Plasma characterization of an atmospheric microwave plasma torch using diode laser absorption studies of the argon 4s ³P₂ state

Diode laser absorption experiments have been performed on atmospheric microwave induced argon plasmas produced by the `Torche à Injection Axiale' in order to study the 4s ³P₂ metastable densities and deduce plasma properties. It is found that the density is maximal in the active plasma zone (∼1.5×10¹⁸ m⁻³) and rapidly decreases in the recombining plasma zone. In strongly ionizing atmospheric plasmas the 4s population can be used to estimate the electron temperature. The obtained value (1.3 eV) is considerably lower than the temperature found with Thomson scattering in a previous study (∼2.0 eV). The very strong scattering of the laser spot on the plasma in the expansion zone just above the nozzle suggests that in this region the plasma is filamentary. Above this zone lateral measurements have been Abel-inverted. Hollow profiles, like those determined for the electron density in previous studies, have not been observed for the 4s distribution. It is found that the plasma zone with high 4s density is extended further downstream with increasing gas flow whereas the introduction of typically a few per cent of CO₂ already results in a significant quenching of the metastables. The width of the absorption profile varies from typically 10 GHz in the active zone, where Van der Waals broadening is dominant, to 4 GHz in the recombining zone, where Doppler broadening becomes significant. The position of the central absorption frequency is found to be dependent on among other things the position in the plasma. If this shift is attributed to the Stark effect it is obtained that the maximum electron density is within (2–4)×10²¹ m⁻³. Since this latter value is rather high, it must be concluded that the Van der Waals effect induces a shift as well. The width and shift have been used to study trends in plasma properties (i.e. gas temperature and electron density) as a function of various parameters (i.e. position in the plasma, gas flow and gas composition). The observed tendencies are in agreement with findings in previous studies.