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Molten layer characteristics of W materials and film coated W by pulsed laser irradiation
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FUSION-9341; No. of Pages 5
Fusion Engineering and Design xxx (2017) xxx–xxx
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Molten layer characteristics of W materials and film coated W by pulsed laser irradiation Daisuke Inoue, Kenzo Ibano ∗ , Satoru Yoshikawa, Takeru Maeji, Yoshio Ueda Graduate School of Engineering, Osaka University, Osaka, Japan
h i g h l i g h t s • • • • •
Laser induced heat flux up to 6.6 GW/m2 formed >100 m surface deformation on W. W and W-10wt%Re showed similar deformations in both macro- and micro- scales. W-2wt%Ta showed severe erosions via bumping. Suppressed deformation was seen for the Al coated W targets. Alloying of Al-W was not observed.
a r t i c l e
i n f o
Article history: Received 3 October 2016 Received in revised form 27 March 2017 Accepted 28 March 2017 Available online xxx Keywords: Liquid tungsten Plasma facing materials Transient heat loads Nd:YAG laser heating
a b s t r a c t Molten layer characteristics were investigated for four W materials; pure W, W-10wt%Re, W-2wt%Ta and W with Al coating. Specimens were irradiated by a Nd:YAG laser (wavelength 1064 nm) simulating transient heat loads such as ELMs and disruption. The power density of the heat loads were varied in the 2.3–6.6 GW/m2 range. Surface profiles were observed with a laser microscope. The central part of irradiated W-2wt%Ta became dented deeply in a wider range with increasing the power density in comparison with pure W and W-10wt%Re. This could be attributed to the release of Ta droplets by bumping (rapid boiling), closely related to microstructure and/or impurity concentration determined by the materials production processes. We also observed surface morphology changes of pure W with and without Al coating after irradiation in order to investigate the effect of the protective coatings. Melting of W with Al coating seems to be suppressed by a protective effect of Al film when compared with pure W. © 2017 Published by Elsevier B.V.
1. Introduction Tungsten (W) is a primary candidate of plasma-facing materials for future fusion reactors and W is planned to be used for divertor armor of ITER [1]. Although W has high thermal conductivity [2], low tritium retention [3] and low sputtering yield [4], its poor mechanical properties are the weakest point of this material. In particular, transient heat loads caused by disruption and edge localized modes (ELMs) severely influence W materials [5]. In ITER, transient heat loads such as Type I ELM (0.2–3 MJ/m2 for 0.1–1 ms) could cause submillimeter melting on W surface [6]. High temperature gradients and high thermal stresses developed during these transients lead to material damages, such as formation of melt layers, material erosion/ejection and crack formation. These damages can
∗ Corresponding author. E-mail address: [email protected] (K. Ibano).
decrease the lifetime of plasma-facing components and increase the contamination of plasma in subsequent operations [7]. During the reactor operation, the W surface encounters continuous bombardments by fusion neutrons as well as energetic plasma loads. It is expected that the W surface will be covered by the first wall originated Beryllium (Be) [8]. It is also expected that the nuclear transformation leads Rhenium (Re) inclusion [9]. Plus, some studies indicate that W-alloy such as W-Tantalum (W-Ta) is suitable for the first wall because of its improved mechanical property [9]. Thus, the erosion characteristic by the transient heat loads should be studied for these W materials. In order to simulate the transient heat loads, plasma guns [3,5], quasi-steady-state plasma accelerator [10,11], electron beams [12], and pulse lasers [13] have been used to study the effect of ELMs and disruptions on PFC materials. Within these methods, the pulsed laser experiments are suitable for the heat flux test of various materials in a controlled manner for a wide heat flux range. Thus we irradiated various types of W specimens to investigate macroscopic
http://dx.doi.org/10.1016/j.fusengdes.2017.03.160 0920-3796/© 2017 Published by Elsevier B.V.
Please cite this article in press as: D. Inoue, et al., Molten layer characteristics of W materials and film coated W by pulsed laser irradiation, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.160
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and microscopic changes of each specimens. We also investigated whether there are suppressing effects of morphology changes due to thin film coatings on W, or there are enhancing effects of morphology changes due to a depression of the melting point via alloying [13].
