Radial mass fraction distributions, in Ar + N plasmas produced in a wall-stabilized arc

At the beginning of the 90s last century A.B. Murphy from CSIRO (Lindfield, Australia) presented a new treatment of the demixing effect in plasmas (demixing means separation of plasma components in the presence of plasma parameter gradients). In this theory the diffusion is described in terms of gases instead of separated species. In the paper published in 1994 Murphy predicted that in arc plasmas created from Ar+N2 mixtures the mass fraction of argon increases with the temperature, when the mass fraction of argon is much less than that of nitrogen, and decreases with temperature in the opposite case. The experimental data confirmed quantitatively this prediction. Unfortunately the unstable plasma source (free-burning arc) and the unsophisticated measurement technique applied in this experiment caused rather large scatter of experimental points and consequently makes the results somewhat doubtful. In order to check and proof these theoretical predictions an experiment with a wall-stabilized arc was performed. The diameter of discharge channel was 4 mm and the discharge current was 40 A. Plasma radiation, emitted perpendicular to the arc axis has been registered using a grating spectrograph and a 2-dimensional CCD camera. After Abel inversion the radial distributions of radiation for several Ar I and N I lines have been determined. After solving a set of equations describing the plasma (applying the local thermal equilibrium model) radial distributions of plasma parameters have been calculated. The obtained results confirmed that the radial distributions of Ar mass fractions strongly depend on the plasma chemical composition. In plasmas containing large amount of Ar the distributions have local minima at the arc axis (in high temperature plasma regions), whereas in plasmas consisting mainly of nitrogen the distributions reveal local maxima on the discharge axis. (author)