# Grain orientation and temperature dependences of bubbles at tungsten surfaces upon helium plasma exposure

Mounir Alfazzaa, Federica Pappalardo, Andrea Campos, Gilles Cartry, Marco Minissale, Céline Martin

► To cite this version:

Mounir Alfazzaa, Federica Pappalardo, Andrea Campos, Gilles Cartry, Marco Minissale, et al. Grain orientation and temperature dependences of bubbles at tungsten surfaces upon helium plasma exposure. Nuclear Materials and Energy, 2025, 10.1016/j.nme.2025.101883. hal-04932310

HAL Id: hal-04932310

https://hal.science/hal-04932310v1

Submitted on 6 Feb 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Grain orientation and temperature dependences of bubbles at tungsten surfaces upon helium plasma exposure

Mounir Alfazzaa<sup>a,†</sup>,*, Federica Pappalardo<sup>a</sup>, Andrea Campos<sup>b</sup>, Gilles Cartry<sup>a,*</sup>, Marco Minissale<sup>a,†</sup>, Céline Martin<sup>a,*</sup>

## ARTICLE INFO

Keywords:

Tungsten

Helium

Bubbles

SEM

Grain orientation

Temperature

## ABSTRACT

The impact of grain orientation and surface temperature on helium bubble growth at tungsten surfaces exposed to low-pressure helium plasmas was investigated by analyzing the nanoholes formed at the surface. Polycrystalline tungsten samples were exposed to a 2 Pa RF plasma at 400 W and 79 eV positive ion bombardment energy. Through detailed SEM imaging before and after plasma exposure and a developed image analysis protocol, we assessed quantitatively the size and density of helium bubbles that have burst on the surface upon plasma exposure. The results demonstrate a clear temperature dependence, with surface bubble density and size increasing at higher temperatures, and a dependence on grain orientation, with the (111) orientation giving slightly larger and denser surfaces of burst bubbles.

## 1. Introduction

Nuclear fusion, a carbon-free energy source, is planned to be tested on the ITER tokamak. The experiment will focus on burning plasmas created by the fusion reaction between hydrogen isotopes, deuterium and tritium, producing helium nuclei and neutrons. At the strikeline the tungsten divertor will be exposed to a continuous particle flux of around $10^{24}$ particles m$^{-2}$ s$^{-1}$ [1,2] consisting of helium and hydrogen isotopes. The interaction of these (mainly ionized) particles with the divertor can induce changes in the surface morphology, thus modifying the properties of the irradiated material. Tungsten, with its body-centered cubic crystalline structure, is known to accumulate helium atoms in interstitial sites, forming helium clusters. When the number of atoms in the cluster exceeds a temperature-dependent threshold, the accumulated stress in the lattice becomes sufficient to displace a tungsten atom from its original lattice position to an interstitial site. This displacement creates a vacancy and a self-interstitial tungsten atom, together forming a Frenkel pair. At this point, the cluster will continue to grow in size by absorbing smaller clusters of helium, acting as a nucleus for bubble formation [3]. Thus, helium accumulation can lead to bubble growth near the free surface, where over-pressurized bubbles burst to form surface nanoholes [4–7]. This can increase the roughness of the material and promote the formation of nanostructured fuzz layers [8,9], which are undesirable due to their potential to generate tungsten dust. In addition, the formation of helium bubbles affects not only the

surface morphology of the irradiated material but also the retention of hydrogen isotopes in tungsten [10,11], a potential major concern for ITER operation. It has been well explored in the plasma-material interactions community that ion bombardment significantly impacts polycrystalline samples, with the extent of this effect depending on various physical parameters. For instance, in the extensively studied case of $D_2$ blister formation, these parameters encompass the sample temperature [12], the flux and fluence of the implanted deuterium [13, 14] as well as intrinsic and extrinsic characteristics of the selected sample, such as grain orientation and defect concentration [15]. The number of studies focusing on helium is less, highlighting the need for more detailed studies with He plasmas. Just to mention a few, the work of Ialovega et al. [16], shows that round helium bubbles became faceted during post-implantation thermal annealing. They observed that the shape evolved from spherical to square-faceted in the case of holes formed on grains close to the (100) surface orientation, while no significant changes were found for the holes on grains close to the (111) grain orientation. Notably, Ohno et al. [17] and Sakamoto et al. [18] found that grains close to the (101) orientation tend to develop undulating structures. It has to be noted that the relationship between the crystallographic parameters and the size and shape of the nanoholes (i.e., burst helium bubbles), is not straightforward and may be influenced by factors such as implantation depth, helium diffusion, bubble growth, and surface burst strength. Hence, due to the

