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Micro ion beam analysis for the erosion of beryllium marker tiles in a tokamak limiter
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Micro ion beam analysis for the erosion of beryllium marker tiles in a tokamak limiter ⁎
Y. Zhoua, , H. Bergsåkera, I. Bykovb, P. Peterssona, V. Panetac, G. Possnertc, JET contributorsd a
Department for Fusion Plasma Physics, School of Electrical Engineering, Royal Institute of Technology, S-10405 Stockholm, Sweden Centre for Energy Research, University of California (San Diego), La Jolla, CA 92093, USA c Department of Physics and Astronomy, Uppsala University, Box 516, SE-75120 Uppsala, Sweden d See the Appendix of F. Romanelli et al., Proceedings of the 25th IAEA Fusion Energy Conference 2014, Saint Petersburg, Russia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Microbeam Limiter Beryllium marker tile Joint European Torus (JET) Plasma-facing components (PFC)
Beryllium limiter marker tiles were exposed to plasma in the Joint European Torus to diagnose the erosion of main chamber wall materials. A limiter marker tile consists of a beryllium coating layer (7–9 μm) on the top of bulk beryllium, with a nickel interlayer (2–3 μm) between them. The thickness variation of the beryllium coating layer, after exposure to plasma, could indicate the erosion measured by ion beam analysis with backscattering spectrometry. However, interpretations from broad beam backscattering spectra were limited by the non-uniform surface structures. Therefore, micro-ion beam analysis (μ-IBA) with 3 MeV proton beam for Elastic backscattering spectrometry (EBS) and PIXE was used to scan samples. The spot size was in the range of 3–10 μm. Scanned areas were analysed with scanning electron microscopy (SEM) as well. Combining results from μ-IBA and SEM, we obtained local spectra from carefully chosen areas on which the surface structures were relatively uniform. Local spectra suggested that the scanned area (≈600 μm × 1200 μm) contained regions with serious erosion with only 2–3 μm coating beryllium left, regions with intact marker tile, and droplets with 90% beryllium. The nonuniform erosion, droplets mainly formed by beryllium, and the possible mixture of beryllium and nickel were the major reasons that confused interpretation from broad beam EBS.
1. Introduction Thermonuclear fusion power is a potentially clean and sustainable energy source for human society in the future. Presently, the most advanced concept for a fusion reactor is the tokamak, a torus shaped device that contains hot plasma, formed by hydrogen isotopes and confined by a strong magnetic field in a vacuum vessel [1]. Even though confined by a strong magnetic field, a part of energetic particles is still able to escape from the plasma edge and then interacts with the surrounding plasma facing components (PFC) in the vessel. These plasma-material interactions include erosion of PFC and re-deposition of eroded particles. The erosion, generated by neutral particles and ions, will limit the lifetime of PFC. Deposition may change the properties of materials and bring safety and economic issues [2]. In order to reduce the effect from those interactions, tokamaks are designed to direct most of the plasma-surface interactions (PSI) towards specifically designed structures, divertors and/or limiters which stay away from the core plasma. Extensive studies were carried out to survey the mechanisms and patterns of the erosion and deposition. Especially, the ITER-like-wall
⁎
project was set up in the Joint European Torus (JET) with deuterium plasma [3] for testing erosion and deposition on candidate materials for ITER. Complicated net erosion and deposition distribution patterns have been found from previous experimental results in the JET tokamak [4,5]. The limiter tiles suffered from strong erosion after exposure to plasma. In order to measure the amount of eroded Be, several Be limiter marker tiles have been developed as a diagnostic [6]. A Be limiter marker tile consists of a Be coating layer on the top of a bulk Be, with a nickel interlayer between them. The manufacture method was thermionic vacuum arc deposition which could produce high-density layers [7]. The thickness variation of the Be coating layer was intended to be measured by Elastic backscattering spectrometry (EBS) so that the amount of erosion Be should be determined. However, during the exposure to plasma, the melting, arching, erosion and deposition roughen the limiter tile surface, especially at the ends of tiles [8]. Non-uniform structures at rough surfaces make the interpretation from broad beam EBS ambiguous due to the ambiguity between the surface roughness effects and the concentration variation [9].
Corresponding author. E-mail address: [email protected] (Y. Zhou).
https://doi.org/10.1016/j.nimb.2018.08.028 Received 30 October 2017; Received in revised form 14 July 2018; Accepted 21 August 2018 0168-583X/ © 2018 Published by Elsevier B.V.
