S. Jovićević

Parametric study of an atmospheric pressure microwave-induced plasma of the mini MIP torch } I. Two-dimensional spatially resolved electron-number density measurements

Abstract The results of spatial distribution measurements of the electron-number density, N e , in an atmospheric pressure microwave-induced plasma in argon are presented. Electron-number density is determined from the width of the hydrogen H β 486.13-nm line. The influence of the microwave input power in the range 80–150 W on the spatial distribution of N e is first determined. For the selected input power of 100 W, the influence of molecular hydrogen in wet and desolvated nebulizer and support gas, and the corresponding changes in the electron density distributions are studied. The influence of potassium as a low ionization-potential element on the spatial distribution of N e is also studied, with the result that a considerable lowering of the electron number density is detected.

Optical emission spectroscopy for simultaneous measurement of plasma electron density and temperature in a low-pressure microwave induced plasma

The simple optical emission spectroscopy technique for diagnostics of low pressure microwave induced plasma (MIP) in hydrogen or in MIP seeded with hydrogen is described and tested. This technique uses the Boltzmann plot of relative line intensities along Balmer spectral series in conjunction with the criterion for partial local thermodynamic equilibrium for low electron density (Ne) plasma diagnostics. The proposed technique is tested in a low pressure MIP discharge for simultaneous determination of electron density Ne (1017–1018 m−3) and temperature Te.

Parametric study of an atmospheric pressure microwave-induced plasma of the mini MIP torch — II. Two-dimensional spatially resolved excitation temperature measurements

Abstract Two-dimensional mapping of the excitation temperatures have been carried out in the microwave-induced plasma torch with tangential argon flow. It is detected that the presence of wet aerosols in a nebulizer gas increases axial excitation temperature while desolvation acts in the opposite direction. The influence of low hydrogen concentration in the support gas and potassium in the nebulizer gas on the temperature distribution is studied in detail. The comparison of excitation temperature and electron number density distributions clearly indicates non-equilibrium plasma conditions.

Plasma diagnostics using the He I 447.1 nm line at high and low densities

The broadening of the He I 447.1 nm line and its forbidden components in plasmas is studied using computer simulation techniques and the results are compared with our and other experiments. In these calculations wide ranges of electron densities and temperatures are considered. Experimental measurements are performed with a high electron density pulsed discharge and with a low electron density microwave torch at atmospheric pressure. Both calculations and experimental measurements are extended from previous works towards low electron densities in order to study the accuracy of plasma diagnostics using this line in ranges of interest in different practical applications. The calculation results are compared with experimental profiles registered in plasmas diagnosed using independent techniques. The obtained agreement justifies the use of these line parameters for plasma diagnostics. The influence of self-absorption on line parameters is also analysed. It is shown that the separation between the peaks of the allowed and forbidden components exhibits a clear dependence upon plasma electron density free of self-absorption influence. This allows the peak separation to be used as a good parameter for plasma diagnostics. From the simulation results, a simple fitting formula is applied that permits obtaining the electron number density plasma diagnostics in the range 5 × 1022–7 × 1023 m−3. At lower densities the fitting of simulated to experimental full profiles is a reliable method for Ne determination.

Influence of the target material on secondary plasma formation underwater and its laser induced breakdown spectroscopy (LIBS) signal

The goal of this study was to investigate whether a secondary plasma can be formed on a non-metallic target under water and to give insight into the related processes. The material of choice here was alumina, since its physical, thermal and mechanical properties are substantially different from those of pure Al, only for which secondary plasma formation was recently demonstrated. To achieve this, plasma and bubble formation on alumina under water after single pulse laser excitation were studied using fast photography, shadowgraphy, Schlieren and LIBS techniques. The results show that the secondary plasma caused by backward heating of the target and successive slow target evaporation into the growing vapour bubble also occurs for alumina. The secondary plasma formed on alumina involves only a narrow interaction region on the target resulting in an almost spherical plume shape. In contrast, on thermally conductive and easily melting/evaporating aluminium, the secondary plasma is intense, with a large volume which is flattened on the target surface. Inside the expanded bubble above the alumina target, glowing particles were not observed. Due to less efficient secondary plasma formation on alumina compared to aluminium, its optical emission only slightly increases at a delay of 400 ns from the laser pulse but emission persists during three bubble cycles with a total duration of about 650 μs. The LIBS spectra related to the secondary plasma are almost free from any continuum component and show narrow emission lines from low excited states. Here we discuss the observed differences in the plasma’s spatial, temporal and spectral evolution on the two considered target materials. The obtained results indicate that under water a secondary plasma might be formed on very different materials and that its detection produces a good quality LIBS signal from single pulse excitation using a commercial nanosecond laser source.

The Beenakker's Cavity for Uniform Column of Nonequilibrium Argon Plasma Generation: Experiment and 3-D Modeling

In this experiment, a uniform column of nonequilibrium argon plasma (p = 0.5 torr) generated by the Beenakkers cavity has been studied and self-consistent nonlocal 3-D modeling has been applied. The results of modeling allowed explaining uniformity of long plasma column outside of the cavity. It is found that the nonlocality provides axial homogeneity of such parameters as mean energy density of electrons and concentrations of all plasma particles. Images of plasma properties are presented.

Separation between Allowed and Forbidden Component of the He I 447 nm Line in High Electron Density Plasma

Stark broadened He I 447.1 nm line is measured and the dependence of the separation between its allowed and forbidden components upon electron density is analyzed. Experimental results are compared with computer simulation results and with former experimental results.