<footnote><sup>a</sup> Aix Marseille Univ, CNRS, PIIM, Marseille, France</footnote>

<footnote><sup>b</sup> Aix Marseille Univ, CNRS, Centrale Med, FSCM, CP2M, Marseille, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: mounir.alfazzaa@cnrs-orleans.fr (M. Alfazzaa), gilles.cartry@univ-amu.fr (G. Cartry), marco.minissale@univ-amu.fr (M. Minissale), celine.martin@univ-amu.fr (C. Martin).</footnote>
<figure><img src="image_3.png" /><figcaption>Fig. 1. SEM image in secondary electron mode of three adjacent grains of different orientations showing the nanoholes formed after He plasma exposure at a pressure of 2 Pa, RF power of 400 W, surface temperature of 550 °C, ion flux of 2 × 10¹⁹ particles m⁻² s⁻¹, fluence of 4.5 × 10²³ m⁻² and maximum incident ion energy of 79 eV.</figcaption></figure>

complexity of the problem and the many parameters that influence the formation and bursting of bubbles on the surface of tungsten, isolating all the processes involved is a major scientific challenge. In a laboratory plasma, the fluxes are much lower than in a fusion device, but the control on the experimental parameters (surface temperature, ion energy...) is much greater. Therefore, in this study, we use detailed SEM imaging to quantitatively investigate helium nanoholes formed on tungsten surfaces under low-pressure laboratory plasma exposure, with a focus on the effects of grain orientation and surface temperature. As an example, Fig. 1 shows an SEM image of a tungsten surface after plasma irradiation. Qualitatively, a (101)-oriented grain can be seen where the density of nanoholes is clearly different from that of the other two grains, both of which follow a (111) orientation. This observation prompted a comprehensive statistical analysis to examine the distribution of holes on the surface.

We present the investigation of the evolution of the size distribution of nanoholes left by helium bubbles as a function of surface temperature and its dependence on the crystallographic orientation of the grains. In Section 2, we present the sample preparation and characterization protocols detailing the experimental conditions for implantation and the image analysis procedure we designed to examine and quantify the SEM images obtained after helium plasma exposure. In Section 3, we discuss the main results and observations. In Section 4, we draw the main conclusions and perspectives.

## 2. Experimental methods

### 2.1. Sample preparation and pre-characterization

Experiments were performed using recrystallized polycrystalline tungsten samples from A.L.M.T. Corp. (Japan) with 99.99 wt% purity and with dimensions of 10 × 10 × 0.5 mm³. Prior to plasma exposure, samples were electro-polished in order to have a mirror-like surface. Then, the samples were cleaned using ultrasonic baths, followed by outgassing and annealing for 2 h under a vacuum of 1 × 10⁻⁷ mbar at a temperature of 800 °C. The surface roughness estimated using Atomic Force Microscopy (AFM NT-MDT), is reduced from $S_q = 10.24$ nm to $S_q = 6.21$ nm after electro-polishing. Moreover, this protocol allows for a proper observation of the crystallographic grains.

Before exposing the samples to a helium plasma, they were examined using Scanning Electron Microscopy (Zeiss Gemini 500 FE-SEM,

<figure><img src="image_4.png" /><figcaption>Fig. 2. (a) Example of a SEM image of a crystallographic grain; (b) Typical EBSD mapping revealing several clear grain orientations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

Electron-Backscattered Diffraction (EBSD-EDAX), and an in-situ Spectroscopic Ellipsometer (iSE, J.A. Woollam). Fig. 2 shows an example of an SEM image and EBSD maps taken after sample preparation and before plasma exposure. The SEM picture 2(a) presents in its center a (111) grain that was identified thanks to the EBSD measurement. The picture 2(b) is an example of an EBSD mapping. Grain orientations can be clearly distinguished according to the color scale. On average, considering all the samples used in this study, EBSD mappings reveal that the textures of the samples are predominantly (111)-oriented. Some grains exhibit slight variations in color, possibly indicating the presence of sub-grains with slightly different (hkl) parameters and minor disorientation, as shown in Fig. 2(b) (evidenced by small areas of different color orientations). Other grains display a uniform, distinct color, corresponding to a unique grain orientation. In this work, we focus exclusively on the latter, i.e., grains with a unique crystalline orientation.

