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Laser-induced breakdown spectroscopy measurement of a small fraction of rhenium in bulk tungsten
Spectrochimica Acta Part B 141 (2018) 94–98
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Research note
Laser-induced breakdown spectroscopy measurement of a small fraction of rhenium in bulk tungsten D. Nishijima a, * , Y. Ueda b , R.P. Doerner a , M.J. Baldwin a , K. Ibano b a b
Center for Energy Research, University of California at San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0417, USA Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita 565-0871, Japan
A R T I C L E
I N F O
Article history: Received 14 June 2017 Received in revised form 18 December 2017 Accepted 24 January 2018 Available online 2 February 2018 Keywords: LIBS Rhenium Tungsten-rhenium alloy Boltzmann plot Calibration-free
A B S T R A C T Laser-induced breakdown spectroscopy (LIBS) of bulk rhenium (Re) and tungsten (W)-Re alloy has been performed using a Q-switched Nd:YAG laser (wavelength = 1064 nm, pulse width ∼4–6 ns, laser energy = 115 mJ). It is found that the electron temperature, Te , of laser-induced Re plasma is lower than that of W plasma, and that Te of W-Re plasma is in between Re and W plasmas. This indicates that material properties affect Te in a laser-induced plasma. For analysis of W-3.3%Re alloy, only the strongest visible Re I 488.9 nm line is found to be used because of the strong enough intensity without contamination with W lines. Using the calibration-free LIBS method, the atomic fraction of Re, cRe , is evaluated as a function of the ambient Ar gas pressure, PAr . At PAr < 10 Torr, LIBS-measured cRe agrees well with that from EDX (energydispersive X-ray micro-analysis), while cRe increases with an increase in PAr at >10 Torr due to spectral overlapping of the Re I 488.9 nm line by an Ar II 488.9 nm line. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Tungsten (W) and its alloys are promising candidates for plasmafacing materials (PFMs) in future fusion reactors. In ITER, pure W will be used as the divertor PFM. The advantages of W as a PFM are the highest melting temperature of metals, high thermal conductivity, low physical sputtering yield, and low hydrogen isotope retention [1–4]. Deuterium (D)-tritium (T) fusion reactions produce 14 MeV neutrons, which irradiate W PFM. The high energy neutron irradiation can lead to the transmutation of W, producing neighboring atoms in the periodic table as well as hydrogen (H) and helium (He). The main product of W-transmutation is rhenium (Re), the concentration of which is calculated to be around 0.2 at.% after 2 years DD and 12 years DT ITER experiments and around 4 at.% after 5 years irradiation under first wall fusion power-plant conditions [5]. The ability of laser-induced breakdown spectroscopy (LIBS) for in situ diagnostics of PFMs in fusion reactors has been examined (see e.g. [6–8]), since post-mortem analyses of PFMs by removing PFM tiles will be largely restricted in future radioactivated reactors. In
* Corresponding author. E-mail address: [email protected] (D. Nishijima).
https://doi.org/10.1016/j.sab.2018.01.013 0584-8547/© 2018 Elsevier B.V. All rights reserved.
recent years, LIBS has been examined for analyses of both bulk W materials [9–12] and deposited W layers [13–16]. The purpose of this work is to assess whether LIBS can quantitatively measure a small fraction (a few at.%) of Re in W, which mimics W PFMs irradiated by high-energy neutrons in fusion reactors. First, laser-induced Re plasmas produced from a pure Re sample are characterized. It should be noted that laser-induced Re plasmas have not been extensively studied so far compared to W. Next, a W sample containing a small fraction of Re is analyzed. The fraction of Re is quantitatively measured based on the calibration-free LIBS method [17], and then compared with EDX (energy-dispersive X-ray micro-analysis) of the sample. 2. Experimental setup The LIBS experimental setup is briefly described below. The details can be found in Refs. [10,11]. A Q-switched Nd:YAG laser (Continuum Surelite III-10) was used in this experiment. The output laser wavelength and pulse width are 1064 nm and ∼4–6 ns, respectively. The output laser energy was set to 115 mJ. A laser pulse was focused onto a target surface with a plano-convex lens (focal length = 250 mm). A target was placed inside a vacuum chamber, which is pumped down with a rotary pump. Thus, the minimum ambient gas pressure was ∼5 mTorr, and Ar gas was injected to investigate the effect of the ambient gas pressure, PAr .
