Comparative studies of compact kHz and MHz driven low pressure plasmas by emission and laser spectroscopy

Abstract

The temporal evolution of a low pressure argon plasma driven by a combination of radio frequency and kilohertz frequency power sources was characterized by phase resolved optical emission and laser absorption spectroscopy. The compact, low pressure plasma was formed in a hybrid surface/jet geometry and powered by a high voltage power supply operating at 31 kHz and by a lower voltage RF supply operating at 13.56 MHz. An accurate evaluation of the 1s state density dynamics was performed by means of laser absorption measurements of the 1s5 and 1s4 sublevels. The emission spectra were recorded with a submicrosecond temporal resolution and evaluated using an extended Corona model. In comparison with the modeling results, the phase dependent ratio of different Ar(2p) → Ar(1s) emission lines was used to determine temporal profiles of electron temperature and relative electron density. The results were compared with the results from time averaged emission measurements by performing an identical analysis. Strong spatial and temporal variations of plasma parameters were observed when the plasma was driven solely by the 31 kHz source, resulting in nonequilibrium conditions for long living states. Inconsistent observations in the reconstructed temporal development of plasma energy and density were further investigated by illustrating full spatiotemporal reconstruction of plasma parameters. These results permit clear identification of strong spatial gradients through the tube.

Diagnostics of a high-pressure DC magnetron argon discharge with an aluminium cathode

“…Such a method was developed and discussed in details in Ref. [46] and compared with a complete CRM [44] for similar plasma conditions. Rate coefficients for electron impact excitation were calculated using the same set of excitation cross-sections as in Refs.…”

“…[43,44] and downloaded from the LXCAT database [52] Ref. [46]. Evaluated electron temperatures are relatively low (∼ 1.5 eV) and show systematic dependence on pressure and power.…”

“…Both spectroscopic models have their drawbacks that could explain observed discrepancies in electron temperature. Due to the low density of Ar(1s) states, the excitation from ground state Ar by energetic electrons is the dominant process and secondary excitation are negligible making the whole method impervious to low energetic electrons [46,49] and resulting in large uncertainties in electron temperature. The evaluated electron temperatures were used for an estimation of relative changes of electron density resulting in a different overall response of the system.…”

“…In the complete model for argon [44,45] the populations of the first 14 excited states (Ar(1s) and Ar(2p) branches) were modelled including electron impact excitation from ground state, electron coupling between and within the Ar(1s) and Ar(2p) levels, and electron quenching and radiative processes including light trapping. It was shown that in the case of low temperature plasmas with moderate electron densities and temperatures, the collisional-radiative model (CRM) can be simplified to an extended corona model including excitation from ground and 1s states and radiation transfer processes [44,46]. As an outcome of such a description, the population of Ar(2p) excited states linearly depends on the electron density making the analysis only dependent on the electron temperature.…”

“…Similar to our previous works [44,46] the argon spectra was first analysed using the line branching method in order to estimate the argon 1s state densities that play an important role in the whole CRM and are non-trivial to model. The line branching model compares measured intensities of lines originating from the same excited level, to the theoretically estimated ratios (ratios of the Einstein coefficients for spontaneous emission), as in [44,49].…”