No abstract

The influence of the content and nature of impurities on the rotational temperature of three probe molecules (OH, NH and ) 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).

A collisional–radiative model is proposed to describe the behaviour of a helium plasma sustained in a resonant cavity at atmospheric pressure. A set of rate constants is proposed at 2450 K. Indeed, most of the available data in the literature are reliable below 500 K. It is shown that the time post-discharge is mainly controlled by ambipolar diffusion during the first ten µs, next by electron-assisted recombination of helium ions and finally by chemionization from the excited state. This post-discharge lasts for hundreds of µs. It is mainly due to the slow recombination of electrons together with chemionization. Then, at steady state in CW mode, the electron temperature is found to be lower than 1 eV since low reduced fields are needed to sustain the discharge.

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