**2. Carbon materials**

### **2.1. Context**

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 cam‐ paign through particle balance [24], 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.

The *post‐mortem* analyses used techniques devoted to the estimation of the D inventory [25, 26] and to structural and chemical characterizations [27], Raman microscopy being one of them [26, 28, 29]. Due to the different rates of erosion/deposition associated to the plasma footprint, the TPL surface exhibited a pattern which combined deposition‐dominated zones (labeled thick and thin depending on the amount of matter found) and erosion‐dominated zones. The shapes of these patterns were reproduced by simulating plasma wall interactions from the micrometric to the metric scale [30, 31].

Temperatures of the TPL were determined (with the help of thermographic measurements [24]) 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 nano‐ crystalline graphite (nc‐G) to amorphous carbons, hydrogenated or not (a‐C:H or a‐C). It probes the structure [32] 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 car‐ bon atoms [33]. The G band, close to *σ*G=1600 cm−1, is assigned to the bond stretching of both aromatic and aliphatic C‐C pairs, whereas the D band, close to 1350 cm−1 with a 514‐nm laser (associated with a D' band close to 1620 cm−1) is assigned to the breathing mode of aromatic rings. The Raman analysis of graphite and nc‐G clearly shows that this D band exists only when there is disorder [32]. Implanted graphite also displays this D band [34]. For a‐C and a‐C:H, the G bandwidth is related to disorder (sp2 cluster size, cluster size distribution, chemi‐ cal bonding) or stress [35] and presence of C(sp<sup>3</sup> ). For disordered multilayer graphene, the G bandwidth evolution can also be related to disorder [36].

Spectra of a‐C and nc‐G are clearly distinct [37] (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 *Γ*<sup>G</sup> ∼80–200 cm−1) than for nc‐G (width ∼15 to 40 cm−1). Moreover, the intensity ratio (D over G band intensity) depends on the amount of disorder. Generally, to reproduce experimental nc‐G spectra, bands at ∼1500 cm−1 and at ∼1200 cm−1 are very often needed, interpreted as sp3 or out‐of‐plane defects, or as an additional amorphous contribution [38–43], see the discussions in Refs. [29, 43]. Raman spectra thus contain informa‐ tion on disorder [35] such as the size clusters, L<sup>a</sup> , [32, 44], the C(sp2 )/C(sp<sup>3</sup> ) ratio, the hydrogen content [45–47], etc. All these information will help trying to understand the complex thermo/ chemistry story of samples extracted from tokamaks.

#### **2.2. Laboratory experiments**

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

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 cam‐ paign through particle balance [24], 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

The *post‐mortem* analyses used techniques devoted to the estimation of the D inventory [25, 26] and to structural and chemical characterizations [27], Raman microscopy being one of them [26, 28, 29]. Due to the different rates of erosion/deposition associated to the plasma footprint, the TPL surface exhibited a pattern which combined deposition‐dominated zones (labeled thick and thin depending on the amount of matter found) and erosion‐dominated zones. The shapes of these patterns were reproduced by simulating plasma wall interactions

Temperatures of the TPL were determined (with the help of thermographic measurements [24]) 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

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

it can be released easily in tungsten oxide.

**2. Carbon materials**

6 Raman Spectroscopy and Applications

actively cooled during discharges.

the top surface of thick deposit tiles.

from the micrometric to the metric scale [30, 31].

**2.1. Context**

#### *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 [48]. 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 [46]. 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 [49]. 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 [50].

**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. *σ*G is not shown here but it reaches a plateau for *T* > 600°C, meaning that the number of C(sp<sup>3</sup> ) neighboring C(sp2 ) no longer changes.

**Figure 2.** TDS, IBA and Raman microscopy measurements of a heated a‐C:H sample.

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 *T* > 600°C, the slope is close to zero. If we compare to the H content, between RT and 300°C, it is constant (H/H+C ∼ 30 at.%). Then, the increase of the m/HG spectroscopic parameter is due to aromatization (and defect passiv‐ ation) that leads to an increase of the photoluminescence background. As a consequence, m/ HG cannot be related easily to H/H+C in this thermal range. For higher temperatures, the H content decreases from 30 down to approximately 2%, without reaching a plateau.

The bottom part of **Figure 2** shows that HD/HG and H/H+C evolve in a remarkably sym‐ metrical 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 [50], showing a good agreement with what is known: when the H content increases, the sample becomes more transparent, because of the diminu‐ tion 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 [23], 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].

