Numerical and experimental investigation of thermal regimes of thermionic cathodes of arc plasma torches

Abstract

The modelling method based on decoupling the simulation of the cathodic part of the arc (the cathode and the near-cathode non-equilibrium plasma layer) from the simulation of the arc on the whole has been extended to cathodes of arc plasma torches, consisting of an insert with a conical tip, made of pure or doped tungsten, and a surrounding water-cooled copper holder. The method was validated by comparison with the experiment, performed on a 200 A DC arc in atmospheric-pressure argon. Standard work function of polycrystalline tungsten of 4.54 eV was used for modelling of pure-tungsten insert and a good agreement with the experiment was found with respect to both the insert tip shape and the temperature distribution in the tip, recorded in the stable operation mode. There are no unambiguous data on the work function for arc cathodes made of doped tungsten, although in situ measurements of the effective work function of cathodes of high-pressure arc discharges provide useful hints. On the other hand, the experiments reported in this work show that the tip temperatures of inserts made of tungsten doped with of thorium, or lanthanum, or yttrium, recorded during the stable-mode operation at the arc current of 200 A, vary in a rather narrow range 3100–3200 K. This suggests that the work functions of doped tungsten inserts, operated in the stable mode, are close to each other as well. Indeed, the results of modelling with the same value of the work function of 3 eV give a reasonably good agreement with the experiment in all three cases.