Application of Calibration-Free Boltzmann Plot Method for Composition and Pressure Measurement in Argon Free-Burning Arcs

Arc plasmas have been extensively studied, experimentally and by means of modeling, due to their wide range of applications. In this article, a calibration-free Boltzmann plot method was applied to measure the species composition and pressure in the arc plasmas based on the assumption of local thermodynamic equilibrium and only single ionization of atoms. Experiments were performed on the argon free-burning arcs at two different pressures. Firstly, several Ar I and Ar II spectral lines were utilized to draw the Boltzmann plot. Then, the plasma temperatures were obtained by three different methods, the Boltzmann plot method, two-line Saha–Boltzmann plot method, and the Fowler–Milne method. The species concentration was calculated using the intercepts with ordinate in the Boltzmann plot. Combined with the electron number density determined from the Stark broadening of Ar I 696.54 nm, arc pressures are calculated based on the equation of state. The measured species concentration and pressures are in good agreement with the theoretical results, indicating that the used method is reliable for further arc plasma applications.