Please cite this article as: Zhou, Y., Nuclear Inst, and Methods in Physics Research B (2018), https://doi.org/10.1016/j.nimb.2018.08.028
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
Y. Zhou et al.
15° from the sample surface normal with a solid angle around 0.5 sr. The characteristic X-rays from metal elements were detected by a Si(Li) detector which was installed at 45° from the normal and subtended a solid angle about 0.03 sr. The total 256 × 256 steps were used as the position indexes for events from detectors. When detectors were trigged by a particle or an X-ray signal, the position indexes were stored at the same time. For each event in the data file, the information about detector, energy and position were recorded. Through MATLAB, we could gather the EBS and PIXE spectra from the whole scanned region by specifying the detector type with the full spectrum energy range. Moreover, using the position indexes, one can choose interesting regions within the scanned area and gather local spectra. Then by counting the number of events from each position with determined detector and energy range, a map of event numbers versus position can be drawn as well. This map was used to show the distribution of events over the sample surface. EBS spectra were simulated by the SIMNRA 6.06 [13] with cross section of the 9Be(p,p)9Be and 9Be(p,d)8Be reactions from [14]. The current (≈2.48 × 1010 particles·sr) used in fitting was calculated from the PIXE and EBS spectrum of the attached copper grid. For the copper grid the scattering cross section can be taken as Rutherford and this method is more convenient than current integration and geometric measurement of the detector solid angle and is sufficiently accurate (estimated < 5% error). Where required, the areal density for both Be and nickel had been converted into the thickness of elemental material with the mass density from SIMNRA 6.06 with ρBe = 1.803 g cm−3 and ρNi = 8.897 g cm−3 respectively. SEM images were collected by a FEI Quanta 3D focused ion beam device at KTH, Sweden. Those SEM images were overlaid with the distribution map for comparing the sample topography with results from EBS spectra. Using the distribution maps as reference, several small regions were chosen and spectra from them were extracted for fitting.
In this report, we analysed samples from one of the Be marker tiles by EBS and particle-induced X-ray emission (PIXE) with proton micro beam, and scanning electron microscopy (SEM). By comparing micro EBS and SEM, relatively uniform areas have been selected. The spectra from those areas (local spectra) could offer the erosion information with less effect from the sample topography.
2. Experiments Samples were taken from limiter tiles in JET with ITER-like wall (JET-ILW). Samples contain a sandwich structure with a Ni (2–3 μm) interlayer between a bulk Be substrate and a coating Be layer (7–9 μm), which had been produced by the thermionic vacuum arc deposition method. Limiter tiles were exposed to the plasma during the first JETILW campaign with overall 19 h plasma duration [10]. The typical plasma density in JET was about 1019 m−3 and the deuterium impact energy varied in the range of 35–200 eV under the local plasma condition near limiter tiles[11]. Following exposure to plasma, the whole marker tile was cut into small cubes of 12 mm × 12 mm × 12 mm for post mortem analysis. Two of them, numbered as 74 and 170, from the right-hand-wing segment of the same tile were investigated by micro proton beam and imaged by SEM. As shown in Fig. 1, sample 74 had been located at the middle part of this segment and sample 170 came from the upper edge. The micro EBS and PIXE experiments were carried out at the Tandem Laboratory in Uppsala University, Sweden. A 3 MeV proton beam was produced in a 5 MV Tandem accelerator with beam spot size in the range of 3–10 μm. By magnetic steering coils the proton beam can scan over sample surface. The scanning was done step by step in both horizontal and vertical directions with 256 steps in each direction [12]. In this report, the total scan area, identified by a 100 mesh copper grid, was about 600 μm × 1200 μm on one sample and about 600 μm × 600 μm on the other. Two detectors were mounted in the vacuum chamber (10−6 mbar). One annular detector, measured the backscattering protons and nuclear reaction products, e.g. deuterium from 9Be(p,d0)8Be. It is mounted at
3. Results Fig. 2 shows the EBS spectrum of sample 74 from the whole scanned region. The scanned region is indicated by the black dashed line in Fig. 3 together with the SEM image for sample surface. The fitting result in Fig. 2 came from an ideal model which consisted of a 9 μm coating Be layer on the top of bulk Be with a 3 μm nickel interlayer between them. Compared to the fitting result, two ambiguous regions in the
Fig. 1. Sketch of the cross section of the vessel chamber in JET and one maker limiter tile, which consisted of several segments. After exposure to plasma, the whole tile was cut into small cubes of 12 mm × 12 mm × 12 mm. Sample 74, which was located at the middle of the right-hand-wing segment and sample 170 at the upper edge were chosen for micro ion beam analysis.
