Comprehensive Study Of The Confinements Effects On The Spectroscopic Performance Of A Laser-Induced Magnesium (Mg) And Titanium (Ti) Plasmas

Laser-induced breakdown spectroscopy（LIBS）is a quantitative analytical technique that is uti-lized to investigate the spectroscopic performance of chemical elements.Laser-induced break-down spectroscopy is also referred to as laser-induced plasma spectroscopy（LIPS）,laser optical emission spectroscopy（LOES）,and laser spark spectroscopy.The field of laser-induced break-down spectroscopy has gained a lot of scientific attention over the past decades.This technique is a non-destructive technique as compared with other spectroscopy techniques such as electric sparks,microwave induced plasmas,and atomic absorption spectroscopy（AAS）.LIBS method has a lot of advantages over other spectroscopy techniques such as conventional atomic spectroscopy methods:which require little or no sample preparations for both conduct-ing and non-conducting samples.LIBS has been used to investigate several samples such as liquids,solids,and gasses.In LIBS the plasma expansion and the plasma properties such as electron temperature and electron density can also be influenced by the nature of the ambient environment.In this dissertation,a comprehensive study of different confinements effects on the laser-induced Mg and Ti plasmas are investigated.The study was conducted by using an Nd:YAG laser sys-tem focused onto the surface of the samples under different cavities,magnetic and electric field confinements.The study investigated the effects of these different confinements on the emission intensities,electron temperatures,and electron densities of the laser-induced plasmas.The aim of this thesis is to improve the sensitivity of the LIBS technique under these different conditions.In the first study,we have investigated the plasma generated by a Q-switched Nd:YAG laser operating at its fundamental wavelength of 1064 nm focused on magnesium（Mg）and titanium（Ti）target samples in the air under atmospheric pressure.We employed different circular cav-ities radii（2.5,3.0,and 3.5 mm）and a square cavity to investigate their confinement effects on the spectral emission intensities of the plasmas.From the study,we observed that the cir-cular cavity of radius 2.5 mm had the maximum signal enhancement,and this was attributed to the compression of the plasma and reheating by the reflected shock waves.The maximum enhancement factor was approximately 3.8,3.4,and 2.8 with a circular cavity of radius 2.5,3.0,and 3.5 mm,respectively,for the Mg I-518.4 nm line at a delay time of 350 ns and laser energy of 350 m J.From our results,we observed that the radius of the cavity had a tremendous effect on the enhancement of the emission signal intensities.We also investigated the magnetic field confinements on the plasma,by sandwiching two permanent magnets with magnetic fields of0.47 T,0.62 T,0.91 T,and 1.23 T perpendicular to the plasma expansion.We also confirmed the existence of the magnetic confinement of the plasma by calculating the value ofβ,which is the ratio of the plasma pressure and the magnetic pressure.When the value ofβ=1,the ablated plasma would be stopped entirely by the magnetic field.Hence,the magnetic pressure becomes equal to the thermal pressure.However,ifβ>1 there would be no confinement in the plasma because the magnetic confinement would be ineffective.But whenβ<1 there would be effective confinement in the plasma.From our calculations,we obtained the values ofβto be less than 1 for all the magnetic fields（0.47 T,0.62 T,0.91 T,and 1.23 T）employed in our study.Which confirms the existence of plasma confinements at the presence of the magnetic fields.The second study investigated the effect of the sample temperature on the magnesium（Mg）and titanium（Ti）plasmas generated by a Q-switched Neodymium-doped Yttrium Aluminum Garnet（Nd:YAG）laser operating at its fundamental wavelength of 1064 nm.We found that increasing the sample temperature significantly enhanced the emission intensities of the plas-mas.Comparing the emission peak intensities of the case of 100℃ to the case of 300℃,we recorded a substantial enhancement of the peak intensities of the latter compared to the former.From these results,it can be observed that increasing the sample temperature had a significant effect on the emission intensities of the plasmas.We further studied the plasma dy-namics and found that increasing the sample temperature also decreases the air density around the Mg sample surface.Hence,a decline in the air density lowers the collision between the air and the plasma plume,which causes the plasma pressure to decrease.This decrease in the plasma pressure results in a drop in the collision probability and the radiation process which resulted in an increase in the plasma plume expansion of the Mg plasma.Therefore,the size of the plasma plume increases with higher sample temperatures as compared to that with lower sample temperatures.From our results,we found that the plasma plume expansion exhibited a linear relationship with the sample temperature.Hence,as the sample temperature increases the plasma plume expansion also increases.In addition,we also employed circular and square cavities to confine the titanium plasma and investigated the effect of the sizes of the circu-lar and square cavities on the titanium plasma.We observed a general improvement in the emission intensities with both the circular and square cavities and attributed this improvement to the plasma compression effect of the shock waves produced by the plasma within the cavities.Lastly,we studied the effect of an applied electric field on the laser-induced titanium（Ti）plasma.We then analyzed the influence of the electric field on the spectral intensities of the Ti plasma.From this study,we found that the Ti spectral intensities at the presence of the negative biasing voltages were much higher than those of the positive biasing voltages.The enhancement of the intensity of the-15 k V voltage was 2.5 times higher as compared to the one without an electric field.At an applied electric field of 15 k V,the spectral intensity of the Ti plasma recorded a 0.9 times decrease in the intensity with reference to the zero voltage.This was attributed to the decrease in the recombination rate by the strong attractive field between the negative front electrode and the positive ions within the plasma.The electrical characteri-zation of the Ti plasma was also investigated.From the results,we found that there was a sharp decrease in the electrical signal as the distance between the plates increased.From the Saha-Boltzmann equation,the plasma temperature was calculated.At the presence of the electric field of-15 k V,the electron temperature was deduced to be 9152 K with laser energy of 100 m J.We found that the plasma temperatures were more enhanced under the negative biasing voltage case as compared with the positive biasing voltage case.The electron density of the Ti plasma was also investigated,and the results predicted a general trend of increasing electron density with increasing laser energy.At the presence of an applied electric field of-15 k V and laser energy of 300 m J,the highest electron density of the Ti plasma was recorded as 1.5×10¹⁸cm^-3.