D. Nishijima et al. / Spectrochimica Acta Part B 141 (2018) 94–98
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Fig. 1. Typical single-pulse (SP) LIBS spectrum of laser-induced Re plasma at P ∼ 5 mTorr. Only major peaks of Re I lines are labeled. The spectrum is corrected for the spectral response.
Laser-induced plasma light was collected with a lens at the front of the target surface. An optical fiber was coupled to the lens, and the other end of the fiber was connected to the entrance slit of an Echelle type spectrometer (Andor ME5000). An ICCD camera (Andor iStar DH334T) was attached to the exit port of the spectrometer. The spectral sensitivity of the entire spectroscopic system including a vacuum window was calibrated with an integrating sphere (Gooch & Housego OL 455-12) at a wavelength, k, range of 350 ≤ k ≤ 1000 nm, while the spectrometer can simultaneously cover a wider range of 200 ≤ k ≤ 1000 nm. Since the vacuum chamber is not large enough to accommodate the integrating sphere, the sphere was placed outside the chamber. Then, the calibration of the spectroscopic system was done by putting the vacuum window in between the exit port of the sphere and the lens. In this experiment, 20 plasma emission spectra were accumulated for better accuracy. The ICCD camera was triggered with a TTL signal from the laser. The ICCD delay time, tdelay , and the gate width, twidth , were set to 0.13 ls, to exclude strong continuum emission, and 20 ls, to cover most of the plasma lifetime, respectively. Thus, the measured quantities presented here are averaged over the gate width. Note that tdelay is relative to the timing when a laser pulse hits the target surface.
A Re I Boltzmann plot for P ∼ 5 mTorr is presented in Fig. 3 (a), which is obtained from the spectrum shown in Fig. 1 and the spectroscopic parameters listed in Table 1. The data points are well fit to the straight line, giving Te = 0.44 ± 0.03 eV. Since the electron density was not measured in this study, we cannot judge whether if the plasma is in LTE (local thermodynamic equilibrium) or not based on the McWhirter criterion [22]. However, the plasma is expected to be close to LTE, because, as demonstrated in Fig. 3, the population well obeys the Boltzmann distribution [17]. It is seen from Fig. 3 that the intensity of the resonance transitions (488.9 nm and 527.6 nm) becomes gradually lower than the fitted lines as PAr increases. This behavior is similar to W I lines, which suffer from self-absorption [10]. Thus, it is considered that self-absorption of the resonance transitions may occur in the laserinduced Re plasmas at higher PAr , while the effect of self-absorption on Te seems to be small. Here, we compare the database [19] that we employed here with another data set [23], which was recently reported. In the newer data set [23], Aki = 1.16e + 7 s- 1 was given for the 488.9 nm line,
3. Laser-induced pure Re plasma In this section, properties of laser-induced Re plasmas are explored. Fig. 1 shows a typical laser-induced Re plasma spectrum with no Ar gas injection, i.e. P ∼ 5 mTorr. In the figure, only major Re I peaks are labeled. The Re I 488.9 nm line is found to be strongest in the presented wavelength range, which is one of the resonance transitions. The PAr dependence of the Re I 488.9 nm and another resonance transition 527.6 nm line emission intensity is plotted in Fig. 2. As PAr increases, the intensity increases and peaks at PAr ∼ 10–30 Torr. Then, the intensity drops at PAr > 30 Torr. This behavior as well as the peak PAr are very similar to laser-induced W plasmas with Ar background gas. For Boltzmann plot analysis to obtain the electron temperature, Te , 21 Re I transitions were carefully selected from laser-induced Re plasma spectra. Since necessary atomic data of Re I are not currently available in the NIST database [18], we took them from another database [19]. This database compiles Re I data from Refs. [20,21]. Spectroscopic parameters of selected 21 Re I transitions are listed in Table 1.
Fig. 2. Ar pressure dependence of the line emission intensity of the Re I resonance transitions 488.9 nm and 527.6 nm. Note that the intensity is corrected for the spectral response.
