**1. Introduction**

Structural materials for nuclear fusion applications will have to withstand a very hard environment in the future reactor. Very energetic neutrons which will produce displacement cascades, transmutation into light atoms, nuclear activation and nuclear heating will be produced during operation. These neutron reactions, along with stresses produced by the reactor weight itself, and cyclic loads due to thermal and electromagnetic stresses draw a very harsh panorama for these materials [1–4].

Neutrons from nuclear fusion reaction induce elastic and inelastic nuclear reactions. The very first atoms from the matrix displaced by the incident neutron are denominated Primary Knock-on Atoms (PKA) spectrum, and this PKA spectrum is generated by both elastic and inelastic reactions. Generally speaking, depending on the isotope considered, the larger contribution to the total displacement is due to the elastic reactions (90%). However, atoms initially displaced for their lattice site

by neutrons (PKA spectrum) will induce elastic reactions, which produce additional displacement damage (displacement cascades) because they eventually will produce large collision cascades, since the secondary atom impacted by the PKA has enough energy to move a third one and so on.

Therefore, this PKA spectrum will be responsible to deposit the displacement damage in the material. On the other hand, if the neutron produced an inelastic reaction, an induced transmutation by neutron collision will be produced, generating light atoms as helium and hydrogen until hundreds of atomic parts per million (appm) in the whole life service of the reactor along with PKAs depending on the energy deposited [5].

Regarding helium atoms, one of the main issues in terms of structural material degradation is the nucleation and growth of He bubbles at grain boundaries which would produce a reduction of service lifetime. This degradation comes due to helium atoms produced by transmutation reaction of Fe; around 4 MeV of neutron energy may produce the following reaction generating alpha particles 56Fe (n.a) 53Cr [6]. It is well known that the combination between helium atoms and vacancies is very energetically favorable, so it will form eventually He bubbles. For that reason a deeper understanding of the effect of those bubbles in the evolution of microstructure and further degradation of mechanical properties are critical. However, several parameters have to be controlled and studied to determine properly the evolution of He bubbles during irradiation such as temperature, He production rate, displacement rate, and dose (accumulation of He).

Candidate materials for being used as structural materials are reduced activation ferritic martensitic (RAFM) steels, since they show a high resistance to irradiation damage, higher thermal conductivity, good corrosion resistance, and good liquid metal compatibility than austenitic steels. Several studies have shown that grain boundaries or phase boundaries may also act as sinks for radiation-induced point defects and the cluster formed during irradiation [7].

On the other hand, collision cascades known as accumulation of atoms displaced will form a complex form of Frenkel pair defects such as interstitial clusters which may turn into large dislocation loops or vacancy-type voids. Those irradiationinduced defects act as barriers to the dislocation movement, so they produce a significant hardening and hence a ductility reduction.

Neutron irradiation is not the most used technique to irradiate materials since the nuclear activation of the specimens makes necessary to have available hot cells to characterize the samples. On the other hand, nowadays there is no facility to emulate neutron fusion environment; for that reason ion beam irradiations are used to emulate neutron irradiation, of course, having into consideration the main differences such as shallow depth of irradiated material, elevated dose rate, and, clearly, not transmutation.

Current neutron sources with an energy spectrum typical of fusion reactions (14 MeV neutrons) are far from the fluence expected in a fusion reactor, and they are only useful for a few cases involving functional materials not exposed to high radiation doses [8, 9]. Therefore, a common approximation to test fusion materials consists of using ion irradiation to emulate the effects of neutron irradiation [10]. Ion irradiation can yield higher damage rates generating negligible activation levels in the irradiated samples at a reduced cost than other approximations to the problem like fission reactor irradiations. These elevated dose rates are very interesting to obtain samples submitted to an accelerated damage aging that would take several years to be achieved by fission irradiation. It is necessary to consider, however, that these accelerated tests may be driven by aging mechanisms very different from the real processes taking place under the low damage rate produced by real fusion conditions.

**17**

evolution.

**2. Accelerator description**

*Ion Beam Experiments to Emulate Nuclear Fusion Environment on Structural Materials…*

thicknesses in order to achieve different mitigations of the beam energy [11].

As stated before, to date, most of the studies on these structural materials have been focused on material behavior as a function of different parameters as irradiation dose, particle energy, and irradiation temperature, among others [12]. However, nextstep fusion devices, such as the International Thermonuclear Experimental Reactor (ITER) [13] and demonstration power plant (DEMO) [2, 14, 15], are magnetically confined devices, and the performance of the materials under reactor conditions when high magnetic fields are present is still unexplored. Besides, theoretical predictions suggest that magnetism can be a non-negligible factor in defining the defect properties induced by He irradiation or in determining the atomic distribution in FeCr alloys [16, 17]. Material microstructural properties can be modified by defect propagation due to irradiation. It is considered that such propagation may be affected by external magnetic fields, as recently pointed in Ref. [18]. For this reason, more detailed experimental knowledge of structural materials is sought, in particular with regard to mobility and clustering, as well as helium and hydrogen accumulation in reactor conditions. Thus, expanded experimental knowledge of structural material response to irradiation