2. Experimental Four types of tungsten specimens were investigated: Pure W (99.99% at. purity) with and without Al coating, W-10wt%Re, and W-2wt%Ta. The size of specimens was 10 × 10 × 1 mm3 , and all specimens were mechanically polished to a mirror quality. The thermal shock tests were performed by applying transient heat loads in a vacuum of ∼10−7 Torr by Nd:YAG laser (wavelength 1064 nm, irradiation diameter 0.6 mm). The laser pulse width can be varied from 0.25 ms to 5 ms, and the maximum peak power is 7 kW. The laser absorption rate of pure W was measured as 26%. That of W-10wt%Re and W-2%Ta were 36% and 30%. Following laser irradiation, surface profiles of the irradiated specimens were observed using a laser microscope (VK-9710, KEYENCE Co). Then, a focused ion beam (FIB, FB2200, HITACHI Co) was used to expose the cross section of central irradiated point. The areas of 50 m × 50 m located in the center of the samples were exposed to Ga ions (40 keV). Secondary Ion Microscope images of the cross section microstructure was obtained (FIB–SIM). The element distribution was also measured by the energy dispersive X-ray spectrometry (EDX) analysis of a FE-SEM (Ultra55, Zeiss Co) attachment. 1 m Al coatings was prepared by using a magnetron sputtering device. Al itself cannot be used as a protective coating in a fusion reactor because of its neutron reactivity. However, Al is often used as a proxy to Be because of their similar vapor pressure [14]. One concern for this proxy is the alloying related enhanced erosion known for Be coated W[13]. Although both materials are known to form alloys with W (Be22 W, Be12 W and Be2 W [15] for Be. Al12 W, Al5 W and Al4 W[16] for Al), Al coating cannot mimic this characteristic due to the melting point difference (Be 1560 K and Al 933 K). Thus Al-W alloying was studied in detail.
3. Results and discussions 3.1. Macroscopic deformation of laser irradiated spots The laser pulsed irradiation by using Nd:YAG laser were performed to Pure W, W-10wt%Re and W-2wt%Ta. The power density of the heat loads were varied in the 2.3-6.6 GW/m2 range and the pulse width was 0.5 ms. The heat flux is similar with expected heat flux of Type I ELM at ITER. Surface profiles of the irradiated specimens measured by the laser microscope are shown in Fig. 1. In these plots, the height of non-irradiated surface was used as the origin. We observed surface during the irradiation by a high speed camera and concluded that these surface modification were caused by unstable melt layer motion. [17] After irradiation in 2.3–3.7 GW/m2 to a pure W sample, the central part became dented in comparison with unirradiated surface. However, after irradiation in 5.2 GW/m2 to W, the central part became a convex shape. The cause of this protruded shape was not clear, and further microscopic analysis was taken and discussed in the next chapter. The surface profile of W-10wt%Re became a similar form of pure W. In contrast, the central part of irradiated W-2wt%Ta became dented deeply in wider range with increase in the power density. In addition, the surface profile of W-2%Ta is asymmetric unlike the ones of pure W and W-10wt%Re. Fig. 2 summarizes volume changes after irradiation. These volume changes
Fig. 1. The surface profiles of W specimens after laser irradiation with 0.5 ms pulse width for various power density: (a)pure W (b) W-10wt%Re (c) W-2wt%Ta. Legends in Fig. 1 show absorbed heat road to W specimens. Blue dashed lines indicate the laser irradiation spot. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: D. Inoue, et al., Molten layer characteristics of W materials and film coated W by pulsed laser irradiation, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.160
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Fig. 2. Volume change of W specimens after the Nd:YAG laser irradiation. Pure W and W-10wt%Re shows volume gains. In contrast, W-2wt%Ta shows volume losses. It should be emphasized that the increments of the y-axis of positive side and negative side are different.
