*3.2.1 Defocused beam*

For this research, a defocused beam was used to implant He into the RAFM steel. This kind of beam configuration has a Gaussian-like profile in terms of ion current with its consequent reduction in density. To establish a valid methodology which allow repetitive and successful experiments, it is necessary to fix some experimental parameters such as ion current, the size of the beam and integration time of the camera that acquires the images from the sample camera because the higher the integration time is, the brighter the image is and the beam size measurements can be wrongly calculated and hence the implanted dose. In order to diminish the numbers of variables to study, irradiation temperature was fixed as room temperature. Hence, the relationship between ion energy, implanted dose, and steel microstructure was studied in terms of radiation defects observed afterward in transmission electron microscopy (TEM).

Regarding the sample holder, as observed in **Figure 8a**, STD line offers a very high flexibility in terms of geometry and size of the samples. Two different squared specimens of steel were attached to the holder along with a wire that was placed underneath to measure the ion current properly and with a current integrator, the implanted dose was calculated accurately. The characteristics of the He ion beam were evaluated by means of ionoluminescence on fused silica, which was used to set up the beam properly: size and stability of the beam prior to irradiation (**Figure 8b**).

### **Figure 8.**

*(a) Irradiation sample holder for implantation beamline. (b) Helium beam irradiating quartz used to measure the quality and size of the beam.*

**29**

**Figure 9.**

*simulation. Published in [36].*

identify.

*3.2.2 Raster beam*

the whole area.

only 15–20°C maximum.

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

Before irradiation, simulation code MARLOWE [34, 35] was used to determine the maximum irradiation depth (obtained with 15 MeV in this experiment) and the resultant concentration of He for each energy. It is well known that

ion energy, the peak experiences a broadening, therefore the concentration is the same (integrating the area under the peaks) but the maximum value decreased. As ion implantation depth is quite shallow in comparison to nuclear irradiation, a stair-like profile of He concentration was used, starting for 15 He MeV and ending with 2 MeV, decreasing the energy in steps of 1 MeV. After implantation, samples were cut and polished through its cross section and were etched in diluted Marble as described before. The etching revealed some lines which represented each and one of the He implantation peaks. Afterwards, the microstructure was observed with a scanning electron microscope (SEM) in order to measure the depth and eventually compare with the simulated position of the stopping peaks for all the implanted energies. The result was that the simulation (**Figure 9a**) and the experiment (**Figure 9b**) matched completely [36]. In addition, it is possible to observe a clear difference with **Figure 5**, where the lines are more diffuse because of the use of a degrader; however, in this experiment, the irradiation peaks are very clear to

The other possibility to perform He irradiation by means of ion beams is using a raster beam. In this case, the beam swept a certain area with a very high frequency. **Figure 10** showed a fused silica emitting light because of the He beam. On the image on the left, the beam swept only the x-axis, the image of the center swept only the y-axis, and on the image on the right, the beam scanned both axes covering

Regarding beam heating, as the beam is more focused than the over-focused beam, it is possible that the material experiences a heating which is critical to be

*(a) Depth and helium ion concentration profiles as obtained using MARLOWE code. b) SEM micrograph of EUROFER97 steel implanted with He ions from 15 to 2 MeV, showing the ion stopping region matches with* 

A thermographic camera was used to control the aforementioned heating, using as reference a small sample of fused silica because its emissivity and its dependence with temperature are very well known. In **Figure 11**, images taken from the camera are shown after some minutes of irradiation. No important heating was measured,

measured since the irradiation defects are very temperature sensitive.

) when increasing the

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

although the fluence is the same (1.65 × 1015 He ions/cm2

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

Before irradiation, simulation code MARLOWE [34, 35] was used to determine the maximum irradiation depth (obtained with 15 MeV in this experiment) and the resultant concentration of He for each energy. It is well known that although the fluence is the same (1.65 × 1015 He ions/cm2 ) when increasing the ion energy, the peak experiences a broadening, therefore the concentration is the same (integrating the area under the peaks) but the maximum value decreased.