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 *T* > 600°C, the slope is close to zero. If we compare to the H content, between RT and 300°C, it is constant (H/H+C ∼ 30 at.%). Then, the increase of the m/HG spectroscopic parameter is due to aromatization (and defect passiv‐ ation) that leads to an increase of the photoluminescence background. As a consequence, m/ HG cannot be related easily to H/H+C in this thermal range. For higher temperatures, the H

The bottom part of **Figure 2** shows that HD/HG and H/H+C evolve in a remarkably sym‐ metrical 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

content decreases from 30 down to approximately 2%, without reaching a plateau.

**Figure 2.** TDS, IBA and Raman microscopy measurements of a heated a‐C:H sample.

different value at the origin.

8 Raman Spectroscopy and Applications

**Figure 3.** Thermal evolution of HD/HG for six a‐C samples. (a) Optical constant in function of H content. (b) Comparative linear ramp study. Green stars in the top figure are from Ref. [50].

**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 com‐ pared two sets of data: one by switching off the laser between acquisitions and one with a contin‐ uous 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. [56], 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.

**Figure 4.** Example of in situ measurement isothermal annealing.

Then, to resume, we have shown that


#### *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 experimen‐ tally ([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 [68]. 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) [43]. 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

**Figure 5.** Raman spectra of bombarded HOPG and CFC.

**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 com‐ pared two sets of data: one by switching off the laser between acquisitions and one with a contin‐ uous 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. [56], we

allows to probe long‐term hydrogen release. We will use it on tokamak samples in Section 2.3.

• 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 pro‐

• The HD/HG parameter is quasi‐linear in the full range of H content and can thus be used

The modification by ion impact of graphite has been studied both theoretically and experimen‐ tally ([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

).

cesses and second because it is not sensitive to H bonded to C(sp2

*2.2.3. H, D and He implantation in graphite: Starting the amorphization regime*

• HD/HG can be used in situ to retrieve the H content evolution under heating.

. Then, this technique

studied that we had no power dependence in the range 0.01–1 mW/μm2

Then, to resume, we have shown that

10 Raman Spectroscopy and Applications

**Figure 4.** Example of in situ measurement isothermal annealing.

to estimate the H content.


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, *σ*G, in function of its width, *Γ*G, for many samples collected in the Tokamak Tore Supra (deposition zones and erosion zones of the TPL, and on the NTR) and compared to pristine CFC and to heated laboratory deposited a‐C:H (see Sect. 2.2 of this chapter for more information). The data points form a wide range of carbon materials from nc‐G to a‐C, all described in more details in [28, 69, 70]. Pristine CFC data points are all situated at *Γ*Gv∼ 25 cm−1 and *σ*<sup>G</sup> ∼ 1580 cm−1. NTR deposits form a data point cloud continuously spread either along the positive slope straight line (*Γ*G in the range 20−80 cm−1, *σ*G in the range 1580–1600 cm−1) or along the negative slope straight line for the TPL deposited and eroded samples (*Γ*G in the range 80–180 cm−1, *σ*G in the range 1600–1520 cm−1). The deposited TPL points are very close to a‐C:H data points, at slightly lower frequency. Note that the shift between heated a‐C:H and the TPL deposits can be attributed to an isotopic effect as TS samples are deuterated samples, which downshifts the G band position. A proof of that can be found in Ref. [23] where both synthetic a‐C:H and a‐C:D were heated and compared. The a‐C:H data were heated under vacuum from room temperature (right down corner up to 1000°C, left up corner). Then this line from right to left means that the carbons locally organize under heating. This is in agreement with the temperature measured in situ by thermography. The NTR deposits display lower *Γ*G values, meaning they are more struc‐ tured. This is in agreement with the fact that the NTR was not actively cooled, being able to reach temperatures higher than 1000°C. As it is know that the deuterium is released at these tempera‐ tures (see Section 2.2 of this chapter, below), one can say that the D is mainly trapped in deposits found on the TPL and not on the NTR. The eroded TPL samples are less spread than the others, meaning the structure of the implanted carbons is more homogeneous and more amorphous. It can retain more deuterium, relatively. However, the thickness of that layer was found to be few nanometers only [69], which is low compared to the hundreds of microns found for Tore Supra deposits, meaning the D is essentially trapped in these deposits or deeper in the porosities of the initial prisitine CFC. These points are investigated in a next subpart.