were calculated from the measured surface profiles assuming the circular symmetry of these profiles. The volume of W-2wt%Ta greatly decreases after irradiation in comparison with that of pure W and W-10wt%Re. Their volume gains can be explained by a cavity formation or decreasing density. In the meantime, the volume reduction of W-2wt%Ta can be explained by a release of Ta droplets by bumping (rapid boiling). Bumping can be caused by inhomogeneous microstructure and/or impurity nucleation originated from the material production process. Smaller weight percent specimens should be tested in future. The laser pulsed irradiation by using Nd:YAG laser were also performed to pure W with and without the Al coating. The laser pulse width was 0.5 ms and the laser power was varied in the 3–7 kw range (1.5–3.0 J). The absorbed heat loads to the Al coating specimen could not be precisely determined because of its inconstant reflectivity. However, the total energy needed to evaporated Al was negligible (∼10 mJ) to the incident laser energy. Thus, the heat flux was determined based on the W surface reflectivity. The surface profiles of the irradiated specimens are shown in Fig. 3. The figure shows that morphology changes of W with Al coating after laser irradiation seem to be suppressed compared with the pure W without Al. These observations indicated that the alloying enhanced erosion did not happen for the Al/W samples in the macroscopic scale. 3.2. Macroscopic deformation of laser irradiated spots Cavities formation were suspected in the protruded part (Fig. 1(a), (b)), because the volume gains were observed for pureW and W-10wt%Re specimens as shown in Fig. 2. Thus, the cross section of central irradiated point was exposed by using the FIB. However, cavities were not observed at least for the 50 m depth. The volume gains are <5% of melted volume. Thus, up to 5% of the density decrease of re-solidified regions explain the gains. Obtained SIM images were summarized in Fig. 4. Central part of pure W with and without Al coating irradiated with 5.2 GW/m2 , W-10wt%Re irradiated with 6.6 GW/m2 , W-2wt%Ta irradiated with 4.0 GW/m2 were shown here. Within these cross sectional SIM images, only W-2wt%Ta did not show significant changes of its grain size. This observation also suggests that the melting layer
Fig. 3. The surface profiles of pure W with and without Al after laser irradiation with 0.5 ms pulse width for various laser power: (a)pure W without Al (b)pure W with Al. Legends in Fig. 3 show laser power irradiated to pure W with and without Al.
of W-2wt%Ta was quickly scattered by bumping. In contrast, other specimens show large grain sizes after laser irradiation. Recrystallization or re-solidification caused these changes of grain sizes. In addition to these observations, EDX analysis was performed to investigate whether W and Al were alloyed after laser irradiation (Fig. 5). EDX analysis of central part of melting spot confirmed that no Al or Al-alloy remained at the part. Previous study of W with Be coating showed that the Be-W alloying occurred at the central part of melting spot [13]. W with Be coating have shown a larger damage than non-coating W because of the formation of Be-W alloy, which has a lower damage threshold compared to W. In contrast, in the case of Al in this study, the macroscopic damage was suppressed. These observations indicate that Al evaporates or melted Al scattered quickly and Al-W alloy was not be formed in this experiment. As a result, the Al coating effectively suppress the morphology changes of underlying W. 4. Conclusion In this study, four W materials; pure W with and without Al coating, W-10wt%Re, W-2wt%Ta were irradiated by Nd:YAG laser simulating transient heat loads such as ELMs and disruption. Sur-
Please cite this article in press as: D. Inoue, et al., Molten layer characteristics of W materials and film coated W by pulsed laser irradiation, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.160
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Fig. 4. SIM cross section images of laser irradiated and not irradiated region. Pure W with 5.2 GW/m2 , Al coated W with 5.2 GW/m2 , W-10wt%Re with 6.6 GW/m2 and W-2wt%Ta with 4.0 GW/m2 were compared. The maximum energy densities within the spots of symmetric erosion profile were chosen.
