Raman microscopy, which is sensitive to chemical bonds, defects, structure, is a suitable tool that can give information on how a material can be modified by interacting with ions. We will first give concrete examples on how it can be used to characterize with a micrometric resolution samples extracted from tokamaks. We will then give concrete examples on what information can be obtained by doing a study on laboratory synthesized materials, benchmarking Raman microscopy with quantitative techniques. The first part of the chapter is focused on carbon‐based material analysis. We will show how Raman spectra are sensitive to the presence of hydrogen, a major safety issue in the field. The second part of the chapter will be focused on beryllium‐ and tungsten‐based material analysis. We will show that hydrogen can be stored as an hydride after ion implantation and that it can be released easily in tungsten oxide.
- plasma wall interaction
- hydrogen isotope implantation
Raman microscopy, which is sensitive to chemical bonds, defects, and structure, is a suitable tool that can give information on how a material can be modified by interacting with ions. We first gave concrete examples on how it can be used to characterize with a micrometric resolution samples extracted from tokamaks. We then gave concrete examples on what information can be obtained by doing a study on laboratory‐synthesized materials, comparing Raman microscopy with quantitative techniques. The first part of the chapter is focused on carbon‐based material analysis. We showed how Raman spectra are sensitive to the presence of hydrogen, a major safety issue in the field. The second part of the chapter is focused on beryllium‐ and tungsten‐based material analysis. We showed that hydrogen can be stored as a hydride after ion implantation and that it can be released easily in tungsten oxide.
1.1. Plasma‐wall interactions in tokamaks
Tokamaks are machines made to study the possibility to make energy from nuclear fusion reactions by confining hot plasma (1–100 keV ions) magnetically. The future international reactor
Then, in tokamaks, understanding and being able to predict the evolution of the plasma‐facing components (PFCs) that interact with both heat loads and the cold edge plasma is one of the keystones to make nuclear fusion a way of producing energy in the future . The material chosen for the
1.2. Raman microscopy as a technique to probe the top of deposits
In this field of research, due to their isotope selectivity, ion beam analyses (IBA) play a prominent role in this characterization, as discussed in Refs. [15, 16]. Thermal desorption spectroscopy (TDS) also plays a key role as it can give access to the characterization of D or T trap energies in metals such as Be [17–19] or W . However, even if TDS and IBA techniques give quantitative information about hydrogen isotope concentration, they only give indirect information about chemical bonding. A direct way to probe that chemical bondings is to probe them spectroscopically by means of their vibrational spectrum using Raman microscopy.
Moreover, the in‐depth resolution reached by IBA is not enough to describe properly the top layer of deposits called the
The aim of this chapter is to give concrete examples on how Raman microscopy can be used to characterize samples extracted from tokamaks, from the micrometric to the macroscopic scale of the machine. We then gave concrete examples on what information can be obtained by doing a study on laboratory‐synthesized materials, benchmarking Raman microscopy with quantitative techniques such as TDS or IBA. The first part of the chapter is focused on carbon‐based material analysis as all the previous tokamaks were using this element in the past. We showed how Raman spectra are sensitive to the presence of hydrogen, a major safety issue in the field. The second part of the chapter is focused on beryllium and tungsten based material analysis. We showed that hydrogen can be stored as a hydride after ion implantation, and that it can be released easily in tungsten oxide.
2. Carbon materials
The Tore Supra tokamak had the ability to perform long plasma discharges with actively cooled PFC, which was a good opportunity to study fuel retention, a major concern in the plasma‐wall interaction community. The study devoted to that was the Deuterium Inventory in Tore Supra (DITS) campaign. It was aimed at studying and linking the erosion of carbon PFCs and the fuel retention. The in‐vessel D inventory was followed after the D loading campaign through particle balance , a part of the toroidal pump limiter (TPL), which was actively cooled during discharges, was dismantled for an extensive postmortem analysis of a few tens of carbon fiber composite tiles (CFC). The TPL was situated on top of a part of the machine called the neutralizer (NTR), which was also composed of CFC, but which was not actively cooled during discharges.