As ion implantation depth is quite shallow in comparison to nuclear irradiation, a stair-like profile of He concentration was used, starting for 15 He MeV and ending with 2 MeV, decreasing the energy in steps of 1 MeV. After implantation, samples were cut and polished through its cross section and were etched in diluted Marble as described before. The etching revealed some lines which represented each and one of the He implantation peaks. Afterwards, the microstructure was observed with a scanning electron microscope (SEM) in order to measure the depth and eventually compare with the simulated position of the stopping peaks for all the implanted energies. The result was that the simulation (**Figure 9a**) and the experiment (**Figure 9b**) matched completely [36]. In addition, it is possible to observe a clear difference with **Figure 5**, where the lines are more diffuse because of the use of a degrader; however, in this experiment, the irradiation peaks are very clear to identify.

### *3.2.2 Raster beam*

*Ion Beam Techniques and Applications*

als damage.

**3.2 He implantation**

*3.2.1 Defocused beam*

microscopy (TEM).

intense magnetic fields are required for plasma confinement. Experiments with higher B and higher sample temperature are currently in progress in order to elucidate if external magnetic fields are a key parameter in the structural materi-

Acquiring knowledge concerning fusion transmutation product (He and H) effects on structural materials is difficult to study because of the lack of proper facilities. Using an ion beam accelerator, nuclear fusion-related amount of He was incorporated into the structural material, EUROFER97, using two different ways in terms of beam configuration—defocused beam and raster beam. This is a critical matter to consider because different defect evolution has been detected depending on which method has been taken into consideration to perform the irradiations [32, 33].

For this research, a defocused beam was used to implant He into the RAFM steel. This kind of beam configuration has a Gaussian-like profile in terms of ion current with its consequent reduction in density. To establish a valid methodology which allow repetitive and successful experiments, it is necessary to fix some experimental parameters such as ion current, the size of the beam and integration time of the camera that acquires the images from the sample camera because the higher the integration time is, the brighter the image is and the beam size measurements can be wrongly calculated and hence the implanted dose. In order to diminish the numbers of variables to study, irradiation temperature was fixed as room temperature. Hence, the relationship between ion energy, implanted dose, and steel microstructure was studied in terms of radiation defects observed afterward in transmission electron

Regarding the sample holder, as observed in **Figure 8a**, STD line offers a very high flexibility in terms of geometry and size of the samples. Two different squared specimens of steel were attached to the holder along with a wire that was placed underneath to measure the ion current properly and with a current integrator, the implanted dose was calculated accurately. The characteristics of the He ion beam were evaluated by means of ionoluminescence on fused silica, which was used to set up the

beam properly: size and stability of the beam prior to irradiation (**Figure 8b**).

*(a) Irradiation sample holder for implantation beamline. (b) Helium beam irradiating quartz used to measure* 

**28**

**Figure 8.**

*the quality and size of the beam.*

The other possibility to perform He irradiation by means of ion beams is using a raster beam. In this case, the beam swept a certain area with a very high frequency. **Figure 10** showed a fused silica emitting light because of the He beam. On the image on the left, the beam swept only the x-axis, the image of the center swept only the y-axis, and on the image on the right, the beam scanned both axes covering the whole area.

Regarding beam heating, as the beam is more focused than the over-focused beam, it is possible that the material experiences a heating which is critical to be measured since the irradiation defects are very temperature sensitive.

A thermographic camera was used to control the aforementioned heating, using as reference a small sample of fused silica because its emissivity and its dependence with temperature are very well known. In **Figure 11**, images taken from the camera are shown after some minutes of irradiation. No important heating was measured, only 15–20°C maximum.

### **Figure 9.**

*(a) Depth and helium ion concentration profiles as obtained using MARLOWE code. b) SEM micrograph of EUROFER97 steel implanted with He ions from 15 to 2 MeV, showing the ion stopping region matches with simulation. Published in [36].*

### **Figure 10.**

*Ionoluminescence produced by raster beam in different sweeping axes during beam setup. The following images were taken when beam was moving along a) x axis, b) y axis and c) x and y axes together.*

### **Figure 11.**

The sample holder belonging to this line is very flexible, so several specimens can be mounted as observed in **Figure 12a** On the other hand, **Figure 12b** was taken during irradiation. For raster beams, there is no need to measure the beam size before irradiation, since it was determined with the sweeping parameters. However, a way to observe if the beam experienced any kind of sparkling (or any other phenomena which can indicate malfunction) a piece of fused silica was placed along with the specimens in order to observe the beam during irradiation (**Figure 12b**) by means of ionoluminescence as it was done with the defocused beam experiment.