### 2.2. Sample implantation

Samples were exposed to a helium plasma in the diffusion chamber of an Inductively Coupled Plasma (ICP) reactor. Briefly, this reactor consists of a top cylindrical plasma source (100 mm long, 150 mm diameter) and a bottom spherical diffusion chamber (200 mm diameter) separated by a grounded mesh (the mesh reduces plasma potential
fluctuations and lowers electron temperature in the diffusion chamber compared to the without-grid configuration). The plasma is created using a Huttinger PFG 1600 RF generator coupled to a Huttinger PFM 3000 attached to one end of a 3-turn coil on top of a dielectric ceramic window (150 mm diameter). On the other end, the coil is connected to a variable capacitance to set the voltage value at the coil center to 0, and further reduces the capacitive coupling in the ICP discharge, responsible for plasma potential fluctuations. The plasma parameters were measured using an RF-compensated Impedans Langmuir probe placed in the diffusion chamber close to the sample holder. The sample holder sits in the middle of the diffusion chamber, 100 mm below the mesh grid. The sample can be biased. Its temperature can be measured thanks to a thermocouple attached to the holder and controlled with the help of a PID controller and a heating filament placed at the back of the sample. A sketch of the experimental arrangement and the sample holder design can be found in [19,20]. Three samples were separately subjected to the inductively coupled helium plasma at exposure temperatures, relevant for edge plasma loading divertor in tokamaks, set at 400 °C, 550 °C and 700 °C which were selected to remain below the threshold for fuzz formation [18]. With the sample holder biased at -75 V and the plasma potential assessed at 4 V, the resulting maximum energy of the incident helium ions was 79 eV, which is below the energy sputtering threshold (~120 eV) [21,22]. The low plasma potential fluctuation level in the diffusion chamber and the low pressure employed ensure a limited spread of the ion energy impacting the sample and keep the energy below the threshold required for causing displacement damage and sputtering in W when impacted by He [23]. The incident flux measured by the Langmuir double probe was close to $2 \times 10^{19}$ He $m^{-2}$ s$^{-1}$ and the total incident fluence was $4.5 \times 10^{23}$ He $m^{-2}$ achieved after an exposure time of 7 h and 40 min and is below the fuzz formation fluence threshold (~$4.5 \times 10^{24}$ m$^{-2}$) [24,25].

## 2.3. Data analysis procedure for bubble characterization

After the exposure, high-resolution SEM images of the sample surfaces were obtained. The In Lens secondary electron detector and a 1 kV incident electron beam were used to display the detailed surface structure of the samples, with magnification between 80 and 100K, and most of the pixel sizes and image areas being 700 pm and 1.53 µm², respectively. Subsequently, an image analysis protocol was devised and applied to a wide range of SEM images using the ImageJ software. The aim was to develop a robust method for analyzing hole size distributions to identify evolution trends based on various parameters in order to provide input to models describing bubble growth. The designed algorithm, illustrated in Fig. 3 displaying each step on an image cut over one-quarter of its surface, consists of the following steps:

1. Apply on the as-acquired image (3(a)) an FFT bandpass filter with limits at 3 and 30 nm. This filter eliminates background noise corresponding to structures smaller than 4 pixels (corresponding to 3 nm) and structures larger than 30 nm, as the bubbles are smaller than this upper limit. We note that the brightness and contrast of the image get automatically adjusted during this step (3(b)).

2. Choose an appropriate threshold defined by a specific gray-level value. This threshold creates a mask that retains only pixels with gray-level values at or below the defined threshold (3(c)). The correct threshold is determined manually by measuring several hole diameters and comparing them to the automatic measurement with the chosen threshold.

3. Apply the watershed function to separate groups of merged bubbles that have been counted as a single one in the SEM images (3(d)). This function basically identifies the center of each particle, and then creates a distance map from these centers

<figure><img src="image_5.png" /><figcaption>Fig. 3. (a) A raw SEM image displaying holes at a tungsten surface; (b) The image post-application of an FFT bandpass filter (Including the adjustment of brightness and contrast); (c) The red mask obtained by the thresholding; (d) The resulting segmented image used for the particle analysis. The inset displays the application of the Watershed function to a bubble, providing a closer view obtained by zooming in on the image. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

to the particle edges. Filling this map with imaginary water forms watersheds. Where watersheds meet, dams are built to separate them, effectively segmenting the objects.

4. Produce an image with the segmented particles for the Analyze particles function of the ImageJ software. The Analyze particles function determines and extracts various shape parameters from the segmented particles, such as area, major radius, minor radius, and aspect ratio. In our analysis, the diameter of each hole was determined from their area by approximating the shape as circular. Subsequently, we exclude all structures with a diameter less than 2 nm and an aspect ratio greater than 3, as these are likely to be surface defects rather than He nanoholes.

