Preliminary results of in situ laser-induced breakdown spectroscopy for the first wall diagnostics on EAST

Post-mortem methods cannot fulfill the requirement of monitoring the lifetime of the plasma facing components(PFC) and measuring the tritium inventory for the safety evaluation.Laserinduced breakdown spectroscopy(LIBS) is proposed as a promising method for the in situ study of fuel retention and impurity deposition in a tokamak.In this study,an in situ LIBS system was successfully established on EAST to investigate fuel retention and impurity deposition on the first wall without the need of removal tiles between plasma discharges.Spectral lines of D,H and impurities(Mo,Li,Si,...) in laser-induced plasma were observed and identified within the wavelength range of 500-700 nm.Qualitative measurements such as thickness of the deposition layers,element depth profile and fuel retention on the wall are obtained by means of in situ LIBS.The results demonstrated the potential applications of LIBS for in situ characterization of fuel retention and co-deposition on the first wall of EAST.     

Diagnostics of First Wall Materials in a Magnetically Confined Fusion Device by Polarization-Resolved Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy(LIBS) is a powerful analytical tool for realtime diagnostics and detection of multiple elements deposited at the first wall of magnetically confined plasma fusion devices. Recently,we have tested LIBS in our laboratory for application to in situ real-time diagnostics in the fusion device EAST. In this study,we applied polarizationresolved LIBS(PR-LIBS) to reduce the background continuum and enhance the resolution and sensitivity of LIBS. We used aluminium(Al)(as a substitute for Be) and the first wall materials tungsten(W) and molybdenum(Mo) to investigate polarized continuum emission and signal-tobackground ratio(SBR). A Nd:YAG laser with first,second and third harmonics was used to produce plasma. The effects of the laser polarization plane,environmental pressure and polarizer detection angle were investigated. The spectra obtained without using a polarizer(i.e. LIBS)were compared with those obtained with a polarizer(PR-LIBS). Distribution of emission spectral intensity was observed to follow Malus' law with respect to variation in the angle of detection of the polarizer. The spectra obtained by PR-LIBS had a higher SBR and greater stability than those obtained by LIBS,thereby enhancing the reliability of LIBS for quantitative analyses.A comparison of Al,Mo and W showed that W exhibited a higher continuum with stronger polarization than the low-Z elements.     

Temporal and spatial dynamics of optical emission from laser ablation of the first wall materials of fusion device

Laser-induced breakdown spectroscopy(LIBS) has been developed to in situ diagnose the chemical compositions of the first wall in the EAST tokamak. However, the dynamics of optical emission of the key plasma-facing materials, such as tungsten, molybdenum and graphite have not been investigated in a laser produced plasma(LPP) under vacuum. In this work, the temporal and spatial dynamics of optical emission were investigated using the spectrometer with ICCD.Plasma was produced by an Nd:YAG laser(1064 nm) with pulse duration of 6 ns. The results showed that the typical lifetime of LPP is less than 1.4 μs, and the lifetime of ions is shorter than atoms at ~10~(-6)mbar. Temporal features of optical emission showed that the optimized delay times for collecting spectra are from 100 to 400 ns which depended on the corresponding species. For spatial distribution, the maximum LIBS spectral intensity in plasma plume is obtained in the region from 1.5 to 3.0 mm above the sample surface. Moreover, the plasma expansion velocity involving the different species in a multicomponent system was measured for obtaining the proper timing(gate delay time and gate width) of the maximum emission intensity and for understanding the plasma expansion mechanism. The order of expansion velocities for various species is V_C~+ V_H V_(Si)~+ V_(Li) V_(Mo) V_W.These results could be attributed to the plasma sheath acceleration and mass effect. In addition, an optimum signal-to-background ratio was investigated by varying both delay time and detecting position.     

Electron Cyclotron Emission Imaging on the EAST Tokamak

An upgraded electron cyclotron emission imaging (ECEI) system consisting of new optics lenses with necessary electronics for receiving and processing signals for two dimension (2D) ECEI diagnostics was installed on EAST. Hyperboloid lens were adopted in the new system to optimize the spatial resolutions. The mixers array of sixteen elements measured the plasma electron cyclotron emission at eight frequencies simultaneously, and the profiles of the electron temperature and its fluctuation in an area of 20 cm (vertical) × 6 cm (horizontal) could then be analyzed. Eevolution of sawtooth precursor and crash in EAST was observed.     

