The excitation theory of arcs with evaporating cathodes

Abstract

Most metals when used as the cathode of an arc discharge cannot reach temperatures sufficiently high to emit electrons thermionically. However, the temperatures are high enough to produce considerable evaporation. Mercury and copper are examples. The older theories suggest that here the current at the cathode is carried by electrons extracted by field emission, photo-electric emission or secondary emission by positive ions, or that the entire current is carried by positive ions produced by thermal ionization in the gas. All the theories are shown to be quantitatively inconsistent with observations. A new theory is suggested: electrons are released from the cathode by the impact of excited atoms. The electrons gain energy in the cathode fall and produce excited atoms in the dense vapour. The radiation from the excited atoms diffuses, mainly in the direction of the cathode, by successive absorption and re-emission in the vapour and is ultimately absorbed by atoms which strike the cathode. Positive ions are formed in the vapour by collisions between excited atoms, and by electrons colliding with excited atoms. The positive ions have three functions; their space charge provides the cathode fall in potential, they supply energy for evaporation and they transfer momentum to the evaporated atoms. The majority of the latter are back-scattered and in this way a vapour density is set up close to the cathode which is many orders of magnitude larger than elsewhere. An exceptionally high density, however, is a necessary condition for a low cathode fall and a high current density. The new picture is also consistent with the observed force on the cathode and the evaporation rate. The energy balance also supports the theory.