DISTRIBUTION OF PLASMA PARAMETERS IN A NITROGEN TRANSFERRED DC ARC

Spatial distributions of plasma temperature and electron density have been measured in a transferred direct current are running at amospheric pressure in 500 sec s(-1) nitrogen gas flow. The measurements were made at 100-250 A current for two are lengths of 100 and 200 mm. Optical spectroscopy techniques have been used to determine the plasma parameters using local emissivities of atomic and ion spectral lines, as well as continuum radiation. In the are positive column the parameters agree well, that is the plasma state is close to local thermodynamic equilibrium. Near the cathode some variation has been observed in the measured data, showing a possible deviation from the equilibrium state: emissivities of atomic and ion lines give very different results. The phenomenon can be explained, if one supposes that the plasma ionization composition does not correspond with an equilibrium model of the plasma in this spatial region, with the electron density being higher than would follow from the Saha equation for pure nitrogen plasmas. This is supported by our measurement data of the plasma electron density.