**Figure 6.** Carbon deposits spectroscopic parameters: G band position in function of its full width at half maximum.

#### *2.3.2. Micrometric inhomogeneities and long‐term release of H*

○ 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

○ Both bombarded Raman spectra display two kind of environment: in‐plane defects and

○ No laser probe polarization effect of IPD and OPD on the HOPG sample, whereas there

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

**Figure 6** displays the G band position, *σ*G, in function of its width, *Γ*G, for many samples collected in the Tokamak Tore Supra (deposition zones and erosion zones of the TPL, and on the NTR) and compared to pristine CFC and to heated laboratory deposited a‐C:H (see Sect. 2.2 of this chapter for more information). The data points form a wide range of carbon materials from nc‐G to a‐C, all described in more details in [28, 69, 70]. Pristine CFC data points are all situated at *Γ*Gv∼ 25 cm−1 and *σ*<sup>G</sup> ∼ 1580 cm−1. NTR deposits form a data point cloud continuously spread either along the positive slope straight line (*Γ*G in the range 20−80 cm−1, *σ*G in the range 1580–1600 cm−1) or along the negative slope straight line for the TPL deposited and eroded samples (*Γ*G in the range 80–180 cm−1, *σ*G in the range 1600–1520 cm−1). The deposited TPL points are very close to a‐C:H data points, at slightly lower frequency. Note that the shift between heated a‐C:H and the TPL deposits can be attributed to an isotopic effect as TS samples are deuterated samples, which downshifts the G band position. A proof of that can be found in Ref. [23] where both synthetic a‐C:H and a‐C:D were heated and compared. The a‐C:H data were heated under vacuum from room temperature (right down corner up to 1000°C, left up corner). Then this line from right to left means that the carbons locally organize under heating. This is in agreement with the temperature measured in situ by thermography. The NTR deposits display lower *Γ*G values, meaning they are more struc‐ tured. This is in agreement with the fact that the NTR was not actively cooled, being able to reach temperatures higher than 1000°C. As it is know that the deuterium is released at these tempera‐ tures (see Section 2.2 of this chapter, below), one can say that the D is mainly trapped in deposits found on the TPL and not on the NTR. The eroded TPL samples are less spread than the others, meaning the structure of the implanted carbons is more homogeneous and more amorphous. It can retain more deuterium, relatively. However, the thickness of that layer was found to be few nanometers only [69], which is low compared to the hundreds of microns found for Tore Supra deposits, meaning the D is essentially trapped in these deposits or deeper in the porosities of the

(see **Figure 1**).

12 Raman Spectroscopy and Applications

is one for IPD of the CFC.

benefit of the out‐of‐plane defects.

out of plane defects (see Ref. [43] for details).

**2.3. Information retrieved from tokamak samples**

*2.3.1. Raman measurements inside the tokamak Tore Supra*

initial prisitine CFC. These points are investigated in a next subpart.

In Ref. [23], 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 mecha‐ nism [71] 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 dis‐ cussing **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 *σ*G which is in agreement with what we know about the thermal stability of a‐C:H [23, 49, 56]. The his‐ tograms of a TS deposit heated isothermally at 120°C for three times (120, 300 and 640 h) and at 250°C allows to show that the sample still evolve under long time scales (the average value of m/HG increases slightly with time). The problem is that we need to average spectra on large zones because of large inhomogeneities found at the micrometric scale. However, using the mapping mode allows to identify different kind of defects of chemical environments which is needed to better characterize the sample. The average value of HD/HG (not shown here) leads to a value of H close to 20 %.

**Figure 7.** Raman spectroscopy imaging of a unheated Tore Supra (TS) deposit. (a) m/HG mapping of this TS sample. (b) *σ*G mapping. (c) Histograms of heated TS samples (120°C during 120, 300 and 630 h, and 250°C).

#### *2.3.3. Hydrogen depth concentration and history of Tore Supra deposits*

In Ref. [72], 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 [31]. 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. [73] where IBA and cross‐section electron micros‐ copy 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, *σ*G is displayed for three lines of sight. Then on top of this deposit, the H content is close to 20%, whereas it decreases close to zero at the interface between the deposit and the top of the CFC. The diminution is rapid in the 20 first microns. Same trends are seen with the structure probed with Raman spectroscopy: carbon close to the surface is less organized (*σ*G close to 1520 cm−1), whereas *σ*G increases (meaning an increase of order) and then reaches a plateau 20 μm deep in the deposit, at the value of *σ*<sup>G</sup> = 1580 cm−1. This deuterium impoverishment of the deep layers may be caused by long‐term release mechanisms, as discussed in the previous part.

**Figure 8.** H‐content impoverishment in deep deposits compared to ordering of the carbon. H content is extracted from Ref. [73].