Fig. 5. EDX mapping analysis for the pure W specimen with the Al coating after the laser irradiation of 3.7 GW/m2 .
face profiles were measured by a laser microscope. Similar forms were observed for the pure W and W-10wt%Re after irradiation. On the other hand, the central part of irradiated W-2wt%Ta became dented deeply in a wider range for higher power density. Moreover, the crystal grain size of W-2wt%Ta hardly changed after irradiation. These observations suggest that the melting layer of W-2wt%Ta was quickly scattered via the droplet ejection by bumping. When comparing the surface profiles of pure W with and without Al, morphology changes of W with Al coating after laser irradiation seem to be suppressed. In addition, the EDX element analysis did not find any Al-W alloys. Thus, Al-W alloying induced erosion enhancement was not found in this study. Acknowledgements This work is partly supported by JSPS KAKENHI Grant Number 25249132 and by ZE Research Program, IAEZE28B-30 and IAEZE28A-19.
References [1] T. Hirai, et al., ITER tungsten divertor design development and qualification program, Fusion Eng. Des. 88 (2013) 1798–1801. [2] J.W. Davis, et al., Assessment of for use in the ITER plasma facing components, J. Nucl. Mater. 258–263 (1998) 308–312. [3] V. Philipps, Tungsten as material for plasma-facing components in fusion devices, J. Nucl. Mater. 415 (2011) S2–S9. [4] G. Federici, et al., Effects of ELMs and disruptions on ITER divertor armour materials, J. Nucl. Mater. 337–339 (2005) 684–690. [5] A. Zhitlukhin, et al., Effects of ELMs on ITER divertor armour materials, J. Nucl. Mater. 363–365 (2007) 301–307. [6] Loewenhoff, Thorsten Werner, Combined Steady State and High Cycle Transient Heat Load Simulation with the Electron Beam Facility JUDITH 2, Ph.D. Thesis, RWTH, Aachen Germany, 2012. [7] A. Suslova, et al., Material ejection and surface morphology changes during transient heat loading of tungsten as plasma-facing component in fusion devices, Nucl. Fusion 55 (2015) 033007. [8] S. Brezinsek, JET-EFDA contributors, Plasma-surface interaction in the Be/W environment: conclusions drawn from the JET-ILW for ITER, J. Nucl. Mater. 463 (2015) 11–21. [9] M.R. Gilbert, J.-Ch Sublet, Neutron-induced transmutation effects in W and W-alloys in a fusion environment, Nucl. Fusion 51 (2011) 043005. [10] V.I. Tereshin, et al., Application of powerful quasi-steady-state plasma accelerators for simulation of ITER transient heat loads on divertor surfaces, Plasma Phys. Controlled Fusion 49 (2007) A231–A239.
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[11] I.E. Garkusha, et al., The latest results from ELM-simulation experiments in plasma accelerators, Phys. Scr. T138 (2009) 014054. [12] G. Pintsuk, et al., Investigation of tungsten and beryllium behavior under short transient events, Fusion Eng. Des. 82 (2007) 1720–1729. [13] J.H. Yu, et al., ITER-relevant transient heat loads on tungsten exposed to plasma and beryllium, Phys. Scr. T159 (2014) 014036. [14] I. Sakuma, et al., Experimental investigation of vapor shielding effects induced by ELM-like pulsed plasma loads using the double plasma gun device, J. Nucl. Mater. 463 (2015) 233.
[15] A. Wiltner, C.h. Linsmeier, Formation of a surface alloy in the beryllium-tungsten system, J. Nucl. Mater. 337–339 (2005) 951. [16] H. Okamoto, Desk Handbook, Phase Diagrams for Binary Alloys, ASM, International, Materials Park, OH, 2000, pp. 48. [17] T. Maeji et al., this conference.
