Characteristics of the xenon plasma sustained by lasers in different incidence directions

Xenon plasma was sustained using continuous fiber lasers with varying incident directions, and the stability of the plasma was analyzed through both experimental and theoretical approaches. High-speed cameras were employed to analyze the plasma's shape and size, while photodetectors recorded the plasma's brightness fluctuations. It was found that plasma sustained by lasers incident from below exhibited greater stability, as indicated by smaller standard deviations of the centroid and axial diameter, as well as reduced brightness fluctuation amplitudes. The Schlieren method revealed an upward circulating hot bubble surrounding the plasma. Fluid dynamic analysis of gas flows surrounding the high-temperature plasma demonstrated turbulent flow in the region above the plasma and laminar flow below it, as determined by Reynolds number evaluation. This observation provides a fundamental explanation for the superior stability observed in plasmas sustained by laser irradiation from below. A direct relationship between Kelvin-Helmholtz instability and the formation of toroidal vortex structures was confirmed. Based on combined experimental and simulation results, the optical configuration for plasma maintenance was optimized. By employing bottom laser irradiation with F=0.8, a highly stable, small-diameter plasma with nearly perfect circular symmetry was achieved.