#### **3. Beryllium‐ and tungsten‐based materials**

#### **3.1. Context**

*2.3.3. Hydrogen depth concentration and history of Tore Supra deposits*

(b) *σ*G mapping. (c) Histograms of heated TS samples (120°C during 120, 300 and 630 h, and 250°C).

release mechanisms, as discussed in the previous part.

to erosion zones.

14 Raman Spectroscopy and Applications

In Ref. [72], 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

**Figure 7.** Raman spectroscopy imaging of a unheated Tore Supra (TS) deposit. (a) m/HG mapping of this TS sample.

Such things were simulated recently [31]. 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. [73] where IBA and cross‐section electron micros‐ copy 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, *σ*G is displayed for three lines of sight. Then on top of this deposit, the H content is close to 20%, whereas it decreases close to zero at the interface between the deposit and the top of the CFC. The diminution is rapid in the 20 first microns. Same trends are seen with the structure probed with Raman spectroscopy: carbon close to the surface is less organized (*σ*G close to 1520 cm−1), whereas *σ*G increases (meaning an increase of order) and then reaches a plateau 20 μm deep in the deposit, at the value of *σ*<sup>G</sup> = 1580 cm−1. This deuterium impoverishment of the deep layers may be caused by long‐term

Tritium retention, occurring in a magnetically confined deuterium plus tritium plasma, can be obtained by erosion, retention, material modification, dust formation (and more gener‐ ally 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 m<sup>2</sup> ITER's beryllium inner walls because of erosion [75]. 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 satura‐ tion (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 [85]. The exis‐ tence of BeH2 was suggested in [17] but never observed before.

Raman microscopy is a nondestructive, noncontact and local (≈1 μm2 lateral resolution [86]) technique that has been proved to be sensitive to Be stretching modes [87], beryllium oxide modes [88], bending and stretching tungsten oxide modes [89], Bex Wy ‐mixed samples density of states [90], BeCW‐irradiated samples [91] and give information when a pristine material is implanted by hydrogen ions [90, 92, 93]. First Raman analyses in ILW‐tokamaks were per‐ formed on some molybdenum JET mirrors [90], 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 iden‐ tify 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. [94], we identified on laboratory experiments the growth of BeD2 with dendritic forms appearing sub‐ sequently 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 [95], who determined recently the structure by using additionally synchrotron X‐ray diffraction. In [94], 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 impu‐ rity 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. [96] 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 [94].

Hydrogen isotope retention in beryllium can also play a role in the lifetime estimation of the ≈700

technique that has been proved to be sensitive to Be stretching modes [87], beryllium oxide

of states [90], BeCW‐irradiated samples [91] and give information when a pristine material is implanted by hydrogen ions [90, 92, 93]. First Raman analyses in ILW‐tokamaks were per‐ formed on some molybdenum JET mirrors [90], 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 iden‐ tify 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. [94], we

sequently 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 [95], who determined recently the structure by using additionally synchrotron X‐ray diffraction. In [94], we showed that these dendrites appear when the amount of deuterium in the material is higher than ≈2 × 1017

Below, we compare defective Be samples that have been produced by Be deposits in an impu‐ rity 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. [96] and

10−3 Pa partial pressure) has been used during the deposit. The amount of N being ≈5 atomic %.

was suggested in [17] but never observed before.

Raman microscopy is a nondestructive, noncontact and local (≈1 μm2

modes [88], bending and stretching tungsten oxide modes [89], Bex

identified on laboratory experiments the growth of BeD2

D.cm−2, when the ≈40 nm under surface layer is saturated by D.

references therein). To introduce disorder, a partial pressure of N2

**3.2. Laboratory experiments**

*3.2.1. Defect‐induced bands in beryllium*

 ITER's beryllium inner walls because of erosion [75]. 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 satura‐ tion (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 [85]. The exis‐

lateral resolution [86])

‐mixed samples density

Wy

with dendritic forms appearing sub‐

(in the range from 10−2 to

m<sup>2</sup>

16 Raman Spectroscopy and Applications

tence of BeH2

**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. *Γ*E2G for the reference sample is close to 8 cm−1. For the defective sample, it is higher (27 cm−1). The logarithmic scale used here allows to distinguish several additional broad bands (413, 544 and 616 cm−1) that were first evidenced in Ref. [90] and called defect‐induced bands. These bands are attributed to the phonon density of states (PDOS), as can be seen by comparing the Raman spectrum (where the E2G band has been removed) to the PDOS of Be measured in the literature. Their intensity reaches values as high as 23% of the Raman active E2G mode heights and they are related to the defect content, defect being a term which can be impurity, vacancy or other kind of defects.

