Investigation of the bilayer region of metal vapor in a helium tungsten inert gas arc plasma on stainless steel by imaging spectroscopy

A blue bilayer region presents in a helium tungsten inert gas (TIG) arc plasma on stainless steel. The blue region indicates the presence of metal vapor. The spectral intensity distributions of metal vapors, including ions, in a steady state helium TIG arc plasma on stainless steel were experimentally investigated by imaging spectroscopy. The spectral intensity distributions of the ionized metal vapors were clearly observed in the arc plasma. The ionized metal vapor was affected by the electric field, which suggested that it was transported through the inside of the arc plasma to the tungsten cathode by cataphoresis. Chromium vapor was widely distributed in the arc plasma because more was generated and it was more easily ionized than iron vapor. As the arc plasma was cooled by the strong radiation heat loss of chromium, not only the chromium vapor spectral intensity distributions but also iron vapor spectral intensity distributions were observed near the center of the arc plasma. In both the iron and the chromium vapor, the metal atoms were observed at the arc fringe, and the metal ions were observed in the arc. A blue bilayer region was observed in the arc plasma because the temperature ranges in which the metal atoms and the metal ions strongly emit light are different.