Abstract. Plasmas created by the microwave torch Torcheà Injection Axiale (TIA), which are around 2 mm in diameter and 15 mm long, are investigated. In these plasmas large gradients are present so that the edge is supposed to play an important role. Using global Thomson scattering measurements, in which global refers to the fact that the size of the laser beam is approximately equal to the diameter of the plasma, the electron densities and temperatures were determined. However, these results lead to discrepancies in the particle balance: the production of free electrons is much larger than the classical losses due to recombination, convection and diffusion. Radially resolved Thomson scattering measurements show the plasma has a hollow structure. Although this enhances the losses due to diffusion, still a large discrepancy remains between production and destruction of free electrons in the argon plasmas. Probably some molecular processes are significant as well. A good candidate is the charge transfer between argon ions and nitrogen molecules, since mixing with the surrounding air has a large impact on the plasma.

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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 3 P 2 metastable densities and deduce plasma properties. It is found that the density is maximal in the active plasma zone (∼1.5 × 10 18 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 CO 2 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) × 10 21 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.