Study On Effect Of Spatial Confinement On Laser-induced Breakdown Spectroscopy

During the operation of tokamak,a series of plasma-wall interactions(PWI)between the edge plasma and plasma-facing material are inevitable,which induces sputtering,erosion of the first wall,deposition and retention of tritium.The analysis of PWI is one of the most important tasks for the future fusion device and largely determines the lifetime and availability of fusion reactors.Laser-induced breakdown spectroscopy(LIBS)has become an attractive tool for in-situ diagnostic of the first wall materials.But currently there are disadvantages such as low spectral line intensity under vacuum conditions,unfavorable diagnosis of trace elements,low detection sensitivity,short spectral line lifetime,and low accuracy of quantitative analysis.In this thesis,the effects of the spatial confinement on the spectrum of the laser ablated tungsten material have been investigated in detail,as a prerequisite for improving the accuracy of quantitative analysis in laserinduced breakdown spectroscopy in vacuum in the laboratory.The main contents of this paper are as follows.(1)The time evolution of the LIBS with and without spatial confinement is compared experimentally.The time evolution of tungsten atom and ion line intensities and continuous background radiation during the experiments using spatial constraints is analyzed.The time evolution of the spectral line signal-to-back ratio is calculated to obtain the measurement time that could better improve the spectral quality.The plasma parameters(including electron temperature and density)at different delay times are calculated,and it is found that the spatial confinement determines the plasma parameters to some extent.It is the increase of electron temperature and electron density that leads to the enhancement of the spectral line intensity.(2)LIBS experiments with different confinement sizes are studied.The temporal evolution of integrated spectral intensities with different spatial confinement size is studied.The optimal spatial constraint sizes are obtained for different delay times.The results show that at the early stage of laser-induced plasma generation(the first 200 ns),the cylinder with 2 mm inner diameter has the greatest enhancement effect on the discrete spectral intensity and electron temperature,and the smaller the inner diameter of the cylinder used for confinement,the lower the signal-to-back ratio of the spectral lines.Before 200 ns later,the enhancement ratio and signal-to-back ratio of the spectral lines are greater when using a cylinder with 4 mm inner diameter is used for confinement than when using other inner diameter cylinders for confinement.In addition,experiments on the effect of different cylinder inner diameter sizes on the total intensity of LIBS spectra are conducted.The results show that the maximum enhancements of the plasma spectral intensity and electron temperature can be reached at an inner diameter of 4 mm of the cylinder.(3)By using cylinders made of different materials for spatial confinement the LIBS spectra are analyzed.The time evolution patterns of spectral line intensity and electron temperature do not change when using three metals(Cu,Al and Fe)as the spatial constraint materials,compared to those without spatial constraints.When using nonmetallic PTFE for spatial confinement,the continuous background radiation,discrete spectral lines and electron temperature show a secondary enhancement peak during the time evolution.The maximum enhancement of the total intensity of the spectrum of electron temperature can be obtained under spatial confinement when the cylinder is made of Al.The enhancement effect of spatial confinement on the spectral line intensity may be related to the thermal conductivity of the confining material.The enhancing effect of the spectral line intensity is more obvious when using materials with lower thermal conductivity for spatial confinement.In addition,the reflectivity of Al is much larger than several other materials at the wavelength of the spectral lines studied in this thesis,which may be another reason for the enhancement of the spectral line intensity.