Dynamic measurements of cathodic emission in a moving arc

A new technique has been developed for the measurement of the fine structure of the cathodic emission in a moving arc. The results obtained give evidence for the initiation and extinction of individual emitting sites and allow the determination of the current they carry. The lifetime of the sites is determined from the exponential-like decay in the number of extinction of emitting sites following a perturbation of the arc. On copper, in air, for a 700 A arc typical values are: microcurrent 0.6 A, lifetime 1 µs, dI/dt 3 × 10⁷A/s, current density 10¹⁴A/m². In addition, an electron microscope study of the surface using a replica technique - magnification 24000 X - has revealed the existence of 0.1 µm craters and of 1 µm ring-clusters made up of 0.1 µm unit craters. There is no directional motion of the craters. The motion of the discharge is accompanied by the formation of an excess of emitting sites over extinguishing sites located at the front of the arc, while the opposite is true at the rear of the arc.