Abstract
For volatile cathodes (e.g., Hg), the ratio of the work function and the latent heat of vaporization is such that vaporization cooling overwhelmingly exceeds ion heating at moderate cathode temperatures. This fact was an obstacle in cathode spot modeling for cathodes of such metals. Analysis of the emission process at higher temperatures revealed the effect of “phase transition” that takes place at some critical temperatures: 1360 K for mercury and 2040 K for cadmium. During a very sharp transition that occurs within a few degrees or less, there is a simultaneous rise of electron emission, the arrival of an intense ion flux at the cathode, and a reduction or even reverse of emission cooling (the Nottingham effect), which leads to a completely new cathode state. Heat flux to the cathode goes from being highly negative to highly positive. This makes it possible for the arc to burn. The characteristics of that new state are similar to that of a regular cathode spot of a vacuum arc. A scenario of cathode spot cycling with this new state is suggested.
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