Impact of helium and hydrogen plasma exposure on surface damage and erosion of tungsten

The impact of helium plasma exposure on the tungsten surface damage structure development and erosion has been investigated by comparing the impact of hydrogen plasma exposure. Crystal orientation dependence of the undulating surface structure formation and erosion rate is observed on the plasma-exposed tungsten surface independently from the plasma species. The top surface of the plasma exposed tungsten has a tendency to {100} plane independently from the initial surface orientation. Although hydrogen and/or helium cause no erosion in tungsten under incident ion energy exposure conditions below the sputtering threshold, inevitable minute impurities, like oxygen, play an essential role in erosion, and significant erosion can be observed even at 30 eV.


Introduction
In a magnetically confined fusion reactor that will use a deuterium-tritium reaction, tungsten is a primary candidate for plasma-facing materials due to its excellent hightemperature properties, high sputtering threshold energy, low hydrogen retention and acceptable induced radioactivity [1].
Even in the first wall, where the magnetic field lines are not connected directly with high-temperature plasma, the plasmafacing materials are exposed to radiation and charge exchange particles from the core plasma and also exposed to scrap-off layer plasma.The incident particles are not only fuel hydrogen isotope, but also several % of helium ash and the other impurities, and the total flux is estimated as 10 20 − 10 22 (m 2 s −1 ) −1 [2].From the viewpoint of the protection of a blanket behind the first wall, the first wall must be thick and robust.On the other hand, from the viewpoint of a reasonable tritium breeding ratio (TBR) in a blanket, a thick first wall will not be allowed in order to minimize neutron attenuation.As a mutual compromise, only a thin tungsten coating layer which is submm to a few mm thick is envisaged as the first wall to protect the blanket from the incident heat and particles [3,4].Therefore, the erosion rate of tungsten is a critical issue not only for the lifetime of the plasma-facing materials but also for the TBR and lifetime of the blanket.
A considerable number of studies have been carried out on tungsten as a plasma-facing material from the viewpoint of both theoretical and experimental aspects for more than a decade [5,6].Many studies have shown strong impacts of helium exposure on mechanical properties and surface modification of tungsten, i.e. surface hardening, bubble, hole, and fuzz nanostructures [7][8][9][10][11][12].Furthermore, hydrogen isotope retention is significantly affected by the presence of helium in the tungsten surface layer [13].These previous results motivate us to investigate helium exposure effects.
In the previous helium plasma exposure experiments, we have indicated the formation of a nano-scale undulating surface structure, which shows a crystal orientation dependence and effective erosion even at low energy below the sputtering threshold [14,15].Our novel discovery was that heliumplasma exposure tends to cause specific crystal orientations to appear on the surface, and the nano-scale undulating surface structure is spontaneously developed to resolve the mismatch between the crystal orientations at the initial and exposed surface.In this paper, we have tried to confirm the relationship between surface crystal orientation and surface structure directly.Furthermore, we have investigated the role of helium in surface structure formation and effective erosion by comparing pure helium plasma exposure and pure helium or heliumcontaining plasma exposure.

Experimental setup
High-purity tungsten (>99.995%,Toho Kinzoku Co. Ltd) samples were mechanically mirror-polished and then annealed at 1773 K under low-pressure high-purity hydrogen conditions (>99.995%, 0.03 MPa) for 2 h to obtain a several 10 µm grain without surface oxidation layer for the surface analyses.
Pure hydrogen, pure helium, and hydrogen/helium mixed plasma exposure experiments have been carried out in the linear device PSI-2 [16].The incident energy to the sample was varied between 30-200 eV by adjusting the bias voltage.A radially scanning Langmuir probe was employed to measure plasma parameters, namely, electron density, electron temperature, space potential, and ion flux.Typical values of these parameters were 1.0 × 10 18 m −3 , 9 eV, −20 V and 0.6 × 10 22 (m 2 s −1 ) −1 , respectively.The plasma exposure experiment was carried out up to an ion fluence of 1.0 × 10 26 m −2 .The sample temperature was actively controlled at 773 K by a combination of water cooling and electric heating, in addition to plasma heating, based on an infrared camera temperature measurement with the support of thermocouple measurements.
After the plasma exposure experiments, the surface erosion and nanostructure development were analyzed using diverse methods, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), confocal laser microscopy, electron backscatter diffraction (EBSD), and precision microbalance with an accuracy of 1 µg.A focused ion beam (FIB) method was employed to fabricate crosssectional samples for a depth profile observation of the damaged structure and surface morphology.