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Table 1 Spectroscopic parameters of Re I transitions selected for Boltzmann plot analysis. Wavelength, k, Einstein A coefficient, Aki , energy level, E, statistical weight, g. The subscripts i and k designate lower and upper states, respectively. Source: Data are taken from [19]. k (nm)
Aki (s- 1 )
Ei (eV)
Ek
gi
gk
368.9510 372.5761 373.5314 414.4363 422.7466 425.7600 435.8678 441.5824 451.3311 451.6610 488.9136 492.3911 527.1001 527.5549 566.7905 583.4323 630.7726 632.1903 635.0750 681.3424 682.9967
6.191e+07 1.521e+09 3.624e+08 4.775e+06 3.226e+08 7.896e+06 3.648e+06 5.698e+06 2.522e+08 1.955e+08 1.154e+06 2.378e+06 1.469e+08 3.028e+05 1.199e+06 5.356e+05 6.546e+07 6.517e+07 2.631e+07 2.260e+05 3.443e+05
1.762666 2.929972 2.929972 2.148748 2.349513 1.762666 1.436228 1.866978 2.535204 3.577462 0.000000 1.762666 2.929972 0.000000 1.955283 1.457375 3.577462 3.581869 3.590775 1.762666 1.762666
5.122162 6.256781 6.248273 5.139539 5.281512 4.673914 4.279965 4.673914 5.281512 6.321765 2.535204 4.279965 5.281512 2.349513 4.142155 3.581869 5.542511 5.542511 5.542511 3.581869 3.577462
8 10 10 8 6 8 6 8 8 6 6 8 10 6 6 10 6 8 4 8 8
6 12 10 10 8 6 8 6 8 8 8 8 8 6 6 8 6 6 6 8 6
which is about an order of magnitude higher than Ref. [19] used in this study (see Table 1). Judging from the nicely fit Boltzmann plot in Fig. 3, Ref. [19] is considered to be more accurate. Moreover, since the lifetime of the upper state of the 488.9 nm line was measured to be 412 ns in Ref. [23], which is on the same order as 860 ns in Ref. [21], it seems that the higher value of Aki in Ref. [23] is just a typographic error. Fig. 4 shows the PAr dependence of Te in laser-induced Re plasmas in comparison with W plasmas. W I lines used to obtain Te are detailed in Ref. [10], and are free from self-absorption [10]. It is found that Re plasmas possess lower Te than W plasmas, and that the PAr dependence is very weak for Re plasmas. The discrepancy of Te between Re and W plasmas will be further discussed in Section 4 together with Te in W-3.3%Re plasmas. 4. Quantification of a small fraction of Re in bulk W Here, a W-Re alloy is analyzed; the Re fraction, cRe , in W was measured with EDX (energy-dispersive X-ray micro-analysis) to be ∼3.3 ± 0.6 at.%. It is, hereafter, denoted as W-3.3%Re. From comparison of laser-induced plasma spectra between pure W and W-3.3%Re, only the strongest Re I 488.9 nm line was found to be usable for LIBS analysis of Re in the W-3.3%Re alloy. In Fig. 5, spectra of laser-induced W and W-3.3%Re plasmas are compared at
Fig. 4. Ar pressure dependence of Te in laser-induced Re (circles), W-3.3%Re (squares), and W (diamonds) plasmas.
a wavelength range of k = 488–489.5 nm. The Re I 488.9 nm line is clearly seen right next to a W I line at 488.7 nm. The calibration-free (CF) LIBS method [17] is applied to evaluate cRe in the W-3.3%Re alloy. The temperature-dependent partition function is necessary for the CF-LIBS method, and was taken from the NIST database [18] for both W I and Re I. Boltzmann plots of W I and Re I from a laser-induced W-3.3%Re plasma at PAr = 0.3 Torr are shown in Fig. 6. Since there is only one line available for Re I as mentioned above, Te obtained from W I is applied for Re I. Here, the plasma is again expected to be close to LTE, because the population obeys the Boltzmann distribution, as mentioned above. Since the fraction of Re in the alloy is only around 3.3%, the effect of selfabsorption of the resonance transition at 488.9 nm is much weaker than in the pure Re plasmas. As demonstrated in Fig. 4, Te of laser-induced W-3.3%Re plasmas is higher than that of pure Re plasmas, and is slightly lower than that of pure W plasmas over the entire PAr range. This indicates that Te in a laser-induced plasma may be sensitive to some material properties. For instance, addition of a small fraction of Re to W leads to material property changes such as improved ductility, a lower brittle to ductile transition temperature, a higher recrystallization temperature, a lower thermal conductivity, and a lower melting temperature. One, or some, of these changes may affect Te in a laser-induced plasma. From Boltzmann plots of W I and Re I, the intercept of each species is derived, which is proportional to the logarithm of the species fraction. It should be noted that the extrapolation is made
Fig. 3. Re I Boltzmann plots at (a) P ∼ 5 mTorr, (b) PAr ∼ 3 Torr, and (c) PAr ∼ 100 Torr.
