Spatiotemporal measurement of electron number density in high density laser-induced plasmas using laser absorption

Laser absorption measurements were conducted on a high-density, laser-induced plasma produced in atmospheric-pressure air to investigate the spatiotemporal evolution of its electron number density. Measurements taken both along and perpendicular to the plasma’s symmetric axis showed that, upon formation, the plasma propagates in the direction opposite to the laser beam used for plasma generation, while expanding rapidly radially. The spatiotemporal evolution of the electron density was further analyzed from the measurements taken perpendicular to the plasma’s symmetric axis through tomographic reconstruction. Notably, the reconstruction was achieved using a genetic algorithm, as a probe laser beam used for absorption measurement is non-negligible in size compared to the plasma. Importantly, our measurements could reveal that the electron density reaches 4.99 × 1019 cm−3 immediately after the plasma formation at the center; moreover, there is a development of a pressure wave with high electron density, propagating outward radially due to the rapid expansion of the produced plasma.