抄録
The demixing in gas tungsten arc plasma was investigated by measuring of the helium concentra-tion at a water-cooled copper anode using a mass spectrometer in argon-helium mixed gas shielding arc. It was expected from a study on arc pressure that helium gas, which is lighter than argon gas, is efficiently absorbed near the electrode tip by the plasma stream.
In this measurement, a small amount of plasma gas was sampled through a hole (ID: 0.5 mm) in the anode plate by using a setup similar to that of a leak test for a vacuum chamber using mass spectrometer. The helium concentration of the gas was determined from a ratio of the helium spectrum peak value to that of argon in argon-helium arc.
From measured concentration distribution of helium at the anode, it was found that helium concentrated in the arc center at the anode surface and it was suggested that argon concentrated at the arc periphery in argon-helium arc.
Furthermore, it was shown that blow holes were formed in argon-helium arcs upon introduction of nitrogen. Blow hole formation occurred at lower nitrogen content when argon content in the shielding gas was high. These results could be understood from mass spectroscopic measurements which showed that helium gas concentrated at the core in helium-nitrogen arcs as well as in argon-helium arcs, but both nitrogen and argon distributed uniformly throughout the arc in argon-nitrogen arcs.
