  The focusing of a secondary shock wave was experimentally recorded during the
implementation of an extended nanosecond high-current discharge of a
cylindrical configuration in air. Using high-speed shadowgraph recording (up to
300 000 frames/s) of the emerging unsteady flow, cylindrically symmetric
gas-dynamic discontinuities were visualized: a diverging shock wave moving from
the discharge plasma boundary, a diverging contact discontinuity separating the
shock-heated gas from the nonequilibrium region excited by the discharge
plasma, and also a converging compression wave (secondary shock wave) moving
from the contact surface to the axis of symmetry and focusing on it 50-60
microseconds after discharge with the formation of a heated long-lived cord.
