Shielding and diagnostics of laser-induced air plasmas generated in collinear double pulse configuration

Abstract

Plasma shielding in collinear double-pulse laser-induced breakdown spectroscopy was characterized in terms of its time-resolved absorptivity, photographic images, and emission spectra. In experiments performed with the double pulse beam configuration, one to generate plasma and another a probe beam, the transmission of the probe beam was measured at various inter-pulse delay times up to 1500 ns as the plasma opacity evolves rapidly with time. Photographic images obtained at the same time delay indicate that the plasma absorptivity becomes weaker as plasma evolves. At times up to 500 ns after plasma initiation, plasma absorptivity rose above 70% but later decreased markedly. When viewing the transmission and photographic images, the beam-plasma interaction realized with the double-pulse configuration is explainable as laser-supported absorption waves. To clarify the phenomenon and plasma properties when the probe beam and plasma are coupled, Stark broadening was used to obtain the electron number density, which is of the order of 1018 cm−3. Also, emission spectra under single- and double-pulse experimental configuration were obtained and analyzed using the Boltzmann-plot method to provide the plasma temperatures. The intensities from the double-pulse experiments were slightly stronger, in agreement with the laser energy absorption data. Moreover, from emission signal enhancements obtained from the double-pulse experiments, the increase in the intensity of the ionic emission was more than that obtained from the atomic emission for which a larger fraction of the probe beam energy was absorbed.