Study Of Nanosecond Laser-Induced Breakdown Spectroscopy For Diagnosis Of The Wall Materials On EAST

During Tokamak operation,the heat flux and particles flux from core-plasma bombard the plasma-facing material?PFM?,which causes plasma wall interaction?PWI?processes.In Tokamak device,the PWI problems are very serious.On the one hand,the PFM is bombarded and irradiated by particle flux,which leads to the PFM surface erosion,impurities deposition and fuel retention.The series of problems greatly affact the performance and lifetime of the PFM.On the other hand,impurities generated from PFM can easily enter the core plasma,which may cool down and dilut the thermal plasma.In severe cases,the plasma discharge is extinguished.Therefore,it is very improtant to inveatigate the PWI process in order to realize long-pulse and high parameters plasma operation of Tokamak.While,real-time monitoring elemental composition on PFM surface is one of the key steps for understanding the PWI processes.For the above problems,an in-situ laser-induced breakdown spectroscopic?LIBS?technique has been developed for diagnosis of PWI processes on EAST device.The LIBS diagnostic system can work in a very critical condition?pulsed discharge and strong magnetic field?on Tokamak device.In the first part of this thesis,the influence of pulsed discharge and magnetic field on the diagnostic ability of LIBS have been investigated in the laboratory.In the second part,the in-situ LIBS diagnostic system has been successfully built on the Experimental Advanced Superconducting Tokamak?EAST?based on the previous experimantal data.Using this in-situ diagnostic system,the investigations of lithium-wall conditioning,impurties deposition,fuel retention,ion cyclotron range of frequency?ICRF?discharge cleaning process have been carried out.The contents of this thesis are as follows:In chapter 2,the working principle of LIBS and scheme diagram of the LIBS system are introduced.The mechanism of plasma formation during the interaction of laser with sample material,the characteristics of plasma radiation and the spectral line broadening mechanism are discussed The methods of determining plasma parameters?electron temperature and density?are also discribed.For temporal dynamics of laser ablation plasma,the changes of plasma spectral intensity,electron temperature and density as a function of gate delay time of detection were studied.Experimental results show that with the increase of detection delay time,the optical emission intensity and spectral line signal to background ratio?SBR?present a trend of increasing at first and then decreasing,while the continuous background,electron temperature and density are gradually decreasing.In chapter 3,in order to test the effect of Tokamak discharge on the LIBS signals,the effect of a pulsed spark discharge on measuring ability of LIBS has been investigated in the laboratory.The results show that the pulsed spark discharge can significantly enhance intensity of LIBS signals,electron temperature and electron density,and improve the detection sensitivity of LIBS.To simulate the magnetic condition of EAST device,an investigation of the effect of magnetic field on the LIBS signal was carried out.The results show that magnetic field confinement can increase the LIBS spectral intensity,and the enhancement factor incrases with the increase of the magnetic field.Increasing the laser power density,the LIBS signal enhancement factor also increases.For the influence of background pressure on LIBS signal,the result shows that when changing the pressure from 1×10^-5 mbar to 0.1 mbar,magnetic field confinement has a significant increase in the spectral signal.However,when the pressure is higher than 0.1 mbar,the magnetic field confinement effect can be almost negligible.This experimental data also presents that the confinement effect of magnetic field on the laser-produced plasma is the most pronounced under helium gas and is the weakest under argon gas.A detailed discussion is given in the thesis.To simulate the vacuum and magnetic field conditions of EAST device,the influence of a steady magnetic field?1.1 T?on laser-induced Molybdenum?Mo?,Silicon?Si?and Carbon?C?plasma spectral signals has been investigated.It is found that spectral intensity from atomic and ionic species under magnetic confinement are all enhanced.The enhancement factor increases at first and then decreases with increasing delay time.A theoretical model combined with the magnetic fluid dynamical analysis is used to explain these phenomena,and it is concluded that the spectral signal enhancement factor is mainly determined by the electron temperature,electron density,the lifetime of upper energy level and magnetic field strength.The phenomenon of selective enhancement of the LIBS signal from lithium plasma by magnetic field was observed for the first time.The temporal and spatial resolution characteristics,the electron temperature and the ratio of Li ionic number density to atomic number density under the different laser fluence,the electron-ions recombination were discussed.It is concluded that magnetic field enhances the recombination process of the electrons and Li ions in the plasma,this results in the increase of Li atomic spectral intensity,the decrease of Li ionic number density and Li ionic spectral intensity.Temporal resolution of plasma images recorded by ICCD proves the experimental results.We believe that the special phenomenon of Li element is due to the special electronic configurations of Li ions?1s¹2s¹?as compared to Li atoms?1s²2s¹?.Li ions has more lowest unoccupied atomic orbit?LUAO?,which leads to the bigger recombination cross section of Li ions.In chapter 4,based on the above results,a LIBS in-situ diagnostic technique has been combined with the EAST device for the first time for on-line monitoring the wall lithiation treatment,impurities deposition,fuel retention and ICRF discharge cleaning process.The total Li film thickness measured by the experiment is about 1.3 ?m,and the average deposition rate of the Li film is about 0.522 ?m/hour.The amount of Li is approximately 41.65 g and the rate of average deposition is approximately 4.65 mg/s.The changes of Li-deuterium?D?/hydrogen?H?co-deposited layer and H/?H+D?ratio on PFM after lithium-wall conditioning in EAST device were also studied.The variation trend of the spectral emission intensities of Mo,Na,Ca,Li,D and H with the number of laser pulses were obtained.The experiment also obtained the change of the H/?H+D?value on PFM with the days of EAST operation,and the D retention rate on PFM every day.The results indicate that Li can absorb H isotopes and significantly reduce the H/?H+D?ratio on EAST.LIBS technique has been applied to on-line monitor ICRF discharge cleaning process in the fusion device for the first time.The cleaning performance of ICRF discharge for deposited impurties on Mo and W tiles has been investigated.The experimental data shows that ICRF discharge cleaning using helium gas as a working gas can effectively remove lithium,deuterium?D?,and hydrogen?H?on the wall surface,and it is not selective for the removal of D and H.Experimental results also present that more Li is retained on the Mo tile than W tile,and the removal rate of Li on the W tile is faster than on the Mo tile.The above experimental data indicates that LIBS can be used to in-situ and on-line diagnose the wall states on fusion device,and it has a singnificant application prospect for studing the PWI process in fusion device.