Measurements of the arc velocity and of the erosion rate are reported for a copper cathode. The 100 A arc, burning in nitrogen, is driven by a magnetic field B, varying from 5.1 to 171.0 mT, between concentric copper electrodes having an inter-electrode gap of 4 mm. The arc velocity varied with B0.60 throughout the range investigated. The erosion rates dropped from 9.0 to 1.0 mu g C-1 as the arc velocity was increased from 15 to 135 m s-1.

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Erosion measurements on a copper cathode are reported. The 100 A arc, driven by a magnetic field, runs continuously for up to 30 min between two concentric cylindrical electrodes. Argon-nitrogen gas mixtures in various proportions are blown through the electrode gap. The erosion rate in argon is drastically reduced by the addition of only 1% nitrogen and is further reduced as the nitrogen content increases in the gas mixtures. The decrease in erosion rate is found to be correlated to an increase in arc velocity.

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The arc movement was examined for tubular copper electrodes using different plasma gases (argon, helium, nitrogen, air, chloride and mixtures of these gases). The normal arc current was 100 A; the arc was moved using an external magnetic field which varied between 10 and 1500 G. The arc velocity followed an aerodynamic type of equation when the cathode surface was contaminated. For clean or heavily contaminated cathode surfaces another opposing force becomes important, the surface drag force. The arc movement was also investigated by using high speed filming and following the arc current fluctuations. The cathode surface was examined using Auger, ESCA and SEM; work function values were obtained using a Kelvin probe. The results were consistent with the model presented for the arc movement.

Current distributions of the arc foot on the surface of the electrodes in a geometry simulating a plasma torch are reported. Most of the experiments were to determine the arc current distribution at the cathode. The results were obtained using a novel technique and represent experiments using different plasma gases (Ar, He, Ar+0.3% CO, He+0.4% CO, He+0.4% N2, N2, CO) and operating conditions (magnetic field and arc velocity). It is shown that all three, the surface composition due to contamination, the transverse magnetic field used to move the arc, and the arc velocity have a strong influence on the current distribution. Higher erosion rates were found for higher current densities of the arc foot.

In the modelling of non-transferred plasma torches, the electrical potential is normally used in the same way as in the simulation of transferred plasma torches. This approach results in physically questionable current density profiles. A modification is proposed in the manner that the electrical potential is used in the simulation of non-transferred plasma torches. The new model provides in physically acceptable results. A comparison of the results obtained from the two models (the previously used and the presently proposed), shows differences of up to 50% in the velocity and 20% in the temperature were found. Tests were performed with the new model showing the possibility of using it for a broad range of parameters.