D. Nishijima et al. / Spectrochimica Acta Part B 141 (2018) 94–98
Fig. 5. Spectra of laser-induced pure W (circles) and W-3.3%Re (square) plasmas, showing W I 488.7 nm and Re I 488.9 nm lines.
to obtain the intercept, which is prone to errors. Fig. 7 shows the LIBS-measured cRe plotted as a function of PAr . The scattering of the data points mainly represents an error of cRe . An error of cRe caused by an error of Te is relatively small (≤0.1%). At PAr < 10 Torr, the LIBS-measured cRe agrees well with that from EDX, while the LIBSmeasured cRe starts to increase at PAr ∼ 10 Torr. This deviation at high PAr is caused by the spectral overlapping of the Re I 488.9 nm line by an Ar II 488.9 nm line. Although the Ar II 488.9 line intensity is very weak, the contribution of this line cannot be neglected at high PAr when a small fraction of Re is measured. To avoid the contamination, He background gas can be used instead of Ar, when He is not an element of interest. For laser-induced W plasmas, while the signal intensity is reduced by a factor of ∼2 compared to Ar, the signal to noise ratio is improved by ∼30% [11]. It should be noted that this good agreement of cRe with EDX also proves that the Aki value of the
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Fig. 7. LIBS-measured cRe in the W-3.3%Re alloy against PAr , showing good agreement at PAr < 10 Torr with that from EDX. The deviation at PAr > 10 Torr is due to the spectral overlapping of Re I 488.9 nm line by an Ar II 488.9 nm line.
Re I 488.9 nm transition from Ref. [19] is more accurate than that from Ref. [23]. 5. Conclusion This paper demonstrated a new potential of LIBS as an in situ diagnostic method of W PFM in fusion reactors. Neutron irradiation of W results in W-transmutation, which mainly produces Re. In this study, a small fraction (∼3.3 at.%) of Re in W was successfully quantified using the CF-LIBS method. This will enable evaluation of neutron damage to W PFM and also estimate the neutron fluence to W PFM from the fraction of Re in fusion reactors. At the beginning of fusion reactor operations, the Re fraction in W is even lower than 3.3% used in this study. Thus, the detection limit of Re in W will be explored in the future. Furthermore, the quantification of a small fraction of tantalum (Ta) and osmium (Os) in W, which are the next main products of W-transmutation, will be also attempted. From comparison of Te between laser-induced pure W, W-Re alloy, and pure Re plasmas, it is speculated that material properties may affect Te in laser-induced plasmas. This implies a new possibility of detecting changes in material properties from laser-induced plasma Te . Acknowledgments This work is supported by the U.S. Department of Energy Grant No. DE-FG02-07ER54912.
References
Fig. 6. Boltzmann plots of W I (circle) and Re I (square) from a laser-induced W3.3%Re plasma at PAr = 0.3 Torr. Te is obtained from the W I Boltzmann plot, and is assumed to be the same as Re I.
