A measurement of axial velocity and temperature in a free-burning arc using Thomson scattering

High-resolution lineshape analysis of Thomson-scattered laser light was used to determine the centreline temperature and velocity of ions in a free-burning arc. The arc was maintained at in atmospheric-pressure argon. The technique allows direct measurement of plasma gas velocity without disturbing conditions within the plasma - the values found were lower than previously reported using indirect methods and increased with time, over 2 h. On the other hand, the ion temperature was found to drift little over the same period and its value was higher than previously reported for Thomson scattering, yet lower than found by emission spectroscopy. Possible reasons for these differences are examined and, in particular, the role of small concentrations of oxygen is discussed. This preliminary study highlights experimental difficulties that are inherent in this type of measurement.