Zeeman-resolved TDLAS using metastable levels of Ar in the weakly magnetized plasma of the linear plasma device PSI-2

Abstract Tunable diode laser absorption spectroscopy was applied at the linear plasma device PSI-2 to measure the magnetic field, temperature of argon and density of metastable species in a low density gas discharge. The measurements on the two metastable levels of Ar were performed by scanning the plasma column of PSI-2 at different radii. The obtained magnetic field using the lines at 763 and 772 nm (Ar) was found to be systematically lower (by 5% to 17%) than the calculated vacuum field. Part of the deviation arises from the line integration of the absorption signal. The radial gradient of the magnetic field strength combined with the radial metastable density determines the magnitude of this contribution (2%–3%). The temperature of the neutral gas was found to be essentially constant within the discharge chamber. The gas temperature rises with increasing cathode current and magnetic field due to an increase in the plasma density and, consequently, an increase in the energy transferred to the neutral gas by collisions with the charged particles. The density of the 4 s metastable level with J = 2 was found to be 8–9 times higher than that of the level with J = 0 similarly to observations by others in non-magnetized plasmas. To understand this trend a simple collisional-radiative model for the metastable argon 4s J = 2 level was developed. Depending on the treatment of the 4p levels it predicts a lower and an upper limit of the metastable density. The experimental values are within the limits predicted by the model indicating that the complex kinetics of the excitation and deexcitation collisional-radiative processes lead to this deviation from the statistical equilibrium.