To determine the mean hole diameter ($x_c$ in nm) and the integrated hole density ($A$ in $\mu m^{-2}$) for a given experimental condition, histograms of hole density against their diameter were plotted and fitted using a Gaussian Function, as illustrated in Fig. 4. It is important to note that these distributions and the results of their fits are influenced by the choice of the threshold in the image analysis process. The choice of threshold is a critical step, and its variation may lead to either an over-estimation or an underestimation of $A$ and $x_c$. This error was therefore assessed using two additional threshold values, adjusted by $\pm 10$ (over 256) gray levels. Subsequently, the relative deviations of the measured mean hole size and bubble density – determined from the maximum and area of the Gaussian functions fitted to the bubble distributions – were calculated at 15% and 20%, respectively as can be seen in Fig. 4. To avoid deviations from thresholding, the same threshold value was applied to all images taken during the same SEM session with the same SEM acquisition parameters. However, it was considered important to estimate these deviations for comparisons with measurements taken under different experimental conditions (e.g., different microscopes, samples, and exposure conditions) using the same image processing protocol.

# 3. Results and discussion

## 3.1. Grain orientation dependence

Following the established image processing protocol described in Section 2.3, we proceeded to analyze the images acquired from the
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Three bubble distributions, each fitted with a Gaussian function obtained by applying different gray-level thresholds.</figcaption></figure>

<table><caption>**Table 1**

Bubble distribution parameters of the (100)-oriented grains of the sample exposed at 550 °C with A, the hole density; $x_c$, the mean diameter and $\sigma$ the standard deviation.</caption><thead><tr><th>Orientation (100)</th><th>A (holes µm<sup>-2</sup>)</th><th>x<sub>c</sub> (nm)</th><th>σ (nm)</th></tr></thead><tbody><tr><td>Image 1</td><td>1153</td><td>5.8</td><td>2.7</td></tr><tr><td>Image 2</td><td>1740</td><td>6.1</td><td>2.8</td></tr><tr><td>Image 3</td><td>1672</td><td>6.2</td><td>3.1</td></tr><tr><td>Image 4</td><td>1998</td><td>6.1</td><td>2.8</td></tr><tr><td>Image 5</td><td>1944</td><td>6.8</td><td>3.2</td></tr><tr><td>Average</td><td>1701</td><td>6.2</td><td>2.9</td></tr><tr><td>Standard deviation</td><td>≈300 (17% from average)</td><td>0.6 (10%)</td><td>0.6</td></tr></tbody></table>

samples. We first started by checking the visual dependence on grain orientation observed in Fig. 1, by analyzing the displayed image. Despite limited statistics from processing a single image with reduced representative areas per grain, the hole size and density for both (111) grains showed less than 10% deviation from their mean values (with a 20% difference between the two grains). The mean density of burst bubbles on (111) grains is found to be approximately 64% higher than on the (101) grain, with the average hole size on (111) grains being about 40% larger than the one on the (101) grain, confirming the qualitative visual assessment. These partial results clearly show that the density and hole size can depend on the grain orientation, while no significant variation is observed for grains with the same orientation.

Afterward, we carried out a statistical analysis based on images of the sample exposed at 550 °C, selecting five images of (100) zones, intentionally including one zone of less well-defined grain orientation (showing slight variation of colors on the EBSD mapping). This approach was taken to account for slight deviations that may occur within the same grain or among different selected grains for different images, and by including grains of lower crystallographic quality to estimate the highest mistake that could occur when using only one image. The corresponding integrated hole densities and mean bubble sizes are summarized in Table 1.

The average diameter of all the grains is about 6.2 nm and the average hole density is found to be about 1701 holes µm⁻². The standard deviations obtained for (100) grains are quite low, about 17% for the hole density and 10% for the diameter, giving confidence in the method as long as several images are used and averaged to determine mean diameters and hole densities. All numbers presented in this paper have been obtained by averaging results from several images of the same crystalline orientation, except for the experimental conditions at 400 °C which lead to very small nanohole diameters and consequently to a challenging image analysis. Therefore, for this condition, only the best image has been chosen for analysis.