Temperatures of the TPL were determined (with the help of thermographic measurements ) to be 500°C for gap surfaces of both thick deposit tiles and eroded tiles, 200°C for top surfaces of eroded tiles and 120°C for both gap and top surfaces of CFC tiles situated in the so‐called thin deposit region. Temperature was estimated to vary in the range 500–900°C for the top surface of thick deposit tiles.
In Figure 6, we display Raman spectroscopic parameters of several samples, but before detailing the figure, we need to remember some well‐known facts about the Raman spectroscopy of disordered and amorphous carbons. They are briefly given below but are not detailed.
Raman microscopy is routinely used to characterize rapidly C‐based materials, from nanocrystalline graphite (nc‐G) to amorphous carbons, hydrogenated or not (a‐C:H or a‐C). It probes the structure  and is highly sensitive to bonding properties by interpreting the 1000–1800 cm−1 region, dominated by the G and D bands due to C(sp2) hybridization of carbon atoms . The G band, close to
Spectra of a‐C and nc‐G are clearly distinct  (spectrum 1 from Figure 8(a) belongs to a‐C, whereas spectrum 3 belongs to nc‐G, whereas spectrum 2 is in between): for example, G and D bands are much broader for a‐C (width
2.2. Laboratory experiments
2.2.1. H‐content determination in a‐C:H layers
The Raman parameters generally used to probe the structure are the peak wavenumber and the width of the G and D bands, respectively, and the peak height ratio of these two bands, HD/HG . An additional spectral feature has to be taken into account: a photoluminescence background which is superimposed to the Raman spectrum and is correlated to the H content . The slope of this background, m, is calculated between 800 and 2000 cm−1. The higher m, the higher is the photoluminescence intensity. Figure 2 displays ultra‐high vacuum heating effect on a 220‐nm thick hard amorphous, hydrogenated carbon thin films (a‐C:H) that were deposited on a Si . The mass density of such typical hard a‐C:H layers, with H/H+C ∼ 30 at.% is ∼1.9 g cm−3 are formed .
Figure 2 displays the thermal evolution of m/HG and HD/HG compared to the H content determined by IBA and the H2 release determined by TPD.
We see that m/HG increases by ∼40% between room temperature and 300°C, decreasing between 300 and 600°C (at that point, m/HG ∼0.05). For
The bottom part of Figure 2 shows that HD/HG and H/H+C evolve in a remarkably symmetrical way. This suggests that the two parameters are correlated and that HD/HG could be used to probe the hydrogen content. The relation obtained that works with a 514 nm laser is (H/H+C = 0.54−0.53 HD/HG) in the range H/H+C = 2–30 at .%. Such a linear law, with the same slope, can be obtained for other wavelengths (407–633 nm has been tested), but with a different value at the origin.
2.2.2. Long‐term hydrogen release revealed by in situ Raman spectroscopy
The thermal stability of six 200‐nm‐thick plasma enhanced chemical vapor deposited a‐C, a‐C:H and a‐C:D layers ranging from soft to hard layers has been studied. The imaginary part of the refractive index at 633 nm and the corresponding H or D contents have been displayed together and compared to results from the literature , showing a good agreement with what is known: when the H content increases, the sample becomes more transparent, because of the diminution of aromatic C(sp2) bonds. Figure 3(b) displays the thermal evolution of HD/HG for these six samples heated with a linear ramp (3°C min−1) under a 1‐bar argon atmosphere from room temperature to 600°C.Without entering into details, that can be found in , let us only focus on H data. One can see that the more hydrogen there is on the as deposited sample, at a lower temperature, the HD/HG ratio starts to increase. Because the evolution of HD/HG for a H‐free amorphous carbon is small and because of what was shown in the previous part, most of the evolution in that thermal range is due to hydrogen release from C(sp3). This is consistent with what is known in the literature about the bad thermal stability of hydrogen‐rich a‐C [51–55].
