Physics of Spotless Mode of Current Transfer to Cathodes of Metal Vapor Arcs

Abstract

A fresh attempt is made to clarify the physics of the diffuse, or spotless, mode of current transfer that may occur on cathodes of vacuum arcs if the average cathode surface temperature is high enough, about 2000 K. It is shown that in the case of chromium cathode the usual mechanism of current transfer to arc cathodes cannot sustain current densities of the order of 10 5 -10 6 A · m −2 observed in the experiment, the reason being that the electrical power deposited into electron gas in the nearcathode space-charge sheath is insufficient. It is hypothesized that the electrical power is supplied to the electron gas primarily in the bulk plasma, rather than in the sheath, and a high level of electron energy in the vicinity of the sheath edge is sustained by electron heat conduction from the bulk plasma. Estimates of the current of ions diffusing to the sheath edge from the quasineutral plasma gave values comparable with the experimental current density, which supports the above hypothesis. On the contrary, the spotless attachment of vacuum arcs to gadolinium cathodes may be interpreted as a manifestation of the usual mechanism of current transfer to arc cathodes. Results given for gadolinium cathodes by a model of near-cathode layers in vacuum arcs conform to available experimental information.

Detailed Numerical Simulation of Cathode Spots in Vacuum Arcs—I

“…[This happens when the average temperature of the cathode surface is high enough, typically around 2000 K (see [1] and the references therein). It is interesting to note that the physics of this regime, while supposedly being relatively simple, still has not been fully understood (see [2] and the references therein.)] On the other hand, in most cases, the current on the cathode of a vacuum arc is localized in bright, narrow regions, or cathode spots.…”

Validation of evaporation-determined model of arc-cathode coupling in the peak current phase in pulsed GMA welding

“…Detailed complete models for e.g. a vacuum arc with a chromium cathode [7] and also for a tungsten inert gas welding arc [20] have shown that a considerable part of energy required in the cathode layer is provided by the plasma bulk and transferred by electron energy flux.…”

“…[4] or [5] are not yet well established and investigated. One of the reasons is that the most prominent approaches for the consideration of the plasma-cathode interaction tend to predict cathode surface temperatures that are considerably higher than boiling temperature >3144 K [6][7][8]. However, in the actual process, neither boiling, nor the highly constricted cathode spots, which might justify such local overheating, can be observed.…”

Arc-cathode attachment in GMA welding

“…Although there have been many models developed for cathodic attachment, that have been very successful in their prediction of the conditions at the cathode in an arc discharge, these models usually refer to either vacuum arcs [7] or tungsten cathodes [8]. Both these conditions are not present in the case of GMAW.…”