Time-integrated, spatially resolved plasma characterization of steel samples in the VUV

A Nd:YAG laser working at its fundamental wavelength with a pulse duration of 7 ns and energy of 820 mJ was focused onto a steel sample placed in vacuum (similar to10(-6) mbar) to produce a plasma. The excitation temperature and electron number density of the plasma were determined using a time-integrated, spatially resolved spectroscopic system. Light elements (S and C) in low concentrations (0.38% and 1.32%) in the steel samples have been used as the diagnostic elements. The strongest spectral lines of these elements are emitted from their ions in the deep vacuum ultraviolet (VUV) spectral region. Using the spectral lines emitted from the ions during the early phase of the plasma, excitation temperature and electron density were measured for different heights above the steel target surface. The electron excitation temperature was determined using the Boltzmann plot technique for S IV and S V, while the electron density was measured by the Stark broadening of two S V and one C II emission lines. Different values of the excitation temperatures of S IV and S V lines were obtained. The excitation temperature determined with S V was higher than that with S IV for all positions above the sample surface.