Temporal and spatial evolution measurement of laser-induced breakdown spectroscopy on hydrogen retention in tantalum

Fuel retention measurement on plasma-facing components is an active field of study in magnetic confinement nuclear fusion devices.The laser-induced breakdown spectroscopy(LIBS)diagnostic method has been well demonstrated to detect the elemental distribution in PFCs.In this work,an upgraded co-axis LIBS system based on a linear fiber bundle collection system has been developed to measure the hydrogen(H) retention on a tantalum(Ta) sample under a vacuum condition.The spatial resolution measurement of the different positions of the LIBS plasma can be achieved simultaneously with varying delay times.The temporal and spatial evolution results of LIBS plasma emission show that the H plasma observably expands from the delay times of 0-200 ns.The diameter of Ta plasma is about 6 mm which is much less than the size of H plasma after 200 ns.The difference in the temporal and spatial evolution behaviors between H plasma and Ta plasma is due to the great difference in the atomic mass of H and Ta.The depth profile result shows that H retention mainly exists on the surface of the sample.The temporal and spatial evolution behaviors of the electron excited temperature are consistent with that of the Ta emission.The result will further improve the understanding of the evolution of the dynamics of LIBS plasma and optimize the current collection system of in situ LIBS in fusion devices.     

The Enhanced Effect of Optical Emission from Laser Induced Breakdown Spectroscopy of an Al-Li Alloy in the Presence of Magnetic Field Confinement

In this paper,the influence of magnetic field strength on laser-induced breakdown spectroscopy(LIBS) has been investigated for various pressures.The plasma plume was produced by employing Q-switch Nd:YAG laser ablation of an Al-Li alloy operating at a 1064 nm wavelength.The results indicated that the LIBS intensity of the Al and Li emission lines is boosted with an increase of magnetic strength.Typically,the intensity of the Al Ⅰ and Li Ⅰ spectral emissions can be magnified by 1.5-3 times in a steady magnetic field of 1.1 T compared with the field-free case.Also,in this investigation we recorded time-resolved images of the laser-produced plume by employing a fast ICCD camera.The results show that the luminance of the plasma is enhanced and the time of persistence is increased significantly,and the plasma plume splits into two lobes in the presence of a magnetic field.The probable reason for the enhancement is the magnetic confinement effect which increases the number density of excited atoms and the population of species in a high energy state.In addition,the electron temperature and density are also augmented by the magnetic field compared to the field-free case.     

Numerical simulation of laser ablation of molybdenum target for laser-induced breakdown spectroscopic application

Laser-induced breakdown spectroscopy has been recognized as a significant tool for element diagnostics in plasma–wall interaction. In this work, a one-dimensional numerical model is developed to simulate the laser ablation processes of a molybdenum(Mo) target in vacuum conditions. The thermal process of the interaction between the ns-pulse laser with wavelength of 1064 nm and the Mo target is described by the heat conduction equation. The plasma plume generation and expansion are described by Euler equations, in which the conservation of mass density, momentum and energy are included. Saha equations are used to describe the local thermal equilibrium of electrons, Mo atoms,Mo~+ and Mo~(2+) Plasma shielding and emission are all considered in this model. The mainly numerical results are divided into three parts, as listed below.Firstly, the rule of the plasma shielding effect varying with laser intensity is demonstrated quantitatively and fitted with the Nelder function. Secondly, the key parameters of plasma plume,such as the number density of species, the propagation velocity and the temperature, are all calculated in this model. The results indicate that the propagation velocity of the plume center increased with time in a general trend, however, one valley value appeared at about 20 ns due to the pressure gradient near the target surface leading to negative plasma velocity. Thirdly, the persistent lines of a Mo atom in the wavelength range from 300 nm to 600 nm are selected and the spectrum is calculated. Moreover, the temporal evolutions of Mo's spectral lines at wavelength of 550.6494 nm,553.3031 nm and 557.0444 nm are given and the results are compared with experimental data in this work.     