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[14] S. Almaviva, L. Caneve, F. Colao, G. Maddaluno, N. Krawczyk, A. Czarnecka, P. Gasior, M. Kubkowska, M. Lepek, Measurements of deuterium retention and surface elemental composition with double pulse laser induced breakdown spectroscopy, Phys. Scr. T167 (2016) 014043. [15] D. Nishijima, E.M. Hollmann, R.P. Doerner, Spatially-offset double-pulse laser-induced breakdown spectroscopy: a novel technique for analysis of thin deposited layers, Spectrochim. Acta B 124 (2016) 82–86. [16] K. Piip, H.J. van der Meiden, L. Hämarik, J. Karhunen, A. Hakola, M. Laan, P. Paris, M. Aints, J. Likonen, K. Bystrov, J. Kozlova, A. Založnik, M. Kelemen, S. Markelj, LIBS detection of erosion/deposition and deuterium retention resulting from exposure to pilot-PSI plasmas, J. Nucl. Mater. 489 (2017) 129. [17] A. Ciucci, M. Corsi, V. Palleschi, S. Rastelli, A. Salvetti, E. Tognoni, New procedure for quantitative elemental analysis by laser-induced plasma spectroscopy, Appl. Spectrosc. 53 (1999) 960–964. [18] A. Kramida, Yu. Ralchenko, J. Reader, NIST ASD Team, NIST Atomic Spectra Database (Version 5.4), 2016, [Online]. Available: http://physics.nist.gov/asd [Fri Jun 02 2017], National Institute of Standards and Technology, Gaithersburg, MD, 20l6. [19] 1995 Atomic Line Data (R.L. Kurucz and B. Bell), Kurucz CD-ROM no. 23. Cambridge, Mass.: Smithsonian Astrophysical Observatory 1995, https://www. cfa.harvard.edu/amp/ampdata/kurucz23/sekur.html. [20] C.H. Corliss, W.R. Bozman, Experimental transition probabilities for spectral lines of seventy elements, NBS Monograph 53 (1962) [21] D.W. Duquette, S. Salih, J.E. Lawler, Radiative lifetimes in Re I, J. Phys. B: At. Mol. Phys. 15 (1982) L897–L901. [22] G. Bekefi, C. Deutsch, B. Yaakobi, Principles of Laser Plasmas, in: G. Bekefi (Ed.), (New York, Wiley). 1976, chapter 13. [23] P. Palmeri, P. Quinet, É. Biémont, S. Svanberg, H.L. Xu, Radiative lifetime measurements and semi-empirical transition probability calculations in neutral rhenium, Phys. Scr. 74 (2006) 297–303.
Contents lists available at ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Research note
Laser-induced breakdown spectroscopy measurement of a small fraction of rhenium in bulk tungsten D. Nishijima a, * , Y. Ueda b , R.P. Doerner a , M.J. Baldwin a , K. Ibano b a b
Center for Energy Research, University of California at San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0417, USA Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita 565-0871, Japan
A R T I C L E
I N F O
Article history: Received 14 June 2017 Received in revised form 18 December 2017 Accepted 24 January 2018 Available online 2 February 2018 Keywords: LIBS Rhenium Tungsten-rhenium alloy Boltzmann plot Calibration-free
A B S T R A C T Laser-induced breakdown spectroscopy (LIBS) of bulk rhenium (Re) and tungsten (W)-Re alloy has been performed using a Q-switched Nd:YAG laser (wavelength = 1064 nm, pulse width ∼4–6 ns, laser energy = 115 mJ). It is found that the electron temperature, Te , of laser-induced Re plasma is lower than that of W plasma, and that Te of W-Re plasma is in between Re and W plasmas. This indicates that material properties affect Te in a laser-induced plasma. For analysis of W-3.3%Re alloy, only the strongest visible Re I 488.9 nm line is found to be used because of the strong enough intensity without contamination with W lines. Using the calibration-free LIBS method, the atomic fraction of Re, cRe , is evaluated as a function of the ambient Ar gas pressure, PAr . At PAr < 10 Torr, LIBS-measured cRe agrees well with that from EDX (energydispersive X-ray micro-analysis), while cRe increases with an increase in PAr at >10 Torr due to spectral overlapping of the Re I 488.9 nm line by an Ar II 488.9 nm line. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Tungsten (W) and its alloys are promising candidates for plasmafacing materials (PFMs) in future fusion reactors. In ITER, pure W will be used as the divertor PFM. The advantages of W as a PFM are the highest melting temperature of metals, high thermal conductivity, low physical sputtering yield, and low hydrogen isotope retention [1–4]. Deuterium (D)-tritium (T) fusion reactions produce 14 MeV neutrons, which irradiate W PFM. The high energy neutron irradiation can lead to the transmutation of W, producing neighboring atoms in the periodic table as well as hydrogen (H) and helium (He). The main product of W-transmutation is rhenium (Re), the concentration of which is calculated to be around 0.2 at.% after 2 years DD and 12 years DT ITER experiments and around 4 at.% after 5 years irradiation under first wall fusion power-plant conditions [5]. The ability of laser-induced breakdown spectroscopy (LIBS) for in situ diagnostics of PFMs in fusion reactors has been examined (see e.g. [6–8]), since post-mortem analyses of PFMs by removing PFM tiles will be largely restricted in future radioactivated reactors. In
* Corresponding author. E-mail address: [email protected] (D. Nishijima).
https://doi.org/10.1016/j.sab.2018.01.013 0584-8547/© 2018 Elsevier B.V. All rights reserved.