Fig. 2. EBS spectrum from the whole scanned region on sample 74. Fitting data from model with Be-Ni-Be (9 μm-3 μm-bulk) was used to compare with experimental results for identifying the ambiguous part. The inset shows the details of the spectrum from the Ni peak with two extra plains and slope at high energy side. 2
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experimental spectrum were found. A continuous Be spectrum appears instead of separate peaks. The nickel spectrum becomes broad with two extra plains and a smooth slope at the high energy side, marked by C, B, and A respectively. Therefore the Ni peak can be divided into three energy sections. By counting the numbers of backscattering particles in each energy section and their position indexes, distribution maps of backscattering particles over sample surface were obtained, shown in Fig. 4a–c. The energy section A corresponds to backscattering protons from nickel near the sample surface. The distribution map in Fig. 4a suggests that the near surface nickel appears on two damaged regions, appearing in light blue colour (except for the copper grid). The EBS spectrum from the damaged region with a small area (≈40 μm × 40 μm, named as 741 and marked by red dashed line), is shown in Fig. 4d. Fig. 5a and b shows the nickel and beryllium depth profile from 74-1. The Be concentration varied in the range of ≈0.9–0.7 when depth ≤2 × 1019 atoms/cm2 and the Ni concentration in the same depth was smaller than 0.3. This suggests that in the near surface region a thin beryllium layer (1.6 μm, Table 2) stays in front of the Ni interlayer (1.99 μm, listed in Table1). For energy section C, the distribution map in Fig. 4c shows that the corresponding area locates at the bridge-like region extending from the upper left corner to the lower right. Unlike the damaged zone, this area has a smooth and thick appearance. The depth profile from the local region (marked by red dashed line and named as 74-4) shows a pure beryllium layer (concentration ≈1) appears on top of the Ni interlayer
Fig. 3. SEM image for sample 74. The black dashed line indicates the scanned region in the micro ion beam measurement. Coloured dashed regions mark the locations for local EBS spectra. The copper grid stays on the left hand side. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. A SEM image was overlaid with distribution maps of backscattering protons from three energy sections A, B, and C. Colour codes indicate the proton yields at each pixel from the specified energy regions. (a) Shows the distribution map of proton from section A, which suggests that Ni near surface appears on the damaged region appearing in light blue. (b) Shows the map of intermediate section B, which corresponds to the transition area between damaged region and intact region. (c) Shows the smooth and thick layer indicating an intact marker tile structure. (d) Shows the EBS spectra and simulation result from 74-1, 74-3, and 74-4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 3
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Fig. 6. SEM image for sample 170. The black dashed line shows the microbeam scanned area and red areas mark the position of local EBS spectra. The copper grid attaches on the left hand side. 170-4 indicates the smooth region while 170-2 exemplifies the damaged region. 170-1 and 170-3 are droplets consist of 90% Be. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
as shown in Fig. 5b. It suggests that in this smooth and thick region the marker structure remained intact, with a deposition layer above it. The distribution map from the intermediate section B (Fig. 4b) shows that the marker tile in this region appears like the transition from the erosion zone to the intact region. The Be thickness calculated from the 74-3 spectrum in Table 2 also indicates that more Be coating layer remains than 74-1. It means that this region suffered from less erosion than the damaged region. For sample 170, similar smooth and damaged zones on the surface were found as shown in Fig. 6, marked by 170-4 and 170-2 respectively with the depth profiles of Be and Ni in Fig. 7a. Additionally, the surface topography includes droplets of around 100 μm, shown in Fig. 6 and marked by 170-1 and 170-3. Different from the marker tile, the Ni concentration in droplets was lower than 10% as represented in Fig. 7b. From the fitting results, those droplets were mainly composed of Be with the concentration higher than 90%. The depth profiles from small regions at sample 74 and 170 showed a common phenomenon: in most of the sample areas, the pure Ni interlayers disappeared and were replaced with the mixture of Be and Ni. This disappearance is followed by the connection between the coating Be layer and the bulk Be.
Fig. 5. Depth profiles of Be and Ni from SIMNRA for sample 74. A pure Ni interlayer only appears in area 74-1 with Be coating layer in front of it. On 74-3 and 74-4, the pure Ni interlayers disappear and are replaced with the mixture of Be and Ni.
Table 1 Thickness of the Ni interlayer from simulation of sample 74. Uncertainty comes from the stopping power and statistics. The areal density for both Be and nickel had been converted into the thickness of elemental material with the mass density from SIMNRA 6.06 with ρBe = 1.803 g cm−3 and ρNi = 8.897 g cm−3 respectively. Region
Ni 1015 atom/cm2
Thickness ± 5%
74-1 74-3 74-4
18,198 21,423 22,820
1.99 μm 2.34 μm 2.50 μm
