Numerical Simulations Of Plasma Dynamics And Optical Emission Of High-Z Tokamak Wall Materials Ablated By Pulsed Laser

Nuclear fusion provides an ideal solution to the crises of both energy and environment.However,in a modern tokamak device,the intensive heat and particle fluxes can not be confined completely by the magnetic field due to various kinds of plasma instabilities.This has led to a number of harmful outcomes arising from the plasma-wall interaction(PWI),such as wall erosion,impurity production,fuel retention,material migration and redeposition.The extreme working conditions make the study of plasma-wall interaction on site particularly important and necessary for the assessment of the wall damage and guarantee of the operation safety of current and especially,future devices.Laser-induced breakdown spectroscopy(LIBS)has been recognized as an important tool for element diagnosis in PWI and has been applied for in-situ PWI diagnosis in EAST device.In recent years,LIBS diagnosis research on linear plasma devices has also made progress by many researchers.The key physical process of LIBS is determined by the parameters of laser ablated plasma.Due to its small scale,short time,and extreme conditions of high temperature and high pressure,it is difficult for traditional experimental methods to obtain detailed parameters of laser ablation plasma.Especially in the early stage of plasma evolution,the spectral lines are submerged in the bremsstrahlung emission,which caused great difficulties to the experimental measurement and greatly affected the accuracy of the quantitative analysis of LIBS.Therefore,it is necessary to establish a self-consistent model to provide detailed parameters for the plasma dynamics and emission during the processes of laser and tokamak high Z-wall materials interaction.While optimizing the LIBS experiment technology,it also can be a way to better understand the mechanism of laser-material interaction.At present,there are various researches on the PLA model,but most of the physical processes described by the models are not comprehensive.These researches only focus on the ablation of materials,or the description of plasma plume.There is little investigation on the numerical simulation of the interaction between the laser pulse and the high-Z tokamak first wall material.In large-scale fusion devices,the model of laser ablation plasma mainly focuses on the field of inertial confinement fusion(ICF).The numerical simulation of the plasma dynamics and optical emissions of laser ablation high-Z materials is lacking.The numerical simulation work on double pulse LIBS is also extremely scarce.Therefore,to better understand the physical mechanisms in the processes of laser ablation,this thesis has developed a self-consistent theoretical model to invesitgate the plasma dynamics and emission spectrum in the laser ablation of the tokamak wall material.The details are as follows:In chapter 2,the one-dimensional fluid model of the dynamics and spectral emission in nanosecond pulse laser ablation of tokamak high-Z wall materials under vacuum conditions has been established.The physical processes such as the phase change of the laser ablation target,the evolution of the plasma plume,the local thermal equilibrium(LTE),the plasma shielding effect and the plasma emission were considered in the model.The finite difference method,Riemann solution and Newton's iterative method were used to solve the equations.The code was written by C++language.In chapter 3,the numerical simulations of plasma dynamics and spectral emission in laser ablation of high-Z tokamak wall material molybdenum(Mo)and tungsten(W)were carried out.First,the numerical results of the shielding effect of the Mo plasma varied with the laser irradiances,the main parameters of the Mo plasma plume(particle number density,velocity and temperature),and the temporal evolution of the spectral line of the Mo were given.The experiment data were compared with numerical results.Then,the relationship between laser ablation W plasma temperature and W plasma bremsstrahlung intensity,the time evolution of W atomic and ionic line,as well as the temporal evolution of W plasma bremsstrahlung were studied.A comparative analysis was carried out with the experimental data.Finally,the density,temperature and velocity of W plasma and Mo plasma corresponding to different laser irradiances were calculated.In Chapter 4,the one-dimensional fluid model of the dynamics and emission spectrum in the double-pulsed laser ablation of tokamak high-Z wall materials under vacuum conditions was established.Considering two Gaussian-type pulsed lasers with the same pulse duration in the order of nanoseconds,for the continuous case of the interval between two pulses being less than the pulse width of the first laser,the boundary condition was the same as that in single pulse model;but for the discontinuous case of the interval between two pulses being more than the pulse width of the first laser,the Knudsen layer was adopt as the boundary condition.For different time intervals of double pulses and different laser irradiances,the coupling process of the second pulsed laser energy in the plasma and the target was discussed.For the discontinuous and continuous double-pulse ablation of tokamak high Z-wall material models,the formation of shock waves in both plasmas was considered,and the conservative scheme and the Riemann solution were used to capture the shock wave formed in the interaction process of the two plasmas.In Chapter 5,the numerical simulation of plasma dynamics and spectral emission in the laser ablation of EAST wall material Mo and W under the conditions with pulse interval of 50 ns and 100 ns were carried out.First,the parameters of W plasma(particle number density,velocity and temperature),plasma shielding effect,and W plasma emission under double pulse conditions were calculated.Then,the comparative study on Mo plasmas under double pulse and single pulse ablations,the comparison of W plasma and Mo plasma under double pulse,and the temporal evolution of Mo plasma corresponding to the different laser irradiances were calculated.In chapter 6,the conclusions were summarized for the whole dissertation,and the outlook for the following research of further development of LIBS theoretical model was also discussed.