Sweeping electrostatic probes in atmospheric pressure arc plasmas-Part II: temperature determination

In a previous paper, it was shown that only the ion portion of the probe characteristics curve (V-I) can be used to estimate the temperature from probes sweeping in the column of a flowing atmospheric pressure arc plasma. Methods relating the measured ion current directly to the temperature can be used, if a particle density-temperature relationship is available. However, several sources of "disturbance" need to be considered in the application of these methods. The main focus of this paper is in establishing how the experimental data permit an identification of the correct model taking into account these limitations. It is shown that some previously published criteria for the establishment of cooling induced by probes in atmospheric pressure arc plasmas and of recombination in the perturbed region are inadequate and their modification is consistent with the gathered data. Because of the thin and collisionless sheath, the temperatures reconstructed by probes belong to the edge of a sheath which is fully embedded in a perturbation region (PR) and not to the bulk of the plasma. A range of arc currents is studied and comparison with data from emission spectroscopy is used to assess and quantify the degree of depression shown by the probe determined temperatures. The consideration of flow velocity within the expressions for the probe currents shows that ad hoc corrections, in absence of direct experimental evidence on flow velocity distributions, do not lead to substantial benefit for the temperature corrections. "Cooling" mechanisms are considered and between ion energy loss and ion-electron recombination, the first dominates at temperatures below about 7500 K, whereas the latter dominates within the PR in the temperature range 9-13 000 K. For higher temperatures, the two contributions are very close to each other. Despite the severe interpretation difficulties, the practical reproducibility of the results and their empirical relationships with spectroscopic values assumed "exact," permits the reconstruction of reasonable electron temperature by the probe method in these plasmas even if experimental data on flow velocity distribution are not available.