| Laser-induced breakdown spectroscopy(LIBS)has been applied in remote,in-situ,online monitoring of erosion/deposition processes and D-retention on the first wall of the Experimental Advanced Superconducting Tokamak(EAST).While,the quantitative analysis of LIBS on plasma wall interaction(PWI)is still considered as its primary challenge,due to the complex nature of the laser ablation processes and the laser induced plasma particle interaction processes,as well as due to the extreme conditions of tokamak like vacuum,magnetic field,plasma ambient,etc.Further improvement of the quantitative aspects of the LIBS technique may only be expected from a significantly understanding of laser ablation and induced plasma formation mechanism.In this work,a systematical investigation of the species including electrons,atoms,singly charged ions,multi-charged ions in the laser produced plasma and their dynamics have been carried out.Three approaches including optical emission spectroscopy(OES),time of flight mass spectrometry(TOF-MS),fast imaging photograghy were developed to diagnose the laser produced high Z metal plasma.The thesis is organized as follows:In chapter 2,laser induced tungsten(W)plasma process were investigated in time scales ranging from nano-seconds to micro-seconds in vaccum using time-resolved optical emission spectroscopy.It was found that in the different temporal evolution process of the laser produced W plasma,the dominant spectral radiation was different.The time scales of the continuum radiation,ionic and atomic optical emission were about 10-100 ns,60-300 ns,100-800 ns,respectively under the experimental conditions.This suggested that the delay time should be set as 100 ns to restrain the influence of strong continuum radiation for LIBS measurement.The electron density and electron temperature were determined as 4×1017 cm-3,1.3 eV respectively at the delay time of 100 ns and the values decreased along with time.It was also found that the plasma has comparatively high ionizaion ratio up to 90%at the early stages of less than 200 ns.In chapter 3,the effects of gas pressure and magnetic field on the optical emission specreoscopy(OES)and plasma plume dynamics were systematically investigated.The pressure varied from 4×10-5 mbar to 1000 mbar.Temporal evolution of the intensities of continuum radiation,ionic and atomic optical emission at various pressures showed that the continuum radiation,ionic and atomic optical emission begin to increase significantly when the pressure increases to 0.1 mbar.The intensities of ionic and atomic spectral lines both increased to their maximum values at the pressure of several tens of mbar,while the continuum radiation increased with pressure further increase.According to analysis of the signal quality assessment factor(FSq)and the plasma plume expansion,the best LIBS signal appeared at 1 mbar.With the ambient pressure increase,the plasma size became smaller due to the confinement of the ambient gas,the electron temperature and electron density of the plasma increased as the gas pressure increase,and the radiation lifetime also became longer.At ambient gas,the laser produced plasma plume expansion presented complex behaviours such as plume splitting,sharpening,confinement,and shockwaves formation as well as internal plume structures etc.Temporal evolution of the optical emission showed the Bremsstrahlung emission presented little change at early stages in the presence of a magnetic field while the ionic and atomic emission lines of tungsten showed significant enhancement at the delay times of>200 ns,>400 ns,respectively.Optical time of flight dynamics at different positions showed that W ion and W atom velocities in the magnetic field were about 4 and 2 times lower respectively due to the magnetic confinement.Fast imaging photography confirmed that magnetic field confined the plasma,reducing the velocity of the plasma by about 2.6 times.The enhancement of the optical emission is due to the higher electron density and electron temperature in the presence of magnetic field compared to the magnetic field-free case.The time dependent dynamics of the plasma plume in the presence of a magnetic field was correlated to the temporal variation of the plasma parameter?(the ratio of plasma pressure to magnetic pressure).In chapter 4,the dynamics of prompt electrons,ions and neutrals of ns-laser ablation of tungsten were investigated using time and space resolved optical emission spectroscopy,fast imaging.The results showed that not only there were continuum,neutral and singly ionized W atoms optical emission in the laser induced plasma,but also the spectra of ambient gas Nz and N2?were observed which appear at very early times(<30 ns).The prompt electrons ejected from the target collisionally excited and ionized ambient N2 molecules.The prompt electrons show a temporal profile with a width that was very close to the duriation of laser pulse.Time of flight dynamics of the laser-produced W plasma presents four time-scales of plasma processes including prompt electrons emission,continuum radiation,ionic and atomic spectral emission in the range of 1-20 ns,10-100 ns,50-1000 ns and 100-2000 ns,respectively.The hemispherically distributed feature of the prompt electrons and the space separation between fast electrons and ions identified the formation of a near-surface transient plasma sheath.The observed acceleration of ions further verified the existence of the plasma sheath.In chapter 5,the diagnositic study of laser produced high-Z metal plasma using optical spectroscopy and time-of-flight mass spectroscopy were carried out.The optical and particle emissions were systematically investigated using Laser-induced breakdown spectroscopy and time-of-flight mass spectrometry respectively.The results showed that not only there were neutral and singly charged ions in the laser induced plasma,but quite a number of multi-charged ions were observed.The ion charge state was even up to 7.The generation of the multi-charged ions is attributed to step ionization mechanisms due to plasma shielding effect during the laser plasma interaction process.The acceleration mechanisms for the multi-charged ions are attributed to the transient dynamic plasma sheath during laser plasma interaction process.Energy saturation phenomenon with the laser power density increasing was observed and the mechanism was discussed.Temporal evolution of the multi-charged ions was found to follow Shifted-Maxwell Boltzmann(SMB)distribution and the center of shifted velocities for the multi-charged ions were also obtained.The electron density and electron temperature values and their variations with time are determined by fitting Saha-ionization equation using the observed ion factions.The values of electron density and electron temperature declined from about 1021 CIm-3 to 1018 Cm-3 and 12 eV to 2 eVrespectively after the plasma formation and expansion at the initial several tens of nanoseconds. |
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