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Under the slow varying ambient electric field, positive leader propagation exhibits steps characterized by intense reilluminations and abrupt elongations. These steps are currently not well understood. In this work, we investigate these steps in laboratory atmospheric discharges, using a high‐speed video camera and a synchronized electrical parameter measurement system. The discharge, emitting weak light and preceding the intense reillumination, is discovered. This finding suggests that the leader channel actually restarts and extends forward before the intense reillumination, which deepens our understanding of the dynamic process of the positive leader step. The discharge before the intense reillumination contributes to the corona inception from the electrode, leading to the intense reillumination of the leader channel and the emergence of an intense corona streamer burst from the leader tip.

Knowledge of leader corona is the basis for long spark discharge modelling, which is critical to understand the underlying physics of lightning flashes and dielectric breakdown of long air gap widely utilized in power systems. This paper presents an accurate observation of the leader corona. The morphology of the leader corona region during the discharge was obtained using a high-speed camera and an image-enhanced charge-coupled device camera. Statistical results showed that the morphology of the streamer region varied at different discharge sub-phases. The first corona was a conical region with the average apex angle of 118.91°; the leader corona region behaved as a cone with an average vertex angle of 75.87° during the leader propagation process; the streamer area in the final jump was a combination of a cylinder and a cone with the average vertex angle of 82.61°. Noting that the obtained cone apex angle of the leader corona during the leader propagation phase was significantly smaller than that in photographical reports, and the observation results were related to the selected temporal resolution of the optical image apparatus. The discussion on the effect of the single-frame exposure time on observation results found that the average vertex angle of the conical streamer region gradually decreased with the decrease of the single-frame exposure time, and eventually tended to a stable value of about 75.96°. For comparison purposes, discharge experiments under two gap-distances demonstrated that on the premise of stable leader, the statistical distribution characteristics of the leader corona vertex angle were consistent, indicating the universal applicability to the simulation researches. Finally, it is found that the distortion of the electric field caused by the space charge explained the variation of the streamer area during the discharge process.

Microparticle initiated pre-firing of high pressure gas switches for fast linear transformer drivers (FLTDs) is experimentally and theoretically verified. First, a dual-electrode gas switch equipped with poly-methyl methacrylate baffles is used to capture and collect the microparticles. By analyzing the electrode surfaces and the collecting baffles by a laser scanning confocal microscope, microparticles ranging in size from tens of micrometers to over 100 μm are observed under the typical working conditions of FLTDs. The charging and movement of free microparticles in switch cavity are studied, and the strong DC electric field drives the microparticles to bounce off the electrode. Three different modes of free microparticle motion appear to be responsible for switch pre-firing. (i) Microparticles adhere to the electrode surface and act as a fixed protrusion which distorts the local electric field and initiates the breakdown in the gap. (ii) One particle escapes toward the opposite electrode and causes a near-electrode microdischarge, inducing the breakdown of the residual gap. (iii) Multiple moving microparticles are occasionally in cascade, leading to pre-firing. Finally, as experimental verification, repetitive discharges at ±90 kV are conducted in a three-electrode field-distortion gas switch, with two 8 mm gaps and pressurized with nitrogen. An ultrasonic probe is employed to monitor the bounce signals. In pre-firing incidents, the bounce is detected shortly before the collapse of the voltage waveform, which demonstrates that free microparticles contribute significantly to the mechanism that induces pre-firing in FLTD gas switches.

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To study corona discharge in air, it is important to measure the electric field in the discharge space, which is directly related to the corona discharge process. In this paper, we present a method for measuring the electric field with electro-optical modulation. This method uses space laser modulation based on Bi4Ge3O12 (BGO) crystals, and we suggest using the dual optical path difference method to improve the measurement performance. The measurement error is analyzed from theory, including the measurement error of a highly inhomogeneous electric field and the error due to theoretical approximation. In system calibration, the system noise and the dual optical path difference method were analyzed. The dynamic linear range is determined, and measurement effectiveness in the severely nonuniform electric field is verified by comparison with a finite element numerical simulation. We measured the electric field in Trichel pulses under negative dc voltage and found electric field pulses. These measurement results show that the characteristics of Trichel pulses are directly related to the electric field. Therefore, this method can provide strong support for the study of corona discharges.

Both positive and negative leaders, which are the fundamental discharge processes underlying lightning, could propagate through a series of steps, characterized by a fast elongation of the channel and a transient current pulse (Dwyer & Uman, 2014;Gallimberti, 1979;Williams, 2006). Knowledge of leader steps is critical to deepening the understandings of the radio-frequency electromagnetic radiation from lightning (