Oxide films on arc cathodes and their emission and erosion

Evidence is discussed which suggests that there are significant differences between the processes occurring when the films are <5-10 nm thick and those occurring when they are >10 nm thick. For the former, electron emission can occur by tunnelling at an average electric field of about 10 9 V m -1 whereas for the latter some evidence suggests that emission occurs when conducting filaments are electroformed through the film at an average field of 10 7 -10 8 V m -1 . The effects of UV irradiation on charge production and differences in crater rims between these two film thicknesses, and possible differences in positive ion current in the cathode fall region are discussed.