2003 IEEE Conference on Electrical Insulation and Dielectric Phenomena
Several methods have been combined to measure conductivity and charge storage in insulating spacecraft materials. In order to avoid insulator problems, the motions of conducting electrons and holes must prevent the development of large electric fields exceeding 1E5 V/cm, where problems occur in spacecraft insulators. Approximate knowledge of the electric fields is important. One must consider generation of mobile electrons and holes, their trapping, thermal de-trapping, mobility and recombination.
Classical methods to measure thin film insulator conductivity apply a constant voltage to two electrodes on the sample and measure the resulting current for tens of minutes. Under constant voltage, a gradually increasing dielectric constant may occur to produce a polarization current that may be misinterpreted as a conduction current. For spacecraft charging, conductivity is actually measured as the "decay" of charge deposited on the surface of an insulator. Therefore, conductivity values based on classical methods are inappropriate for such applications.
Our methods use a metal electrode on one side of the insulator, and, in vacuum, expose the other side to sequences of charged particles, light, and plasma. Currents to the electrode are monitored. The most interesting data are obtained by capacitive coupling to measure both the resulting voltage on the open surface and emission of electrons from the exposed surface. Also interesting are: the effect of varying electron beam energies (1-75 keV) on the resulting time dependent surface voltage, and lightactivated removal of surface voltage. A primary component of the methods described in this paper is a long time duration for the measurements, weeks to months. In space the insulating materials are charged by radiations that slowly vary during the life of the spacecraft.
We find that dark conductivity is orders of magnitude smaller than classical ASTM and IEC methods find, that penetration profiles of radiation and light are exceedingly important, and high-field conduction by trapped electrons must be considered for space applications.
Frederickson, A. R. and Dennison, JR, "Measurement of Conductivity and Charge Storage in Insulators Related to Spacecraft Charging" (2003). 2003 IEEE Conference on Electrical Insulation and Dielectric Phenomena. Conference Proceedings. Paper 30.