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Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Molten layer characteristics of W materials and film coated W by pulsed laser irradiation Daisuke Inoue, Kenzo Ibano ∗ , Satoru Yoshikawa, Takeru Maeji, Yoshio Ueda Graduate School of Engineering, Osaka University, Osaka, Japan
h i g h l i g h t s • • • • •
Laser induced heat flux up to 6.6 GW/m2 formed >100 m surface deformation on W. W and W-10wt%Re showed similar deformations in both macro- and micro- scales. W-2wt%Ta showed severe erosions via bumping. Suppressed deformation was seen for the Al coated W targets. Alloying of Al-W was not observed.
a r t i c l e
i n f o
Article history: Received 3 October 2016 Received in revised form 27 March 2017 Accepted 28 March 2017 Available online xxx Keywords: Liquid tungsten Plasma facing materials Transient heat loads Nd:YAG laser heating
a b s t r a c t Molten layer characteristics were investigated for four W materials; pure W, W-10wt%Re, W-2wt%Ta and W with Al coating. Specimens were irradiated by a Nd:YAG laser (wavelength 1064 nm) simulating transient heat loads such as ELMs and disruption. The power density of the heat loads were varied in the 2.3–6.6 GW/m2 range. Surface profiles were observed with a laser microscope. The central part of irradiated W-2wt%Ta became dented deeply in a wider range with increasing the power density in comparison with pure W and W-10wt%Re. This could be attributed to the release of Ta droplets by bumping (rapid boiling), closely related to microstructure and/or impurity concentration determined by the materials production processes. We also observed surface morphology changes of pure W with and without Al coating after irradiation in order to investigate the effect of the protective coatings. Melting of W with Al coating seems to be suppressed by a protective effect of Al film when compared with pure W. © 2017 Published by Elsevier B.V.
1. Introduction Tungsten (W) is a primary candidate of plasma-facing materials for future fusion reactors and W is planned to be used for divertor armor of ITER [1]. Although W has high thermal conductivity [2], low tritium retention [3] and low sputtering yield [4], its poor mechanical properties are the weakest point of this material. In particular, transient heat loads caused by disruption and edge localized modes (ELMs) severely influence W materials [5]. In ITER, transient heat loads such as Type I ELM (0.2–3 MJ/m2 for 0.1–1 ms) could cause submillimeter melting on W surface [6]. High temperature gradients and high thermal stresses developed during these transients lead to material damages, such as formation of melt layers, material erosion/ejection and crack formation. These damages can
∗ Corresponding author. E-mail address: [email protected] (K. Ibano).
decrease the lifetime of plasma-facing components and increase the contamination of plasma in subsequent operations [7]. During the reactor operation, the W surface encounters continuous bombardments by fusion neutrons as well as energetic plasma loads. It is expected that the W surface will be covered by the first wall originated Beryllium (Be) [8]. It is also expected that the nuclear transformation leads Rhenium (Re) inclusion [9]. Plus, some studies indicate that W-alloy such as W-Tantalum (W-Ta) is suitable for the first wall because of its improved mechanical property [9]. Thus, the erosion characteristic by the transient heat loads should be studied for these W materials. In order to simulate the transient heat loads, plasma guns [3,5], quasi-steady-state plasma accelerator [10,11], electron beams [12], and pulse lasers [13] have been used to study the effect of ELMs and disruptions on PFC materials. Within these methods, the pulsed laser experiments are suitable for the heat flux test of various materials in a controlled manner for a wide heat flux range. Thus we irradiated various types of W specimens to investigate macroscopic
http://dx.doi.org/10.1016/j.fusengdes.2017.03.160 0920-3796/© 2017 Published by Elsevier B.V.
Please cite this article in press as: D. Inoue, et al., Molten layer characteristics of W materials and film coated W by pulsed laser irradiation, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.160
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and microscopic changes of each specimens. We also investigated whether there are suppressing effects of morphology changes due to thin film coatings on W, or there are enhancing effects of morphology changes due to a depression of the melting point via alloying [13].
