An optical emission spectroscopy line-ratio method for determining the plasma parameters of krypton plasma based on the excitation cross-section cliff regularity and the ground-state and metastable-state competition mechanism

The rising demand for low-cost propellants in commercial aerospace and for heavy-ion etching at ultra-high aspect ratios drives research on krypton plasmas. However, previous studies focus on the diagnostics of plasma with a low degree of ionization. There is a lack of an effective optical emission spectroscopy method to determine the electron temperature, electron density, and ground-state atom density in highly ionized krypton plasmas. In this work, we present an OES line-ratio method to determine the above parameters simultaneously based on the excitation cross-section characteristics, the kinetics of the 2p emitting states, and the self-absorption effect of 4d₈ emitting states of atomic krypton. The line ratio used for determining the electron temperature is based on the ground-state excitation cross-section cliff regularity. The ratio for electron density is based on the ground- and metastable-state competition mechanisms. The self-absorption effect of the 4d₈ emitting state is found, and it is combined with the 2p emitting states dominated by the excitation from the ground state to construct the line ratio for determining the ground-state density of atomic krypton. Additionally, we define a coupling-degree parameter and adopt a coupling-degree criterion of less than 30% to select the optimal line ratios using the above method. The optimal line-ratio scheme is validated using a krypton HET-P100 Hall thruster to determine the electron temperature, electron density, and krypton atom density along the axial direction of the discharge channel. In future work, this method can be further combined with imaging techniques to invert the spatial distribution of plasma parameters.