<table><caption>**Table 2**

Bubble distribution parameters for different sample temperatures and grain orientations: A is the hole density; $x_c$ is the mean diameter; $\sigma$ is the standard deviation.</caption><thead><tr><th>Temperature (°C)</th><th>Grain orientation</th><th>A (holes µm<sup>-2</sup>)</th><th>x<sub>c</sub> (nm)</th><th>σ (nm)</th></tr></thead><tbody><tr><td>400</td><td>100</td><td>933</td><td>3.25</td><td>2.8</td></tr><tr><td>550</td><td>100</td><td>1700</td><td>6.2</td><td>3.0</td></tr><tr><td>550</td><td>111</td><td>2115</td><td>7.0</td><td>2.4</td></tr><tr><td>700</td><td>100</td><td>3113</td><td>7.0</td><td>3.2</td></tr></tbody></table>

The influence of the crystalline orientation is assessed by comparing (100) and (111) grains at 550 °C. This work could not be conducted on grains with (101) orientation due to their texture, which included numerous sub-grains that were not well-resolved in the EBSD maps and appeared incompletely colored in Fig. 2. The hole density distributions are presented in Fig. 5(b) and (c). The hole density and diameter values are summarized in Table 2 for (100) and (111) grains. The average diameter is assessed to 7.0 nm and the average hole density to 2115 holes µm⁻² for the (111) orientation versus 6.2 nm and 1700 holes µm⁻² for the (100) orientation. Definitive conclusions will require more analysis to get higher statistics and to include other grain orientations but the differences observed appear significant enough to state that the hole density and the hole mean diameter are higher for the (111) orientation compared to the (100). A priori, the size of subsurface bubbles is not expected to vary with grain orientation, as helium atoms are incorporated into a three-dimensional lattice structure independently of the surface orientation. This was confirmed by a recent analysis of the kinetic growth of helium bubbles within W (100) and (111) single crystals showing that bubble sizes within the subsurface appear to be independent of the orientation [26]. However, we note that surface bursting and nanostructures might be dependent on crystallographic direction due to the energetic effect leading to the formation of a concentrated layer of helium bubbles below the surface as already shown theoretically by Hammond and Wirth [27]. Our experimental results indeed confirm that the crystalline orientation plays a role in surface bubble bursting.

## 3.2. Evolution with temperature

To study the evolution with temperature, the same analysis was repeated with (100)-oriented grains as the (111) grains exhibit at 700 °C nano-structures that complicate the image processing due to roughness contrasts that add to those of the nanoholes. As illustrated in the SEM image of Fig. 1, pointy shapes on (111)-oriented grains were identified while the (100) grains were smooth after exposure at 700 °C.

Three (100) grains from the sample exposed at a temperature of 700 °C were chosen and analyzed. The average distribution parameters are displayed in Table 2. As already mentioned, analyzing samples exposed at 400 °C was challenging due to the small diameter of the nanoholes.

Fig. 5 showcases three size distributions of (100) grains gathered from the He-exposed samples at the three chosen temperatures, with A and $x_c$ plotted as a function of the temperature in Fig. 6.

These results reveal that integrated density and hole diameter increase with higher exposure temperatures. The increase in hole density is directly proportional to temperature whereas the size is clearly increasing faster between 400 and 550 °C than between 550 °C and 700 °C. This result on hole density is in agreement with modeling [3] indicating that the He bubble inventory strongly depends on temperature. Although we do not have access to kinetics by conducting post-exposure studies, our results highlight the role of temperature in the growth of helium bubbles. As demonstrated in recent literature [26], the growth kinetics are dominated by temperature-activated mechanisms such as He and/or vacancy diffusion, as well as bubble migration and coalescence.
<figure><img src="image_7.png" /><figcaption>Fig. 5. Size distributions acquired from samples exposed at a temperature of: (a) 400 °C on a (100)-oriented grain; (b) 550 °C on a (100)-oriented grain; (c) 550 °C on a (111)-oriented grain; (d) 700 °C on a (100)-oriented grain.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. Size distribution parameters (hole density and mean diameter) evolution with temperature for the (100) grain orientation.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. Variation of the reflectivity (derived from the ellipsometric quantities, $Ψ$ and $Δ$) for the considered surface states over a wavelength range from 400 to 1000 nm.</figcaption></figure>

