![]() ![]() ![]() Calibration was carried out using a spectral Hg lamp. The system is equipped with an Echelle grating and the spectral resolution capability is 22,000 (17.3–35.7 pm). The collected radiation was transferred through an achromatic lens and an optical fiber to the 50 μm entrance slit of the spectrometer, and then detected by a 1024 × 1024 CCD array. Plasma radiation was detected through a borosilicate glass window in the process chamber. The spectrometer applied for OES was the ARYELLE 200 (Laser Technik Berlin (LTB), Berlin, Germany) scanning a wavelength range of 381–786 nm. Particular difficulties in applying some OES-based methods due to the low electron densities are discussed in detail.įull size table Optical Emission Spectroscopy Furthermore, local variations of the plasma gas composition were investigated. Analyses of the intensities and the broadening of emission lines were performed to determine electron densities and plasma temperatures. In this work, optical emission spectroscopy (OES) was applied to study such phenomena in low pressure plasma jets. Under such conditions, hydrogen affects fundamental mechanisms like diffusion and recombination while this is less relevant under atmospheric conditions at equilibrium. Here, the low electron densities imply a disposition to thermodynamic non-equilibrium. As a consequence, the arc resistance, which is inversely proportional to the effective arc cross section, and thus the voltage and power are increased.Īn essential characteristic of some novel plasma spray processes like very low pressure plasma spraying (VLPPS) and plasma spray-physical vapor deposition (PS-PVD) is the small plasma density in the jet. This arc constriction is controlled mainly by the thermal conductivity of the plasma gas and thus by the amount of hydrogen admixture. Furthermore, hydrogen permits to increase the voltage and therefore the plasma power since the plasma arc in the torch adjusts itself to a size which minimizes the heat flux from the core to the cooled anode wall. However, it is assumed that the transport properties of the plasma were hardly affected by this, since the electron densities and thus the ionization degrees were generally small due to the low pressure conditions.Īr–H 2 mixtures are widely used in experimental devices and industrial processes, such as for example in plasma spraying, because they provide to particles injected in the plasma jet, on the one hand, a sufficiently high momentum through the argon gas, and on the other hand, a high heat transfer through the hydrogen gas. Clear indications were found, that higher hydrogen contents promoted the fast recombination of electrons and ions. The local hydrogen–argon concentration ratios revealed an accumulation of hydrogen atoms at the jet rims. Variations in the radial temperature profiles were related to the jet structure and radial thermal conductivity. In the jet cores, the lowest temperatures were found for the highest hydrogen admixture because the energy consumption due to the dissociation of molecular hydrogen outbalanced the increase of the plasma enthalpy. Moreover, the jet expanded radially as the reactive part of the thermal conductivity was enhanced by recombination of atomic hydrogen so that the shock waves were less reflected at the cold jet rims. Adding hydrogen to the plasma gas effected an increased plasma enthalpy. The small electron densities under the investigated low pressure conditions implied specific difficulties in the application of several OES-based methods which are discussed in detail. This was investigated for argon–hydrogen mixtures by optical emission spectroscopy (OES). Under such conditions, fundamental mechanisms like diffusion and recombination are affected while this is less relevant under atmospheric conditions. In particular under low pressure, there are strong effects on the plasma jet characteristics even by small hydrogen percentages. In plasma spraying, hydrogen is widely used as a secondary working gas besides argon. ![]()
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