All Physics Faculty Publications

Document Type

Article

Journal/Book Title/Conference

Proceedings of the 12th International Spacecraft Charging and Technology Conference

Publication Date

2012

Abstract

The conductivity of the material is a key transport parameter in spacecraft charging that determines how deposited charge will redistribute throughout the system, how rapidly charge imbalances will dissipate, and what equilibrium potential will be established under given environmental conditions. As the requirements for space missions extend to new regions of space and more stringent requirements are placed on spacecraft performance, it becomes necessary to better understand the underlying conduction mechanisms that determine the dynamic response of insulators to temperature, electric field dose rate, and sample conditioning and history. This study performed detailed measurements of the transient conductivity of representative highly disordered insulating materials using the constant voltage method and analyzed the data with dynamic models for the time, temperature, and electric field dependant conductivity. We describe substantial upgrades to an existing Constant Voltage Chamber (CVC), which improved the precision of conductivity measurements by more than an order of magnitude. A battery operated voltage source supplied a highly stable applied voltage. Data acquisition and analysis algorithms and the interfaces between electronics and the data acquisition system were optimized for higher precision and accuracy. Painstaking attention to ground loops, shielding, filtering and other associated issues greatly reduced electrical noise in the extremely low (<0.2 fA) current measurements. . Mechanical systems, including vacuum and cryogenic equipment, were also modified to eliminate excessive noise. To insure sufficient, uniform and repeatable contact between the electrodes and the sample surface, an adjustable spring clamping mechanism adhering to ASTM D 257-99 standards was added that maintains electrical isolation between the electrode plate assembly and the cooling reservoir; this system also significantly reduced uncertainties associated with contact area reproducibility. Stable measurements can now be made over temperatures ranging from 100 K to 400 K. At room temperature and above and at higher applied voltages (approaching typical breakdown potentials of thin film samples of ~2-6 keV at fields of >50 MV/m), the ultimate instrument conductivity resolution can increase to ≈4•10-22 (Ω-cm)-1 corresponding to decay times of more than a decade; this is comparable to both the thermal Johnson noise of the sample resistance and the radiation induced conductivity

Comments

Paper presented at the 12th International Spacecraft Charging and Technology Conference. PDF available for download through link above.

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