### 3.3. Ellipsometry measurements

In the previous sections, we studied how morphology evolves at the submicrometric scale. This morphological evolution should be reflected at a larger spatial scale with a change in the optical properties of the material. To validate this hypothesis, we performed optical measurements on some samples used to conduct the grain-by-grain studies. In particular, we conducted ex-situ spectroscopic ellipsometric (400–1000 nm) measurements on four tungsten samples each exhibiting a unique surface state—as received, electropolished, electropolished and annealed, and electropolished, annealed, and subsequently exposed to helium plasma at a temperature of 550 °C. Spectroscopic ellipsometry is widely used for thin film characterization, for example, growth monitoring in real-time and process diagnosis including etching and thermal oxidation. Ellipsometry measures two values, $Ψ$ and $Δ$. $Ψ$ ranges from 0° to 90° and is related to the ratio of the parallel and perpendicular components of the total reflection coefficients while $Δ$, ranging from 0° to 360°, represents the phase shift between these parallel and perpendicular components of the detected light [28]. The $Ψ$ and $Δ$ measured from ellipsometry are defined from the ratio of the amplitude coefficients for p- and s-polarizations, $r_p$ and $r_s$, respectively:

$$ \rho = \frac{r_p}{r_s} = \tan^2(\Psi) e^{i\Delta} \quad (1) $$

It is possible to extract analytically from $Ψ$ and $Δ$ the pseudo-optical constants (refractive index $n$ and extinction coefficient $k$) through the following relation:

$$ \begin{aligned} \epsilon &= \epsilon_1 + i\epsilon_2 = (n + ik)^2 \\ &= \sin^2(\theta) + \left[ 1 + \tan^2(\theta) \left( \frac{1 - \rho}{1 + \rho} \right)^2 \right] \end{aligned} \quad (2) $$

where $θ$ is the incident angle of light (with respect to the normal to the surface) and $ρ$ is defined in Eq. (1).

The term “pseudo” derives from the fact that we do not take into account surface roughness or the presence of overlayers. This assumption can be valid in the case of bulk materials with a negligible native oxide layer (<5 nm) and surface roughness (<10 nm), as it is the case of our samples. From the pseudo-optical constants $n$ and $k$, it is then possible to calculate the reflectivity for s and p polarized light using Fresnel equations.

Fig. 7 represents the reflectivity spectra (of the unpolarized light, obtained as an average of s and p polarizations) of the sample after the different stages of preparation and exposure. We observe a super-position of reflectivity curves among the as-received, electropolished, and annealed states, whereas a significant deviation is evident in the Helium-exposed sample. In particular, the interaction with He plasma induces a decrease in the reflectivity, possibly due to the combined effect of bubble formation in the sub-surface (~30 nm) layer and the increase of roughness caused by the bursting of such bubbles on the surface. This change in behavior could be monitored in real-time by installing the ellipsometer in-situ on the plasma chamber, already used to monitor tungsten oxide reduction during plasma exposure [29], initiating a pathway towards real-time tracking of bubble growth during plasma exposure.

### 4. Conclusion

The impact of grain orientation and surface temperature on nanohole formation on tungsten surfaces exposed to low-pressure helium plasmas was investigated. Highlighting the influence of grain crystalline orientation, we demonstrated that the (111) orientation leads to bubble bursting at the surface in greater numbers and larger sizes compared to the (100) orientation, with a more pronounced impact on surface roughness. This indicates that, under strictly identical exposure conditions, crystallographic directions play a significant role,
either in subsurface bubble formation mechanisms – due to deeper or shallower implantation – or in the mechanisms of bubble bursting and atom diffusion at the surface. Furthermore, we experimentally demonstrated that increasing the surface exposure temperature from 400 °C to 700 °C leads to an increase in both the density and diameter of nanoholes, from 1000 µm⁻² to 3000 µm⁻² and from 3 nm to 6 nm, respectively. This observation suggests that, at fixed flux and fluence, the thermal diffusion mechanism plays a critical role in the formation of numerous, larger nanobubbles near the surface. Additionally, ellipsometric measurements reveal significant changes in optical properties following plasma exposure, pointing towards potential avenues for in-situ monitoring and real-time analysis of bubble dynamics. As the ellipsometric data are averaged over the different crystalline orientations of our W polycrystals, in future works, it could be possible to perform ellipsometry on single crystal W samples, to acquire complementary information to the local SEM analysis.

### CRediT authorship contribution statement

**Mounir Alfazzaa:** Writing – original draft, Visualization, Investigation, Formal analysis, Data curation. **Federica Pappalardo:** Visualization, Investigation, Data curation. **Andrea Campos:** Writing – review & editing, Investigation, Formal analysis, Data curation. **Gilles Cartry:** Writing – review & editing, Visualization, Project administration, Methodology, Funding acquisition, Formal analysis, Data curation. **Marco Minissale:** Writing – review & editing, Validation, Supervision, Funding acquisition, Formal analysis. **Céline Martin:** Writing – review & editing, Visualization, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

This work received support from the French government under the France 2030 investment plan, as part of the Initiative d'Excellence d'Aix-Marseille Université-A*MIDEX (AMX-19-IET-013-ISFIN). This work was supported by the ANR under grants Nos. ANR-20-CE08-0003 and ANR-23-CE08-0026.

