Oscillations at the Anode of Alkali-Vapor Arc Plasmas

The theory of the anode sheath for quasineutral collision-dominated arc plasmas is developed, both for the steady state and for behavior following emission of a burst of ions from the anode surface. It is shown that the decay time constant for such a burst is related to the ambipolar diffusion time, τ=Λ2/Da, where Λ is the appropriate diffusion length. The ion burst, a consequence of field emission from the anode surface, causes a local sharp decrease in the electric field strength, followed by a gradual increase as the ion cloud dissipates. A new burst of field-emitted ions begins another cycle. A summary is given of experimental work on anode-spot oscillations, some of which supports the predictions of the theory.