Experimental Study of the Discharge Characteristics of a Stepped-Nozzle Arc Plasma Torch

The stepped-nozzle arc plasma torch (SNAPT) is a promising arc source due to the advantages of high stability and high energy density, but the micro-time scale discharge characteristics have not yet been adequately investigated. In this study, a SNAPT was designed and studied, in order to investigate the effects of discharge current, gas flow rate, and nozzle morphology on the discharge characteristics. The results show that the volt-ampere characteristics of the SNAPT have an increasing curve when the gas flow rate is greater than 4 Nm³/h, and a decreasing curve when the gas flow rate is 3 Nm³/h. This result is related to the position of the arc root, and an arc constriction phenomenon occurs at high current and low gas flow rate. The thermal efficiency of the SNAPT decreases with the increasing current and decreasing gas flow rate, ranging from 69.6 to 82.3% within the experimental parameters. The optical emission spectroscopy results show that there are many active particles inside the plasma. The experimental parameters of the heavy particle temperature of 4851.5 to 9189.3 K and the electron temperature of 7846.9 to 10185.6 K both correspond to the Boltzmann distribution and are close to the local thermodynamic equilibrium state. Comparative experimental results show that the cylindrical-nozzle has the decreasing volt-ampere characteristics and higher voltages, but has large voltage fluctuations and poor stability compared with the stepped-nozzle. The results of the study are of guiding significance for design, selection of operating parameters, and application scenarios for this type of plasma torch.