Undulating surface structure
In the series of pure helium plasma exposure experiments in PSI-2, the nano-scale undulating surface structure, which shows crystal orientation dependence, is typically observed at temperatures below the fuzz formation threshold [14,15].Near the {100} surface, the undulating surface structure cannot develop.As a grain surface tilt from the {100} surface, the interval of the undulating surface structure becomes narrower.A possible explanation for this formation is that the top surface of the undulation tends to {100} plane independently of the original surface orientation as shown in figure 10 of the [15].To verify the relationship between the original surface orientation and the top surface of the undulation, EBSD measurements are applied to the cross-sectional sample, which is fabricated by using FIB processing.Figure 1(a) shows an SEM image of a helium plasma exposed tungsten surface at a fluence of 1 × 10 26 m −2 .A cross-sectional sample is fabricated along the white line, which crosses the two-grain boundaries.Therefore, three grains, i.e.Grain A, B, and C, are included in the cross-section.Enlarged images of the undulating surface structure on each grain are also shown at the bottom of figure 1(a).Figure 1(b) shows crystal orientation which is measured by EBSD at the cross-section.The measured crystal orientation of each grain is shown by a cube.The cubes which show crystal orientations are also indicated in the enlarged surface images in figure 1(a).One of the {100} planes which is indicated by the gray color, corresponds to the top surface of the undulation in all grains.This observation result is evidence that the top surface of the undulation tends to be {100} plane independently of the original surface orientation.

Damage structure formation under hydrogen and helium plasma exposure
To verify the role of helium in the undulating surface structure formation, hydrogen, and helium mixed hydrogen plasma exposure experiments have been carried out comparatively.SEM images of the pure-hydrogen, helium-hydrogen mixture, and pure-helium plasma exposed tungsten surface at a fluence of 1 × 10 26 m −2 are shown in figure 2. The undulating surface structure, which is different from grain to grain, is formed not only at the pure-helium or helium-mixed plasma exposed surface but also at the pure-hydrogen exposed surface.It becomes clear that the undulating surface structure is not a peculiar effect of the helium-plasma exposure but a common effect of the plasma exposure, because similar undulating surface structures are formed in the pure-hydrogen  plasma.However, it should be noted that there is a clear difference between pure hydrogen plasma exposure and exposures containing helium in hole structure formation, which is developed from an aggregation of helium bubbles.Figure 3 shows SEM images of tungsten {100} surface with hydrogen plasma exposures containing helium.The hole structure, which is ∼10 nm size black dot in SEM images, is never observed under pure hydrogen plasma exposure.However, it is a common damage structure under plasma exposures in which helium exists.Since the hole structure is formed by the aggregated helium bubbles reaching the surface, the bubbles are not observed under the pure hydrogen exposure condition in the cross-sectional TEM observation, as shown in figure 4.

