Extensive investigations of the dependence of the relative ionic composition of a high-frequency plasma on the discharge conditions (particularly the pressure) and of the relative ionic composition of the plasma as a function of the translational energy of the ions issuing from the orifice of the sampling probe at constant pressure were made by the use of a quadrupole mass spectrometer employing a retarding-potential grid system. The results indicate that the ions collected and observed are formed in a narrow region in the plasma (approximately 1 mm in depth) in front of the probe orifice and that these ions gain most of their energy from the electric field of the sheath in a distance that is much less than a reaction mean-free path (∼0.01 mm). The average time available for reaction (reaction time) was a few tenths of a microsecond and independent of the vessel geometry and dimensions. The reaction time was relatively insensitive to changes in pressure and was reasonably constant for ions in several different gases. An explanation is given in terms of a distrubance to the electric fields of the plasma in the vicinity of the probe aperture.
This paper summarizes the progress of an extensive study of the ac voltage endurance, extending from a single cycle to more than a year, for cast silicafilled epoxy resin, with recessed silver paint electrodes and cast-in aluminum electrodes, giving a nearly uniform electric field. Although the specimens were prepared to avoid gas cavities, and partial discharges were usually undetected, the ac strength declines regularly with time in a manner similar to that due to partial discharges. Therefore, it is inferred that undetected partial discharges are responsible. A derivation is given for a suggested new analytic relation for the ac volt-failure time curve. It is suggested that the distribution of ac dielectric strength values for a given test time is correlated with the distribution in failure times at a given test voltage level.
This paper describes the factors influencing the voltage or electric stress-failure time relation of cast and molded resins. It also discusses the reasons for similar failure times at equal stresses with surface and internal discharges, when the discharges are continuous, and presents data supporting those reasons. The phenomena of intermittency of internal discharges is also described. Voltage endurance with tiny micro-cavity defects, which may be practically undetectable, is also discussed.
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