Figure 4 displays an example of isothermal evolution in function of time. This evolution cannot be due to the effect of the laser on the sample, accumulating heat with long times, as we compared two sets of data: one by switching off the laser between acquisitions and one with a continuous flux of heat. The position of the G band (not shown) no more evolves after 50 min, meaning there is no order increase due to an overestimated heat load by the probe laser. In Ref. , we studied that we had no power dependence in the range 0.01–1 mW/μm2. Then, this technique allows to probe long‐term hydrogen release. We will use it on tokamak samples in Section 2.3.
Then, to resume, we have shown that
The m/HG parameter, often used to estimate the H content in the literature, should be used with care, first because it is sensitive to various photoluminescence‐quenching processes and second because it is not sensitive to H bonded to C(sp2).
The HD/HG parameter is quasi‐linear in the full range of H content and can thus be used to estimate the H content.
HD/HG can be used in situ to retrieve the H content evolution under heating.
2.2.3. H, D and He implantation in graphite: Starting the amorphization regime
The modification by ion impact of graphite has been studied both theoretically and experimentally ([57–67] and references therein). Inside the sample, collision cascades are created by ions that knock on the surface and penetrate. It creates defects and vacancies, ions being implanted after they slow down . Protusions (height lower than 1 nm), called hillocks, are observed for low fluencies (<1014 ions cm−2) [57, 58]. They are due to stress created by the first collision under the near surface region. More precisely, interstitial carbon clusters or coalescence of interstitials and vacancies are at the origin of these hillocks. At fluencies >1015 ions cm−2 [61, 62] and in the 400–800 K range, domes with a height one order of magnitude higher were observed.
We have exposed graphite to hydrogen deuterium and helium plasma (from 1016 to 1018 cm−2) . The energies were tuned from 40 to 800 eV. The ion incidence was parallel to the basal plane for CFC (carbon/carbon composite) and perpendicular for HOPG (highly oriented pyrolitic graphite). The changes of the material were studied by means of both Raman and atomic force microscopies.
We display the 400 eV/D implantation Raman spectra in Figure 5 as an illustration. The main differences or common points between HOPG and CFC are
Pristine HOPG contains less defects than CFC (D band at 1350 cm−1 is close to zero for HOPG, whereas HD/HG is close to 2 for CFC, indicating nanometric crystal size domains)
Implantation produces an amorphization for both materials (broad bands at 1500–1300 cm−1 appearing in the spectra, looking like the spectrum 1 in Figure 8(a))
The underlying pristine sample is visible can be seen in both cases. This is because the ions penetrate only 15–20 nm and the penetration depth of light is few tens of nanometer (see Figure 1).
Both bombarded Raman spectra display two kind of environment: in‐plane defects and out of plane defects (see Ref.  for details).
No laser probe polarization effect of IPD and OPD on the HOPG sample, whereas there is one for IPD of the CFC.
When increasing the impinging ion energy, the growth of nanometric domes at the surface has been observed by atomic force microscopy (AFM) and the incident kinetic energy has been found as the parameter determining their height. The Raman study has also revealed that both the defect‐defect distance in the IPD and OPD are typically 1 nm. When the number of vacancies created in the material increases, the number of in‐plane defects decreases to the benefit of the out‐of‐plane defects.
2.3. Information retrieved from tokamak samples
2.3.1. Raman measurements inside the tokamak Tore Supra
Figure 6 displays the G band position,
2.3.2. Micrometric inhomogeneities and long‐term release of H
In Ref. , we compared with Raman microscopy the thermal evolution of reference samples to samples extracted from the Tore Supra tokamak, as it was shown that a long‐term mechanism  occurred even at low temperature (low means here 120°C, which correspond to the thermostat temperature of the PFC cooling loop, staying at this values for days and weeks even when the plasma is off) and as we wanted to distinguish the Raman signature of this heating. Figure 7 displays the Raman data of the Tore Supra samples. Figure 7(a) and (b) is mappings of m/HG and sG, respectively, for an as received sample. One can see that the deposit is inhomogeneous, the sample being more organized in the upper right part. The high value of m/HG does not mean here that there is more hydrogen there, as mentioned by discussing Figure 2. In fact, this is the opposite. The value of HD/HG in the blue and red regions (not shown here) give typically values of 28 down to 17%, using the linear relation mentioned in Sect. 2.1. The zone where hydrogen has been outgassed displays a higher value of
2.3.3. Hydrogen depth concentration and history of Tore Supra deposits
In Ref. , lock‐in thermography, scanning electron microscopy and confocal microscopy were used to study the erosion and depositions on the TPL of the Tore Supra tokamak. Taking into account, all the methods allowed the authors to perform a complete mapping of the mass of carbon that has been eroded and deposited. The foundings were 520 g of deposits and 920 g of eroded carbon. It shows that more than a half of the carbon that is eroded is redeposited on the TPL. It was also found that the zones containing the highest deposits were found close to erosion zones.
