The influence of the spatial dependence of the line profile on the shape of a line with asymmetric self-reversal emitted from inhomogeneous discharge plasmas is investigated by solving the equation of radiative transfer using relative distribution functions for the source function and the line broadening including complex overlapping lines. The theoretical results show that the features of an asymmetric lineshape are related to the broadening mechanisms and enable us to infer from the recorded line the dominant broadening mechanism. These results are experimentally confirmed using self-reversed lines emitted from high-pressure lighting plasmas. Our detailed analysis suggests that the one-parameter-approximation (OPA) model is able to reproduce satisfactorily the spectral emissivity as in the case of a spatially constant profile. An improved technique for deducing the emissivity from line-reversal is developed and applied to plasma temperature determination of metal-halide lamps. The obtained results are optimized taking into account the functional dependence of the line width and shift on the position.
The excitation temperatures of mercury as well as the electron and gas temperature in two high-pressure pure-Hg discharges operated on ac have been determined by measuring the ground- and excited-state densities. The excitation temperature depends on the excited-state energy, is significantly lower than the electron temperature, and higher than the gas temperature. The differences in the temperatures are higher at the maximum current phase and for the lower pressure discharge. Comparison with the Saha densities shows that the plasma at the maximum current is in ionizing phase, whereas it is close to local thermodynamic equilibrium at the voltage zero crossing.
This paper investigates the reliability of deducing the emissivity at the peaks of a self-reversed emission line from a simple empirical one-parameter approximation for the source function. The theory of spectral line shape formation owing to self-absorption in inhomogeneous axially symmetric plasma layers was reformulated and readily calculable expressions were obtained. In the case of self-reversed lines, the emissivity at the line maximum was calculated using different relative distributions for the densities of the absorbing and independently emitting atoms. The results were compared with those calculated using a source function described by a simple exponential law, the exponent of which is known as the inhomogeneity parameter. The obtained difference in the emissivity is less than 3%, which implies a difference in the (density-ratio) temperature between the line-levels better than 0.5%. Therefore, if the inhomogeneity parameter is known, the line emissivity can be deduced from the one-parameter approximation for the source function with reasonable accuracy. The effect of the structure of the plasma layer on the emissivity as well as the lateral dependence of the inhomogeneity parameter was also studied through numerical simulation.
The temperature of a high-pressure gas discharge is determined from the line intensity at the self-reversal maximum of a symmetric self-reversed spectral line, the depth of the valley at the centre of the line and the distance between self-reversal maxima, on the basis of the Cowan and Dieke model (1948). This procedure takes account of the actual inhomogeneity of the plasma structure. A comparison is made with Bartels' method (1950).
The distribution temperature between excited atomic states in an arc plasma can be determined using self-reversed lines. This technique was applied to a high-pressure mercury arc. The electron temperature was deduced from the distribution temperature between the 63P2,0 metastable levels. The population temperatures of the 63P1, 73S1 and 63D3 levels were also determined as well as the lowering of the population of the 63P1 resonance level caused by resonance radiation. The electron density was deduced from the Saha law, the known electron temperature and the measurement of the density of the 63D2 level. At the arc axis the difference between the electron temperature and the 'equilibrium temperature' deduced from the optically thin line technique assuming local thermodynamic equilibrium (LTE), is of the order of 1000 K and 450 K at the moments of maximum and minimum current phase, respectively. The difference found between the population temperatures of the different levels indicates that collisionally dominated plasma is not a sufficient condition for the presence of LTE in the central region of an inhomogeneous arc where the diffusion of charged particles can disturb the Saha balance.
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