4. Discussion We measured the EBS spectra and calculated the thickness of the Ni interlayer and the coating beryllium. The spectrum from the whole scanned area cannot be used to calculate the Ni and Be amount since the spectrum contains several ambiguous parts. Those ambiguous parts, the broadened Ni peak, the continuous beryllium spectrum, and the smooth Ni slope, could be interpreted by the local spectra data. The first two ambiguous parts in the EBS experimental spectrum resulted from the nonuniform erosion as shown in Fig. 4. The intact marker tile zone, the serious erosion zone and the transition zone correspond to spectra with variated shapes. The combination of them finally can create the broad and continuous spectrum. Besides, droplets shown in Fig. 6 confused the EBS result as well because droplets were formed by almost pure beryllium. The regions covered by droplets could be interpreted incorrectly by EBS spectrum as the bulk beryllium with the complete erosion of coating layer and interlayer. Two potential explanations could be provided for the smooth Ni slope. Firstly, the front side of Ni interlayer mixes with the rear side of the beryllium coating layer as shown in the depth profile from the simulation. This mixture could dilute Ni and beryllium layer with each other which lead to the concentration of Ni at the front surface
Table 2 Thickness of left coating Be layer from simulation. Except region 74-1, the thickness was calculated by Be areal density before the layer with maximum Ni concentration. Uncertainty mainly comes from the cross section data. Region
Coating Be 1015 atoms/cm2
Thickness ± 10%
74-1 74-2 74-3 74-4 170-2 170-4
19,275 28,332 55,715 124,759 29,650 70,390
1.60 μm 2.35 μm 4.62 μm 10.35 μm 2.46 μm 5.84 μm
4
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distribution maps in Fig. 4 and SEM images in Fig. 2. The erosion data on right-hand wing of the same limiter tile in Ref. [15] had an error bar in the range of ± 100 μm and our thickness data on 74 helps to reduce this error. However the extrapolation on sample 170 is not straightforward since the topography is more complicated. Sampling more regions for local spectra will be necessary and helpful. The method described above could be used in other marker limiter tiles as well. But the selection of energy section in EBS spectrum for mapping should be careful, especially when dealing with the slope part. As discussed earlier, the slope in the marker limiter tile EBS spectrum may come from the concentration variation of target element. An overlarge energy section will make the extrapolation of thickness from small area to whole measure region less accurate. 5. Conclusions Samples from Be limiter marker tiles were investigated by micro ion beam and SEM. The distribution maps of backscattered protons from specified energy sections in EBS spectrum were compared with the sample topography. Local EBS spectra from small areas were simulated and information was obtained about the tile components. Results from the simulations suggest that serious erosion areas appeared at sample 74 with only 2–3 μm coating Be left, adjacent to an intact region. The nonuniform erosion, droplets mainly formed by Be, and the possible mixture of Be and Ni are the major reasons complicating the interpretation of EBS spectra from the whole scanned area. Acknowledgements This work was supported by EURATOM and carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References Fig. 7. Depth profiles of Ni and Be of 170-1, 170-2, 170-3, and 170-4 from simulation with SIMNRA. Depth profiles from 170-2 to 170-4 show that the mixture of Ni interlayer with Be coating layer appears on 170 sample as well. Unlike marker tiles, Ni concentrations in droplets are less than 10%.
[1] H.P. Furth, Tokamak research, Nucl. Fusion 15 (1975) 487–534. [2] J. Roth, et al., Recent analysis of key plasma wall interactions issues for ITER, J. Nucl. Mater. 390 (2009) 1–9. [3] G.F. Matthews, et al., Overview of the ITER-like wall project, Phys. Scr. T 128 (2007) 137–143. [4] M. Mayer, et al., Erosion at the inner wall of JET during the discharge campaigns 2001–2009, J. Nucl. Mater. 438 (2013) S780–S783. [5] S. Krat, et al., Erosion and deposition on JET divertor and limiter tiles during the experimental campaigns 2005–2009, J. Nucl. Mater. 438 (2013) S742–S745. [6] M.J. Rubel, et al., Beryllium plasma-facing components for the ITER-Like Wall Project at JET, J. Phys. Conf. Ser. 100 (2008). [7] C.P. Lungu, et al., Beryllium coatings on metals for marker tiles at JET: development of process and characterization of layers, Phys. Scr. T 128 (2007) 157–161. [8] A. Widdowson, et al., Material migration patterns and overview of first surface analysis of the JET ITER-like wall, Phys. Scr. 2014 (2014) 014010. [9] M. Mayer, et al., Computer simulation of ion beam analysis: possibilities and limitations, Nucl. Instr. Meth. B 269 (2011) 3006–3013. [10] A. Widdowson, et al., Overview of the JET ITER-like wall divertor, Nucl. Mater. Energy 12 (2017) 499–505. [11] S. Brezinsek, J.-E. Contributors, Plasma-surface interaction in the Be/W environment: conclusions drawn from the JET-ILW for ITER, J. Nucl. Mater. 463 (2015) 11–21. [12] I. Bykov, et al., Combined ion micro probe and SEM analysis of strongly non uniform deposits in fusion devices, Nucl. Instr. Meth. B 342 (2015) 19–28. [13] M. Mayer, SIMNRA, a simulation program for the analysis of NRA, RBS and ERDA, AIP Conference Proceedings, AIP, 1999, pp. 541–544. [14] S. Krat, et al., The 9 Be (p, p 0) 9 Be, 9 Be (p, d 0) 8 Be, and 9 Be (p, α 0) 6 Li crosssections for analytical purposes, Nucl. Instr. Meth. B 358 (2015) 72–81. [15] K. Heinola, et al., Tile profiling analysis of samples from the JET ITER-like wall and carbon wall, Phys. Scr. 2014 (2014) 014013.
