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

Cong LI (李聪),Jiajia YOU (游加加),Huace WU (武华策),Ding WU (吴鼎),Liying SUN(孙立影),Jiamin LIU (刘佳敏),Qianhui LI (李千惠),Ran HAI (海然),Xingwei WU (吴兴伟) and Hongbin DING (丁洪斌)

Key Laboratory of Materials Modification by Laser,Ion and Electron Beams (Ministry of Education),School of Physics,Dalian University of Technology,Dalian 116024,People’s Republic of China

Abstract

Keywords:temporal and spatial evolution,fuel retention,tokamak,laser-induced breakdown spectroscopy

1.Introduction

Plasma-wall interaction (PWI),which can result in fuel retention,wall erosion,and redeposition,is still one of the key issues in the magnetic confinement nuclear fusion devices.In the International Thermonuclear Experimental Reactor [1],the in-vessel tritium inventory must be minimized and limited to ＜700 g due to the safety requirement [2].Moreover,the fuel retention will change the property of the material surface,influence the recycling of the particles,and even degrade the performance of the fusion plasma.Therefore,elemental diagnosis and monitoring,especially for the fuel elements,on the plasma-facing components (PFCs) are very important to understand the behavior of fuel retention for PWI research.

Laser-induced breakdown spectroscopy (LIBS) is a valuable and well-established technique to determine the elemental composition for not only solids but also gases and liquids[3–7].Several works with post-mortem LIBS setups in the labs [8–13]have demonstrated that the LIBS technique has an excellent capability to study the fuel retention in PFCs.Since the 2014 campaign,a remote and in situ LIBS system has been installed in the Experimental Advanced Superconducting Tokamak to real-time measure the fuel retention and impurity distributions on the first wall[14,15].However,it is well known that quantitative analysis is one of the most major obstacles to analyzing LIBS data due to the transient process of LIBS plasma.The fuel elements of low-Z(such as H,D,and T)and the wall elements of high-Z(such as W,Mo,Ta etc) expand with various velocities under the vacuum condition.Therefore,temporal and spatial measurements for LIBS plasma are essential to study the evolution of the dynamics of both the elemental emission intensity and plasma condition as well as to further understand the characterization of laser-induced plasma.Tantalum(Ta)is a high-Z metal with high fracture toughness,low neutron activation,and high radiation resistance.Although Ta is a rare commodity and difficult for bulk application on large fusion devices,Ta alloy is a potential candidate for some parts of the PFCs[16].Many works related to the spatial and temporal evolution of LIBS measurement with respect to the efforts in the theoretical,experimental,and background environment have been reported [17–20].In our previous works,the temporal and spatial dynamics of LIBS plasma with respect to H,Li,C,Si,Mo,and W have been investigated to show the different expansion velocities for various species [21,22].However,up to now,no study using the temporal and spatial LIBS measurement on a Ta sample with H retention has been reported.

In the current study,an upgraded co-axis LIBS system based on a linear fiber bundle collection system has been developed to measure PFCs under the vacuum condition of about 10–7mbar.The spatial resolution measurement of the different positions of the LIBS plasma can be achieved simultaneously for each laser ablation.The temporal and spatial evolution of the H intensity,Ta intensity,and electron excitation temperature are investigated.The results 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.

2.Experiment

Figure 1.Schematic of the LIBS experimental system.

A schematic of the co-axial LIBS system in the lab is shown in figure 1.A nanosecond pulsed Q-switched Nd:YAG laser(CFR200,Quantel) at a wavelength of 1064 nm and an energy of 120 mJ was used.The laser was focused onto the surface of the sample by using a lens with a focal length of 200 mm to produce the LIBS plasma.The diameter of the laser spot size and the energy density of the laser were 1.5 mm and 6.8 J cm−2,respectively.By using an off-axis parabolic mirror with a through hole in the center,the superposition of the laser beam and the collected signal light were achieved.This kind of design could also be used in the in situ LIBS system in a nuclear fusion device because the optical windows are usually quite limited.The diameter of the mirror,the diameter of the center hole,and the focal length were 50.8,10,and 152.4 mm,respectively.Pure Ta samples were mounted on a two-dimensional XY piezo stage inside a vacuum chamber equipped with quartz windows.The vacuum chamber was pumped down to a pressure of 10–7mbar to avoid the influence of the water vapor from the air.The emission light from the LIBS plasma was reflected by the parabolic mirror which was placed 152.4 mm away from the sample,and then the emission light was collected by a lens with a diameter of 50.8 mm and a focal length of 60.2 mm onto a fiber bundle.This fiber bundle included ten fibers with a core diameter of 300 μm by linear arrangement,so each fiber corresponded to the different collected regions of the LIBS plasma.The magnification of the signal collection system was 3.23,therefore a spatial resolution of 0.97 mm was achieved.The other side of the fiber bundle with a linear arrangement of ten fibers was coupled to a high resolution spectrometer (Shamrock 750,Andor) with a gated intensified charge coupled device (ICCD) camera (iStar 340,Andor).The wavelength range covered 647–665 nm with a full width at half maximum resolution of better than 0.05 nm by a grating of 1200 lines/mm.The response factors of the intensity for each pixel on the ICCD were calibrated using a radiometric calibration source (Labsphere).A digital delay/pulse generator (DG645,Stanford) was used to synchronize the laser and ICCD as well as control the delay time to obtain the time-resolved spectra.The exposure time of the ICCD was set as 200 ns.Thirty LIBS spectra were repeated and calculated the error bars in this work.