Such things were simulated recently . The gap deposition contribution is estimated at 23%, mostly from the erosion zones and with a main contribution from the low field side of the tile toroidal gap surfaces. This deuterium impoverishment in the deep layer has also been investigated more systematically in Ref.  where IBA and cross‐section electron microscopy where done on 15 samples. The mean in‐depth hydrogen isotope content is plotted in Figure 8(b) and scaled to the 120 µm of the deposit shown. The surface content has been divided by 2 in less than 20 µm. In parallel,
3. Beryllium‐ and tungsten‐based materials
Tritium retention, occurring in a magnetically confined deuterium plus tritium plasma, can be obtained by erosion, retention, material modification, dust formation (and more generally plasma wall interactions). This is a big safety issue because of tritium radioactivity [7, 74]. Hydrogen isotope retention in beryllium can also play a role in the lifetime estimation of the ≈700 m2 ITER's beryllium inner walls because of erosion . This explains why several studies based on ion irradiation of Be samples have been devoted to D behavior to ensure that tritium retention will not be a limiting issue in ITER operations. Previous studies found D/Be ratio in the range 0.1–0.7 using energies in the range 0.6–20 keV/D and fluency in the range 1016–1019 cm−2 [76–82].
At fluencies close to 2 × 1017 cm−2 (in the range 0.6-1 keV/D), the existence of the D-content saturation (D/Be close to 0.25) has been shown [13, 83–84]. For fluencies lower than 0.7 × 1017 cm−2 TDS display only one peak at 900 K, whereas for fluencies between 0.7 and 1 × 1017 cm−2, a second peak lies at 750 K. For fluencies higher than 1.2 × 1017 cm−2, two extra peak rise close to 500 K. These last peaks were explained by structural modification of the sample. Moreover, the role of crystal orientation and D diffusion along grain boundaries was investigated in . The existence of BeH2 was suggested in  but never observed before.
Raman microscopy is a nondestructive, noncontact and local (≈1 μm2 lateral resolution ) technique that has been proved to be sensitive to Be stretching modes , beryllium oxide modes , bending and stretching tungsten oxide modes , BexWy‐mixed samples density of states , BeCW‐irradiated samples  and give information when a pristine material is implanted by hydrogen ions [90, 92, 93]. First Raman analyses in ILW‐tokamaks were performed on some molybdenum JET mirrors , showing that the technique is sensitive to thin ≈10 nm deposited layer composed of ≈33% Be, ≈33% C and ≈33% O and the underlying molybdenum oxidized mirror. C‐O and C=O modes have been detected in that layer and defective or beryllium mixed with O and/or C have been found without possibility to identify the phases rigorously up to now, because of a poor benchmarking of the technique for these materials. Raman microscopy can also be able to give information about the hydrogen isotope behavior by combining IBA, Raman microscopy, atomic force microscopy (AFM) and with the help of DFT modeling. As an illustration of this complementarity, in Ref. , we identified on laboratory experiments the growth of BeD2 with dendritic forms appearing subsequently to 2 keV ion implantation when the Be layer is saturated by implanted D ions. The width of the bands recorded by Raman microscopy suggested that this hydride has grown in a crystalline form which seems to be close to a body‐centered orthorhombic structure with Ibam symmetry as the spectra look like to the ones reported in , who determined recently the structure by using additionally synchrotron X‐ray diffraction. In , we showed that these dendrites appear when the amount of deuterium in the material is higher than ≈2 × 1017 D.cm−2, when the ≈40 nm under surface layer is saturated by D.
