occur. The possibility that a given arc might stimulate arcs in adjacent “glue dots” was investigated through coincidence correlation analysis. The dependence of such correlations with “glue dot” separation was also studied.
The data for this project were collected at Marshall Space Flight Center. The epoxy “glue dot” samples were mounted inside a high vacuum (<10-4 Pa) chamber on a Black Kapton substrate attached to a large grounded metal plate, cooled with liquid nitrogen to ~120 K. An electron gun was used to bombard the sample with electrons of a known energy (12 to 40 keV) and flux density (0.3 to 5 nA/cm2), similar to what would be seen in a typical space environment. Light emitted from the samples was monitored with a high sensitivity visible to near-infrared (400 nm to 900 nm) CCD video camera,calibrated using a NIST traceable light source. Analysis of multiple regions for each glue dot and background regions of sequential frames after correction for stray light contamination, created an array of calibrated intensities (absolute spectral radiance) versus time for each region.
Arcs appear as localized phenomena, which are mostly random. Coincidence was defined by arcs which occurred within ± 1 frame (±33 ms) of each other. The correlation of coincidence events versus the distance between samples was tested. For low energies little to no correlation was observed; however, at 40 keV some correlation was observed. It is suggested that at higher energies more samples are charged close to the breakdown field at any given time. A discharge in one “glue dot” may cause a sudden spike in the electric field of neighboring “glue dots” which could trigger premature arcing. Such stimulated arc rates might reasonably be expected to scale with electric field intensity. If confined to a 2D surface the field would fall off inversely with separation distance, for charges spreading out across a 2D conducting surface. A power law fit to the arc data found a power of -1.06 ± 0.09, consistent with this model.
*Supported through funding from NASA Goddard Space Flight Center.
Christensen, Justin; Dennison, JR; and Dekany, Justin, "Temporal and Spatial Correlations in Electron-induced Arcs of Adjacent Dielectric Islands" (2016). 14th Spacecraft Charging Technology Conference. Posters. Paper 34.