Influence of Nozzle Diameter on Electric Arc Dynamics in a Cascaded-Anode Plasma Torch

Abstract

Electric arc dynamics in plasma torch affects plasma jet stability and consequently, coating properties. Depending on plasma torch design, voltage fluctuations can vary from 100 % to only a few percent of the mean voltage. Particularly, cascaded-anode plasma torch leads to very low voltage fluctuation owing to the presence of neutrodes that limit the amplitude of arc fluctuations. However, electric arc dynamics and electrode erosion process in this type of plasma torch are still poorly understood. The aim of this work is to deepen the knowledge on the influence of nozzle diameter on electric arc dynamics for two plasma forming gas compositions by means of several diagnostics devices (end-on imaging, current and voltage time monitoring, plasma jet brightness fluctuations and thermal balance determination). Reducing nozzle diameter from 9 mm to 6.5 mm results in higher voltage fluctuations, lower mean voltage and lower plasma torch thermal efficiency, probably due to a more evenly distributed warm plasma gas in the anode nozzle volume, as suggested by the higher plasma brightness. Nozzle observations after testing show significant wear in a 6.5 mm diameter nozzle, which may be evidence of a longitudinal movement of the electric arc on the anode surface, leading to high voltage fluctuations.