3.Results and discussion

3.1.Temporal and spatial dynamics of H emission

Figure 2.LIBS spectra obtained from the different fibers at the delay times of (a) 0 ns and (b) 200 ns of the surface of the H retention Ta sample.The exposure time is 200 ns,and the laser energy density is 6.8 J cm−2.

Figure 3.LIBS spectra obtained at different delay times from 0–1200 ns from (a) fiber No.6 and (b) fiber No.10 on the surface of the H retention Ta sample.The exposure time is 200 ns,and the laser energy density is 6.8 J cm−2.

The typical spectra of LIBS on the surface of Ta samples from different fibers are shown in the figure 2.The LIBS spectra from the first laser shot on the surface of the sample show obvious H I (656.28 nm) and C II (657.81 and 658.29 nm)signals.H retention accumulated on the surface of the Ta samples due to the absorption of H2O and other impurities from the air.The C is a trace impurity and mainly exists on the surface of the sample.The very high content of H and other impurities on the surface results in the Ta signal being suppressed in the detected region of 647–665 nm.The ten collected fibers numbered 1–10 correspond to the different radial positions of the LIBS plasma.The collected positions of the No.5 and No.6 fibers are near the center of the plasma.The collected positions of the No.1 and No.10 fibers are about 4.5 mm away from the center.As shown in figure 2(a),the spectra at the delay time of 0 ns from the center of the plasma show very strong continuum spectrum which is produced by the bremsstrahlung and recombination radiation.The intensities of the H signal and continuum background decrease from the center to the edge of the plasma.Figure 2(b)shows the LIBS spectra at a delay time of 200 ns.The intensity of the continuum background decreases dramatically due to the cooling of the plasma.The spatial distribution of the H intensity shows that the size of the H plasma at the delay time of 200 ns is much larger than the plasma size at the delay time of 0 ns due to the plasma expansion under the vacuum condition.

Figure 3 shows a comparison of LIBS spectra at the delay times of 0,200,400,600,800,1000,and 1200 ns.Fiber No.6 and fiber No.10 correspond to the center of the plasma and the edge of the plasma,respectively.The spectra from the center of the plasma(figure 3(a))show that both the H signal and continuum background intensity decrease with the delay time.However,the spectra from the edge of the plasma(figure 3(b)) show that the intensity of the H signal increases from the delay times of 0–200 ns,and then decreases with the delay time.

Figure 4.Temporal and spatial dynamics of the H intensity at different(a)radial positions and(b)delay times.The exposure time is 200 ns,and the laser energy density is 6.8 J cm−2.

Figure 5.(a)LIBS spectra with different depths on the Ta sample,(b)H and Ta depth profiles on the Ta sample.The exposure time is 200 ns,the delay time is 200 ns,the fiber number is 6,and the laser energy density is 6.8 J cm−2.

Figure 4 summarizes the temporal and spatial dynamics of the H intensity in the LIBS plasma.Fiber Nos.1–10 are converted to the radial positions of the LIBS plasma from–4.5 to+4.5 mm by considering the diameter of the fiber core and the magnification of the signal collecting system.Figure 4(a)shows the H intensity with different delay times at different radial positions.At the very early times of 0–200 ns,the diameter size of the H plasma is only about 6 mm and this results in the H intensity dramatically decreasing from the center to the edge of the plasma.After 200 ns,the H intensity distributions at different radial positions are more and more homogeneous because the plasma has expanded larger than 9 mm.Figure 4(b) shows that only the H intensities from the center of the plasma decrease monotonously with the delay time.The H intensities from the other positions have the maximum values at the delay time of 200 ns due to the dynamic expansion of the plasma.

