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The leader plays an important role in long-air-gap discharges. In this paper,Mach-Zehnder interferometry and a high-speed video camera were used to observethe dynamic expansion process of positive leaders near the anode in a 1 m airgap. The leader diameters under lightning and switching impulse are obtainedthrough the analysis of interference fringes. The influences of the appliedvoltage, including the amplitude and the front time, as well as the electrodesizes on leader expansion are obtained and analysed. For a 0.5-cm-diameter coneelectrode, when the applied voltage amplitudes are 330-419 kV, the diameters ofthe leaders are 1.5-2.5 mm at time scales of less than 195 {\mu}s, and thediameters increase as the voltage rises. The diameters of the leaders arelarger and the expansion rates are higher for shorter front times. The averageexpansion rates are 72.30-9.54, 28.09-5.05, 14.38-3.02 and 5.73-1.44 m/s forfront times of 1.2, 40, 100 and 250 {\mu}s for a 0.5-cm-diameter coneelectrode. A larger electrode size leads to a wider diameter. A numerical modelwas employed to analyse the expansion of the leaders, and the calculatedresults are in good agreement with the experimental data. Based on the model,the mechanism underlying the leader expansion is discussed in detail.

The dynamic expansion of positive leaders observed using Mach-Zehnder interferometry in a 1-m air gap