Electron Temperature and Density of the Plasma Measured by Optical Emission Spectroscopy in VLPPS Conditions

Measurements of electron temperature and electron density have been carried out on a plasma plume under very low pressure conditions (P = 1 mbar) using optical emission spectroscopy. The plasma torch was operated at a power level of 20-55 kW corresponding to an Ar/H₂ flow rate of 40/0 to 40/8 L/min and current intensity variation from 500 to 700 A. The electron temperature (T _e) was determined based on the calculation of the relative intensity of two spectral lines, H_α and H_β. It was found that T _e decreased from 0.2 to 0.9 eV with increasing detection distance from the nozzle exit from 40 to 60 cm. Changes in the current and the flow rate of H₂ also had a significant impact on it. There are significant fluctuations in the hydrogen emission intensity when using current intensity of 700 A or with increasing hydrogen fraction in the mixture. However, it seemed that there was no evident change in T _e when different powder sizes were injected. Correspondingly, electron densities ranged from 0.76 × 10¹⁴ to 1.41 × 10¹⁵ cm⁻³ and were obtained using Stark-effect broadening of the H_β (486 nm) line under the assumption of local thermodynamic equilibrium (LTE).