**1. Introduction**

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 infor‐ mation can be obtained by doing a study on laboratory‐synthesized materials, comparing

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 *ITER*<sup>1</sup> (International Thermonuclear Experimental Reactor), now in construction at Cadarache (south of France), is one of them. One of its aims is to produce, with the help of the D+T nuclear reaction, 500 MW by injecting 50 MW. The D+T nuclear reaction has been chosen because it has the highest cross section in the relevant domain of energy (one to two orders of magnitudes higher than the D+D reactions). The D+T reaction will produce 14‐MeV neutrons and 3.52‐MeV He nuclei, which will maintain the temperature of the D+T reacting plasma to an equilibrium state. Magnetic field lines are however connected to the inner walls of the ITER torus, which leads to the existence of cold edge plasma (composed of He, D, T, impurities such as oxygen or eroded metals) interacting with them. The heat produced by the hot plasma is received mainly on a part of the wall called a *divertor*. The material that will compose this part will have to work with heat loads in the range 10–20 MW/m2 .

Then, in tokamaks, understanding and being able to predict the evolution of the plasma‐fac‐ ing 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 [2]. The mate‐ rial chosen for the *divertor* is tungsten as it drives well the heat to the underlying cooling loops present in the PFCs, it is difficult to erode, and its melting/fusion temperatures are very high, and hence creating bonding with hydrogen isotope is supposed to be difficult. The other parts of the PFCs are composed of beryllium [3]. Some tokamaks are already working nowadays in an ITER‐like wall (ILW) configuration to prepare the ITER project, such as the Joint European Torus/JET tokamak, with some PFCs composed of tungsten‐coated carbon tiles [4–6], car‐ bon tiles, mounted on *limiters* and/or *neutralizers*, being used in the ancient carbon area of tokamaks, now abandoned, because of tritium retention (tritium being radioactive and being easily trapped in carbon) [7, 8]. In that framework, the surface composition and morphology modifications under operation will lead to changes in the material properties that can address safety issues. These changes will have to be measured and then understood. In more details, the migration [9] and/or melting of elements in the machine [10, 11], the production of dust, the hydrogen isotope retention [12, 13], the impurity contamination (traces of carbon and/or oxygen, nitrogen seeding [14], etc.) leading to formation of new mixed materials/phases (such as oxides or alloys) and the complex surface erosion will all influence the lifetime of these PFCs and their fuel retention properties. Surface characterization techniques are then neces‐ sary to measure the elemental changes in JET and in ITER PFCs erosion zones and deposits.

<sup>1</sup> 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 [1].

#### **1.2. Raman microscopy as a technique to probe the top of deposits**

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

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

Cadarache (south of France), is one of them. One of its aims is to produce, with the help of the D+T nuclear reaction, 500 MW by injecting 50 MW. The D+T nuclear reaction has been chosen because it has the highest cross section in the relevant domain of energy (one to two orders of magnitudes higher than the D+D reactions). The D+T reaction will produce 14‐MeV neutrons and 3.52‐MeV He nuclei, which will maintain the temperature of the D+T reacting plasma to an equilibrium state. Magnetic field lines are however connected to the inner walls of the ITER torus, which leads to the existence of cold edge plasma (composed of He, D, T, impurities such as oxygen or eroded metals) interacting with them. The heat produced by the hot plasma is received mainly on a part of the wall called a *divertor*. The material that will compose this

Then, in tokamaks, understanding and being able to predict the evolution of the plasma‐fac‐ ing 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 [2]. The mate‐ rial chosen for the *divertor* is tungsten as it drives well the heat to the underlying cooling loops present in the PFCs, it is difficult to erode, and its melting/fusion temperatures are very high, and hence creating bonding with hydrogen isotope is supposed to be difficult. The other parts of the PFCs are composed of beryllium [3]. Some tokamaks are already working nowadays in an ITER‐like wall (ILW) configuration to prepare the ITER project, such as the Joint European Torus/JET tokamak, with some PFCs composed of tungsten‐coated carbon tiles [4–6], car‐ bon tiles, mounted on *limiters* and/or *neutralizers*, being used in the ancient carbon area of tokamaks, now abandoned, because of tritium retention (tritium being radioactive and being easily trapped in carbon) [7, 8]. In that framework, the surface composition and morphology modifications under operation will lead to changes in the material properties that can address safety issues. These changes will have to be measured and then understood. In more details, the migration [9] and/or melting of elements in the machine [10, 11], the production of dust, the hydrogen isotope retention [12, 13], the impurity contamination (traces of carbon and/or oxygen, nitrogen seeding [14], etc.) leading to formation of new mixed materials/phases (such as oxides or alloys) and the complex surface erosion will all influence the lifetime of these PFCs and their fuel retention properties. Surface characterization techniques are then neces‐ sary to measure the elemental changes in JET and in ITER PFCs erosion zones and deposits.

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 [1].

(International Thermonuclear Experimental Reactor), now in construction at

.

a hydride after ion implantation and that it can be released easily in tungsten oxide.

part will have to work with heat loads in the range 10–20 MW/m2

**1.1. Plasma‐wall interactions in tokamaks**

4 Raman Spectroscopy and Applications

reactor *ITER*<sup>1</sup>

1

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 [20]. 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 bond‐ ings 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 *hydrogen isotope supersaturated layer*, which sizes in the range of tens of nanometers, and is composed of few atomic percent to few tens of atomic percent of hydrogen isotopes [13]. Depending on the material, Raman microscopy can be a well‐suited technique adapted to that depth, as can be seen in **Figure 1**. In that figure, the transmittance coding the effectiveness of a material in transmitting incident light is plotted for *λ*L = 514 nm and for Be, W, C (graphite and amorphous form). The depth at which the transmitted energy reaches 10% of the initial incident energy is 30, 35, 70 and 310 nm for, respectively, Be, W, C\graphite and amorphous carbon. The last value depends on the local organization of the amorphous structure, as detailed in Refs. [21–23]. Note that as the Raman measurements are generally done in back scattering geometry, one as to take into account an additional factor 2 in the argument of the exponential, as mentioned in [21, 22].

**Figure 1.** Transmittance in function of the depth for Be, W, C.

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.
