Analysis Of ITER-like R St Wall Materials In Tokamak By Laser Induced Breakdown Spectroscopy And Mass Spectrometry

Study and characterization of the wall erosion, material deposition and the associated tritium retention by co-deposition with eroded wall material are of high importance for a fusion reactor operation and, in particular, to fulfil safety requirements for tritium retention. In-situ monitoring and analysis of the composition of the first wall material can provide guidance for the fusion operation. Due to special constraints of fusion environments, many analytical tools can not be used for online analysis, while laser based methods, such as laser induced breakdown spectroscopy(LIBS) can be employed for in-situ diagnosis and removal of the deposition on the first wall components in fusion devices. This work was carried out with respect to these issues with the following aspects:Firstly, the properties the interaction between laser and first wall material were studied, including the time evolution of the plasma and the velocity of plasma plume, the electron temperature and electron density of plasma and their dependence on laser power density, ambient gas type and pressure. It is found that both electron temperature and electron density increase with the laser power density. Particles produced by laser ablation were used to simulated dust particles generated on the edge plasma during transient event. The ablation rates and ablation thresholds of materials under different laser power densities were investigated for studying the laser ablation mechanism, which indicates that three phases of the ablation process with the increasing laser power density, that is, normal evaporation, plasma shielding and phase explosion. Comparing the photon emission efficiency as well as the spectral behavior of the plasma produced by different laser power densities, it is recommend that the plasma shielding stage is better for operating laser-induced breakdown spectroscopy for in-situ analysis of the deposition layer on first wall material. For laser removal, it is suggested that an energy density larger than the ablation threshold of the deposition layer but smaller than the damage threshold of the substrate material should be selected after the determination of the composition of the deposited layer by LIBS. In this study, a first mirror with deposits retrieved from HL-2A tokamak and the ITER like deposit layer were well removed by laser ablation under proper laser energy density without damage of the substrate.Secondly, qualitative and quantitative analysis of ITER-like deposits layers were firstly carried out in the laboratory without magnetic field by laser-induced breakdown spectroscopy. The depth distribution of the different components of layers and the ablation rate of different materials under different laser power density conditions were investigated. The mechanisms of the fuel retention and its removal were analyzed for the selection of a suitable laser energy density for characterization and removal of the fuel and deposits. The in-situ monitoring of the spectral characteristics were used to control the removal process to prevent damage of the substrate. As for the quantitative analysis, three methods namely slope method, calibration-free method and the developed absolute calibrated method were used for analysis of the composition of the deposited layer. By comparing the results with other analytical methods, it is found that the choice of analytical line is of high importance to get reasonable results. Generally, we should select lines with non-resonant absorption or high excitation energies of upper levels and no other spectral interference lines for the analysis. Comparing the three methods, the first two can provide relative concentrations of each component, while the third one can provide the absolute content of each component in the sample. Due to the spectrometer used in this work does not support the function of selection of time delay and gate window, which are necessary to meet the basic assumptions of calibration-free method, the results of calibration-free method were in large errors, while the other two gave relatively reasonable quantitative results.This work also investigates the effects of magnetic fields on the properties of the space distribution of emission lines and corresponding its influence on the edge plasma in TEXTOR tokamak device. The results show that with the presence of magnetic field, the emission intensity of the laser ablation plasma were enhanced compared with absence of magnetic field. This is due to increased confinement by the magnetic field. The variation of the different ionization states of the element is not the same, spectral intensity of neutral component with increasing magnetic field changes almost unchange, while the intensity of CII located on the center of the plasma enhanced with increasing magnetic field and CIII line slightly reduced with the magnetic field increases. However, for the whole LIBS plasma, the enhancement of CII line is not obvious while the total intensity of CIII line increase with magnetic field strength. This is due to different constraints of the magnetic field for the components with different ionization degree.Finally, time-of-flight mass spectrometry and laser microprobe mass spectrometry were used for species determination of the plasma produced by laser ablation of first wall material carbon, tungsten and the deposition layer on the first mirror which was retrieved from HL-2A tokamak. The results show that the use of molecular beam sampling can improve the resolution of the mass spectra and provide more abundant species information. The microprobe mass spectrometry can provide the compositions distribution and depth profile of different regions, which can provide a reference information to infer the source of deposition and fuel retention. From the width of the mass peak, we can get the transverse velocity distribution of different species. A velocity limiter was designed to select the species generated by the laser ablation process, which allows species with stable structure and a specific direction of the speed get through. From the geometric parameters of the velocity limiter, the initial emission speed of the C10particle is～104m/s, which is in good agreement with the speed of the plasma deduced from CCD picture of the plasma. The mass spectra results show that the interaction between the laser and the material will produce a lot of particles, which may enter the edge plasma with large velocity and induce some degree of local disturbance. Therefore the laser ablation method for characterization of the first wall materials is recommended to proceed between plasma discharge.