Study of deuterium retention on lithiated tungsten exposed to high-flux deuterium plasma using laser-induced breakdown spectroscopy

Tungsten is under consideration for use as a plasma-facing material in the divertor region of ITER. Lithiation can significantly improve plasma performance in long-pulse tokamaks like EAST. The investigation of lithiated tungsten is important for understanding the lithium conditioning effects for EAST, where tungsten will be used as a plasma-facing material. In this paper, a few important issues of lithiated tungsten interacting with high-flux deuterium plasma have been studied, such as the effect of lithiation on deuterium retention, the profile of elemental distribution, and the chemical state of lithiated tungsten. Deuterium retention inside both pure and lithiated tungsten has been investigated for the first time in the linear plasma simulator Magnum-PSI by in-situ laser induced breakdown spectroscopy (LIBS). The results indicate that, after deuterium plasma exposure, deuterium retention could be saturated in the lithiation layer, and the lithium in the lithiated layer is chemically bound with deuterium. Moreover, the lithiation can inhibit the blistering on the tungsten surface. These results can be valuable for the application of LIBS as a diagnostic technique for plasma-facing components of tokamaks.     

Microanalysis of Multi-Element in Juncus effusus L. by LIBS Technique

Laser-induced breakdown spectroscopy (LIBS) was used to decipher the unique multi-elemental characteristics of Juncus e?usus L. The spectral fingerprints of Juncus e?usus L. were established based on elemental microanalysis via LIBS. Microanalysis and multimode sam?pling methodologies were designed in this study. The relative standard deviation (RSD) approach was performed to optimize the multi-shot measurements. Taking advantage of the capability with no or minimal sample pre-treatment of LIBS, a thermodynamic chart of four elements (Mg, Ca, Ba, and Na) was created from twelve collection regions. The diagram of elemental distribution on a micro-scale was generated to explore the nature of Juncus e?usus L. by LIBS. The results demon?strated that LIBS is a promising technique for rapid elemental microanalysis of heterogeneous samples.     

Temporal and Spatial Evolution of Laser-Induced Plasma from a Slag Sample

Laser-Induced Breakdown Spectroscopy(LIBS) has been demonstrated to be an effective method for slag analysis.In order to better clarify the nature of the plasma generated from a slag sample,an Nd:YAG pulse laser at 1064 nm wavelength was used to ablate the slag sample in air.The temporal and spatial evolutions of plasma parameters,including emission intensity,electronic density and plasma temperature,have been studied.It is shown that the electron density and plasma temperature drop off rapidly with the delay time as a result of plasma expansion and cooling.It has been found that the electron density of the whole plasma is close to that of the center regions in the plasma.The results of the spatial distributions on the two-dimensional plane have shown that there is a big region with lower electron density values caused by the recombination process in the center of the plasma.The maximum of the plasma temperature takes place at the regions close to the target,and the border of the plasma front-head has higher plasma temperatures than that of the center part.     

Effect of distances between lens and sample surface on laser-induced breakdown spectroscopy with spatial confinement

Spatial confinement can significantly enhance the spectral intensity of laser-induced plasma in air.It is attributed to the compression of plasma plume by the reflected shockwave.In addition,optical emission spectroscopy of laser-induced plasma can also be affected by the distance between lens and sample surface.In order to obtain the optimized spectral intensity,the distance must be considered.In this work,spatially confined laser-induced silicon plasma by using a Nd:YAG nanosecond laser at different distances between lens and sample surface was investigated.The laser energies were 12 mJ,16 mJ,20 mJ,and 24 mJ.All experiments were carried out in an atmospheric environment.The results indicated that the intensity of Si (I) 390.55 nm line firstly rose and then dropped with the increase of lens-to-sample distance.Moreover,the spectral peak intensity with spatial confinement was higher than that without spatial confinement.The enhancement ratio was approximately 2 when laser energy was 24 mJ.     

