The results of a twin-torch transferred dc arc study are presented. The arc system consists of two torches of opposite polarity, and a coupling zone of plasma jets located between them. The torch configuration increases the system reliability and efficiency during material plasma processing. The results of the study present data for the voltage-current characteristics, general behavior of the twin-torch arc, and spatial distribution of the plasma parameters. The plasma parameters have been measured using optical emission spectroscopy for a 200 A (20 k IV) dc arc at atmospheric pressure, with argon and nitrogen introduced as plasma-forming gases hzto the anode and the cathode units, respectively. The measurement technique used has allowed the determination of local electron density and temperature values in an inhomogeneous plasma volume having no axial symmetry. The data obtained illustrate the novel features of the twin-torch transferred dc arc for its applications in plasma processing.

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Spatial distributions of plasma temperature and electron density have been measured in a transferred direct current are running at atmospheric pressure in 500 scc s-1 nitrogen gas flow. The measurements were made at 100-250 A current for two arc 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.

Plasma temperature and electron density have been measured by optical emission spectroscopy in a transferred direct current arc with graphite electrodes. The measurements were realized for the arc running in a 1.7 cm 3 s −1 argon flow at atmospheric pressure, a current of 200 A, a voltage of 48 ± 2 V and with a distance of 85 mm between the electrodes. Specially designed electrodes were developed to minimize displacements of the arc because of movements of the electrode spots. In this way, the measurement errors have been greatly diminished. The measurements were made supposing an optically thin collisionally dominated plasma of axial symmetry. Density distributions of excited argon atoms and ions on their energy levels were determined to evaluate plasma temperature. The half-widths of spectral lines were used to reach the electron density in the plasma. Spatial distributions of the measured parameters are presented for the arc positive column and near the electrodes. Maximum values of 1.5 × 10 17 cm −3 and 13 500 K were found close to the cathode for the electron density and temperature, respectively. The data and possible reasons for some of the variation found in the observed results are discussed.