Electron Temperature And Electron Number Density Study In A Laser Ablation Fast Pulse Discharge Plasma

Laser-induced breakdown spectroscopy (LIBS) technology has been widely used in the trace element detection, environmental protection and space exploration. However, LIBS hasn’t been fully developed and widely accepted in practical application, because of its poor detection limit. As an effect to improve the performance of LIBS technique, a new technique, laser ablation-fast pulse discharge plasma spectroscopy (LA-FPDPS), has been proposed recently in our lab. Similar to that in LIBS technique, the spectroscopy line intensities are used as the criterion to determine the concentrations of elements in LA-FPDPS technique. As the spectral line intensity and continuum emission of the plasmas depends strongly on the plasma formation process, the investigation of the effect of parameters which affect the plasma forming processing on the physical characters of plasma, and understanding the dynamics of plasma plume is therefore essential for achieving improved application of LA-FPDPS technique.The plasma temperature and the electron density are the two most significant parameters of fundamental importance for the characterization of plasma sources. Specifically, the electron number density makes it possible to evaluate how far the plasma is from the local thermodynamic equilibrium (LTE) conditions, which is the precondition for the plasma emission spectroscopy analysis. Therefore, the investigation of the temperature and electron number density of laser ablation-fast pulse discharge plasma has been carried out in this thesis.Firstly, the laser induced breakdown spectroscopy (LIBS) and the laser ablation-fast pulse discharge plasma spectroscopy (LA-FPDPS) technique were introduced respectively. Some basic concepts related to plasma, such as the plasma formation, expansion, LTE, line broaden were introduced as well. The details to evaluate the plasma electron temperature and electron number density using the Saha-Boltzmann plot and Stark broadening linewidth were presented. Then, the experimental setup used to carry this study was introduced. By analyzing the relative standard deviation (RSD) and the signal to noise ratio (S/N) of the spectrum line intensities at different discharge voltage, the optimum discharge voltage in LA-FPDPS was obtained.The electron temperature and electron number density of plasma generated by laser ablation and the combination of laser ablation and discharge were studied in detail. For soil plasma, the electron temperature and electron number density of the spark plasma are higher than that of laser plasma. Along with the increment of discharge voltage, the electron temperature and electron number density of soil plasma increase as well. In contrast, for silicon plasma, although the electron temperature becomes higher than laser plasma and it increases with the increment of discharge voltage, similar to that in soil plasma, the electron number density of Si plasma seems does not change when increase the discharge voltage, and almost equal to that in laser plasma of silicon. Possibly, this is due to the higher ionization energy of Si in pure silicon crystal, which limited the free electron generation in the Si plasma. In addition, the spatial distribution of the electron temperature and electron number density of laser plasma and spark plasma of Si were measured as well.Finally, several calibration curves used to measure heavy metals in soil sample were obtained using the LA-FPDPS technique.