Once the experiment is ended, TEM studies were performed to characterize the defects produced on the steel because of He irradiation. In this case, some TEM discs were prepared by electropolishing, keeping the transparent area within the irradiated area. As mentioned in the introduction, He irradiation may produce

### **Figure 12.**

*(a) Sample holder in STD line for raster experiments. (b) Sample holder during irradiation with a circleshape beam.*

**31**

**Figure 13.**

*(poor contrast), and (c) under-focused (white bubbles).*

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

bubbles, and it is well known that a typical way to detect them is through focus serial method, changing the objective length focus distance. When the specimen is found under-focused, the bubbles are observed with white contrast, and on the other hand, when it is over-focused, the bubbles are dark. However, if there are any other microstructural characteristic as secondary phases, grain boundaries, or similar, their contrast remained grayscale-like. In **Figure 13**, three micrographs are presented showing EUROFER97 microstructure with large bubbles within the microstructure (**Figure 13** (a) over-focused, (b) in-focus, and (c) under-focused). It has been demonstrated that both beam configurations are valid to carry out He implantations. However, more experiments have to be conducted to determine

Neutron irradiation produces atomic cascades, as described above, in which the atoms from the matrix moved from their equilibrium positions, generating certain atomic disorder. This disorder stays reflected in dislocation defects as an accumulation of Frenkel pairs. In addition, neutrons also produced transmutation reactions not only generating He and H but radioactive isotopes, thus it would be necessary to keep and test the samples in hot cells. For that reason, a safer and more afforadable way to emulate defects produced by neutrons is to irradiate iron-based alloys such as steels, with Fe ions. Those ions, although they alter a very shallow layer of materials, do not produce transmutation nor modify the chemical composition of the samples so they are called self-ions. There are some examples of irradiations with larger atoms as Xe [37, 38] or Kr [39], although the objective is not the emulation of nuclear fusion environment, since

Unlike He irradiation, the effect of self-ion irradiation in the microstructure is hard to characterize since the dislocation loops are a very complex features to observe properly and it needs long time and effort, along with a great knowledge of TEM (microscope operation, exquisite sample preparation, and insight of on the theory on irradiation defect generation [40, 41]). In addition, in this field there is a huge gap between simulation models and experiments headed to validate such simulations within the frame of nuclear fusion, so it is an opportunity of irradiating simple alloys which are the base of the complex alloys (i. e. EUROFER97, F82H, ODS steels...). Regarding He bubbles, there is relatively large literature about modeling bubbles in actual steels [42–48], and due to this, the irradiations headed to the understanding of He bubble nucleation and growth carried in this matter are subjected to steels instead

they produce a significant chemical change in the material composition.

of more simple alloys. As in the case of Fe implantation that pure iron is used.

*TEM micrographs showing He bubbles within steel matrix in (a) over-focused (bubbles in black), (b) in-focus* 

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

which one is closer to nuclear fusion environment.

**3.3 Fe implantation**

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

bubbles, and it is well known that a typical way to detect them is through focus serial method, changing the objective length focus distance. When the specimen is found under-focused, the bubbles are observed with white contrast, and on the other hand, when it is over-focused, the bubbles are dark. However, if there are any other microstructural characteristic as secondary phases, grain boundaries, or similar, their contrast remained grayscale-like. In **Figure 13**, three micrographs are presented showing EUROFER97 microstructure with large bubbles within the microstructure (**Figure 13** (a) over-focused, (b) in-focus, and (c) under-focused).

It has been demonstrated that both beam configurations are valid to carry out He implantations. However, more experiments have to be conducted to determine which one is closer to nuclear fusion environment.

### **3.3 Fe implantation**

*Ion Beam Techniques and Applications*

The sample holder belonging to this line is very flexible, so several specimens can be mounted as observed in **Figure 12a** On the other hand, **Figure 12b** was taken during irradiation. For raster beams, there is no need to measure the beam size before irradiation, since it was determined with the sweeping parameters. However, a way to observe if the beam experienced any kind of sparkling (or any other phenomena which can indicate malfunction) a piece of fused silica was placed along with the specimens in order to observe the beam during irradiation (**Figure 12b**) by means of ionoluminescence as it was done with the defocused

*Ionoluminescence produced by raster beam in different sweeping axes during beam setup. The following images* 

*were taken when beam was moving along a) x axis, b) y axis and c) x and y axes together.*

*Thermographic camera images during x-axis (a), y-axis (b), and both axes simultaneously (c) sweeping.*

Once the experiment is ended, TEM studies were performed to characterize the defects produced on the steel because of He irradiation. In this case, some TEM discs were prepared by electropolishing, keeping the transparent area within the irradiated area. As mentioned in the introduction, He irradiation may produce

*(a) Sample holder in STD line for raster experiments. (b) Sample holder during irradiation with a circle-*

**30**

**Figure 12.**

*shape beam.*

beam experiment.

