Narrow Results

The critical-charge criterion of the stem-leader transition and the shielding effects of space charges on the streamer inception are two most contested issues on the inception of positive upward leader–streamer system (LSS). In this paper, a series of simulation experiments on the LSS inception were designed and carried out. A new critical-charge criterion in the range of 0.2–0.3 μC was proposed, and the previous criterion of 1.0 μC was proved to be harsh. The shielding effect of positive space charges on the streamer inception was verified directly by the experiment results. A theoretical formula for calculating the LSS inception voltage by the first-corona inception voltage was obtained. An appropriate first-corona inception time for getting an earlier LSS inception was proved to be existent. At last, the effects of the so-called improved lightning rods, such as the early streamer emission rod (ESE) and the controllable lightning rod (CL), were discussed, and it seemed that they would not help to extend the protective zones of the lightning rods.

Sulphur hexafluoride (SF₆) is used as an insulator in high-voltage systems. An electrical breakdown dissociates (SF₆). Therefore, the insulating efficiency is degraded. If the (SF₆) is not renewed, failure of the system can occur. A monitoring device for (SF₆) loss is required, and an optical fiber sensor for continuous monitoring of (SF₆) degradation in high-voltage switchgear has been developed. Dissociation products of the (SF₆) discharge include HF and atomic F, both of which react readily with silica. This reactivity is utilized in our sensor, where etching of a glass fiber surface modifies the transmission characteristics of the fiber.

This paper describes the thermal gas effluent and ion acoustic pressure wave interaction between the fundamental drive frequency and its harmonics within an atmospheric helium discharge. Deconvolution of the acoustic signal and the electrical signals reveal that the plasma jet undergoes a change in operational mode from chaotic (where the plasma is spatially and temporally inhomogeneous at the electrode surface) to stable (periodic in nature) when the plasma expands away from the electrodes and into the reactor cylinder. This effect is strongly influence by the helium flow and input power. In addition the generated acoustic signals is found to have a frequency response to that of a closed-end cylinder column which supports antinodes of n = 1 and 3. Decoding of the acoustic signal allows the helium thermal gas temperature to be obtained: T_gas ~ 290 K. The signal allows the axial gap distance between the jet nozzle and work surface to be estimated which has technology importance in terms of plasma metrology and in the basic understanding of atmospheric pressure plasma jet physics.