High-speed imaging investigation of transition phenomena in the pilot arc phase in Hf cathodes for plasma arc cutting

The behaviour of Hf cathodes has been investigated with high-speed camera (HSC) imaging techniques during the low-current pilot arc phase to highlight phenomena that take place during the transition from an insulating, non-emissive, cold phase to a conductive, emissive, hot phase for Hf-based materials used in electrodes for plasma arc cutting (PAC) of mild steel plates. The different behaviours during the start-up phases of new or used electrodes have been considered. Such phenomena have been partially described on physical bases in the past but can now be fully seen and analysed using diagnostics. Conclusions can be drawn concerning the particular conditions under which heat transfer transients in the cathode tip during the PAC pilot arc take place, giving additional useful information for future design-oriented simulation of such phenomena under different geometric and operating conditions, with the final aim of optimizing the cathode’s expected service life.