increasing gradually, from zero to maximum. This is reflected by the EBS with a smooth slope at the high energy side instead of a sharp edge. The dilution also contributes to the slope of the beryllium peak just behind the surface beryllium in Fig. 2 by generating a decreasing beryllium concentration at the rear side of coating layer. Another explanation would be the significant thickness variation of the surface as discussed in Ref. [9]. In principle, both the concentration variation and thickness variation could offer a good fit to the experimental EBS spectrum and it is not easy to distinguish them. But the local spectrum in this report, especially 74-4, comes from a relatively uniform region. And the simulation from 74-4 suggests the mixture of the beryllium coating and the Ni interlayer. Therefore we may have more confidence in the first explanation. Despite all this, the amount of Ni and beryllium from EBS spectra still can be trusted. The amount of coating Be left on small areas was calculated from the local EBS spectra and results are summarised in Table 2. The extrapolation of thickness data from small region to the whole scanned area could be done in sample 74. Because on 74, the erosion zone, the intact zone, and the transition zone can be separated clearly by
5
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Micro ion beam analysis for the erosion of beryllium marker tiles in a tokamak limiter ⁎
Y. Zhoua, , H. Bergsåkera, I. Bykovb, P. Peterssona, V. Panetac, G. Possnertc, JET contributorsd a
Department for Fusion Plasma Physics, School of Electrical Engineering, Royal Institute of Technology, S-10405 Stockholm, Sweden Centre for Energy Research, University of California (San Diego), La Jolla, CA 92093, USA c Department of Physics and Astronomy, Uppsala University, Box 516, SE-75120 Uppsala, Sweden d See the Appendix of F. Romanelli et al., Proceedings of the 25th IAEA Fusion Energy Conference 2014, Saint Petersburg, Russia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Microbeam Limiter Beryllium marker tile Joint European Torus (JET) Plasma-facing components (PFC)
Beryllium limiter marker tiles were exposed to plasma in the Joint European Torus to diagnose the erosion of main chamber wall materials. A limiter marker tile consists of a beryllium coating layer (7–9 μm) on the top of bulk beryllium, with a nickel interlayer (2–3 μm) between them. The thickness variation of the beryllium coating layer, after exposure to plasma, could indicate the erosion measured by ion beam analysis with backscattering spectrometry. However, interpretations from broad beam backscattering spectra were limited by the non-uniform surface structures. Therefore, micro-ion beam analysis (μ-IBA) with 3 MeV proton beam for Elastic backscattering spectrometry (EBS) and PIXE was used to scan samples. The spot size was in the range of 3–10 μm. Scanned areas were analysed with scanning electron microscopy (SEM) as well. Combining results from μ-IBA and SEM, we obtained local spectra from carefully chosen areas on which the surface structures were relatively uniform. Local spectra suggested that the scanned area (≈600 μm × 1200 μm) contained regions with serious erosion with only 2–3 μm coating beryllium left, regions with intact marker tile, and droplets with 90% beryllium. The nonuniform erosion, droplets mainly formed by beryllium, and the possible mixture of beryllium and nickel were the major reasons that confused interpretation from broad beam EBS.
1. Introduction Thermonuclear fusion power is a potentially clean and sustainable energy source for human society in the future. Presently, the most advanced concept for a fusion reactor is the tokamak, a torus shaped device that contains hot plasma, formed by hydrogen isotopes and confined by a strong magnetic field in a vacuum vessel [1]. Even though confined by a strong magnetic field, a part of energetic particles is still able to escape from the plasma edge and then interacts with the surrounding plasma facing components (PFC) in the vessel. These plasma-material interactions include erosion of PFC and re-deposition of eroded particles. The erosion, generated by neutral particles and ions, will limit the lifetime of PFC. Deposition may change the properties of materials and bring safety and economic issues [2]. In order to reduce the effect from those interactions, tokamaks are designed to direct most of the plasma-surface interactions (PSI) towards specifically designed structures, divertors and/or limiters which stay away from the core plasma. Extensive studies were carried out to survey the mechanisms and patterns of the erosion and deposition. Especially, the ITER-like-wall
⁎
project was set up in the Joint European Torus (JET) with deuterium plasma [3] for testing erosion and deposition on candidate materials for ITER. Complicated net erosion and deposition distribution patterns have been found from previous experimental results in the JET tokamak [4,5]. The limiter tiles suffered from strong erosion after exposure to plasma. In order to measure the amount of eroded Be, several Be limiter marker tiles have been developed as a diagnostic [6]. A Be limiter marker tile consists of a Be coating layer on the top of a bulk Be, with a nickel interlayer between them. The manufacture method was thermionic vacuum arc deposition which could produce high-density layers [7]. The thickness variation of the Be coating layer was intended to be measured by Elastic backscattering spectrometry (EBS) so that the amount of erosion Be should be determined. However, during the exposure to plasma, the melting, arching, erosion and deposition roughen the limiter tile surface, especially at the ends of tiles [8]. Non-uniform structures at rough surfaces make the interpretation from broad beam EBS ambiguous due to the ambiguity between the surface roughness effects and the concentration variation [9].
Corresponding author. E-mail address: [email protected] (Y. Zhou).
https://doi.org/10.1016/j.nimb.2018.08.028 Received 30 October 2017; Received in revised form 14 July 2018; Accepted 21 August 2018 0168-583X/ © 2018 Published by Elsevier B.V.
Please cite this article as: Zhou, Y., Nuclear Inst, and Methods in Physics Research B (2018), https://doi.org/10.1016/j.nimb.2018.08.028
