Radial distribution of electron density, gas temperature and air species in a torch kind helium plasma produced at atmospheric pressure

The radial dependence of the electron density and the gas temperature, two major parameters of the microwave helium plasma produced at atmospheric pressure by the axial injection torch, is presented here. An intensified charge coupled device in combination with an image rotation system allowed to perform laterally resolved emission intensity measurements that were mathematically transformed into radial emissivity using the Abel inversion technique in conjunction with a Smoothing and Interpolation Procedure. When the Abel inversion is not used, only radially averaged values of the electron density and gas temperature values are obtained. The results presented here have been used to obtain the radial range of application of such averaged values. The variation of the radial distribution of these parameters with the incident microwave power and the He flow-rate was also obtained. The entrainment of species from the surrounding air into the plasma was also studied in radial terms. Small amounts of O(I), H(I), N(I) and N2+ were found in addition to He(I); this suggests a high entrainment of these elements from the ambient air into the plasma, which affects the plasma kinetics and the parameter values.