Comparison of nanosecond laser produced brass plasmas under low and moderate pressure air

Nanosecond laser produced brass plasmas are studied at the ambient pressures of 10^-3, 20, 100 and 500 Pa. Spatial and temporal dynamics of the plasmas are captured by fast photography and optical emission spectroscopy (OES). The phenomena of plasma free expansion, splitting, sharpening, and instability are observed, respectively. At different pressures and incident laser fluences, the plasma lengths are found to be in good agreement with the drag force model, and the plasma stopping distance are interpreted with the Dyer's model. By spatially resolved OES, we confirm that the excited target ions (Zn⁺, and Cu⁺) play a growing important role on the emission near plasma front with the increase of the ambient pressure. It also demonstrates that at the late time of 300 ns, there is no significant variation in the temperature distribution on axis in the explored ambient pressure range, while the electron density decreases rapidly in the near-surface region.