Studies Of Ultrashort Pulse Laser-induced Breakdown Spectroscopy And Laser Ablation Features For Diagnosis Of High-Z Wall Materials On Tokamak

In tokamak devices,the first wall is continuously exposed to the steady/transient heat flux and particles bombardment from the core plasma.This will result in the plasma wall interaction(PWI).During the PWI,a series problems,such as fuel retention,impurities formation and wall erosion will occur and then directly affect the working life of the PFMs and even cause the safety problem for tokamak operation.On the other hand,the impurities or dust produced by the PWI will reduce the efficiency of plasma heating and dilute the number density of the fuel.This will lead to the degeneration of plasma confinement and result in the destruction of high parameter and the long-pulse or steady-state operation of tokamak.Therefore,the PWI issue is a key factor to the success of tokamak fusion.Laser-induced breakdown spectroscopy(LIBS)is the best known possibility to in situ obtain safety-relevant nuclear inventory data between and during plasma discharges.The real-time measurements of the first wall information pose a crucial role for understanding the PWI process,revealing the PWI mechanism and even controlling the PWI processes.To these problems,a Remote In Situ Laser-Induced Breakdown Spectroscopic(RISLIBS)system has been developed on Experimental Advanced Superconducting Tokamak(EAST)and applied to investigate the PWI issues,such as fuel retention,impurities formation,wall conditioning and so on.The picosecond(ps)and femtosecond(fs)Laser-Induced Breakdown Spectroscopic(ps-LIBS and fs-LIBS)have been proposed to significantly improve the depth-resolution of LIBS and reduce the ablation mass removal for the diagnosis of the first wall.The picosecond and femtosecond laser ablation features of the high Z wall materials were systematically studied.The optimum range of laser fluence was determined for the diagnosis of the high Z wall materials by ps-LIBS technique.The major works of this thesis are summarized as follows:In chapter 2,according to the characteristic of partial polarization of the radiation from laser produced plasma,a Polarization-Resolved Laser-induced Breakdown Spectroscopic(PRLIBS)method has been developed to reduce the continuum emission and improve the signal-background ratio(SBR).The results show that the PRLIBS can improve the SBR by more than 8 times comparing with the conventional LIBS.For the multielement components analysis in the diagnosis of the first wall by LIBS,the temporal and spatial dynamics of ns-LIBS for the graphite and the high-Z wall materials has been investigated at the simulated pressure of EAST device.The optimized experiment parameters have been obtained for the low-Z and high-Z elements in LIBS measurement.This provides the valuable experience of data base for developing RISLIBS system.Based on these studies,for the first time,a Remote In Situ Laser-Induced Breakdown Spectroscopic(RISLIBS)system has been developed on the EAST.The RISLIBS system,which can be controlled remotely and work between and during the EAST plasma,has the advantages of fast data acquisition and a wide spectral range.The RISLIBS has become an important technique for the diagnosis of the first wall on EAST device.In chapter 3,the picosecond laser-induced breakdown spectroscopic(ps-LIBS)method has been proposed as the high-depth resolution and near non-destruction prober for the diagnosis of the first wall on tokamak devices.The laser ablation features of high-Z first wall material" has been investigated at the simulated pressure of EAST.The results show that there are three ablation regimes as a function of laser fluences in picosecond laser ablation of high-Z wall materials.In the first ablation regime(optical ablation),the line emission intensity increase with the laser fluence increasing;the unfavorable thermally induced complications to the target can be avoided due to the laser energy effectively delivered to the target in short duration of irradiation and a very clean crater surrounded by minimum surface melting is observed.The results suggest that the regime 1 is the superior choice for the diagnosis of the high-Z first wall by the ps-LIBS approach.Moreover,the threshold of the picosecond laser-produced plasma was determined to be 0.3±0.1 J/cm2 for tungsten.The LIBS signal of tungsten was achieved at the depth resolution in the order of 10 nm/pulse and the ablation mass removal in the order of 3 ng/pulse.The present work provides a data guidance for diagnosis of the first wall by ps-LIBS.In particular,this indicates that ps-LIBS method would be applied to diagnose the first wall with the excellent depth-resolution and near non-destruction measurements on tokamak devices.In chapter 4,for improving the depth-resolution of LIBS technique,the femtosecond laser ablation features of high-Z wall materials were investigated using 6 fs laser at the wavelength of 790 nm.For the first time,the five distinct ablation regimes in terms of both the ablation morphology and ablation rate per pulse have been identified.By analyzing the relationship of the ablation rate vs.laser fluence,the morphology of the ablated craters and the penetration depth of laser energy,the first ablation regime(optical ablation regime)and the second ablation regime(thermal electronic ablation regime)are the preferred zone for diagnosing the high-Z first wall by the fs-LIBS approach.The third ablation regime was assigned as the thermal-like ablation regime.A solidified melt layer consisting of a mix of molten particles,nanoparticles,and re-solidified microparticles is observed inside the crater.These show that the thermal processes occur.The fourth ablation regime is identified as phase explosion-like ablation regime.The re-solidified liquid droplets and re-solidified structures within the ablated crater are observed.The fifth ablation regime is assigned as the macro-void explosion regime.The crater depth only weakly depends on laser fluence,the crater diameter becomes the larger than the spot size,transferring the one-dimensional ablation to three-dimensional interaction.The third,fourth and fifth ablation regimes are not suitable for the diagnosis of the first wall by LIBS due to the strong thermal effect and large laser ablation mass.For the propose of improving the precision of quantitative analysis of the LIBS approach,the reflectivity/absorbility and ablation mass removal of molybdenum were investigated using 6 fs laser which can avoid completely the effect of plasma shielding due to the interaction of laser with metal on a timescale faster than plasma formation.The results indicate that the surface reflectivity of target decrease with the increasing of laser ablation pulse,which will affect the pulse laser ablation mass removal due to increasing energy coupling efficiency of laser and target.The change of surface reflectivity must be taken into account to calibrate for the precision of quantitative analysis in LIBS technique.In chapter 5,the main achievements obtained in this thesis are summarized and the future researches are prospected.