Cathode spot formation possibly explained by cathode electron emission from impact of excited state atoms

Abstract

A theory is given for electron emission from electric arcs with non-thermionic cathodes, for example the iron cathodes used in arc welding, proposing that the principal process for electron emission is from the impact of excited atomic states at the cathode. For plasmas in contact with a metal cathode, a sheath of positive ions is formed over the cathode surface because electrons move away from the cathode. A sheath thickness of only 0.1 µm causes secondary electrons produced at the cathode surface to gain ~12 V. The electron transit through this thin sheath is collision-less. These 12 V electrons produce excited states of atoms to a distance of approximately ~10 µm from the cathode. Diffusion and direct transfer of these excited atoms back to the cathode in the almost collision free region next to the cathode is calculated to be in times of ~10−8 s. This time is less than the radiative lifetimes of almost all atomic transitions except for some resonance transitions where there would be an enhancement of lifetimes due to imprisonment of radiation. If these transfer times are less than the radiative lifetimes, secondary emission coefficients of electrons from the cathode are likely to be approximately 1, the measured coefficients for metastable atoms. The energies of these excited atoms are ~12 V or more, much larger than the usual work function of ~4.5 V required to eject electrons. If such secondary electrons, together with secondary electrons produced from the impact of positive ions and photons, replace or exceed the original number of electrons, amplification is possible in successive avalanches to cause electron densities to grow to supply the total arc current. The mechanism would then explain why observed cathode sheath voltages are independent of arc current for currents varying from 10 to 20 000 A for various low melting point electrodes and arc gases. Electron emission from cold cathodes, when temperatures are too low for thermionic emission, would also be explained. Solutions of the densities of electrons, positive ions and excited state atoms as a function of time and distance from the cathode are presented which give a physical explanation for the formation of cathode spots.