Abstract
We illustrate a Stark broadening analysis of the electron density Ne and temperature Te in a laser-induced plasma (LIP), using a model free of assumptions regarding local thermodynamic equilibrium (LTE). The method relies on Stark parameters determined also without assuming LTE, which are often unknown and unavailable in the literature. Here, we demonstrate that the necessary values can be obtained in situ by cross-calibration between the spectral lines of different charge states, and even different elements, given determinations of Ne and Te based on appropriate parameters for at least one observed transition. This approach enables essentially free choice between species on which to base the analysis, extending the range over which these properties can be measured and giving improved access to low-density plasmas out of LTE. Because of the availability of suitable tabulated values for several charge states of both Si and C, the example of a SiC LIP is taken to illustrate the consistency and accuracy of the procedure. The cross-calibrated Stark parameters are at least as reliable as values obtained by other means, offering a straightforward route to extending the literature in this area.
Highlights
We illustrate a Stark broadening analysis of the electron density Ne and temperature Te in a laserinduced plasma (LIP), using a model free of assumptions regarding local thermodynamic equilibrium (LTE)
Invasive technique capable of only limited spatial and temporal resolution, the former is often difficult to apply in practice, while the laser-based methods are preferred as benchmarks but require complex instrumentation and have unintended probe-induced heating effects as a recognized concern[1,2]
Customary analyses usually rely on the assumption of local thermodynamic equilibrium (LTE)—that is, a Maxwellian energy distribution function (EEDF) and Boltzmann population ratios—to evaluate Te from the relative intensities of emission lines according to the “Boltzmann plot” method and, given this information, Ne from the Stark-broadened widths[3,4,5]
Summary
We employ SiC as an example of a material for which the LIP contains two species (C+ and Si+, C II and Si II) with emission lines at similar wavelengths, for both of which there exist well-determined Stark broadening parameters independent of any assumptions regarding LTE15. These and all subsequent images employ a logarithmic false-colour intensity scale, shown at the right of each panel. The plasma must not be so dense that radiation at the wavelengths of interest is unable to escape from the imaged volume, it need not be optically thin if, as here, self-absorption is taken into account While these conditions will be seen to be fulfilled in the present case from the mutually identical Ne and Te values recovered from each species, we emphasize that reliable determinations of unknown Stark parameters require careful attention to these limitations, which may necessitate different experimental conditions or a more sophisticated apparatus than those used in this work.
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