3.2. Laboratory experiments
3.2.1. Defect‐induced bands in beryllium
Below, we compare defective Be samples that have been produced by Be deposits in an impurity atmosphere or by D implantation. In more details, the defective deposit sample has been produced by the thermionic vacuum (TVA) method (more details can be found in Ref.  and references therein). To introduce disorder, a partial pressure of N2 (in the range from 10−2 to 10−3 Pa partial pressure) has been used during the deposit. The amount of N being ≈5 atomic %. The D‐implanted Be sample have been prepared with two impinging ion geometries, with 2 keV/D: normal incidence (90°) and 45° incidence. More details about the implantation of the 90° geometry can be found in .
Figure 9 displays Raman spectra of a deposited Be sample with a high content of defects for four laser wavelengths. Data were acquired with 633, 514, 488 and 325 nm lasers. The band associated to the E2G Raman active mode is downshifted for the defective sample, but the shift depends on the wavelength.
Changing the wavelength does not change the qualitative trends: the band related to the E2G mode is more downshifted to low frequencies and broader. However, quantitative differences can also be seen, with some information. For example, the height of the PDOS diminishes monotonically with the decrease of the laser wavelength (from 23% with 633 nm down to 4% with 325 nm for the defective sample).
In Figure 10, we compare 514 nm spectra of a pristine Be, a Be sample containing 2 × 1017 D cm−2 in the 90° geometry and 514 nm spectrum of Be sample in the 45° geometry. D implantation (Figure 10(b)) and vacancy creation profiles were evaluated for the 90° and 45° geometries using the stopping and range of ions in matter (SRIM) code2 . If we believe in SRIM calculations (which gives trends as many mechanisms and effects are not included), we can then estimate that 16% of the implanted deuterium atoms are implanted in the range 0–19 nm, 31% in the range 0–28 nm and 37% in the range 0–31 nm. For the corresponding ranges, the total amount of vacancies created are, respectively, 37, 59 and 65% of the total vacancies created. The lineal density of vacancies is the higher in the range 0–28 nm, being lower by 7.6% in the 0–19 nm range and by 0.4% in the 0–31 nm range.
We see that the Be implanted by D in the 90° geometry displays a Raman spectrum with a E2G mode downshifted by 5 cm−1 and with an additional broadening of 6.5 cm−1 compared to the pristine sample. The PDOS is present, with a relative height ratio of HPDOS/HBe=3%. If we compare with the Be implanted by D in the 45° geometry, we see that the band corresponding to the E2G mode is downshifted by 5 cm−1, with an additional broadening of 30 cm−1 compared to the pristine sample. The PDOS is rising, with a relative height ratio HPDOS/HBe of 24%. The band position is the same in the two implantation geometries. However, the bandwidth
3.2.2. Characterization of Be nanometric hydrides
By using energetic ions, and with the help of nuclear reaction analysis (NRA), AFM, optical microscopy and quantum calculations compared to Raman spectroscopy, we were able to evidence the formation of crystalline BeD2 . The spectra are typically close to the one found for BeH2 in Ref.  using high pressure to form them. It was shown previously that this latter technique is sensitive to the way hydrogen isotopes are bonded and organized in materials and thus that it can be relevant for fusion  and might be of interest for hydrogen storage by forming hydrides .
Figure 11 shows Raman imaging in the vicinity of a dendrite. In Figure 11(c), the intensities of the band at 1397 cm−1 due to a Be‐D mode are displayed. It is more intense on the dendrite and less intense off the dendrite. The corresponding spectra are displayed in Figure 11(d). Details on how were obtained the spectra can be obtained in . The bands marked by stars are due to Be‐D bonds in BeD2. The fact that their corresponding intensity are higher are more intense on dendrite allows to conclude that the dendrites are made of BeD2. If the formation of such dendrites occurs in tokamaks, it could become a high fuel reservoir.
