Nanosecond laser preheating effect on ablation morphology and plasma emission in collinear dual-pulse laser-induced breakdown spectroscopy

Focus-offset collinear dual-pulse laser-induced breakdown spectroscopy is designed and used to investigate the laser ablation and spectral intensity with an aluminum alloy sample. The laser crater morphologies and ablation volumes were measured. An inter-pulse time delay dependent ablation efficiency on a nanosecond laser-heated sample was observed, which was similar to the trend of spectral intensity versus inter-pulse time delay in the delay time less than 3 μs. Based on the observation, the nanosecond pulse laser preheating effect on subsequent second laser ablation and signal enhancement is discussed, which will be helpful for understanding the ablation and signal enhancement mechanism in the standard collinear DP-LIBS technique.     

Improvement of quantitative analysis of molybdenum element using PLS-based approaches for laser-induced breakdown spectroscopy in various pressure environments

An experimental setup has been designed and realized in order to optimize the characteristics of laser-induced breakdown spectroscopy system working in various pressure environments. An approach combined the normalization methods with the partial least squares(PLS) method are developed for quantitative analysis of molybdenum(Mo) element in the multi-component alloy,which is the first wall material in the Experimental Advanced Superconducting Tokamak. In this study, the different spectral normalization methods(total spectral area normalization,background normalization, and reference line normalization) are investigated for reducing the uncertainty and improving the accuracy of spectral measurement. The results indicates that the approach of PLS based on inter-element interference is significantly better than the conventional PLS methods as well as the univariate linear methods in the various pressure for molybdenum element analysis.     

Stoichiometry distribution of thin films deposited by laser ablation: Monte Carlo simulation

An expansion of a bi-component laser plume into a dilute background gas is simulated using a combined Monte Carlo simulation method. The effects of different types of collisions taking place during the transport of the ablated species on their distributions when arriving at a flat substrate are shown. Furthermore, we demonstrate that the film thickness distributions of both components and, hence, the final stoichiometry distribution, depend strongly on the incorporation probability of the species. This probability is expressed as a function of both deposited particle energy and the substrate absorbing properties. These studies can facilitate the comparison of the simulation results with experiments and are of particular interest for pulsed laser deposition (PLD) of multicomponent materials.     

Numerical simulation of nanosecond laser ablation and plasma characteristics considering a real gas equation of state

Based on the governing equations which include the heat conduction equation in the target and the fluid equations of the vapor plasma,a two-dimensional axisymmetric model for ns-laser ablation considering the Knudsen layer and plasma shielding effect is developed.The equations of state of the plasma are described by a real gas approximation,which divides the internal energy into the thermal energy of atoms,ions and electrons,ionization energy and the excitation energy of atoms and ions.The dynamic evolution of the silicon target and plasma during laser ablation is studied by using this model,and the distributions of the temperature,plasma density,Mach number related to the evaporation/condensation of the target surface,laser transmissivity as well as internal energy of the plasma are given.It is found that the evolution of the target surface during laser ablation can be divided into three stages:(1)the target surface temperature increases continuously;(2)the sonic and subsonic evaporation;and(3)the subsonic condensation.The result of the internal energy distribution indicates that the ionization and excitation energy plays an important role in the internal energy of the plasma during laser ablation.This model is suitable for the case that the temperature of the target surface is lower than the critical temperature.     

Online calibration of laser-induced breakdown spectroscopy for detection of heavy metals in water

In order to reduce the fluctuation of LIBS detection spectrum of liquid sample, the full-spectrum sum method and the internal standardization method is adopted, using an equal-RSD normalization algorithm to calibrate the detection spectrum. Experiment result shows that the full-spectrum sum method reduced the RSD of parallel samples of Cd and Cr to 9.4％ and 11.06％ from 28.32％ and 31.93％ respectively, yielded better overall calibration than the singleelement internal standardization approach, thereby suggesting that the former method is convenient and effective for online calibration of LIBS for detection of aqueous heavy metals.     

