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Gas temperature plays an important role in the characterization of plasma parameters. The temperature evolution of stem roots during the dark period is at the heart of our understanding of the physical mechanism of streamer to the leader transition process in a long spark discharge. The quantitative schlieren system with high spatiotemporal resolution was designed to conduct positive leader discharge experiments with a 1.0 m rod-plate gap at atmospheric pressure, and the amplitude and rise time of positive impulse voltage waveforms were 380 kV and 200 μs, respectively. The time-resolved quantitative schlieren images of the discharge channel near the electrode tip can be captured to gather the temperature data of stem roots for case study. Further statistical tests revealed that due to the dispersion of the first streamer discharge, there were three evolutionary trends of gas temperature at the axis of the stem root in the early dark period. Interestingly, the gas temperature at the axis of the stem root was maintained at 1000–1200 K in regardless of the evolutionary trend in the early dark period. In addition, the statistical results indicate that there is a significant positive correlation between the curvature radius of the discharge electrode tip and the critical charge value Qcrit of the first streamer discharge, which leads to a transition in evolutionary trends of gas temperature at the axis of the stem root. This research has important guiding significance for understanding the physical mechanism of leader inception and the whole process modeling of long air gap discharge. [ABSTRACT FROM AUTHOR]

An interpretable convolutional neural network model is proposed for the classification of very low frequency and low frequency lightning electric field waveforms. This model adopts multi‐scale convolutional kernels and shortcut connections to enhance the ability of lightning waveform classification. Based on the data recorded from five provinces in China, the proposed model achieves an accuracy of 98.56% for a four‐type classification task including return strokes, the intra‐cloud lightning, preliminary breakdown, and narrow bipolar events. The proposed model is validated with another open‐source data set from Argentina with an accuracy of 98.45%, which shows good robustness. To ensure the classification, the features learned by the model are visualized. The class activation mapping (CAM) method is adopted to visualize the class‐specific contribution of different waveform parts by using the feature maps of the final convolutional layer. It is highlighted by the CAM method that the proposed model focuses on waveform parts that align with those areas of interests identified by human experts. The high‐contribution waveform parts are furtherly analyzed, which indicate that the proposed model possesses the capability to associate waveform features with the corresponding lightning discharge processes. Plain Language Summary: Electromagnetic waveforms in very low frequency and low frequency bands are usually used to detect and locate lightning activities. Traditional waveform classification methods have difficulties in distinguishing multiple types of lightning waveforms. Although machine learning models have great potential in multi‐type waveform classification tasks, these models rely on the features proposed by human experts and cannot capture the features of different scales in lightning waveforms. To this end, this paper proposes an improved convolution neural network model, which incorporates modifications to the model structure to better suit the lightning waveform classification task. The data set for model training comes from five provinces in China and contains different meteorological conditions. The proposed model achieves a classification accuracy of 98.56% on this data set and 98.45% on an open‐source data set from Argentina. Meanwhile, the classification process is interpretable by visualizing the convolution outputs. The analysis of the visualization results shows that the high performance of the proposed model is reliable for its ability to focus on waveform parts that align with areas of interests identified by human experts. A closer inspection of these waveform parts suggest that the proposed model possesses the capability to associate waveform features with the corresponding lightning discharge processes. Key Points: The proposed model achieves an accuracy of 98.56% for a four‐type lightning waveform classification task and shows good robustnessThe model is interpretable by visualizing the contribution of different waveform parts to the classification resultsThe proposed model aligns with human‐expert classifications and suggests potential to link waveform features with lightning processes [ABSTRACT FROM AUTHOR]

This paper presents an original investigation into the axial evolution of streamer stem during a dark period in long spark discharge. To obtain thermodynamic morphology and temperature distribution of stems, we set up a quantitative schlieren system with the temporal and spatial resolutions of 0.37 μs and 31 μm/pixel, respectively. The quantitative schlieren observation experiments of positive leader discharge with a 1.0 m rod-plate gap were carried out, and the time-resolved quantitative schlieren images were captured. Furthermore, the temperature distribution of stems and its morphology evolution in the axial direction during a dark period were obtained. Due to the dispersion of first streamer discharge, the gas temperature in stem roots shows two evolutionary trends, namely, rising and falling. It was found that the gas temperature in stem decreased along the axis with the increase in the distance from stem root, and the gas temperature of a thermal thin channel was between 400 and 800 K. There is a significant dependency between axial development parameters of thermal thin channels and the first streamer discharge parameters. The phenomenon of channel abrupt elongation triggered by secondary streamer discharge was observed by the schlieren system, and the influence of characteristic parameters on the inception of secondary streamer was statistically analyzed. The ion current waveform in leader relaxation phase was measured, and it is clarified that the generation mechanism of thermal thin channels is due to the energy transfer between positive ions and neutral particles, which finally leads to the increase in gas temperature in the channels. [ABSTRACT FROM AUTHOR]

