Velocimetry of Laser-Induced Air Plasma With Ultra-High-Speed Camera*

Understanding the phenomenon of laser-driven air breakdown is crucial for modeling and diagnosing problems within expensive laser systems, which are widely used in research, aerodynamics, and manufacturing. In this study, we focused on measuring the expansion speed of a plasma plume generated by laser-induced breakdown of air at 1 atm. We employed a lens to focus a 532 nm, 1 ns, 1 J laser pulse down to a point in order to induce plasma and recorded the subsequent evolution of the plasma plume using a high-speed 1 GHz camera. A total of 19 shots exhibiting plasma growth were captured and analyzed using Python.

During the initial stages of formation, the laser sparks propagated back towards the laser source within 3 ns after their formation, reaching up to 400 km/s in speed. The plasma plume then continued to grow at a slower rate until reaching its maximum width of approximately 5 mm. When fading, the plasma displayed distinctive bead-like structures before undergoing contraction until it faded completely. Additional investigation will focus on examining the patterns and causes of the bead-like structures and identifying the factors contributing to the irregular and rapid increases in plasma growth and width over time.