recent years, LIBS has been examined for analyses of both bulk W materials [9–12] and deposited W layers [13–16]. The purpose of this work is to assess whether LIBS can quantitatively measure a small fraction (a few at.%) of Re in W, which mimics W PFMs irradiated by high-energy neutrons in fusion reactors. First, laser-induced Re plasmas produced from a pure Re sample are characterized. It should be noted that laser-induced Re plasmas have not been extensively studied so far compared to W. Next, a W sample containing a small fraction of Re is analyzed. The fraction of Re is quantitatively measured based on the calibration-free LIBS method [17], and then compared with EDX (energy-dispersive X-ray micro-analysis) of the sample. 2. Experimental setup The LIBS experimental setup is briefly described below. The details can be found in Refs. [10,11]. A Q-switched Nd:YAG laser (Continuum Surelite III-10) was used in this experiment. The output laser wavelength and pulse width are 1064 nm and ∼4–6 ns, respectively. The output laser energy was set to 115 mJ. A laser pulse was focused onto a target surface with a plano-convex lens (focal length = 250 mm). A target was placed inside a vacuum chamber, which is pumped down with a rotary pump. Thus, the minimum ambient gas pressure was ∼5 mTorr, and Ar gas was injected to investigate the effect of the ambient gas pressure, PAr .
D. Nishijima et al. / Spectrochimica Acta Part B 141 (2018) 94–98
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Fig. 1. Typical single-pulse (SP) LIBS spectrum of laser-induced Re plasma at P ∼ 5 mTorr. Only major peaks of Re I lines are labeled. The spectrum is corrected for the spectral response.
Laser-induced plasma light was collected with a lens at the front of the target surface. An optical fiber was coupled to the lens, and the other end of the fiber was connected to the entrance slit of an Echelle type spectrometer (Andor ME5000). An ICCD camera (Andor iStar DH334T) was attached to the exit port of the spectrometer. The spectral sensitivity of the entire spectroscopic system including a vacuum window was calibrated with an integrating sphere (Gooch & Housego OL 455-12) at a wavelength, k, range of 350 ≤ k ≤ 1000 nm, while the spectrometer can simultaneously cover a wider range of 200 ≤ k ≤ 1000 nm. Since the vacuum chamber is not large enough to accommodate the integrating sphere, the sphere was placed outside the chamber. Then, the calibration of the spectroscopic system was done by putting the vacuum window in between the exit port of the sphere and the lens. In this experiment, 20 plasma emission spectra were accumulated for better accuracy. The ICCD camera was triggered with a TTL signal from the laser. The ICCD delay time, tdelay , and the gate width, twidth , were set to 0.13 ls, to exclude strong continuum emission, and 20 ls, to cover most of the plasma lifetime, respectively. Thus, the measured quantities presented here are averaged over the gate width. Note that tdelay is relative to the timing when a laser pulse hits the target surface.
A Re I Boltzmann plot for P ∼ 5 mTorr is presented in Fig. 3 (a), which is obtained from the spectrum shown in Fig. 1 and the spectroscopic parameters listed in Table 1. The data points are well fit to the straight line, giving Te = 0.44 ± 0.03 eV. Since the electron density was not measured in this study, we cannot judge whether if the plasma is in LTE (local thermodynamic equilibrium) or not based on the McWhirter criterion [22]. However, the plasma is expected to be close to LTE, because, as demonstrated in Fig. 3, the population well obeys the Boltzmann distribution [17]. It is seen from Fig. 3 that the intensity of the resonance transitions (488.9 nm and 527.6 nm) becomes gradually lower than the fitted lines as PAr increases. This behavior is similar to W I lines, which suffer from self-absorption [10]. Thus, it is considered that self-absorption of the resonance transitions may occur in the laserinduced Re plasmas at higher PAr , while the effect of self-absorption on Te seems to be small. Here, we compare the database [19] that we employed here with another data set [23], which was recently reported. In the newer data set [23], Aki = 1.16e + 7 s- 1 was given for the 488.9 nm line,
3. Laser-induced pure Re plasma In this section, properties of laser-induced Re plasmas are explored. Fig. 1 shows a typical laser-induced Re plasma spectrum with no Ar gas injection, i.e. P ∼ 5 mTorr. In the figure, only major Re I peaks are labeled. The Re I 488.9 nm line is found to be strongest in the presented wavelength range, which is one of the resonance transitions. The PAr dependence of the Re I 488.9 nm and another resonance transition 527.6 nm line emission intensity is plotted in Fig. 2. As PAr increases, the intensity increases and peaks at PAr ∼ 10–30 Torr. Then, the intensity drops at PAr > 30 Torr. This behavior as well as the peak PAr are very similar to laser-induced W plasmas with Ar background gas. For Boltzmann plot analysis to obtain the electron temperature, Te , 21 Re I transitions were carefully selected from laser-induced Re plasma spectra. Since necessary atomic data of Re I are not currently available in the NIST database [18], we took them from another database [19]. This database compiles Re I data from Refs. [20,21]. Spectroscopic parameters of selected 21 Re I transitions are listed in Table 1.
