  A leader is an electric discharge mechanism in long-air-gap discharges. In
this work, we report the shock wave phenomenon in an air-gap leader discharge
observed using a Mach-Zehnder interferometer with a time resolution of several
microseconds. The continuous temporal evolution of the shock wave and the
plasma channel was recorded and reproduced with a thermo-hydrodynamic model
based on the measured current. The wave propagated at nearly the speed of
sound, and the simulation results for the shock wave front positions and the
plasma channel radius showed good consistency with the experimental
measurements. Detailed thermal parameters obtained through the simulation
showed that continuous energy injection by the current results in a temporary
over-pressure process in the plasma channel and produces the shock wave.
