Measurement of the degree of polarization of the spectra from laser-produced Al plasma

Using a polarization-resolved UV–visible spectrometer, the degree of polarization of the spectra from laser-produced Al plasma was measured. To generate the Al plasma, Q-switched Nd/glass laser was used. The pulse duration of the laser is 3 ns and the energy per pulse is 6 mJ. The degree of polarization of Al III 4s 2S1/2–4p 2P3/20 transition was measured at different positions from a target surface using a space and polarization-resolved spectrometer. To resolve the different polarization components of the emission line, a dichroic polarizer was used in front of the entrance slit of the spectrometer. The degree of polarization was observed to be 2.1±0.13% 220 μm from target surface and decreased as the distance from the target increased, vanishing about 1.3 mm from the target surface. To avoid the possible error due to the shot-to-shot variation of the line intensity, a calcite crystal was used to simultaneously observe the two polarization components. Both measurements yielded the same result. The plasma parameters, such as the electron temperature and density, were estimated by spectroscopic methods. The measured electron temperature was about 3 eV and the density was 2×1017 cm−3 near the target surface and decreased to 4×1016 cm−3 1.3 mm away from the target surface.