Fig. 2. Ar pressure dependence of the line emission intensity of the Re I resonance transitions 488.9 nm and 527.6 nm. Note that the intensity is corrected for the spectral response.
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Table 1 Spectroscopic parameters of Re I transitions selected for Boltzmann plot analysis. Wavelength, k, Einstein A coefficient, Aki , energy level, E, statistical weight, g. The subscripts i and k designate lower and upper states, respectively. Source: Data are taken from [19]. k (nm)
Aki (s- 1 )
Ei (eV)
Ek
gi
gk
368.9510 372.5761 373.5314 414.4363 422.7466 425.7600 435.8678 441.5824 451.3311 451.6610 488.9136 492.3911 527.1001 527.5549 566.7905 583.4323 630.7726 632.1903 635.0750 681.3424 682.9967
6.191e+07 1.521e+09 3.624e+08 4.775e+06 3.226e+08 7.896e+06 3.648e+06 5.698e+06 2.522e+08 1.955e+08 1.154e+06 2.378e+06 1.469e+08 3.028e+05 1.199e+06 5.356e+05 6.546e+07 6.517e+07 2.631e+07 2.260e+05 3.443e+05
1.762666 2.929972 2.929972 2.148748 2.349513 1.762666 1.436228 1.866978 2.535204 3.577462 0.000000 1.762666 2.929972 0.000000 1.955283 1.457375 3.577462 3.581869 3.590775 1.762666 1.762666
5.122162 6.256781 6.248273 5.139539 5.281512 4.673914 4.279965 4.673914 5.281512 6.321765 2.535204 4.279965 5.281512 2.349513 4.142155 3.581869 5.542511 5.542511 5.542511 3.581869 3.577462
8 10 10 8 6 8 6 8 8 6 6 8 10 6 6 10 6 8 4 8 8
6 12 10 10 8 6 8 6 8 8 8 8 8 6 6 8 6 6 6 8 6
which is about an order of magnitude higher than Ref. [19] used in this study (see Table 1). Judging from the nicely fit Boltzmann plot in Fig. 3, Ref. [19] is considered to be more accurate. Moreover, since the lifetime of the upper state of the 488.9 nm line was measured to be 412 ns in Ref. [23], which is on the same order as 860 ns in Ref. [21], it seems that the higher value of Aki in Ref. [23] is just a typographic error. Fig. 4 shows the PAr dependence of Te in laser-induced Re plasmas in comparison with W plasmas. W I lines used to obtain Te are detailed in Ref. [10], and are free from self-absorption [10]. It is found that Re plasmas possess lower Te than W plasmas, and that the PAr dependence is very weak for Re plasmas. The discrepancy of Te between Re and W plasmas will be further discussed in Section 4 together with Te in W-3.3%Re plasmas. 4. Quantification of a small fraction of Re in bulk W Here, a W-Re alloy is analyzed; the Re fraction, cRe , in W was measured with EDX (energy-dispersive X-ray micro-analysis) to be ∼3.3 ± 0.6 at.%. It is, hereafter, denoted as W-3.3%Re. From comparison of laser-induced plasma spectra between pure W and W-3.3%Re, only the strongest Re I 488.9 nm line was found to be usable for LIBS analysis of Re in the W-3.3%Re alloy. In Fig. 5, spectra of laser-induced W and W-3.3%Re plasmas are compared at
Fig. 4. Ar pressure dependence of Te in laser-induced Re (circles), W-3.3%Re (squares), and W (diamonds) plasmas.
