Researches On The Characteristics Of Femtosecond Laser-induced Copper Plasma With Spatial Confinement

At present,laser induced breakdown spectroscopy(LIBS)has some shortcomings,such as poor experimental repeatability and low detection sensitivity,which limits the rapid application of LIBS.Compared with nanosecond laser ablation,femtosecond laser has higher ablation efficiency,smaller heat-affected zone,stable breakdown threshold.The plasma that femtosecond laser induced can emit stable and accuracy spectral signal.The approach of spatial can enhance its emission spectrum intensity and improve the plasma stability.In this paper,combining the advantages of femtosecond laser induced plasma and space confinement,the characteristics of femtosecond laser induced copper plasma with spatial confinement are studied to enhance the spectrum of plasma and improve the spectral stability of plasma,so as to improve the experiment repeatability and sensitivity of LIBS experiment.The emission spectra of copper plasma induced by femtosecond laser are enhanced when it confined by cylindrical cavity with an inner diameter of 6 mm.The enhancement factor is related to the upper energy level of transition.The higher the upper energy level is,the greater the enhancement factor is.The lifetime of copper plasma is prolonged,and the electron temperature and density of plasma are increased.The experimental results show that the parameters of laser power density,spatial confinement size and spatial confinement shape have significant effects on the characteristics of spatial confinement femtosecond laser plasma.The spectral enhancement factor increases with the increase of laser power density,and the spatial confinement time is earlier.The spectral enhancement factor decreases with the increase of the confinement size,and the spatial confinement time appears with a large delay when plasma was confined by cylindrical cavity with an inner diameter of 4,6 and 8mm.The spectral enhancement factor,electron temperature and electron density under cylindrical cavity confinement are larger than those under plate constraints.Based on the fluorescence imaging method,the expansion morphology of copper plasma plume induced by femtosecond laser was studied.The axial expansion distance and velocity of plasma plume were inversely proportional to the distance of two plates,while the radial expansion distance and velocity were proportional to the distance of two plates.On the basis of the research on the characteristics of femtosecond laser plasma with spatial confinement,the quantitative analysis of copper in aluminium alloy fs-LIBS with spatial confinement was carried out.The detection limits(LOD),average relative detection errors(ARE),relative standard deviation(RSD)and Root-Mean-Squares Error of Cross-Validation(RMSECV)were 9.9 ug/g,2.35%,16% and 0.125%,respectively,when the cylindrical cavity with inner diameter of 4 mm was use to confine plasma.Compared with the free condition,it decreased 82.72%,76.85%,78.38% and 41.32% respectively.The experimental proved spatial confined fs-LIBS technology effectively improves the accuracy and repeatability of LIBS detection,laid a theoretical foundation and technical support for rapid quantitative analysis of trace elements by LIBS.