Influence of impurities on the temperature of an atmospheric helium plasma in microwave resonant cavity

The influence of the content and nature of impurities on the rotational temperature of three probe molecules (OH, NH and N2+) in a helium plasma created in a microwave resonant cavity is investigated by the synthetic spectra method and by mass spectrometry. OH is found to be the only reliable probe to obtain a satisfactory estimation of the gas temperature. In our experimental conditions, depending on the impurity, the gas temperature varies strongly and at 500 ppm, for instance, a difference of 700 K is possible (∼30% of the mean temperature)