3.2.Depth profiles of H and Ta

The LIBS spectra with successive laser shots at the same position on the surface of the sample are shown in figure 5(a).An intense H peak is found on the surface of the sample by the first laser shot.From the second laser shot,the H intensity decreases dramatically.Meanwhile,several Ta peaks(648.54,651.44,651.61,661.20,and 662.13 nm)are well defined.The depth profiles of the H and Ta intensities are shown in figure 5(b),and most of the H exists on the surface of the sample.Ta intensities slightly decrease with depth from the second laser shot.This may be due to the effect of the crater created by the laser ablation.

3.3.Temporal and spatial dynamics of the Ta emission

Figure 6.Temporal and spatial dynamics of the Ta intensity (648.54 nm) at different (a) radial positions and (b) delay times.The exposure time is 200 ns,and the laser energy density is 6.8 J cm−2.

Table 1.Spectroscopic parameters of the Ta.

Figure 6(a)shows the Ta intensity(648.54 nm)at the different radial positions with different delay times.In the very early times from 0–200 ns,strong continuum emission but no Ta atomic emission is obtained.During this time,strong bremsstrahlung and recombination radiations by the interaction between the electrons and the charged ions are achieved.Compared to the H emission result shown in figure 4(a),the time of the Ta emission is about 200 ns later than that of the H.The intensity of Ta has the maximum value at the delay time of 200 ns and then decreases with delay time.The radial distribution shows that the diameter of the Ta plasma is always less than 6 mm.Because the mass of the Ta atom is much larger than that of the H atom,the H plasma expands faster than the Ta plasma.This results in the size of the Ta plasma being less than that of the H plasma.Figure 6(b)shows that the Ta intensities from the center of plasma are much higher than those from other positions at the delay time of 200 ns.This means that the emission of the Ta plasma is spatially nonuniform.

In order to further explain the temporal and spatial behaviors of the Ta plasma emission.The electron excited temperature of the LIBS plasma is determined by the Boltzmann method using the following equation [23]:

where I is the light intensity of emission,λ is the wavelength,gkis the statistical weight of the k state,Akiis the transition probability,Ekis the excitation energy of the upper state,kTeis the electron excited temperature,and C is a constant.The spectroscopic parameters of the selected Ta I lines (648.54,651.44,651.61,661.20,and 662.13 nm) are taken from the atomic database of NIST [24]and shown in the table 1.

Figure 7.Temporal and spatial dynamics of the electron excited temperature at different radial positions and delay time.The exposure time is 200 ns,and the laser energy density is 6.8 J cm−2.

The temporal and spatial dynamics of the electron excited temperature at the different radial positions and delay times are shown in figure 7.Because some Ta signal intensities are so weak at the edge of the plasma and after a delay time of 800 ns,only electron excited temperatures between the radial position of –3 and 3 mm at delay times of 200,400,and 600 ns are presented.The electron excited temperatures have the maximum values at the center of the plasma and decrease to the edge of the plasma.Because of the low collision probability and large mean free path of the particles under the vacuum condition of 10–7mbar,the maximum electron excited temperature is only about 0.6 eV at the delay time of 200 ns.With the increase of the delay time,the electron excited temperatures decrease during the cooling of the plasma cooling.The behavior of the electron excited temperature is consistent with the temporal and spatial evolution of the Ta emission.

4.Conclusions

An upgraded co-axis LIBS system based on a linear fiber bundle collection system has been developed.The temporal and spatial behaviors of H and Ta plasma emissions are investigated simultaneously under the vacuum condition of 10–7mbar.The spatial resolution and the maximum detected size are about 0.97 and 9 mm,respectively.The delay times are from 0–1200 ns.The depth profiles of H retention on the Ta sample show that H mainly exists on the surface of the sample.The temporal and spatial evolution results of the LIBS plasma emission show that the H plasma observably expands from a diameter of 6 mm to more than 9 mm during the delay times of 0–200 ns.The diameter of the Ta plasma is about 6 mm which is much less than the size of the H plasma after the delay time of 200 ns.The difference in the temporal and spatial evolution behaviors between the H and Ta plasma emissions is due to the great difference in the mass between the H atom and Ta atom.The electron excited temperatures of the Ta plasma are about 0.35–0.60 eV at the delay times of 200,400,and 600 ns.The behavior of the electron excited temperature is consistent with the temporal and spatial evolution of the Ta plasma emission.The electron density of the Ta plasma will be measured and considered to further improve the understanding of the evolution of the dynamics of LIBS plasma in the near future.

Acknowledgments

This work was supported by National Key R&D Program of China (No.2017TFE0301300),the National Natural Science Foundation of China (Nos.11605023,11805028,11861131010),and the China Postdoctoral Science Foundation (Nos.2017T100172,2016M591423).

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