Figure 11(d) display the intensity of a mode found close to 1580 cm−1 and due to carbon contamination of the sample before it was bombarded (D and G bands in Figure 11(e)), and that has nothing to do with the experimental conditions.
3.2.3. Hydrogen behavior in tungsten oxide
Tungsten alone cannot be seen using standard Raman microscopy because there are no optical phonons that can be probed close to the center of the Brillouin zone. However, as oxygen is one of the impurities that can be found in tokamaks, and as the inner walls will be at temperatures ranging from few hundreds to 1000°C because of heat loads, it could be possible that some oxide will be formed . Then it would be of interest for the plasma‐wall interaction community to take care of D and T behavior in tungsten oxides. Without entering into details, tungsten oxides can exist in various forms (dioxide, trioxide, in between stoichiometry), with many phase transitions that can be affected by nanosize effects  so that a systematic D implantation study could be helpful for the future. Today, only a few studies exist on the subject. In , we report on the formation of thin tungsten oxide layers that have been grown on W surfaces by thermal oxidation (thicknesses up to ∼250 nm) and that have been exposed to low energy deuterium plasma (11 eV/D+). Raman microscopy and X‐ray diffraction show a nanocrystalline WO3 monoclinic structure. We observed that the low‐energy deuterium plasma exposure has induced a phase transition, a change in the sample color and the formation of tungsten bronze (DxWO3). After exposure under ambient conditions, a reversible deuterium retention, due to oxidation of the DxWO3 layer, has been observed. Figure 12 displays the effect of D implantation. The spectral range of tungsten trioxides can be roughly distributed in two kind of spectral regions: the bending modes region, lower than 500 cm−1 and the stretching modes region at higher wavenumbers. Before exposure, we can see the characteristic peaks of the monoclinic structure at 703 and 805 cm−1 which are attributed to symmetric and antisymmetric (O‐W‐O) stretching modes, respectively (see references in Ref. ). The wing in the range 920–970 cm−1 is attributed to terminal oxygen bonding in case of nanocrystalline structures, but other interpretations can be found in the literature that we do not review here. Due to implantation, all the peaks are broadened, the bending modes display bands that are more intense than stretching modes, which can be the sign of some disorder, and two new bands appeared: one at 950 and the other at 388 cm−1. These bands disappeared with time once put in air due to reoxydation.
Because of safety issue, it is important to understand how and where tritium will be stored in tokamaks due to plasma‐wall interaction. In this chapter, we have demonstrated the ability of Rama microscopy to detect and characterize material changes subsequently to interaction with hydrogen, for both carbon‐based tokamaks and for ITER‐like wall materials (tungsten that is oxidized and beryllium). The comparison between well‐controlled samples and sample extracted from tokamak has been used to better characterize some of the tokamaks’ properties.
Among other findings, we report that by changing temperature on various carbon samples, extracted from tokamak (Tore Supra) or comparative reference laboratory samples (a‐C:H) synthesized in controlled conditions, we have shown how Raman spectroscopic parameters can be used to characterize structural and H‐content evolution/changes.
We also report that beryllium hydrides that can form high tritium reservoir in tokamaks can be formed and detected under ion implantation.
To conclude, next studies will have to use different laser wavelengths as they can be used to probe gradients of properties in the beryllium’s depth due to the fact the depth probed is in the range of few tens of nanometer. This is the typical depth at which a high content of hydrogen isotope is found in modern tokamak samples like JET. Future studies have to be driven with this technique to better characterize the so‐called supersaturated layer.
- To give an idea of the size of the project and the machine: the tokamak alone will stay in a room which is 60 m high, all the site will be sprayed on a surface which is 420,000 m2 and the machine itself weighed 23,000 tons .
- This code is widely used to investigate ion-surface interaction phenomena. It is a free access Monte-Carlo computer program based on the binary collision approximation that do not take into account crystal structure or vacancy diffusion (i.e. each single collision event is treated independently between two steps).