Emission Spectrum Analysis of Spatially-Resolved Laser-Induced Plasma

Abstract

A laser-induced plasma (LIP) is one-dimensionally resolved along a laser line and temporal evolution of the emission spectra at each position of LIP are obtained. A plasma is generated by focusing pulsed-laser (2nd harmonics of ND:YAG) just above a jet nozzle supplying dry air during the experiments to avoid any interferences between laser pulses. A 85 mm Nikon camera lens mounted on a translational stage collects the emitted photons from LIP and focuses it onto an entrance slit of a spectrometer with a magnification factor of 10 to improve spatial resolution. The region of interest (ROI)—a section of the LIP of which its emission is illuminated into the spectrometer—is varied by translating the lens. A thickness of the ROI is estimated to 10 μm considering a slit width and the magnification factor. A spatial interval and a total length are 0.1 mm and 3.0 mm, respectively. A gate time of an intensified CCD is fixed at 10 ns and a gate delay is varied from 100 to 400 ns with a constant interval of 10 ns after laser arrival to observe temporal evolution of the emission spectrum. As a result, the intensity matrix is given by a function of wavelength (a spectral range is from 650 to 780 nm), position, and gate delay. The spectral range is chosen because it includes neutral oxygen (O I) lines used for plasma temperature calculation and neutral/singly ionized nitrogen lines (N I/N II) which are useful in evaluating a relative abundance of neutral atoms and ions. A plasma temperature distribution and its temporal evolution are estimated by the local thermodynamic equilibrium (LTE) assumption and the Boltzmann analysis. Two neutral oxygen lines at 715.67 and 777.19 nm are used for the Boltzmann analysis. The result shows a non-uniformity in plasma temperature and a gradual decay of overall temperature during the experiments (gate delay from 100 to 400 ns). On the other hand, a total intensity distribution is also calculated by summing up the spectral intensities. Strong total intensity regions are not matched with that of high temperature. In contrast, high temperature regions are more likely to have a stronger ionized nitrogen line at 661 nm compared to a neutral nitrogen line at 746 nm.