Transition in size and number of emitting sites with increase in arc speed over a copper cathode
A recent scanning-electron-microscope study of the damage caused to copper cathodes with a 100 nm oxide film by low-current static arcs of 3.8 ns to 40μs duration in atmospheric air has revealed unexpectedly low emittingsite lifetimes of only about 4.5 ns. Scanning electron microscopy has now been applied to arcs of 12–68 A which are moving (due to a transverse magnetic field), at speeds between 19 and 241 m/s, along a copper cathode with an oxide film of 44 nm or 100 nm thickness. The paper shows that two emitting-site modes are possible for a 44 nm oxide film. The first, which exists for speeds up to a transition range of 80–120 m/s, is shown to be similar in crater size, crater density and in the product of site current and lifetime to the results obtained for static arcs, and the areas of oxide stripped for the two cases can be related. The other mode, which occurs at speeds above this transition range, does not cause stripping of the oxide, and exhibits craters of ten times the previous diameter which are lower in surface density by between 4 and 5 orders of magnitude. The site lifetimes for this mode are much longer than for the low-speed mode where oxide stripping occurs, so that electroforming rather than switching may have taken place.