2. Experimental Four types of tungsten specimens were investigated: Pure W (99.99% at. purity) with and without Al coating, W-10wt%Re, and W-2wt%Ta. The size of specimens was 10 × 10 × 1 mm3 , and all specimens were mechanically polished to a mirror quality. The thermal shock tests were performed by applying transient heat loads in a vacuum of ∼10−7 Torr by Nd:YAG laser (wavelength 1064 nm, irradiation diameter 0.6 mm). The laser pulse width can be varied from 0.25 ms to 5 ms, and the maximum peak power is 7 kW. The laser absorption rate of pure W was measured as 26%. That of W-10wt%Re and W-2%Ta were 36% and 30%. Following laser irradiation, surface profiles of the irradiated specimens were observed using a laser microscope (VK-9710, KEYENCE Co). Then, a focused ion beam (FIB, FB2200, HITACHI Co) was used to expose the cross section of central irradiated point. The areas of 50 m × 50 m located in the center of the samples were exposed to Ga ions (40 keV). Secondary Ion Microscope images of the cross section microstructure was obtained (FIB–SIM). The element distribution was also measured by the energy dispersive X-ray spectrometry (EDX) analysis of a FE-SEM (Ultra55, Zeiss Co) attachment. 1 m Al coatings was prepared by using a magnetron sputtering device. Al itself cannot be used as a protective coating in a fusion reactor because of its neutron reactivity. However, Al is often used as a proxy to Be because of their similar vapor pressure [14]. One concern for this proxy is the alloying related enhanced erosion known for Be coated W[13]. Although both materials are known to form alloys with W (Be22 W, Be12 W and Be2 W [15] for Be. Al12 W, Al5 W and Al4 W[16] for Al), Al coating cannot mimic this characteristic due to the melting point difference (Be 1560 K and Al 933 K). Thus Al-W alloying was studied in detail.
3. Results and discussions 3.1. Macroscopic deformation of laser irradiated spots The laser pulsed irradiation by using Nd:YAG laser were performed to Pure W, W-10wt%Re and W-2wt%Ta. The power density of the heat loads were varied in the 2.3-6.6 GW/m2 range and the pulse width was 0.5 ms. The heat flux is similar with expected heat flux of Type I ELM at ITER. Surface profiles of the irradiated specimens measured by the laser microscope are shown in Fig. 1. In these plots, the height of non-irradiated surface was used as the origin. We observed surface during the irradiation by a high speed camera and concluded that these surface modification were caused by unstable melt layer motion. [17] After irradiation in 2.3–3.7 GW/m2 to a pure W sample, the central part became dented in comparison with unirradiated surface. However, after irradiation in 5.2 GW/m2 to W, the central part became a convex shape. The cause of this protruded shape was not clear, and further microscopic analysis was taken and discussed in the next chapter. The surface profile of W-10wt%Re became a similar form of pure W. In contrast, the central part of irradiated W-2wt%Ta became dented deeply in wider range with increase in the power density. In addition, the surface profile of W-2%Ta is asymmetric unlike the ones of pure W and W-10wt%Re. Fig. 2 summarizes volume changes after irradiation. These volume changes
Fig. 1. The surface profiles of W specimens after laser irradiation with 0.5 ms pulse width for various power density: (a)pure W (b) W-10wt%Re (c) W-2wt%Ta. Legends in Fig. 1 show absorbed heat road to W specimens. Blue dashed lines indicate the laser irradiation spot. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: D. Inoue, et al., Molten layer characteristics of W materials and film coated W by pulsed laser irradiation, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.160
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Fig. 2. Volume change of W specimens after the Nd:YAG laser irradiation. Pure W and W-10wt%Re shows volume gains. In contrast, W-2wt%Ta shows volume losses. It should be emphasized that the increments of the y-axis of positive side and negative side are different.
