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.