Calibration curve and support vector regression methods applied for quantification of cement raw meal using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy(LIBS) is a qualitative and quantitative analytical technique with great potential in the cement industrial analysis. Calibration curve(CC) and support vector regression(SVR) methods coupled with LIBS technology were applied for the quantification of three types of cement raw meal samples to compare their analytical concentration range and the ability to reduce matrix effects, respectively. To reduce the effects of fluctuations of the pulse-to-pulse, the unstable ablation and improve the reproducibility, all of the analysis line intensities were normalized on a per-detector basis. The prediction results of the elements of interest in the three types of samples, Ca, Si, Fe, Al, Mg, Na, K and Ti, were compared with the results of the wet chemical analysis. The average relative error(ARE),relative standard deviation(RSD) and root mean squared error of prediction(RMSEP) were employed to investigate and evaluate the prediction accuracy and stability of the two prediction methods. The maximum average ARE of the CC and SVR methods is 34.62% instead of 6.13%,RSD is 40.89% instead of 7.60% and RMSEP is 1.34% instead of 0.43%. The results show that SVR method can accurately analyze samples within a wider concentration range and reduce the matrix effects, and LIBS coupled with it for a rapid, stable and accurate quantification of different types of cement raw meal samples is promising.     

Application of laser-induced breakdown spectroscopy for characterization of material deposits and tritium retention in fusion devices

Laser-induced breakdown spectroscopy (LIBS) is discussed as a possible method to characterize the composition, tritium retention and amount of material deposits on the first wall of fusion devices. The principle of the technique is the ablation of the co-deposited layer by a laser pulse with P (power density) ≥ 0.5 GW/cm² and the spectroscopic analysis of the light emitted by the laser induced plasma. The typical spatial extension of the laser plasma plume is in the order of 1 cm with typical plasma parameters of n_e ≈ 3 × 10²² m^?3 and T_e ≈ 1–2 eV averaged over the plasma lifetime which is below 1 μs. In this study “ITER-Like” mixed deposits with a thickness of about 2 μm and consisting of a mixture of W/Al/C and D on bulk tungsten substrates have been analyzed by LIBS to measure the composition and hydrogen isotopes content at different laser energies, ranging from about 2 J/cm² (0.3 GW/cm²) to about 17 J/cm² (2.4 GW/cm²) for 7 ns laser pulses. It is found that the laser energies above about 7 J/cm² (1 GW/cm²) are needed to achieve the full removal of the deposit layer and identify a clear interface between the deposit and the bulk tungsten substrate by applying 15–20 laser pulses while hydrogen isotopes decrease strongly after the first laser pulse. Under these conditions, the evolution of the spectral line intensities of W/Al/C/hydrogen can be used to evaluate the layer composition.     

Remote open-path laser-induced breakdown spectroscopy for the analysis of manganese in steel samples at high temperature

A remote open-path laser-induced breakdown spectroscopy(LIBS) system was designed and studied in the present work for the purpose of combining the LIBS technique with the steel production line. In this system, the relatively simple configuration and optics were employed to measure the steel samples at a remote distance and a hot sample temperature. The system has obtained a robustness for the deviation of the sample position because of the open-path and alloptical structure. The measurement was carried out at different sample temperatures by placing the samples in a muffle furnace with a window in the front door. The results show that the intensity of the spectral lines increased as the sample temperature increased. The influence of the sample temperature on the quantitative analysis of manganese in the steel samples was investigated by measuring ten standard steel samples at different temperatures. Three samples were selected as the test sample for the simulation measurement. The results show that, at the sample temperature of 500 ℃, the average relative error of prediction is 3.1% and the average relative standard deviation is 7.7%, respectively.     

