An optical emission spectroscopy method for determining the electron temperature and density in low-temperature xenon plasma by using a collisional-radiative model considering the hyperfine structure of emission line into metastable state

Optical emission spectroscopy (OES) is an important technique for the diagnostics of plasma parameters in the development of plasma devices and material processing technology. OES depends on the collisional-radiative model which can accurately describe the kinetics of excited species in plasma. Metastable species usually have higher density (2–4 orders of magnitude) and longer lifetime (3–6 orders of magnitude) compared to other excited species, thus play important roles in the reaction kinetics. However, the kinetic of metastable species can be complex due to the transport and self-absorption processes, which may lead to coupling of the plasma reaction kinetics in the adjacent areas. Moreover, the line splitting of some elements make the description of metastable kinetics more complex. This work presents an OES method based on a collisional-radiative model considering the hyperfine splitting of emission lines into metastable levels. The density distribution of metastable state is determined by the self-absorption effect. It is then taken into the rate balance equations of other excited species to make an accurate description of metastable related kinetics. The electron density and temperature are determined based on this model. The OES method is demonstrated on a miniaturized electron-cyclotron-resonance ion thruster. The metastable density, electron density and electron temperature are determined along the radial direction of the ion thruster, which can be useful for revealing the physics of discharge phenomenon and optimizing the performance of the plasma source. This method can be used to determine electron parameters in a large pressure range.