ARC Ignition Phenomena with a Transpiration Cooled Anode

This paper deals with the problem of ignition to the porous transpiration cooled anode of an arc in 1 atm. argon. This was observed for both porous graphite and sintered porous tungsten anodes, in the latter case causing melt spots to form which limited the useful life of the anode. A study was undertaken to overcome this problem in which the transient behavior of the arc current, voltage, anode surface area coverage and gas flow rate of transpiration gas was observed by high speed electronics and cinematography during the ignition period. From these data a computation of the anode current density vs. time showed abnormally high values during the first second or two following ignition; e.g. 1000 amps/cm2 averaged over the first second for a steady state arc current of 70 amps. The damage was attributed to excessive thermal loading during the arc growth period and eliminated by means of a properly control led ignition sequence.