Impacts of laser pulse width and target thickness on laser micro-propulsion performance

In order to optimize the laser ablation performance of a micro-thruster with 1U dimensions, which employs a micro semiconductor laser, the impacts of pulse width and glycidyl azide polymer (GAP) thickness on thrust performance were researched. The results showed that with a GAP thickness of 200 μm, the single-pulse impulse (I) increased gradually with the increase in the laser pulse width from 50 to 800 μs, while the specific impulse (Isp), impulse coupling coefficient (Cm) and ablation efficiency (η) all reached optimal values with a 200 μs pulse width. It is worth noting that the optimal pulse width is identical to the ignition delay time. Both Cm and η peaked with a pulse width of 200 μs, reaching 242.22 μN W−1 and 35.4%, respectively. With the increase in GAP thickness, I and Cm increased gradually. GAP of different thicknesses corresponded to different optimal laser pulse widths. Under a certain laser pulse width, the optimal GAP thickness should be the most vertical thickness of the ablation pit, and the various propulsion performance parameters at this time were also optimal. With the current laser parameters, the optimal GAP thickness was approximately 150 μm, Isp was approximately 322.22 s, and η was approximately 34.94%.     

高速旋转氚钛靶系统设计和靶温度的数值模拟

给出了用于强流中子发生器的高速旋转氚钛靶系统的设计方案,并对靶的温度变化进行了数值模拟,给出了强流中子发生器的运行参数。     

CLAM钢异径管冷成形数值模拟及试验研究

研究高质量CLAM钢管件的成形性能对聚变堆管路系统制造具有重要意义.利用有限元方法对CLAM钢异径管冷成形过程进行了数值模拟.研究了成形过程中速度矢量的变化情况,并分析了异径管成形过程中的应力、应变分布及冲头推力变化;使用摩擦系数较小的表面涂层对管坯润滑处理后进行了冷成形.研究表明,在几何尺寸及厚度分布方面,实验结果和...     

Development of a fiber-coupled laser-induced breakdown spectroscopy instrument for analysis of underwater debris in a nuclear reactor core

To inspect the post-accident nuclear core reactor of the TEPCO Fukushima Daiichi nuclear power plant (F1-NPP), a transportable fiber-coupled laser-induced breakdown spectroscopy (LIBS) instrument has been developed. The developed LIBS instrument was designed to analyze underwater samples in a high-radiation field by single-pulse breakdown with gas flow or double-pulse breakdown. To check the feasibility of the assembled fiber-coupled LIBS instrument for the analysis of debris material (mixture of the fuel core, fuel cladding, construction material and so on) in the F1-NPP, we investigated the influence of the radiation dose on the optical transmittance of the laser delivery fiber, compared data quality among various LIBS techniques for an underwater sample and studied the feasibility of the fiber-coupled LIBS system in an analysis of the underwater sample of the simulated debris in F1-NPP. In a feasible study conducted by using simulated debris, which was a mixture of CeO₂ (surrogate of UO₂), ZrO₂ and Fe, we selected atomic lines suitable for the analysis of materials, and prepared calibration curves for the component elements. The feasible study has guaranteed that the developed fiber-coupled LIBS system is applicable for analyzing the debris materials in the F1-NPP.     

Numerical simulation of carbon arc discharge for graphene synthesis without catalyst

In this study, graphene sheets are prepared under a hydrogen atmosphere without a catalyst, and the growth mechanism of graphene by direct current arc discharge is investigated experimentally and numerically. The size and layer numbers of graphene sheets increase with the arc current.Distributions of temperature, velocity, and mass fraction of carbon are obtained through numerical simulations. A high current corresponds to a high saturation temperature, evaporation rate, and mass density of carbon clusters. When the carbon vapor is saturated, the saturation temperatures are 3274.9, 3313.9, and 3363.6 K, and the mass densities are 6.4×10~(22),8.42×10~(22), and 1.23×10~(23) m~(-3) under currents of 150, 200, and 250 A, respectively. A hydrogen-induced marginal growth model is used to explain the growth mechanism. Under a high current, the condensation coefficient and van der Waals force increase owing to the higher saturation temperature and mass density of carbon clusters, which is consistent with experimental results.     

