Vacuum-UV photoabsorption imaging of laser produced plasmas

Abstract

Complementaiy to the existing VUV/soft X-ray spectroscopic systems, a VUV photoabsorption imaging optical system has been developed.The technique consists in passing a pulsed VUV beam, tuned to an atomic/ionic resonance, through a laser plasma plume and recording the transmitted light or shadowgram. A first optical set-up was designed and built as proof of principle to validate the photoabsorption technique. The promising results obtained, as well as the limitations of the system lead, to the design and fabrication of a new system with improved flux collection, probe beam quality, spatial and spectral resolution. The design o f both systems was done with the aid of a ray tracing code, and the calculated performances from the simulations were compared to experimental measurements. Time and space resolved distributions o f atomic calcium, Ca+, Ca2+ equivalent width were measured using the respective 3p-3d resonances, and information on the plasma expansion dynamics was extracted and compared with an expansion model. With the absolute cross section known, maps of equivalent width could be converted to maps of column density NL; this was achievable when using the 3p-3d Ca+ resonance at 37.43 nm and the 5p-5d Ba+ resonance at 46.7 nm