Time-resolved optical characterization of the interaction between a laser produced plasma and a spark discharge

Abstract

The interaction between a nanosecond laser produced plasma and a unipolar high voltage discharge was analyzed using time-resolved spectroscopy, fast photography with narrow band interference filters and profilometry. Here the laser ablation fluence was varied in the 60–1000 J/cm2 range. The 600-kW electrical discharge was triggered by an ablation plasma on the aluminum target at time 700 ns in order to enhance the laser-induced breakdown spectroscopy (LIBS) emission. Most of the spectra were obtained from a spot 1 mm above the target and these were: time-integrated to gauge overall enhancement and time-resolved to investigate emission enhancement, temperature and density as a function of time. It was found that the time-and-space integrated LIBS signal was enhanced by up to one order of magnitude at the lowest laser fluence employed when compared with a conventional LIBS experiment. Also, that this emission increment could not be mainly attributed to an increased mass removal, but rather to an enhancement of plasma ionization degree as was observed by means of fast photography with filters. This causes longer plasma duration and more enhancement of ionic species than neutrals. It was also found that both the electronic density and the temperature increase when the spark discharge is applied, in comparison with the ablation plasma. Emission enhancement is inversely proportional to laser fluence due to the fact that for higher fluences, by the time the discharge is applied, the laser-produced plasma is already highly excited.