Experimental study of erosion of hafnium electrodes for oxygen plasma arc cutting

Experimental study has been conducted on erosion of hafnium electrodes for oxygen plasma arc cutting. Hafnium (Hf) is used as a refractory cathode material for thermionic emission at high temperature region. Erosion of Hf electrode was investigated by measuring the concavity depth on Hf electrodes after operations. The measurements were done on several operating conditions. The erosion properties have been found to be separated to two portions. One of the Hf erosion happens in transient period including start-up and shut-off phases of plasma arc when arc current is increasing or decreasing. The other erosion happens in the stable arc phase when arc current is kept at operating current. Spectroscopic measurements of the arc plasma jet coming out from a nozzle have been conducted in order to study the behavior of erosion of Hf electrode. An atm line of Hf at λ=340 nm was observed by a monochromator. Characteristic intermittent radiations of Hf in addition to background continuum were observed in transient period when the arc current or gas pressure changes. It is considered that the intermittent radiation comes from droplets of Hf ejected from the melted electrode surface. And the ejection is considered to be induced by a break of balance of fluid dynamic on the melted electrode.