### Data availability

Data will be made available on request.

### References

[1] R.A. Pitts, S. Carpentier, F. Escourbiac, T. Hirai, V. Komarov, S. Lisgo, A. Kukushkin, A. Loarte, M. Merola, A.S. Naik, et al., A full tungsten divertor for ITER: Physics issues and design status, J. Nucl. Mater. 438 (2013) S48-S56.

[2] R. Pitts, X. Bonnin, F. Escourbiac, H. Frerichs, J. Gunn, T. Hirai, A. Kukushkin, E. Kaveeva, M. Miller, D. Moulton, V. Rozhansky, I. Senichenkov, E. Sytova, O. Schmitz, P. Stangeby, G.D. Temmerman, I. Veselova, S. Wiesen, Physics basis for the first ITER tungsten divertor, Nucl. Mater. Energy 20 (2019) 100696, http://dx.doi.org/10.1016/j.nme.2019.100696.

[3] R. Delaporte-Mathurin, M. Ialovega, E.A. Hodille, J. Mougenot, Y. Charles, E. Bernard, C. Martin, C. Grisolia, Influence of exposure conditions on helium transport and bubble growth in tungsten, Sci. Rep. 11 (1) (2021) 14681.

[4] R. Sakamoto, E. Bernard, A. Kreter, N. Yoshida, Surface morphology of tungsten exposed to helium plasma at temperatures below fuzz formation threshold 1073 K, Nucl. Fusion 57 (1) (2016) 016040, http://dx.doi.org/10.1088/1741-4326/57/1/016040.

[5] R. Sakamoto, E. Bernard, A. Kreter, C. Martin, B. Pégourié, G. Pieters, B. Rousseau, C. Grisolia, N. Yoshida, Surface morphology in tungsten and RAFM steel exposed to helium plasma in PSI-2, Phys. Scr. 2017 (T170) (2017) 014062, http://dx.doi.org/10.1088/1402-4896/aa93a2.

[6] M. Ialovega, E. Bernard, R. Bisson, C. Martin, R. Sakamoto, A. Kreter, E. Hodille, T. Angot, C. Grisolia, Hydrogen trapping in tungsten: impact of helium irradiation and thermal cycling, Phys. Scr. 2020 (T171) (2020) 014066, http://dx.doi.org/10.1088/1402-4896/ab68bd.

[7] R. Sakamoto, E. Bernard, A. Kreter, C. Martin, Impact of helium and hydrogen plasma exposure on surface damage and erosion of tungsten, Nucl. Fusion 64 (3) (2024) 036008, http://dx.doi.org/10.1088/1741-4326/ad1fab.

[8] S. Kajita, N. Yoshida, R. Yoshihara, N. Ohno, M. Yamagiwa, TEM observation of the growth process of helium nanobubbles on tungsten: Nanostructure formation mechanism, J. Nucl. Mater. 418 (1-3) (2011) 152-158.

[9] F.W. Meyer, H. Hijazi, M. Bannister, P. Krstic, J. Dadras, H. Meyer, C. Parish, He-ion and self-atom induced damage and surface-morphology changes of a hot W target, Phys. Scr. 2014 (T159) (2014) 014029.

[10] M. Ialovega, E. Bernard, R. Bisson, C. Martin, R. Sakamoto, A. Kreter, E. Hodille, T. Angot, C. Grisolia, Hydrogen trapping in tungsten: impact of helium irradiation and thermal cycling, Phys. Scr. 2020 (T171) (2020) 014066.

[11] E. Bernard, R. Sakamoto, E. Hodille, A. Kreter, E. Autissier, M.-F. Barthe, P. Desgardin, T. Schwarz-Selinger, V. Burwitz, S. Feuillastre, et al., Tritium retention in W plasma-facing materials: Impact of the material structure and helium irradiation, Nucl. Mater. Energy 19 (2019) 403-410.

[12] M. Balden, S. Lindig, A. Manhard, J.-H. You, D2 gas-filled blisters on deuterium-bombarded tungsten, J. Nucl. Mater. 414 (1) (2011) 69-72.

[13] G.-N. Luo, W. Shu, M. Nishi, Incident energy dependence of blistering at tungsten irradiated by low energy high flux deuterium plasma beams, J. Nucl. Mater. 347 (1-2) (2005) 111-117.

[14] M. Balden, A. Manhard, M. Mayer, S. Elgeti, A. Kleyn, P.Z. Van Emmichoven, et al., Surface morphology and deuterium retention of tungsten after low-and high-flux deuterium plasma exposure, Nucl. Fusion 54 (8) (2014) 083014.