In this chapter, the development of a new sample holder located at a vacuum chamber of standard (STD) irradiation line at CMAM is presented. Preliminary results have been obtained in this system for a series of FeCr alloy (10–15% Cr),

field produced by a permanent magnet (0.5 T) at STD irradiation line at CMAM. As sometimes, experiments are costly even with ion beams and are not always available; modeling is one of the key tools to predict long-term defect evolution. However, irradiation damage predictive simulation is still under development and needs many experimental results as inputs to validate their simulations. So, sometimes, irradiation experiments are headed not only to study the degradation of complex microstructure materials such as RAFM candidates but to obtain results of irradiating very simple and pure such as very high pure iron, iron chromium model alloys in order to study the effect of certain alloying elements on irradiation defects

Electrostatic accelerators over the world present a variety of shapes and sizes and can be classified attending to different factors. The fundamental features that characterize a DC accelerator are mainly three: the type of particle that can accelerate, the beam current and the maximum kinetic energy achievable. These defining

In this section, we describe the fundamentals of a 5 MV Tandetron delivered by high-voltage engineering (HVE) for ion beam analysis (IBA) and ion beam modification of materials (IBMM) at operation in CMAM in the Universidad Autónoma de Madrid (Spain) [19]. In **Figure 1** it is possible to observe (a) plan view of the

parameters determine the subsequent research field of application.

ions under the effect of the magnetic

Another important difference that must be considered is the low ion penetration in the material (typically several microns for heavy ion energy in the range of 1–20 MeV) compared to the tenths of centimeters that a neutron can travel before interacting with a material nucleus. Since the particle accelerators have a fixed energy, a selective filter or energy degrader system is required to obtain an almost uniform spectrum of beam energies to allow a particle implantation in the most controlled possible way. The thickness of the filter determines the final energy of the beam, so to obtain a certain energy spectrum, it has been thought of a multifilter revolver-type system, consisting of a design in the form of a rotating daisy, with peripheral aluminum foils of different

*DOI: http://dx.doi.org/10.5772/intechopen.87054*

under magnetic fields has become critical.

alloy specimens irradiated with 1 MeV Fe+

### *Ion Beam Experiments to Emulate Nuclear Fusion Environment on Structural Materials… DOI: http://dx.doi.org/10.5772/intechopen.87054*

Another important difference that must be considered is the low ion penetration in the material (typically several microns for heavy ion energy in the range of 1–20 MeV) compared to the tenths of centimeters that a neutron can travel before interacting with a material nucleus. Since the particle accelerators have a fixed energy, a selective filter or energy degrader system is required to obtain an almost uniform spectrum of beam energies to allow a particle implantation in the most controlled possible way. The thickness of the filter determines the final energy of the beam, so to obtain a certain energy spectrum, it has been thought of a multifilter revolver-type system, consisting of a design in the form of a rotating daisy, with peripheral aluminum foils of different thicknesses in order to achieve different mitigations of the beam energy [11].

As stated before, to date, most of the studies on these structural materials have been focused on material behavior as a function of different parameters as irradiation dose, particle energy, and irradiation temperature, among others [12]. However, nextstep fusion devices, such as the International Thermonuclear Experimental Reactor (ITER) [13] and demonstration power plant (DEMO) [2, 14, 15], are magnetically confined devices, and the performance of the materials under reactor conditions when high magnetic fields are present is still unexplored. Besides, theoretical predictions suggest that magnetism can be a non-negligible factor in defining the defect properties induced by He irradiation or in determining the atomic distribution in FeCr alloys [16, 17]. Material microstructural properties can be modified by defect propagation due to irradiation. It is considered that such propagation may be affected by external magnetic fields, as recently pointed in Ref. [18]. For this reason, more detailed experimental knowledge of structural materials is sought, in particular with regard to mobility and clustering, as well as helium and hydrogen accumulation in reactor conditions. Thus, expanded experimental knowledge of structural material response to irradiation under magnetic fields has become critical.

In this chapter, the development of a new sample holder located at a vacuum chamber of standard (STD) irradiation line at CMAM is presented. Preliminary results have been obtained in this system for a series of FeCr alloy (10–15% Cr), alloy specimens irradiated with 1 MeV Fe+ ions under the effect of the magnetic field produced by a permanent magnet (0.5 T) at STD irradiation line at CMAM.

As sometimes, experiments are costly even with ion beams and are not always available; modeling is one of the key tools to predict long-term defect evolution. However, irradiation damage predictive simulation is still under development and needs many experimental results as inputs to validate their simulations. So, sometimes, irradiation experiments are headed not only to study the degradation of complex microstructure materials such as RAFM candidates but to obtain results of irradiating very simple and pure such as very high pure iron, iron chromium model alloys in order to study the effect of certain alloying elements on irradiation defects evolution.

Electrostatic accelerators over the world present a variety of shapes and sizes and can be classified attending to different factors. The fundamental features that characterize a DC accelerator are mainly three: the type of particle that can accelerate, the beam current and the maximum kinetic energy achievable. These defining parameters determine the subsequent research field of application.
