Study On The Spatiotemporal Evolution Of Optical Emission From The Laser Induced Multi-component Plasma Of Tungsten Carbide Copper Alloy In Vacuum

During the operation of a tokomak device,the first wall and the divertor will be impacted by the heat flux and particle flow from the core,resulting in a series of Plasma Wall Interaction PWI,leading to wall erosion and structural damage,accompanied by fuel retention,impurity deposition and others.Laser-induced breakdown spectroscopy（LIBS）is a non-contact element analysis technology which can realize in-situ,on-line and remotely,detect the multi-elements quickly at the same time.Recently LIBS has been developed to diagnose the PWI processes in EAST tokomak for measuring fuel deuterium retention and impurities deposition of a variety of high-Z and low-Z elements on the first wall.Due to the complexity of the plasma generated by laser ablation of samples,the physical mechanism of the interaction between laser and matter is not completely clear.The emission spectral intensity of laser-induced plasma is affected by many factors,such as the parameters of laser source,the features of plasma transient evolution in the environment of Tokamak,the properties of materials and the concentration of elements,etc.,which also will affect the stability of LIBS spectral signal.Therefore,one of the challenges of online diagnosis of PWI using LIBS technology is how to realize accurately quantitative analysis.Considering the high spatial heterogeneity of multi-component plasma and vacuum environment,a nanosecond pulse laser is used to ablate（WC）₇₀Cu₃₀ and C and W samples.The spectral features of single-component plasma and multi-component plasma are systematically studied.Further,the temporal and spatial evolutions of various species（atoms and ions）are analyzed.The"mass spatial separation"and"ion sheath-acceleration"in plasma expansion have been investigated.Specific contents of this thesis are as follows:In the first chapter,the principle of nuclear fusion and Lawson’s criterion for realizing nuclear fusion are briefly introduced.Then the most promising controllable thermonuclear fusion such as magnetic confinement fusion and tokomak device are introduced,present status and history of magnetic confinement devices are systematically summarized.The PWI and the technical challenges to realize quantitative analysis as well as the research plan of this thesis are presented.In chapter 2,firstly,laser-induced breakdown spectroscopy and the experimental device are introduced.Secondly,the features of the spectrum and basic plasma parameters such as electron temperature and electron density are described.In chapter 3,in vacuum,the temporal evolution and spatial distribution of emission spectra of tungsten and graphite as well as physical mechanisms of spectral lines are studied.The results show that the lifetimes of atomic and ion lines of W are 900 ns and 500 ns,and those of C can reach 650 ns and 380 ns,respectively,but the velocities of carbon atoms and ions are faster than those of tungsten atoms and ions.In chapter 4,the plasma expansion process of a nanosecond laser ablation of ternary alloy（WC）_70%Cu_30%in vacuum is studied by using time-space resolution spectroscopy.The temporal evolution and spatial distribution features of the different spectral lines are studied,and the average velocities of the species in the expansion process are calculated.The results show that during the vacuum expansion of multicomponent plasma,the continuous background emission,atomic and ion spectral lines corresponding to the different elements present the different evolution laws with time,which are related to the generation mechanism,probing position and energy levels of spectral lines.The plasma generated by laser ablation in vacuum has a high spatial heterogeneity in element distribution during expansion,and the spatial distribution of particles of the different elements is significantly different.The expansion velocity of low-Z atoms in laser ablation plasma is higher than that of high-Z,and the velocity of ions is higher than its atomic velocity."Mass separation effect"and"ion sheath-acceleration"are found to be as an important physical mechanism that lead to the significantly spatial distribution of elements during the expansion of laser ablation plasma.The research results could provide a data reference for improving physical understanding of laser ablation multicomponent plasma expansion in vacuum,and also provide some ideas for improving the accuracy of LIBS quantitative analysis.