Dielectric surface flashover in a simulated low earth orbit environment
Virtually all concepts for the use of electrical energy at high voltages (for commercial, scientific or military applications) must utilize solid dielectrics as insulators. In general, surface flashover on insulators sets the limit on the applicable voltage of the system. For practical applications, scaling laws (i.e., empirical curves relating the flashover voltage to the gap distance) were formulated which describe the simplest cases in ideal geometrical configurations on the design of high voltage systems is usually based on unnecessarily high safety factors with severe limitations on high voltage amplitudes and power densities in a system. Understanding the mechanisms of surface flashover in the space environment is critical in increasing the dielectric flashover voltage by applying, for example, electric and magnetic shielding techniques. For this investigation, an experimental apparatus was designed and constructed as a coaxial system in order to operate electrical and optical sensors with high temporal resolution. Current, voltage, and soft x-ray emission are recorded by highly sensitive sensors which have a risetime on the order of one nanosecond. The plasma is generated by an electron cyclotron resonance plasma source and the ultra violet radiation originates from a UV enhanced Xenon arc lamp. This experiment is the first to investigate the early phase of dielectric breakdown in a simulated low earth orbit environment. The results show that the breakdown mechanisms are drastically altered with a plasma and UV environment, compared to the "pure" vacuum case. Dielectric breakdown with a plasma background shows a lower breakdown voltage and a different current amplification mechanism in the early phase of flashover, compared to measurements in vacuum without plasma. With UV illumination, the dielectric flashover voltage amplitude decreases and the pre-flashover current increases at a slower rate, compared to results without UV Low amplitude magnetic fields were applied and several magnetic insulation or shielding techniques were tested. Increases of the breakdown voltage amplitude of up to a factor of four were found with magnetic insulation for the UV case. With a plasma background, the duration of an applied voltage pulse can be increased by a factor of 2 without causing flashover compared to the case without external magnetic fields.