Prediction of properties of free burning arcs including effects of ambipolar diffusion

Abstract

A method is described to predict the two-dimensional distributions of temperature, velocity and potential of free burning arcs and their electrodes for cathodes of tungsten and thoriated tungsten. The effects of non-equilibrium due to the ambipolar diffusion of charged particles are included for the calculation of the plasma electrical conductivity. The electron diffusion current is explicitly included in the solution of the current continuity equation. The plasma for the arc and the electrode sheath regions is treated as a continuum, so that the thickness of the non-equilibrium regions near the electrodes is determined within the model, depending upon the arc current and the arc and electrode configuration. This new treatment allows the calculation of the negative anode fall that may occur across the anode sheath when the electron diffusion current near the anode surface becomes larger than the total arc current. For a thoriated tungsten cathode we take the work function for cooling by electron emission to be that of tungsten, as, for small percentages of thoria in tungsten, cooling effects from electrons passing through the interfaces for tungsten-thoria and then thoria-plasma will add up to be that of a tungsten-plasma interface. Calculations have been made for arcs in argon at currents between 2.5 A and 200 A. For currents above 120 A, we calculate the anode fall voltage to be negative, being -2 V at 200 A. For currents less than 50 A, non-equilibrium effects in the plasma extend across the whole arc and electron number densities can be several orders of magnitude below the values for local thermodynamic equilibrium. Calculated arc voltages, arc temperatures and electrode temperatures are in agreement with experimental measurements to within 20%.

Quasi-neutrality and local thermodynamical equilibrium in atmospheric presure arc discharges

“…It is interesting to observe that the violation of the quasi-neutrality, comparable in the two cases, is accompanied by a change of charge sign at about 1.5 mm from the anode, corresponding to the minimum in the electric field. This feature resembles the result presented in (Sansonnes et al, 2000) where a change of polarity in the potential is predicted, for I = 200 A, from positive to negative, however at a distance z ~ 2.8 mm from the anode. The order of this result, δn~ 10 7 m -3 , is the value of the difference evaluated within one "experimental cell", containing on average ~10 11 particles.…”

“…A further rise occurs at higher radial distances but still n D ~10; v The inner region extending for 3 to 4 mm at these arc currents is considered to be comparable with the current carrying region. So the question is whether the current carrying region may be considered and quasi-neutral 'bulk' plasma in LTE as it is usually assumed in literature (Allum 1983;Haidar, 1997;and Sansonnes et al, 2000). …”

Formation of self-organized anode patterns in arc discharge simulations

“…This fact has motivated the use of a time-dependent and three-dimensional NLTE model in the present work. As stated in Section 1, the NLTE model is based on the chemical equilibrium assumption and uses relatively simple boundary conditions over the anode surface, and does not include the modeling of the bulk electrodes, electrode sheath models, or anode material evaporation effects [62][63][64][65][66][67][68][69]. nodes; therefore, the fine mesh has approximately 2 times the resolution and size of the base mesh.…”

Simulation of electric arc with refractory cathode and evaporating anode

“…In the first publications, devoted to mathematical simulation of the processes of heat, mass and charge transfer in refractory cathode arcs [2][3][4][5][6][7][8][9][10], arc plasma was assumed to be single-component, i.e. containing atoms and ions of just the shielding gas.…”