[15] H. Xu, Y. Zhang, Y. Yuan, B. Fu, A. Godfrey, G. De Temmerman, W. Liu, X. Huang, Observations of orientation dependence of surface morphology in tungsten implanted by low energy and high flux d plasma, J. Nucl. Mater. 443 (1-3) (2013) 452-457.

[16] M. Ialovega, E. Bernard, M.-F. Barthe, R. Bisson, A. Campos, M. Cabié, T. Neius, R. Sakamoto, A. Kreter, C. Grisolia, et al., Helium-induced morphology evolution in tungsten under thermal treatment, Nucl. Fusion 62 (12) (2022) 126022.

[17] N. Ohno, Y. Hirahata, M. Yamagiwa, S. Kajita, M. Takagi, N. Yoshida, R. Yoshihara, T. Tokunaga, M. Tokitani, Influence of crystal orientation on damages of tungsten exposed to helium plasma, J. Nucl. Mater. 438 (2013) S879-S882.

[18] R. Sakamoto, E. Bernard, A. Kreter, N. Yoshida, Surface morphology of tungsten exposed to helium plasma at temperatures below fuzz formation threshold 1073 K, Nucl. Fusion 57 (1) (2016) 016040.

[19] G.J. Smith, J. Ellis, R. Moussaoui, C. Pardanaud, C. Martin, J. Achard, R. Issaoui, T. Gans, J.P. Dedrick, G. Cartry, Enhancing surface production of negative ions using nitrogen doped diamond in a deuterium plasma, J. Phys. D: Appl. Phys. 53 (46) (2020) 465204, http://dx.doi.org/10.1088/1361-6463/aba6b6.

[20] G.J. Smith, L. Tahri, J. Achard, R. Issaoui, T. Gans, J.P. Dedrick, G. Cartry, Surface production of negative ions from pulse-biased nitrogen doped diamond within a low-pressure deuterium plasma, J. Phys. D: Appl. Phys. 54 (43) (2021) 435201, http://dx.doi.org/10.1088/1361-6463/ac18ee.

[21] Y.R. Yamamura, H. Tawara, Energy dependence of ion-induced sputtering yields from monatomic solids at normal incidence, At. Data Nucl. Data Tables 62 (2) (1996) 149-253, http://dx.doi.org/10.1006/adnd.1996.0005, URL https://www.sciencedirect.com/science/article/pii/S0092640X96900054.

[22] W. Eckstein, Calculated Sputtering, Reflection and Range Values, Tech. Rep. IPP-9/132, Germany, 2002, p. 313.

[23] A. Weerasinghe, L. Hu, K.D. Hammond, B.D. Wirth, D. Maroudas, Non-dilute helium-related defect interactions in the near-surface region of plasma-exposed tungsten, J. Appl. Phys. 128 (16) (2020).

[24] F.W. Meyer, H. Hijazi, M.E. Bannister, K.A. Unocic, L.M. Garrison, C.M. Parish, Flux threshold measurements of He-ion beam induced nanofuzz formation on hot tungsten surfaces, Phys. Scr. 2016 (T167) (2016) 014019, http://dx.doi.org/10.1088/0031-8949/T167/1/014019.

[25] D. Dasgupta, R.D. Kolasinski, R.W. Friddle, L. Du, D. Maroudas, B.D. Wirth, On the origin of 'fuzz' formation in plasma-facing materials, Nucl. Fusion 59 (8) (2019) 086057, http://dx.doi.org/10.1088/1741-4326/ab22cb.

[26] L. Corso, S. Curiotto, F. Cheynis, P. Muller, E. Bernard, M. Cabie, C. Martin, F. Leroy, Real-time helium bubble growth in tungsten by in-situ GISAXS, 2024.

[27] K.D. Hammond, B.D. Wirth, Crystal orientation effects on helium ion depth distributions and adatom formation processes in plasma-facing tungsten, J. Appl. Phys. 116 (14) (2014) 143301, http://dx.doi.org/10.1063/1.4897419.

[28] H. Fujiwara, Spectroscopic Ellipsometry: Principles and Applications, John Wiley & Sons, 2007.

[29] F. Pappalardo, L. Rayneau, C. Martin, M. Cabie, E. Salomon, T. Angot, G. Cartry, R. Bisson, M. Minissale, In-situ monitoring of tungsten oxides reduction during deuterium plasma exposure by spectroscopic ellipsometry, Nucl. Mater. Energy 41 (2024) 101751.