Quantitative analysis of saindha salt using laser induced breakdown spectroscopy and cross-validation with ICP-MS

Saindha salt is considered to be more advantageous than the other edible salts for the patients suffering from diabetes,blood pressure and kidney diseases.To explore the constituent elements of this salt,laser induced breakdown spectroscopy(LIBS) has been exploited for its qualitative and quantitative analysis.The third harmonic(355 nm) of a Q-switched Nd:YAG laser has been used to produce the saindha salt plasma and the time integrated optical emission spectra were registered using a set of six miniature spectrometers covering the spectral range of 230-805 nm.The spectroscopic analysis of the emission spectra predominately revealed numerous neutral or singly ionized emission lines of Ca,Mg,Na,K,Fe,Sr,Si,Li and Al.The laser produced plasma was characterized by calculating the electron temperature from the Boltzmann plots and the electron number density from the Stark broadened line profile as a function of laser irradiance and distance from the target sample.The relative concentration of the constituent elements was extracted by the integrated line intensities of the strongest spectral line of each element using the self-calibration-LIBS(SC-LIBS) and one-line calibration free-LIBS(OLCF-LIBS) methods.For cross-validation,the LIBS results have been compared with that obtained from the inductively coupled plasma-mass spectroscopy(ICP-MS) showing good agreement.     

Study of pressure effects on ocean in-situ detection using laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has attracted extensive attention as a new technique for in-situ marine application. In this work, the influence of deep-sea high pressure environment on LIBS signals was investigated by using a compact LIBS-sea system developed by Ocean University of China for the in-situ chemical analysis of seawater. The results from the field measurements show that the liquid pressure has a significant effect on the LIBS signals. Higher peak intensity and larger line broadening were obtained as the pressure increases. By comparing the variations of the temperature and salinity with the LIBS signals, a weak correlation between them can be observed. Under high pressure conditions, the optimal laser energy was higher than that in air environment. When the laser energy exceeded 17 mJ, the effect of laser energy on the signal intensity weakened. The signal intensity decreases gradually at larger delays. The obtained results verified the feasibility of the LIBS technique for the deep-sea in-situ detection, and we hope this technology can contribute to surveying more deep-sea environments such as the hydrothermal vent regions.     

Investigation of the factors affecting the limit of detection of laser-induced breakdown spectroscopy for surface inspection

Laser-induced breakdown spectroscopy(LIBS) was examined to detect a trace substance adhered onto Al alloys for the surface inspection of materials to be adhesively bonded. As an example of Si contamination, silicone oil was employed and sprayed onto substrates with a controlled surface concentration. LIBS measurements employing nanosecond UV pulses(λ?=?266 nm) and an off-axis emission collection system with different detecting heights were performed. Because surface contaminants are involved in the plasma formed by laser ablation of the substrates, the relative contribution of the surface contaminants and the substrates to the plasma emission could be changed depending on the conditions for plasma formation. The limit of detection(LOD) was evaluated under several detecting conditions for investigating the factors that affected the LOD. A significant factor was the standard deviation values of signal intensities obtained for the clean substrates. This value varied depending on the measurement conditions.For the Al alloy(A6061), the smallest LOD obtained was 0.529 μg?·?cm~(-2). Furthermore, an improved LOD(0.299 μg?·?cm~(-2)) was obtained for the Al alloy with a lower Si content.     

An overview of lithium experiments on HT-7 and EAST during 2012

In 2012, lithium coating with an upgraded system on EAST, the first application of lithium granules injection for ELMs pacing on EAST, and the first flowing lithium limiter experiments on HT-7 have successfully been carried out and several new results were obtained. On EAST, it was found that both the Mo first walls and the C divertors were well coated by lithium and the lithium film coverage was increased up to 85%, which greatly contributed to the new achievements of EAST, especially stationary H-mode plasma over 30 s and long pulse plasma over 400 s. And at the same time, ELMs suppression by active lithium conditioning and ELMs pacing using lithium granules injection were demonstrated and reported for the first time on EAST. On HT-7, flowing liquid lithium limiters using the TEMHD concept and using a thin flowing film concept were also initially tested and some references were obtained for the future development. Those experiments show that lithium should be an important material for fusion devices. It could be used for wall conditioning, ELMs mitigation and also provide a self-recovery plasma facing components in future fusion devices.     