Nuclear Inst, and Methods in Physics Research B xxx (xxxx) xxx–xxx
Y. Zhou et al.
15° from the sample surface normal with a solid angle around 0.5 sr. The characteristic X-rays from metal elements were detected by a Si(Li) detector which was installed at 45° from the normal and subtended a solid angle about 0.03 sr. The total 256 × 256 steps were used as the position indexes for events from detectors. When detectors were trigged by a particle or an X-ray signal, the position indexes were stored at the same time. For each event in the data file, the information about detector, energy and position were recorded. Through MATLAB, we could gather the EBS and PIXE spectra from the whole scanned region by specifying the detector type with the full spectrum energy range. Moreover, using the position indexes, one can choose interesting regions within the scanned area and gather local spectra. Then by counting the number of events from each position with determined detector and energy range, a map of event numbers versus position can be drawn as well. This map was used to show the distribution of events over the sample surface. EBS spectra were simulated by the SIMNRA 6.06 [13] with cross section of the 9Be(p,p)9Be and 9Be(p,d)8Be reactions from [14]. The current (≈2.48 × 1010 particles·sr) used in fitting was calculated from the PIXE and EBS spectrum of the attached copper grid. For the copper grid the scattering cross section can be taken as Rutherford and this method is more convenient than current integration and geometric measurement of the detector solid angle and is sufficiently accurate (estimated < 5% error). Where required, the areal density for both Be and nickel had been converted into the thickness of elemental material with the mass density from SIMNRA 6.06 with ρBe = 1.803 g cm−3 and ρNi = 8.897 g cm−3 respectively. SEM images were collected by a FEI Quanta 3D focused ion beam device at KTH, Sweden. Those SEM images were overlaid with the distribution map for comparing the sample topography with results from EBS spectra. Using the distribution maps as reference, several small regions were chosen and spectra from them were extracted for fitting.
In this report, we analysed samples from one of the Be marker tiles by EBS and particle-induced X-ray emission (PIXE) with proton micro beam, and scanning electron microscopy (SEM). By comparing micro EBS and SEM, relatively uniform areas have been selected. The spectra from those areas (local spectra) could offer the erosion information with less effect from the sample topography.
2. Experiments Samples were taken from limiter tiles in JET with ITER-like wall (JET-ILW). Samples contain a sandwich structure with a Ni (2–3 μm) interlayer between a bulk Be substrate and a coating Be layer (7–9 μm), which had been produced by the thermionic vacuum arc deposition method. Limiter tiles were exposed to the plasma during the first JETILW campaign with overall 19 h plasma duration [10]. The typical plasma density in JET was about 1019 m−3 and the deuterium impact energy varied in the range of 35–200 eV under the local plasma condition near limiter tiles[11]. Following exposure to plasma, the whole marker tile was cut into small cubes of 12 mm × 12 mm × 12 mm for post mortem analysis. Two of them, numbered as 74 and 170, from the right-hand-wing segment of the same tile were investigated by micro proton beam and imaged by SEM. As shown in Fig. 1, sample 74 had been located at the middle part of this segment and sample 170 came from the upper edge. The micro EBS and PIXE experiments were carried out at the Tandem Laboratory in Uppsala University, Sweden. A 3 MeV proton beam was produced in a 5 MV Tandem accelerator with beam spot size in the range of 3–10 μm. By magnetic steering coils the proton beam can scan over sample surface. The scanning was done step by step in both horizontal and vertical directions with 256 steps in each direction [12]. In this report, the total scan area, identified by a 100 mesh copper grid, was about 600 μm × 1200 μm on one sample and about 600 μm × 600 μm on the other. Two detectors were mounted in the vacuum chamber (10−6 mbar). One annular detector, measured the backscattering protons and nuclear reaction products, e.g. deuterium from 9Be(p,d0)8Be. It is mounted at
3. Results Fig. 2 shows the EBS spectrum of sample 74 from the whole scanned region. The scanned region is indicated by the black dashed line in Fig. 3 together with the SEM image for sample surface. The fitting result in Fig. 2 came from an ideal model which consisted of a 9 μm coating Be layer on the top of bulk Be with a 3 μm nickel interlayer between them. Compared to the fitting result, two ambiguous regions in the
Fig. 1. Sketch of the cross section of the vessel chamber in JET and one maker limiter tile, which consisted of several segments. After exposure to plasma, the whole tile was cut into small cubes of 12 mm × 12 mm × 12 mm. Sample 74, which was located at the middle of the right-hand-wing segment and sample 170 at the upper edge were chosen for micro ion beam analysis.