a wavelength range of k = 488–489.5 nm. The Re I 488.9 nm line is clearly seen right next to a W I line at 488.7 nm. The calibration-free (CF) LIBS method [17] is applied to evaluate cRe in the W-3.3%Re alloy. The temperature-dependent partition function is necessary for the CF-LIBS method, and was taken from the NIST database [18] for both W I and Re I. Boltzmann plots of W I and Re I from a laser-induced W-3.3%Re plasma at PAr = 0.3 Torr are shown in Fig. 6. Since there is only one line available for Re I as mentioned above, Te obtained from W I is applied for Re I. Here, the plasma is again expected to be close to LTE, because the population obeys the Boltzmann distribution, as mentioned above. Since the fraction of Re in the alloy is only around 3.3%, the effect of selfabsorption of the resonance transition at 488.9 nm is much weaker than in the pure Re plasmas. As demonstrated in Fig. 4, Te of laser-induced W-3.3%Re plasmas is higher than that of pure Re plasmas, and is slightly lower than that of pure W plasmas over the entire PAr range. This indicates that Te in a laser-induced plasma may be sensitive to some material properties. For instance, addition of a small fraction of Re to W leads to material property changes such as improved ductility, a lower brittle to ductile transition temperature, a higher recrystallization temperature, a lower thermal conductivity, and a lower melting temperature. One, or some, of these changes may affect Te in a laser-induced plasma. From Boltzmann plots of W I and Re I, the intercept of each species is derived, which is proportional to the logarithm of the species fraction. It should be noted that the extrapolation is made
Fig. 3. Re I Boltzmann plots at (a) P ∼ 5 mTorr, (b) PAr ∼ 3 Torr, and (c) PAr ∼ 100 Torr.
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Fig. 5. Spectra of laser-induced pure W (circles) and W-3.3%Re (square) plasmas, showing W I 488.7 nm and Re I 488.9 nm lines.
to obtain the intercept, which is prone to errors. Fig. 7 shows the LIBS-measured cRe plotted as a function of PAr . The scattering of the data points mainly represents an error of cRe . An error of cRe caused by an error of Te is relatively small (≤0.1%). At PAr < 10 Torr, the LIBS-measured cRe agrees well with that from EDX, while the LIBSmeasured cRe starts to increase at PAr ∼ 10 Torr. This deviation at high PAr is caused by the spectral overlapping of the Re I 488.9 nm line by an Ar II 488.9 nm line. Although the Ar II 488.9 line intensity is very weak, the contribution of this line cannot be neglected at high PAr when a small fraction of Re is measured. To avoid the contamination, He background gas can be used instead of Ar, when He is not an element of interest. For laser-induced W plasmas, while the signal intensity is reduced by a factor of ∼2 compared to Ar, the signal to noise ratio is improved by ∼30% [11]. It should be noted that this good agreement of cRe with EDX also proves that the Aki value of the
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Fig. 7. LIBS-measured cRe in the W-3.3%Re alloy against PAr , showing good agreement at PAr < 10 Torr with that from EDX. The deviation at PAr > 10 Torr is due to the spectral overlapping of Re I 488.9 nm line by an Ar II 488.9 nm line.
Re I 488.9 nm transition from Ref. [19] is more accurate than that from Ref. [23]. 5. Conclusion This paper demonstrated a new potential of LIBS as an in situ diagnostic method of W PFM in fusion reactors. Neutron irradiation of W results in W-transmutation, which mainly produces Re. In this study, a small fraction (∼3.3 at.%) of Re in W was successfully quantified using the CF-LIBS method. This will enable evaluation of neutron damage to W PFM and also estimate the neutron fluence to W PFM from the fraction of Re in fusion reactors. At the beginning of fusion reactor operations, the Re fraction in W is even lower than 3.3% used in this study. Thus, the detection limit of Re in W will be explored in the future. Furthermore, the quantification of a small fraction of tantalum (Ta) and osmium (Os) in W, which are the next main products of W-transmutation, will be also attempted. From comparison of Te between laser-induced pure W, W-Re alloy, and pure Re plasmas, it is speculated that material properties may affect Te in laser-induced plasmas. This implies a new possibility of detecting changes in material properties from laser-induced plasma Te . Acknowledgments This work is supported by the U.S. Department of Energy Grant No. DE-FG02-07ER54912.
References
Fig. 6. Boltzmann plots of W I (circle) and Re I (square) from a laser-induced W3.3%Re plasma at PAr = 0.3 Torr. Te is obtained from the W I Boltzmann plot, and is assumed to be the same as Re I.
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