| Plasma-facing components(PFCs)of nuclear fusion devices are irradiated with intense particle bombardment and high energy fluxes from the boundary plasma.The divertor has to withstand transient events and high heat loads with a power density of several MW/m2.In these conditions,plasma-facing materials(PFMs)encountered microstructural changes,such as erosion,fuel retention,cracking,and hardening degradations.The potential material that can function well in these harsh conditions is the tungsten due to its high melting point,D/T retention and low thermal expansion rate.These promising properties made it a competitive candidate as a first wall material for future fusion devices.In contrast,the low ductility of tungsten causes inter-granular failure and fracture toughness.During the operational condition,the variation in surface morphology and hardness of tungsten originates from the modification of microstructure induced by high heat loads and particle flux.Even now,it is established that the micro structure and mechanical properties of tungsten-based material play vital roles in its performance.Due to specific conditions of the fusion environment,the online hardness analysis by means of conventional techniques is a challenging task.Therefore,an in-situ monitoring technique,which can measure the hardness of fusion devices,is much preferred.The laserbased methods,such as laser-induced breakdown spectroscopy(LIBS),could be deployed for in-situ monitoring of hardness on PFMs for fusion devices.LIBS would be a promising analytical technique for in-situ monitoring;however,the investigation about the LIBS approach to evaluating the surface hardness of PFMs for tokamak devices has not yet been reported.The objective of the dissertation is to demonstrate the feasibility of the in-situ LIBS technique to analyze the hardness of PFMs.The results of this dissertation are arranged with the following chapters;In chapter 2,the detailed setup of LIBS for hardness analysis of pure tungsten and tungsten heavy alloy(WHA)is discussed.The hardness of WHA was investigated after employing high heat load and transient events.The samples were treated with a linear plasma torch and longpulse laser simulating a practical condition similar to Experimental Advance Superconducting Tokamak(EAST).The characterization was performed using LIBS,scanning electron microscopy(SEM),UV-visible spectroscopy,X-ray diffraction(XRD),and Vickers hardness test.Firstly,the correlation of the hardness of five different WHAs with their microstructural characteristics was analyzed using stand-off LIBS.Then plasma surface interaction and transient heat load events were investigated to examine the impact of the microstructure on hardness using LIBS emission.In chapter 3,the hardness of five different WHA grades was evaluated by a stand-off LIBS.The difference in these samples hardness is associated with their grain size,crystal size,dislocation density and energy band gap.It was observed from the primary analysis that the micro structural and electronic structure properties have an impact on LIBS plasma emission intensity and plasma parameters.The significant enhancement in(WⅠ)and(WⅡ)lines was observed with increasing hardness.The obtained direct relation between ionic to atomic species ratio(WⅡ/WⅠ)and the material hardness is associated with plasma electron temperature.The results from the Boltzmann plot method shows that the plasma electron temperature(Te)increase from 1.76±0.01 to 1.90±0.01 eV as hardness increases from 314±2.2 HV0.5 to 354±1.1 HV0.5.The electron density(Ne)was derived from the stark broadening profile,and the value of Ne decreases exponentially from 4.91 × 1018 to 1.36±1018 cm-3 from soft to hard material.The negative correlation of Ne with surface hardness is mainly related to the mass ablation rate.The band gap(Eg)energy was also evaluated,and the results show that the Eg of these targets increases with hardness and has a linear relationship with plasma electron temperature.Moreover,the ablation efficiency was analyzed from crater depth analysis with increasing laser power density.The results show that the average ablation rate decrease from soft to hard material.In chapter 4,the LIBS used as an in-situ monitoring tool to determine the hardness of WHA(97W-2Ni-1Fe)samples after exposure to various plasma power density irradiations ranging from 0.108 to 1.00 MW/m2.The irradiation of DUT-PSI plasma is a vital factor that displays pronounced changes in crystallographic estimation,microstructural properties,and surface hardness.The X-ray diffraction study reveals that after plasma irradiation,the structure change due to the variation in peak intensity,crystal size,dislocation line density,and micro strains.The change in surface hardness and crystallographic measurements well correlated to the DUT-PSI plasma power densities.In addition,the effect of hardness on WHA LIBS intensity was also investigated.The LIBS calibration curves were plotted by utilizing the ratio of ionic to atomic line intensities of tungsten versus the Vickers hardness.The results indicate that the ratio of ionic to atomic line intensity and Te significantly increases with the hardness.The obtained Pearson correlation coefficient(R2)values in measuring the hardness of all investigated samples have a good approximation,which indicates the reliability of the LIBS approach.The achieved results in this study by the in-situ LIBS system confirm its potential ability to estimate the hardness of WHA much efficiently than other conventional methods.In chapter 5,the transient heat load events such as edge localized modes(ELMs),which induce recrystallization,grain growth,surface roughness and hardening degradations,were investigated.The long-pulse laser beam with power density ranging from 0.165 to 1.909 GW/m2 was used to simulate the ELMs induced transient heat load events on pure tungsten and WHA samples.The threshold parameters for recrystallization,grain growth,and hardening degradation of the samples were carefully determined under ELM-like transient events.The results indicate that recrystallization,grain growth,and hardness changes in PFMs are initiated by transients or destructive deformation in the material.The attained isotropic microstructure texture after recrystallization reveals higher grain growth,mechanical hardness,and increased surface roughening.The obtained results in this chapter indicate that pure tungsten exhibits a higher recrystallization threshold,steady grain growth,and less surface hardness damage compared to WHA.Moreover,the laser-induced shock penning effect was observed with increasing long-pulse laser power density,which ultimately increased the hardness.Besides,the impact of hardness on the LIBS emission intensity was also performed for investigated samples.The variation in hardness was well correlated to the LIBS calibration curve formed by the ionic to atomic line intensity ratio.Despite the different matrix compositions of both samples,the observed increase in Te with hardness justifies the Saha-Eggert relation,mainly describing the dependency of ionic to atomic line ratio on plasma electron temperature.Moreover,the diffused UV-vis spectroscopic characterization results also demonstrate that the electronic properties(band gap)are also directly related to hardness.The results from the standoff LIBS measurements show that the regression coefficient(R2)values for hardness evaluation(i.e.,calibration curve and plasma electron temperature)have good approximation with a value≥0.99,demonstrating the preciseness and potential ability of the LIBS approach for stand-off hardness analysis of plasma-facing components(PFCs)in fusion devices.In chapter 6,the key findings of the experimental works are concluded.The insight into the following research work for the further development of in-situ LIBS hardness characterization is also discussed. |
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