Fig. 2. EBS spectrum from the whole scanned region on sample 74. Fitting data from model with Be-Ni-Be (9 μm-3 μm-bulk) was used to compare with experimental results for identifying the ambiguous part. The inset shows the details of the spectrum from the Ni peak with two extra plains and slope at high energy side. 2
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experimental spectrum were found. A continuous Be spectrum appears instead of separate peaks. The nickel spectrum becomes broad with two extra plains and a smooth slope at the high energy side, marked by C, B, and A respectively. Therefore the Ni peak can be divided into three energy sections. By counting the numbers of backscattering particles in each energy section and their position indexes, distribution maps of backscattering particles over sample surface were obtained, shown in Fig. 4a–c. The energy section A corresponds to backscattering protons from nickel near the sample surface. The distribution map in Fig. 4a suggests that the near surface nickel appears on two damaged regions, appearing in light blue colour (except for the copper grid). The EBS spectrum from the damaged region with a small area (≈40 μm × 40 μm, named as 741 and marked by red dashed line), is shown in Fig. 4d. Fig. 5a and b shows the nickel and beryllium depth profile from 74-1. The Be concentration varied in the range of ≈0.9–0.7 when depth ≤2 × 1019 atoms/cm2 and the Ni concentration in the same depth was smaller than 0.3. This suggests that in the near surface region a thin beryllium layer (1.6 μm, Table 2) stays in front of the Ni interlayer (1.99 μm, listed in Table1). For energy section C, the distribution map in Fig. 4c shows that the corresponding area locates at the bridge-like region extending from the upper left corner to the lower right. Unlike the damaged zone, this area has a smooth and thick appearance. The depth profile from the local region (marked by red dashed line and named as 74-4) shows a pure beryllium layer (concentration ≈1) appears on top of the Ni interlayer
Fig. 3. SEM image for sample 74. The black dashed line indicates the scanned region in the micro ion beam measurement. Coloured dashed regions mark the locations for local EBS spectra. The copper grid stays on the left hand side. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. A SEM image was overlaid with distribution maps of backscattering protons from three energy sections A, B, and C. Colour codes indicate the proton yields at each pixel from the specified energy regions. (a) Shows the distribution map of proton from section A, which suggests that Ni near surface appears on the damaged region appearing in light blue. (b) Shows the map of intermediate section B, which corresponds to the transition area between damaged region and intact region. (c) Shows the smooth and thick layer indicating an intact marker tile structure. (d) Shows the EBS spectra and simulation result from 74-1, 74-3, and 74-4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 3
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Fig. 6. SEM image for sample 170. The black dashed line shows the microbeam scanned area and red areas mark the position of local EBS spectra. The copper grid attaches on the left hand side. 170-4 indicates the smooth region while 170-2 exemplifies the damaged region. 170-1 and 170-3 are droplets consist of 90% Be. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
as shown in Fig. 5b. It suggests that in this smooth and thick region the marker structure remained intact, with a deposition layer above it. The distribution map from the intermediate section B (Fig. 4b) shows that the marker tile in this region appears like the transition from the erosion zone to the intact region. The Be thickness calculated from the 74-3 spectrum in Table 2 also indicates that more Be coating layer remains than 74-1. It means that this region suffered from less erosion than the damaged region. For sample 170, similar smooth and damaged zones on the surface were found as shown in Fig. 6, marked by 170-4 and 170-2 respectively with the depth profiles of Be and Ni in Fig. 7a. Additionally, the surface topography includes droplets of around 100 μm, shown in Fig. 6 and marked by 170-1 and 170-3. Different from the marker tile, the Ni concentration in droplets was lower than 10% as represented in Fig. 7b. From the fitting results, those droplets were mainly composed of Be with the concentration higher than 90%. The depth profiles from small regions at sample 74 and 170 showed a common phenomenon: in most of the sample areas, the pure Ni interlayers disappeared and were replaced with the mixture of Be and Ni. This disappearance is followed by the connection between the coating Be layer and the bulk Be.
Fig. 5. Depth profiles of Be and Ni from SIMNRA for sample 74. A pure Ni interlayer only appears in area 74-1 with Be coating layer in front of it. On 74-3 and 74-4, the pure Ni interlayers disappear and are replaced with the mixture of Be and Ni.
Table 1 Thickness of the Ni interlayer from simulation of sample 74. Uncertainty comes from the stopping power and statistics. The areal density for both Be and nickel had been converted into the thickness of elemental material with the mass density from SIMNRA 6.06 with ρBe = 1.803 g cm−3 and ρNi = 8.897 g cm−3 respectively. Region
Ni 1015 atom/cm2
Thickness ± 5%
74-1 74-3 74-4
18,198 21,423 22,820
1.99 μm 2.34 μm 2.50 μm
4. Discussion We measured the EBS spectra and calculated the thickness of the Ni interlayer and the coating beryllium. The spectrum from the whole scanned area cannot be used to calculate the Ni and Be amount since the spectrum contains several ambiguous parts. Those ambiguous parts, the broadened Ni peak, the continuous beryllium spectrum, and the smooth Ni slope, could be interpreted by the local spectra data. The first two ambiguous parts in the EBS experimental spectrum resulted from the nonuniform erosion as shown in Fig. 4. The intact marker tile zone, the serious erosion zone and the transition zone correspond to spectra with variated shapes. The combination of them finally can create the broad and continuous spectrum. Besides, droplets shown in Fig. 6 confused the EBS result as well because droplets were formed by almost pure beryllium. The regions covered by droplets could be interpreted incorrectly by EBS spectrum as the bulk beryllium with the complete erosion of coating layer and interlayer. Two potential explanations could be provided for the smooth Ni slope. Firstly, the front side of Ni interlayer mixes with the rear side of the beryllium coating layer as shown in the depth profile from the simulation. This mixture could dilute Ni and beryllium layer with each other which lead to the concentration of Ni at the front surface
