We present a simple hydrodynamic model to obtain the profiles of the relevant physical quantities along a nozzle of arbitrary cross-section in a cutting torch. The model uses a two-zone approximation (a hot central plasma carrying the discharge current wrapped by a relatively cold gas which thermally isolates the nozzle wall from the plasma). Seeking for a solution with sonic conditions at the nozzle exit, the model allows expressing all the profiles in terms of the externally controlled parameters of the torch (geometry of the torch, discharge current, mass flow of the gas and plenum pressure) and the values of the arc and gas temperatures at the nozzle entrance. These last two values can be estimated simply appealing to energy conservation in the cathode-nozzle region. The model contains additional features compared with previous reported models, while retaining simplicity. The detailed consideration of an arc region coupled to the surrounding gas dynamics allows determining voltage drops and consequent delivered power with less assumptions than those found in other published works, and at the same time reduces the set of parameters needed to determine the solution.

Spectral emission lines from a low pressure d-c arc discharge with a Ti cathode were studied in various ambient gases; N2, O2, and Ar. Light from the plasma was detected by an optical spectrometer multichannel analyzer (OSMA). The spectral intensities of Ti, Ti+, Ar and N2 were measured as a function of the gas pressure in the range 5 × 10-3 - 0.5 mbar. The measurements were performed in the inter-electrode region at different distances from the cathode. For N2 and O2 as the filling gases, the intensities of Ti and Ti+ increase with the gas pressure up to pressure values of the order of 0:2¡0:4 mbar, while they decrease for higher pressure values. With Ar gas, a different behavior of the Ti+ intensity was found; it presents an increasing general trend. The behavior of the lines was qualitatively analyzed in terms of the most relevant atomic processes that take place in the metallic plasma - gas structure (charge-exchange, electron impact excitation and ionization, etc.). It is found that the behavior of the observed spectral lines can be satisfactorily explained in terms of the relevance of these processes as functions of the neutral gas density and electron temperature.