**Figure 11.**

**Figure 10.**

Neutron irradiation produces atomic cascades, as described above, in which the atoms from the matrix moved from their equilibrium positions, generating certain atomic disorder. This disorder stays reflected in dislocation defects as an accumulation of Frenkel pairs. In addition, neutrons also produced transmutation reactions not only generating He and H but radioactive isotopes, thus it would be necessary to keep and test the samples in hot cells. For that reason, a safer and more afforadable way to emulate defects produced by neutrons is to irradiate iron-based alloys such as steels, with Fe ions. Those ions, although they alter a very shallow layer of materials, do not produce transmutation nor modify the chemical composition of the samples so they are called self-ions. There are some examples of irradiations with larger atoms as Xe [37, 38] or Kr [39], although the objective is not the emulation of nuclear fusion environment, since they produce a significant chemical change in the material composition.

Unlike He irradiation, the effect of self-ion irradiation in the microstructure is hard to characterize since the dislocation loops are a very complex features to observe properly and it needs long time and effort, along with a great knowledge of TEM (microscope operation, exquisite sample preparation, and insight of on the theory on irradiation defect generation [40, 41]). In addition, in this field there is a huge gap between simulation models and experiments headed to validate such simulations within the frame of nuclear fusion, so it is an opportunity of irradiating simple alloys which are the base of the complex alloys (i. e. EUROFER97, F82H, ODS steels...). Regarding He bubbles, there is relatively large literature about modeling bubbles in actual steels [42–48], and due to this, the irradiations headed to the understanding of He bubble nucleation and growth carried in this matter are subjected to steels instead of more simple alloys. As in the case of Fe implantation that pure iron is used.

### **Figure 13.**

*TEM micrographs showing He bubbles within steel matrix in (a) over-focused (bubbles in black), (b) in-focus (poor contrast), and (c) under-focused (white bubbles).*

For these experiments, the sample holder used was the motorized one which allows high temperature heating as shown in **Figure 14**, This study required a wide range of irradiation temperatures because dislocation loops are very temperature sensitive. The type of beam used was defocused, since the specimens were small (TEM discs), and it was not required to move the beam to cover all the area of interest.

The main goal of this research was to determine if there is a difference in the developing of microstructural irradiation defects because of the temperature and the specimen thickness. The energy used was 20 MeV, and the temperature was 300 and 450°C up to a dose of 5 dpa at the irradiation peak. Several irradiations took place, in order to study two thin foils and two discs (bulk samples) at the two aforementioned temperatures. In **Figure 15**, the damage profile obtained with SRIM is shown. The red curve represented the whole damage peak produced in the bulk samples whose thickness was around 100 μm. On the other hand, the damage generated in the thin foils with a thickness approximately of 100–150 nm fabricated by electropolishing, as the regular TEM disc preparation, is showed with the blue curve, because the ions pass through the thin films. For that reason in bulk specimens, the damage reached the maximum, 5 dpa. However, in thin films the damage is much lower (0.1 dpa), although the ion energy was the same.

Once the irradiations were finished, the samples were studied by TEM. Dislocation loops were found in all the specimens (both bulk and thin foils), but size and distribution were completely different between bulk and thin-film experiments. In the first one, although the damage was much higher, the maximum loop observed was 22 nm, and the distribution was quite heterogeneous, being maximum at the damage peak depth (**Figure 16a** and **b**). Nevertheless, in thin films, in spite of the small amount of thickness and the small amount of damage deposited by self-ions, very large loops were detected, even larger than 500–600 nm distributed homogeneously within the material (**Figure 16c** and **d**). In addition, differences between Burger vectors and population density have been found. Deep characterization is being carried out, but those preliminary results proved that the configuration (accelerator device, irradiation parameters, and sample holder) used in CMAM facility provides the tools required to perform high quality experiments whose results will be of a great support for modeling scientists.