Effects of magnetic field on electron power absorption in helicon fluid simulation

In this study, a code, named Peking University Helicon Discharge(PHD), which can simulate helicon discharge processes under both a background magnetic field greater than 500 G and a pressure less than 1 Pa, is developed. In the code, two fluid equations are used. The PHD simulations led to two important findings:(1) the temporal evolution of plasma density with the background magnetic field exhibits a second rapid increase(termed as the second density jump),similar to the transition of modes in helicon plasmas;(2) in the presence of a magnetic field, the peak positions of electron power absorption appeared near the central axis, unlike in the case of no magnetic field. These results may lead to an enhanced understanding of the discharge mechanism.     

电子束辐照钻石热传导过程的数值模拟

应用Matlab程序，对钻石内部和边界采用不同的差分迭代格式，计算模拟了其中的热传导过程。数值计算结果，所用方法与PDE工具箱的计算精度相当，但计算时间明显缩短。同时讨论了辐照时间、吸收剂量率、环境温度等因素对于钻石内部温度分布的影响，这对于钻石的辐照工艺具有重要意义。     

电子加速器扫描盒前端法兰传热的数值模拟

加速器内电子束团在传输过程中因束流传输截面发散或偏离中心,会超出尺寸一定的传输器件.为防止因此导致扫描盒前端法兰盘局部温升过高,在法兰的外侧安装温度传感器以实时监测受照点的温度,从而及时调整系统的电流、电压、聚焦等有关参数.建立了法兰盘两侧热传导的数学模型,用数值模拟的方法计算得出了放置温度传感器的测量点B和受照点A的温度关系.计算结果与实验值符合,证明了数学模型的正确性,并为加速器的调试、运行提供了一种重要的检测手段.     

温度层结下建筑物群周围流场影响的数值模拟研究

采用RNG k-ε湍流模型模拟温度层结下建筑物群对流场结构的影响,并用风洞实验结果对模拟结果进行验证分析。结果表明:CFD模拟结果与Yassin风洞实验结果能较好地吻合,稳定层结建筑尾流区范围内速度和湍流动能减小。通过对稳定、中性和不稳定条件下模拟结果的比较分析,确定了温度层结对规则建筑物矩阵中流动的影响。同时,温度层结对矩阵中街谷涡旋强度和纵向速度垂直分布有着显著影响,特别是当大气处于稳定条件时,建筑物对速度的衰减作用较为明显。     

Numerical Simulation of Gas Flow During Arcing Process for 252 kV Puffer Circuit Breakers

A numerical simulation model for 252 kV puffer circuit breaker is constructed, by using a proven commercial computational fluid dynamics （CFD） package, PHOENICS...     

Numerical simulation of tritium migration in groundwater at Wolsong Plant 1 site

On-site groundwater has been reported to be contaminated by the unplanned release of liquid radioactive material at some nuclear power plants. Thus, it is of utmost importance to implement a timely and effective groundwater protection program at a site based on a good understanding of groundwater flow and a reasonable prediction for the potential impact of the unplanned release. In this study, the tritium migration has been simulated based on the modeling result of groundwater at Wolsong Plant 1 site to assess the potential impact of an unplanned release in the form of a leak. The results indicate that the groundwater eventually flowed into the sea, leaving marine activities as the only possible exposure pathway for receptors. Furthermore, the tritium concentrations in groundwater were simulated to be lowered very quickly with groundwater approaching the sea. Therefore, an additional radiation dose contribution due to the discharge of the contaminated groundwater was estimated to be negligible. Particularly, the draining effect of the dewatering sumps was shown to have a strong influence on the groundwater flow characteristics and the simulation of tritium migration, mainly decreasing the spread rate of the contaminated groundwater, which is advantageous to the protection of groundwater.