were calculated from the measured surface profiles assuming the circular symmetry of these profiles. The volume of W-2wt%Ta greatly decreases after irradiation in comparison with that of pure W and W-10wt%Re. Their volume gains can be explained by a cavity formation or decreasing density. In the meantime, the volume reduction of W-2wt%Ta can be explained by a release of Ta droplets by bumping (rapid boiling). Bumping can be caused by inhomogeneous microstructure and/or impurity nucleation originated from the material production process. Smaller weight percent specimens should be tested in future. The laser pulsed irradiation by using Nd:YAG laser were also performed to pure W with and without the Al coating. The laser pulse width was 0.5 ms and the laser power was varied in the 3–7 kw range (1.5–3.0 J). The absorbed heat loads to the Al coating specimen could not be precisely determined because of its inconstant reflectivity. However, the total energy needed to evaporated Al was negligible (∼10 mJ) to the incident laser energy. Thus, the heat flux was determined based on the W surface reflectivity. The surface profiles of the irradiated specimens are shown in Fig. 3. The figure shows that morphology changes of W with Al coating after laser irradiation seem to be suppressed compared with the pure W without Al. These observations indicated that the alloying enhanced erosion did not happen for the Al/W samples in the macroscopic scale. 3.2. Macroscopic deformation of laser irradiated spots Cavities formation were suspected in the protruded part (Fig. 1(a), (b)), because the volume gains were observed for pureW and W-10wt%Re specimens as shown in Fig. 2. Thus, the cross section of central irradiated point was exposed by using the FIB. However, cavities were not observed at least for the 50 m depth. The volume gains are <5% of melted volume. Thus, up to 5% of the density decrease of re-solidified regions explain the gains. Obtained SIM images were summarized in Fig. 4. Central part of pure W with and without Al coating irradiated with 5.2 GW/m2 , W-10wt%Re irradiated with 6.6 GW/m2 , W-2wt%Ta irradiated with 4.0 GW/m2 were shown here. Within these cross sectional SIM images, only W-2wt%Ta did not show significant changes of its grain size. This observation also suggests that the melting layer
Fig. 3. The surface profiles of pure W with and without Al after laser irradiation with 0.5 ms pulse width for various laser power: (a)pure W without Al (b)pure W with Al. Legends in Fig. 3 show laser power irradiated to pure W with and without Al.
of W-2wt%Ta was quickly scattered by bumping. In contrast, other specimens show large grain sizes after laser irradiation. Recrystallization or re-solidification caused these changes of grain sizes. In addition to these observations, EDX analysis was performed to investigate whether W and Al were alloyed after laser irradiation (Fig. 5). EDX analysis of central part of melting spot confirmed that no Al or Al-alloy remained at the part. Previous study of W with Be coating showed that the Be-W alloying occurred at the central part of melting spot [13]. W with Be coating have shown a larger damage than non-coating W because of the formation of Be-W alloy, which has a lower damage threshold compared to W. In contrast, in the case of Al in this study, the macroscopic damage was suppressed. These observations indicate that Al evaporates or melted Al scattered quickly and Al-W alloy was not be formed in this experiment. As a result, the Al coating effectively suppress the morphology changes of underlying W. 4. Conclusion In this study, four W materials; pure W with and without Al coating, W-10wt%Re, W-2wt%Ta were irradiated by Nd:YAG laser simulating transient heat loads such as ELMs and disruption. Sur-
Please cite this article in press as: D. Inoue, et al., Molten layer characteristics of W materials and film coated W by pulsed laser irradiation, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.160
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Fig. 4. SIM cross section images of laser irradiated and not irradiated region. Pure W with 5.2 GW/m2 , Al coated W with 5.2 GW/m2 , W-10wt%Re with 6.6 GW/m2 and W-2wt%Ta with 4.0 GW/m2 were compared. The maximum energy densities within the spots of symmetric erosion profile were chosen.
Fig. 5. EDX mapping analysis for the pure W specimen with the Al coating after the laser irradiation of 3.7 GW/m2 .
face profiles were measured by a laser microscope. Similar forms were observed for the pure W and W-10wt%Re after irradiation. On the other hand, the central part of irradiated W-2wt%Ta became dented deeply in a wider range for higher power density. Moreover, the crystal grain size of W-2wt%Ta hardly changed after irradiation. These observations suggest that the melting layer of W-2wt%Ta was quickly scattered via the droplet ejection by bumping. When comparing the surface profiles of pure W with and without Al, morphology changes of W with Al coating after laser irradiation seem to be suppressed. In addition, the EDX element analysis did not find any Al-W alloys. Thus, Al-W alloying induced erosion enhancement was not found in this study. Acknowledgements This work is partly supported by JSPS KAKENHI Grant Number 25249132 and by ZE Research Program, IAEZE28B-30 and IAEZE28A-19.
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