### **Figure 14.**

*(a) Motorized sample holder for Fe ion irradiations. (b) Light produced by ionoluminescence because of the irradiation of Fe ions onto MACOR piece.*

**33**

**4. Conclusions**

**Figure 16.**

**Figure 15.**

as DONES [5].

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

*SRIM profile of bulk irradiation (red curve) and thin-film irradiation (blue curve).*

The importance of ion beam accelerators to perform experiments which gain insight with about the possible synergies between radiation damage, microstructure, strain, and magnetic fields regarding degradation of structural materials for nuclear fusion applications has been presented in this chapter. It is well known that there is a gap between neutron irradiation and ion irradiation, but it is still a very important source of knowledge until the scientific community has the possibility of using a facility which emulates the nuclear fusion environment

*Dislocation loops found in pure Fe in different experiments in bulk experiments at 5 dpa and (a) 350°C and* 

CIEMAT has been carrying out for several years numerous experiments in this field generating vast knowledge about irradiation effects on structural materials,

with the help of CMAM facility and its researchers and staff.

*(b) 450°C and in thin foils at 0.1 dpa at (c) 350°C and (d) 450°C.*

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

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

**Figure 15.** *SRIM profile of bulk irradiation (red curve) and thin-film irradiation (blue curve).*

### **Figure 16.**

*Ion Beam Techniques and Applications*

energy was the same.

support for modeling scientists.

*irradiation of Fe ions onto MACOR piece.*

For these experiments, the sample holder used was the motorized one which allows high temperature heating as shown in **Figure 14**, This study required a wide range of irradiation temperatures because dislocation loops are very temperature sensitive. The type of beam used was defocused, since the specimens were small (TEM discs), and it

The main goal of this research was to determine if there is a difference in the developing of microstructural irradiation defects because of the temperature and the specimen thickness. The energy used was 20 MeV, and the temperature was 300 and 450°C up to a dose of 5 dpa at the irradiation peak. Several irradiations took place, in order to study two thin foils and two discs (bulk samples) at the two aforementioned temperatures. In **Figure 15**, the damage profile obtained with SRIM is shown. The red curve represented the whole damage peak produced in the bulk samples whose thickness was around 100 μm. On the other hand, the damage generated in the thin foils with a thickness approximately of 100–150 nm fabricated by electropolishing, as the regular TEM disc preparation, is showed with the blue curve, because the ions pass through the thin films. For that reason in bulk specimens, the damage reached the maximum, 5 dpa. However, in thin films the damage is much lower (0.1 dpa), although the ion

was not required to move the beam to cover all the area of interest.

Once the irradiations were finished, the samples were studied by TEM. Dislocation loops were found in all the specimens (both bulk and thin foils), but size and distribution were completely different between bulk and thin-film experiments. In the first one, although the damage was much higher, the maximum loop observed was 22 nm, and the distribution was quite heterogeneous, being maximum at the damage peak depth (**Figure 16a** and **b**). Nevertheless, in thin films, in spite of the small amount of thickness and the small amount of damage deposited by self-ions, very large loops were detected, even larger than 500–600 nm distributed homogeneously within the material (**Figure 16c** and **d**). In addition, differences between Burger vectors and population density have been found. Deep characterization is being carried out, but those preliminary results proved that the configuration (accelerator device, irradiation parameters, and sample holder) used in CMAM facility provides the tools required to perform high quality experiments whose results will be of a great

*(a) Motorized sample holder for Fe ion irradiations. (b) Light produced by ionoluminescence because of the* 

**32**

**Figure 14.**

*Dislocation loops found in pure Fe in different experiments in bulk experiments at 5 dpa and (a) 350°C and (b) 450°C and in thin foils at 0.1 dpa at (c) 350°C and (d) 450°C.*

## **4. Conclusions**

The importance of ion beam accelerators to perform experiments which gain insight with about the possible synergies between radiation damage, microstructure, strain, and magnetic fields regarding degradation of structural materials for nuclear fusion applications has been presented in this chapter. It is well known that there is a gap between neutron irradiation and ion irradiation, but it is still a very important source of knowledge until the scientific community has the possibility of using a facility which emulates the nuclear fusion environment as DONES [5].

CIEMAT has been carrying out for several years numerous experiments in this field generating vast knowledge about irradiation effects on structural materials, with the help of CMAM facility and its researchers and staff.

Therefore, it has been demonstrated that ion beam accelerators are a fundamental tool to the developing of the future nuclear fusion reactors.
