Radial electron density n e(r) and temperature T e(r) profiles of a microwave argon plasma at intermediate pressure were investigated by Thomson scattering. This method allows one to get n e(r) and T e(r) spatially resolved without any a priori assumption on the shape of the profile. Data were acquired in the pressure range 5–88 mbar where a transition from wall-stabilized to a radially contracted plasma mode was observed. It was found that the fitting of the radial profile can be done with a Bessel function for which the boundary radius R defined by n e(R) = 0 is a free parameter. For pressures above 20 mbar the electron density profile undergoes radial contraction, so R goes down from 3 mm at 5 mbar (wall position) to 2.09 mm at 88 mbar. The electron temperature T e(r) on the other hand is flat in the centre and rises towards the wall. For low pressures, this rise is moderate but for pressures of 20 mbar and above the increase is more pronounced.

The aim of this work is to analyse and discuss a spectroscopic method of diagnosis based on the Stark broadening of emission lines to determine the electron density and temperature in atmospheric-pressure plasmas. Usually, when the electron temperature is previously known, the Stark broadening of certain spectral lines spontaneously emitted by the plasma is used to determine the electron density in a rapid and inexpensive way. However, comparing two or more broadening of lines can allow us to diagnose the electron density and temperature simultaneously. To carry out this cross-point method, we must know the Stark broadening dependence on the electron temperature and density for different lines. In this work we have used the first three Balmer series hydrogen lines, whose Stark broadenings were calculated by means of a recent micro-field model existing in the bibliography. The experimental study was made in argon and hydrogen plasma flames. The plasmas were produced at 2.45 GHz by an axial injection torch, which can operate at atmospheric pressure under different experimental conditions to produce appropriate plasmas in ‘open air’. The flame produced in this way is a two-temperature plasma, so it is not in local thermodynamic equilibrium. Moreover, by means of the Boltzmann-plot modified with the p−6 law, we found for the hydrogen plasma that most of the observable atomic states were ruled by the excitation–saturation balance. With this method we could also determine the electron temperature.

Several active and passive diagnostic methods have been used to study atmospheric microwave induced plasmas created by a surfatron operating at a frequency of 2.45 GHz and with power values between 57 and 88 W. By comparing the results with each other, insight is obtained into essential plasma quantities, their radial distributions and the reliability of the diagnostic methods. Two laser techniques have been used, namely Thomson scattering (TS) for the determination of the electron density, n e , and temperature, T e , and Rayleigh scattering (RyS) for the determination of the heavy particle temperature, T g . In combination, three passive spectroscopic techniques are applied, the line broadening of the H β line to determine n e , and two methods of absolute intensity measurements to obtain n e and T e . The active techniques provide spatial resolution in small plasmas with sizes in the order of 0.5 mm. The results of n e measured with three different methods show good agreement, independent of the plasma settings. The T e values obtained with two techniques are in good agreement for the condition of a pure argon plasma, but they show deviations when H 2 is introduced. The introduction of a small amount (0.3 %) of H 2 into an argon plasma induces contraction, reduces n e , increases T e , enhances the departure from equilibrium and leads to conditions that are close to those found in cool atmospheric plasmas.

In the present work Stark broadening measurements have been carried out on low electron density (n e b 5·10 19 m −3) and (relatively) low gas temperature (T g b 1100 K) argon-hydrogen plasma, under low-intermediate pressure conditions (3 mbar-40 mbar). A line fitting procedure is used to separate the effects of the different broadening mechanisms (e.g. Doppler and instrumental broadening) from the Stark broadening. A Stark broadening theory is extrapolated to lower electron density values, below its theoretical validity regime. Thomson scattering measurements are used to calibrate and validate the procedure. The results show an agreement within 20%, what validates the use of this Stark broadening method under such low density conditions. It is also found that Stark broadened profiles cannot be assumed to be purely Lorentzian. Such an assumption would lead to an underestimation of the electron density. This implies that independent information on the gas temperature is needed to find the correct values of n e .

An absolute intensity measurement (AIM) technique is presented that combines the absolute measurements of the line and the continuum emitted by strongly ionizing argon plasmas. AIM is an iterative combination of the absolute line intensity–collisional radiative model (ALI–CRM) and the absolute continuum intensity (ACI) method. The basis of ALI–CRM is that the excitation temperature T13 determined by the method of ALI is transformed into the electron temperature Te using a CRM. This gives Te as a weak function of electron density ne. The ACI method is based on the absolute value of the continuum radiation and determines the electron density in a way that depends on Te. The iterative combination gives ne and Te. As a case study the AIM method is applied to plasmas created by torche à injection axiale (TIA) at atmospheric pressure and fixed frequency at 2.45 GHz. The standard operating settings are a gas flow of 1 slm and a power of 800 W; the measurements have been performed at a position of 1 mm above the nozzle. With AIM we found an electron temperature of 1.2 eV and electron density values around 1021 m−3. There is not much dependence of these values on the plasma control parameters (power and gas flow). From the error analysis we can conclude that the determination of Te is within 7% and thus rather accurate but comparison with other studies shows strong deviations. The ne determination comes with an error of 40% but is in reasonable agreement with other experimental results.