**Figure 9.** Normalized Raman spectra of a pristine (a) and defective (b) Be deposited sample, using four laser wavelength. (c) The 633 nm spectrum of the defective sample is displayed after removing the E2G Raman active mode and is compared to the PDOS measured by INS and by PCEPI [97]. The bands marked by stars are due to other laser electronic transitions.

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). *σ*E2G does not display a monotonic evolution with the laser wavelength for the defective sample. The reference sample, oppositely, does not dis‐ perse in the range 325−633 nm. Shifts observed may then be due to stress [86] in the deposited layer, the nonmonotonic behavior being caused by two effects: stress gradients existing in the deposited layers and the penetration depth of the laser that is wavelength dependent.

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 implanta‐ tion (**Figure 10(b)**) and vacancy creation profiles were evaluated for the 90° and 45° geometries

**Figure 10.** Varying the in‐depth defects. (a) Normalized Raman spectra of D implanted Be samples. Geometry implantation (90° and 45°) are compared with the 514 nm laser wavelength.

using the stopping and range of ions in matter (SRIM) code2 [68]. If we believe in SRIM calcu‐ lations (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 *Γ*Be is increased by a factor 2.7 and the PDOS relative height HPDOS/HBe is increased by a factor 11 from the 90° to the 45° geometry. How these differences can be interpreted? According to SRIMS calculations in the subsurface slab going from 0 down to 31 nm, there are more defects (implanted D and vacancy) created in the 45° geometry than in the 90° geometry. Then, it explains why HPDOS/HBe and *Γ*Be are higher in the first case than in the second.

#### *3.2.2. Characterization of Be nanometric hydrides*

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). *σ*E2G does not display a monotonic evolution with the laser wavelength for the defective sample. The reference sample, oppositely, does not dis‐ perse in the range 325−633 nm. Shifts observed may then be due to stress [86] in the deposited layer, the nonmonotonic behavior being caused by two effects: stress gradients existing in the

18 Raman Spectroscopy and Applications

deposited layers and the penetration depth of the laser that is wavelength dependent.

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 implanta‐ tion (**Figure 10(b)**) and vacancy creation profiles were evaluated for the 90° and 45° geometries

**Figure 10.** Varying the in‐depth defects. (a) Normalized Raman spectra of D implanted Be samples. Geometry

implantation (90° and 45°) are compared with the 514 nm laser wavelength.

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 evi‐ dence the formation of crystalline BeD2 [94]. The spectra are typically close to the one found for BeH2 in Ref. [95] 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 [92] and might be of interest for hydrogen storage by forming hydrides [98].

**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 [94]. 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 con‐ tamination 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.

<sup>2</sup> 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).

**Figure 11.** Beryllium hydride formation under ion implantation. (a) and (b) Optical microscopy. (c) Be‐D bond intensity mapping. (d) Position of carbon impurities. (d) Comparison off/on dendrites. (e) Raman spectrum of carbon impurities.

#### *3.2.3. Hydrogen behavior in tungsten oxide*

Tungsten alone cannot be seen using standard Raman microscopy because there are no opti‐ cal 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 tem‐ peratures ranging from few hundreds to 1000°C because of heat loads, it could be possible that some oxide will be formed [99]. Then it would be of interest for the plasma‐wall interac‐ tion 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 stoichiom‐ etry), with many phase transitions that can be affected by nanosize effects [100] so that a sys‐ tematic D implantation study could be helpful for the future. Today, only a few studies exist on the subject. In [93], 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<sup>+</sup> ). Raman microscopy and X‐ray dif‐ fraction show a nanocrystalline WO<sup>3</sup> 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 (Dx WO<sup>3</sup> ). After exposure under ambient conditions, a reversible deuterium retention, due to oxidation of the Dx WO<sup>3</sup> 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. [93]). 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.

**Figure 12.** Raman spectra of bombarded WO<sup>3</sup> .

*3.2.3. Hydrogen behavior in tungsten oxide*

20 Raman Spectroscopy and Applications

Tungsten alone cannot be seen using standard Raman microscopy because there are no opti‐ cal 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 tem‐ peratures ranging from few hundreds to 1000°C because of heat loads, it could be possible

**Figure 11.** Beryllium hydride formation under ion implantation. (a) and (b) Optical microscopy. (c) Be‐D bond intensity mapping. (d) Position of carbon impurities. (d) Comparison off/on dendrites. (e) Raman spectrum of carbon impurities.
