A dual-wavelength high-sensitivity laser diagnostic based on a Nd:YAG laser (532 and 1064 nm, pulse duration of 7 ns) was applied to a laboratory high-voltage atmospheric discharge. The applied voltage was 1 MV, and the discharge current reached 10 kA. The discharge gap length was 0.55 m. The electrodes consisted of a pointed cathode inside a smooth-rim cone and a hemispherical mesh anode. The voltage pulse duration was about $1\mu \mathrm{s}$ with a rise time of 150–200 ns. The closure regions of a cathode-directed leader and an anode-directed leader were visualized. Electron density profiles were obtained in both leader channels. Distinct differences in the plasma structure of the two leaders during off-axis merging were revealed.

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• Laser techniques
• Optical interferometry
• Optical plasma measurements