Table 2 Thickness of left coating Be layer from simulation. Except region 74-1, the thickness was calculated by Be areal density before the layer with maximum Ni concentration. Uncertainty mainly comes from the cross section data. Region
Coating Be 1015 atoms/cm2
Thickness ± 10%
74-1 74-2 74-3 74-4 170-2 170-4
19,275 28,332 55,715 124,759 29,650 70,390
1.60 μm 2.35 μm 4.62 μm 10.35 μm 2.46 μm 5.84 μm
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distribution maps in Fig. 4 and SEM images in Fig. 2. The erosion data on right-hand wing of the same limiter tile in Ref. [15] had an error bar in the range of ± 100 μm and our thickness data on 74 helps to reduce this error. However the extrapolation on sample 170 is not straightforward since the topography is more complicated. Sampling more regions for local spectra will be necessary and helpful. The method described above could be used in other marker limiter tiles as well. But the selection of energy section in EBS spectrum for mapping should be careful, especially when dealing with the slope part. As discussed earlier, the slope in the marker limiter tile EBS spectrum may come from the concentration variation of target element. An overlarge energy section will make the extrapolation of thickness from small area to whole measure region less accurate. 5. Conclusions Samples from Be limiter marker tiles were investigated by micro ion beam and SEM. The distribution maps of backscattered protons from specified energy sections in EBS spectrum were compared with the sample topography. Local EBS spectra from small areas were simulated and information was obtained about the tile components. Results from the simulations suggest that serious erosion areas appeared at sample 74 with only 2–3 μm coating Be left, adjacent to an intact region. The nonuniform erosion, droplets mainly formed by Be, and the possible mixture of Be and Ni are the major reasons complicating the interpretation of EBS spectra from the whole scanned area. Acknowledgements This work was supported by EURATOM and carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References Fig. 7. Depth profiles of Ni and Be of 170-1, 170-2, 170-3, and 170-4 from simulation with SIMNRA. Depth profiles from 170-2 to 170-4 show that the mixture of Ni interlayer with Be coating layer appears on 170 sample as well. Unlike marker tiles, Ni concentrations in droplets are less than 10%.
[1] H.P. Furth, Tokamak research, Nucl. Fusion 15 (1975) 487–534. [2] J. Roth, et al., Recent analysis of key plasma wall interactions issues for ITER, J. Nucl. Mater. 390 (2009) 1–9. [3] G.F. Matthews, et al., Overview of the ITER-like wall project, Phys. Scr. T 128 (2007) 137–143. [4] M. Mayer, et al., Erosion at the inner wall of JET during the discharge campaigns 2001–2009, J. Nucl. Mater. 438 (2013) S780–S783. [5] S. Krat, et al., Erosion and deposition on JET divertor and limiter tiles during the experimental campaigns 2005–2009, J. Nucl. Mater. 438 (2013) S742–S745. [6] M.J. Rubel, et al., Beryllium plasma-facing components for the ITER-Like Wall Project at JET, J. Phys. Conf. Ser. 100 (2008). [7] C.P. Lungu, et al., Beryllium coatings on metals for marker tiles at JET: development of process and characterization of layers, Phys. Scr. T 128 (2007) 157–161. [8] A. Widdowson, et al., Material migration patterns and overview of first surface analysis of the JET ITER-like wall, Phys. Scr. 2014 (2014) 014010. [9] M. Mayer, et al., Computer simulation of ion beam analysis: possibilities and limitations, Nucl. Instr. Meth. B 269 (2011) 3006–3013. [10] A. Widdowson, et al., Overview of the JET ITER-like wall divertor, Nucl. Mater. Energy 12 (2017) 499–505. [11] S. Brezinsek, J.-E. Contributors, Plasma-surface interaction in the Be/W environment: conclusions drawn from the JET-ILW for ITER, J. Nucl. Mater. 463 (2015) 11–21. [12] I. Bykov, et al., Combined ion micro probe and SEM analysis of strongly non uniform deposits in fusion devices, Nucl. Instr. Meth. B 342 (2015) 19–28. [13] M. Mayer, SIMNRA, a simulation program for the analysis of NRA, RBS and ERDA, AIP Conference Proceedings, AIP, 1999, pp. 541–544. [14] S. Krat, et al., The 9 Be (p, p 0) 9 Be, 9 Be (p, d 0) 8 Be, and 9 Be (p, α 0) 6 Li crosssections for analytical purposes, Nucl. Instr. Meth. B 358 (2015) 72–81. [15] K. Heinola, et al., Tile profiling analysis of samples from the JET ITER-like wall and carbon wall, Phys. Scr. 2014 (2014) 014013.
increasing gradually, from zero to maximum. This is reflected by the EBS with a smooth slope at the high energy side instead of a sharp edge. The dilution also contributes to the slope of the beryllium peak just behind the surface beryllium in Fig. 2 by generating a decreasing beryllium concentration at the rear side of coating layer. Another explanation would be the significant thickness variation of the surface as discussed in Ref. [9]. In principle, both the concentration variation and thickness variation could offer a good fit to the experimental EBS spectrum and it is not easy to distinguish them. But the local spectrum in this report, especially 74-4, comes from a relatively uniform region. And the simulation from 74-4 suggests the mixture of the beryllium coating and the Ni interlayer. Therefore we may have more confidence in the first explanation. Despite all this, the amount of Ni and beryllium from EBS spectra still can be trusted. The amount of coating Be left on small areas was calculated from the local EBS spectra and results are summarised in Table 2. The extrapolation of thickness data from small region to the whole scanned area could be done in sample 74. Because on 74, the erosion zone, the intact zone, and the transition zone can be separated clearly by
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