Analysis of Pulverized Coal by Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy （LIBS） has been used to detect atomic species in various enviromnents. The quantitative analysis （C, H, O, N and S） of representative coal samples are being carried out with LIBS, and the effects of particle size are analyzed. A powerful pulse Nd：YAG laser is focused on the coal sample at atmosphere pressure, and the emission spectra from laser-induced plasmas are measured by time-resolved spectroscopy, and the intensity of analyzed spectral lines is obtained through observing the laser plasma with a delay time of 0.4 #s. The experimental results show that the slope of calibration curve is nearly 1 when the concentration of the analyzed element is relatively low, and the slope of curve is nearly 0.5 when the concentration of C is higher than other elements. In addition, using the calibration-free model without self-absorption effect, the results show that the decreasing of particle size leads to an increase of the plasma temperature.     

Quantitative analysis of steel and iron by laser-induced breakdown spectroscopy using GA-KELM

According to the multiple researches in the last couple of years, laser-induced breakdown spectroscopy(LIBS) has shown a great potential for rapid analysis in steel industry.Nevertheless, the accuracy and precision may be limited by complex matrix effect and selfabsorption effect of LIBS seriously. A novel multivariate calibration method based on genetic algorithm-kernel extreme learning machine(GA-KELM) is proposed for quantitative analysis of multiple elements(Si, Mn, Cr, Ni, V, Ti, Cu, Mo) in forty-seven certified steel and iron samples.First, the standardized peak intensities of selected spectra lines are used as the input of model.Then, the genetic algorithm is adopted to optimize the model parameters due to its obvious capability in finding the global optimum solution. Based on these two steps above, the kernel method is introduced to create kernel matrix which is used to replace the hidden layer's output matrix. Finally, the least square is applied to calculate the model's output weight. In order to verify the predictive capability of the GA-KELM model, the R-square factor(R~2), Root-meansquare Errors of Calibration(RMSEC), Root-mean-square Errors of Prediction(RMSEP) of GAKELM model are compared with the traditional PLS algorithm, respectively. The results confirm that GA-KELM can reduce the interference from matrix effect and self-absorption effect and is suitable for multi-elements calibration of LIBS.     

Application of Laser Induced Breakdown Spectroscopy in Early Detection of Red Palm Weevil: (Rhynchophorus ferrugineus) Infestation in Date Palm

The Kingdom of Saudi Arabia is one of the leading date producing countries. Un?fortunately, this important fruit crop is under great threat from the red palm weevil (RPW) (Rhynchophorus ferrugineus), which is a highly invasive pest. Several techniques, including vi?sual inspection, acoustic sensors, sniffer dogs, and pheromone traps have been tried to detect the early stages of a RPW infestation; however, each method has suffered certain logistical and im-plementation issues. We have applied laser induced breakdown spectroscopy (LIBS) for the early detection of RPW infestation. Through the analysis of the observed LIBS spectra of different infested and healthy samples, we have found presence of Ca, Mg, Na, C, K elements and OH, CN molecules. The spectra also reveal that with the population growth of the pest, the intensity of Mg and Ca atomic lines in LIBS spectra increases rapidly. Similar behavior is observed in the molecular lines of LIBS spectra. The obtained results indicate that the LIBS technique can be used for the early detection of RPW infestation without damaging the date palms.     

Measurement of the surface morphology of plasma facing components on the EAST tokamak by a laser speckle interferometry approach

The laser speckle interferometry approach provides the possibility of an in situ optical noncontacted measurement for the surface morphology of plasma facing components(PFCs), and the reconstruction image of the PFC surface morphology is computed by a numerical model based on a phase unwrapping algorithm. A remote speckle interferometry measurement at a distance of three meters for real divertor tiles retired from EAST was carried out in the laboratory to simulate a real detection condition on EAST. The preliminary surface morphology of the divertor tiles was well reproduced by the reconstructed geometric image. The feasibility and reliability of this approach for the real-time measurement of PFCs have been demonstrated.     

EAST等离子体图像采集与位置识别技术

本工作采用快速CCD相机采集并保存托卡马克等离子体放电时的图像。根据放电时等离子体图像的位置特点,采用改进的主动轮廓模型算法实时获得等离子体边界位置,并通过最小二乘法对其边界进行拟合,以此获得等离子体的中心位置,为进一步进行等离子体的实时控制提供了基础。该方法已成功应用于EAST装置的放电实验中,并取得了良好的效果。