Thermodynamic Properties of Negative Discharge Channels in a 1-m Air Gap Measured by Optical Interferometry

Lightning is a common natural phenomenon and is a typical air discharge on a large spatial scale. The stepping of lightning leaders and space stems of thunderstorms are similar to those of negative discharges in the laboratory. There have been extensive experimental observations and theoretical research on positive discharge but less research on the negative form. This paper conducts an experiment on a 1-m rod-plane gap under a negative impulse voltage. To observe the thermodynamics of the negative discharge, a Mach-Zehnder interferometer combined with a high-speed video camera was set up. The Abel inverse method was used to quantify the thermal parameters for negative channels. The space stem is of great interest, and its bidirectional propagation and connection with the electrode were captured. When a space stem formed, the gas temperature within the stem ranged 320-550 K. Two negative leaders connected in space and formed one negative channel. The connection region was about 800 K while the temperature on the two sides of the negative channel reached 1500-1800 K. After a negative channel formed, it expanded, and its thermal parameter distributions tended to be uniform owing to thermal conduction and convection losses. The gas density near the discharge center was about 20%-30% of the ambient density, and the corresponding temperature was 1000-1500 K. The experiments provide deep insight into the thermodynamic properties of a negative leader discharge.