A METHOD OF ESTIMATING THE SPACE-CHARGE VOLTAGE DROP FOR THERMIONIC ARC CATHODES

This paper proposes a method that presents simply the relationship between the space-charge voltage drop, the electron temperature at the plasma boundary and the cathode temperature for high-intensity arcs of thermionic cathodes. According to this method, the cathode space-charge voltage drop V(s) is estimated. The results are compared with indirect measurement and numerically analysed results and good agreement is obtained. The results have shown that V(s) ranges from 2 to 4.5 V for the Th-W thermionic cathode of argon arcs at atmospheric pressure. To a great extent V(s) is decided by the boundary electron temperature, which means that the boundary electron temperature plays an important role in controlling the arc cathode energy balance. It has also been found that, under the conditions concerned in this paper, the kinetic energy transfer of electrons emitted from the cathode by elastic and inelastic collisions with heavy particles at the plasma boundary is negligibly small due to the very low collision probabilities and/or the very small mass ratio, while energy transfer by electron-electron collision is dominant. This may be the reason why the cathode space-charge voltage drop is below the ionization voltage of arc atmosphere.