With the rapid development of semiconductor devices, researchers have proposed different types of high-frequency pulse voltage generators based on power electronic converters to meet the requirements of microorganism inactivation, water pollution purification, and piezoelectric and dielectric electroactive polymer drives. The transient modeling of these high-voltage converters aids in understanding the electrical characteristics of the devices and optimizing their performances. This article analyzes the electrical behavior of a semiconductor switching device (i.e., an insulated gate bipolar transistor) used during the switching transient of the converter and constructs an electromagnetic transient model of the flyback converter that considers both the parasitic elements and the ferromagnetic losses of the transformer. The accuracy of the proposed model is then verified by comparing calculation results obtained using the model with experimental measurement results. [ABSTRACT FROM AUTHOR]

Lightning puncture damage on the wind turbine blades can pose a severe threat to the safe operation of large-capacity wind turbines. We investigated its formation by performing long air gap discharge experiments under positive and negative downward leaders. Upward streamers were observed to emit from the sample blade before its electrical breakdown. To explain this phenomenon, a numerical model considering the streamer discharge in air, charge transport in GFRP laminate and surface charge accumulation on air-solid interfaces was established. It is found that streamer discharges can be triggered inside the blade hollow to generate space and surface charges, which strengthen the electric field on the outer blade surface and contribute to the upward streamer initiation. The upward streamer development leads to the accumulation of surface charges with opposite polarities on the two sides of GFRP laminate, which causes the electric field on the GFRP laminate to exceed its electrical breakdown threshold. The electrical breakdown, which occurs before the thermal effect of lightning current appears, is the initial stage of lightning puncture damage of blades. The electrical discharge inside the blade hollow and the surface charge accumulation on the GFRP blade should be suppressed to prevent the puncture damage of blades. [ABSTRACT FROM AUTHOR]

We provide the clearest experimental evidence to date that positive leader velocity decreases with the decline of air density in the troposphere. The experiments are conducted at altitudes of 4,300 m in Tibet and 53 m in Beijing using the long air gap discharge technique. Rigorous comparative studies are performed using synchronized measurement of positive leader velocity and current under different air densities and similar temperature and humidity conditions. The effect of reduced air density on positive leader velocity is analyzed with a detailed thermal‐hydrodynamic model. It is found that the widely used similarity law setting initial leader radius inversely proportional to air density does not hold in estimating positive leader speeds in the troposphere. Plain Language Summary: The effect of reduced air density on leader discharge is a fundamental concern in several subjects such as lightning physics, atmospheric electricity, and electrical engineering. Increasing observation results have shown that lightning leaders seem to propagate slower at higher altitudes. To rigorously investigate the effect of reduced air density on positive leader velocity, electrical discharge experiments were performed at an altitude of 4,300 m in Tibet under the basis of excluding other influencing factors, such as discharge current and other atmospheric conditions. By comparing measurements near sea level and at high altitudes under similar temperature and humidity conditions, we provide reliable evidence to confirm that the positive leader velocity decreases with the decline of air density. Based on a thermal‐hydrodynamic model, it is found that the leader velocity calculated with the use of similarity law deviates significantly from the experimental measurements. A revised scaling formula for the initial conditions of leader stem at reduced air density is proposed. Our work can motivate scientists to revisit the use of similarity law in analyzing lightning discharges and upper‐atmospheric discharges such as gigantic jets. Key Points: We report a rigorous experimental study confirming that positive leaders propagate slower at higher altitudesBased on an improved thermal‐hydrodynamic model, the effect of air density on leader velocity is discussedOur study indicates that the similarity law for leader radius needs to be revised for electrical discharges in the troposphere [ABSTRACT FROM AUTHOR]