Erosion of Thermionic Cathodes in Welding and Plasma Arc Cutting Systems

Thermionic arc cathodes are the cathodes where thermionic emission is the main electron emission mechanism. They are used in welding (Tungsten pure or doped with rare earth oxides) and plasma arc cutting (Tungsten for cutting with inert gas or Hafnium for cutting with Oxygen plasma). There are two different sources of erosion: constant current (CC) erosion and erosion during arc initiation and termination (cycling erosion). Available experimental data for both types of cathode and both types of erosion (CC and cycling) are presented and discussed. For quite some time, it has been clear that CC erosion is due to cathode evaporation. It has been shown that as almost all the evaporated atoms return back to the cathode, the net erosion rate is much lower than the evaporation rate. The existing model allows one to calculate the ratio of these two (escape factor). It is in the range 10^-2-10^-3. The important role of cathode geometry and plasma flow pattern in the cathode proximity is discussed. The nature of cathode erosion during arc start and arc termination is much less understood in spite the fact that the corresponding erosion could be very important and even dominate for multiple cycles. Different processes that lead to this type of erosion are considered.