Surface erosion enhancement with impurity
From the comparison between hydrogen-plasma and heliumplasma exposure, an essential role of helium exposure is the formation of bubble and hole structures.But there is no further  visible effect from the surface observations with SEM and TEM.On the other hand, erosion enhancement with helium exposure is still possible because flaking, which may lead to mass loss, is observed as associated with the hole structure development [15].Mass losses are measured by comparing mass differences before and after the plasma exposure experiments under various plasma exposure conditions to verify the helium effects on erosion enhancement.Figure 5 shows an incident energy dependence of the erosion amounts at a fluence of 1 × 10 26 m −2 .The circle and square symbols denote the measured erosion amount with helium and hydrogen plasma exposure, respectively.Even though the range of incident energy of the plasma particles is lower than the threshold energies of tungsten sputtering (H: ∼500 eV, He: ∼110 eV), the erosion can be observed generally in both helium and hydrogen plasma exposure experiments.
In our experiments, measurable erosions are observed under all experimental conditions down to 33 eV.Another important finding is that there is no significant difference between hydrogen plasma and helium plasma in the erosion amount.Especially at 70 eV, there is almost the same erosion amount among pure hydrogen, 10% helium mixed, and 50% helium mixed plasma exposure.The results mentioned above imply that the major operational gases, i.e. helium and hydrogen, have no impact on the tungsten erosion, but inevitable minute impurities, e.g.oxygen, play an essential role in the erosion.The tungsten erosion with oxygen ions has been estimated from the database of the sputtering yield [17].Since the sputtering yield database is based on the monovalent ion, it is required to compensate for incident energy, considering the multivalent-ion fraction in plasma.Although the plasma electron temperature and density are measured by using a Langmuir probe in our experiments, a large perpendicular anomalous transport [18] makes it difficult to estimate the multivalent ion fraction by assuming an equilibrium state.Here, the temperature dependence of the multivalent-ion fraction, which was measured by the mass-spectrometer of magnetized plasmas (MSMP-03) [19] is applied to estimate the multivalent ion fraction.The MSMP-03 measurements show that the ratio of He 2+ /(He + +He 2+ ) decreases as the electron temperature decreases, and the ratio is 0.4% at 10 eV electron temperature.In our experiments, therefore, the ratio of He 2+ /(He + +He 2+ ) is not more than 0.4% because a typical electron temperature is 9 eV.Since there are no measurements on the multivalent-ion fraction of oxygen, neon data is used for a rough estimation, considering the similar ionic fraction in the equilibrium state [20].Here, the ratio of O 2+ /(O + +O 2+ ) is assumed as 10%, considering the ratio of Ne 2+ /(Ne + +Ne 2+ ) which is measured by the MSMP-03.Since measuring the minute oxygen content in the plasma is difficult, the oxygen content rate is set as a fitting parameter for reproducing experiment results.
The thick solid and thick dashed lines denote incident energy dependences of the total calculated erosion amount for hydrogen and helium plasma, which include the erosion enhancement effect with a 0.13% oxygen impurity, and the dotted and dashed thin lines denote one of each consistent elements, namely, H + , He + , He 2+ , O + and O 2+ .Under low incident energy conditions below the sputtering threshold energy of the main operational gas, the erosion amounts are governed by oxygen impurity independently from the main operational gases, namely, hydrogen and helium.It should be noted that the discrepancy between measured and calculated erosion becomes prominent, as shown by the arrow in the high-energy region above the sputtering threshold energy (∼110 eV) in the helium-plasma exposure.These observations imply that a selective sputtering reduction, namely, the sputtering only by helium, is reduced, while that by oxygen is kept.As shown by the thick dotted line, only the helium sputtering yield must be reduced to ∼1/10 in the erosion calculation to reproduce the experimental observation.In the PISCES linear plasma facilities, approximately one order of sputtering yield reduction has been reported in helium plasma exposure on tungsten [21].The measured values quantitatively agree with our results, implying they exhibit the same phenomenon.A possible explanation of the selective sputtering reduction is due to a range of incident particles.The range and peak of 200 eV helium in tungsten are 8.1 nm and 2.6 nm, respectively, by the TRIM code [22].Also, the range of helium overlaps with the bubble-formative layer in figure 4(b).A numerical simulation shows a reduction of sputtering yield in the presence of a helium bubble because incident helium atoms tend to stop in bubbles, transferring their kinetic energy efficiently to the helium atom of the same mass [23].On the other hand, the range and peak of 200 eV oxygen are as shallow as 3.9 nm and 0.9 nm, in which there are no large bubbles.Therefore, there is no reduction of sputtering yield with oxygen impurity, even in the presence of a helium bubble.Comparative experiments with hydrogen-plasma exposure may be helpful to verify this working hypothesis because there is no bubble formation in the hydrogen-plasma exposure.Nevertheless, it is not clear whether there is a similar effect in the hydrogen-plasma exposure because the sputtering threshold energy (∼440 eV) is sufficiently higher than in the range of the experiments (<200 eV).Therefore, the erosion should be entirely governed by the inevitable minute impurities.

Conclusions
In this study, we have directly confirmed the relationship between surface crystal orientation and surface structure.The top surface of tungsten tends to {100} plane independently of the original surface crystal orientation under heavy plasma exposure beyond the fluence of 1 × 10 26 m −2 in the temperature range below the fuzz formation threshold.The nanoscale undulating surface structure is spontaneously developed to resolve the mismatch between the crystal orientations at the initial and exposed surface.Furthermore, we have shown, by using not only helium plasma but also hydrogen and hydrogen-/helium mixed plasma, that the formation of surface structures appears to be a universal phenomenon.The clear difference between hydrogen plasma exposure and helium-contained plasma exposure is the formation of bubbles and holes, which are developed from the aggregation of the helium bubbles.
The tungsten erosion under both hydrogen and helium plasma exposure is governed by inevitable minute impurities like oxygen in the energy region below the sputtering threshold of hydrogen and helium, and significant erosion can be observed even at 30 eV.On the other hand, in the range of higher energy sufficiently above the sputtering threshold, helium bubbles may selectively reduce erosion with helium plasma exposure.This erosion reduction appears to be a universal phenomenon, and our results confirmed and extended earlier data from PISCES.
These two effects, observed in the ranges below and above the sputtering threshold energy, on erosion should be critical for applying tungsten as a burning plasma-facing material.

Figure 1 .
Figure 1.(a) SEM image of a helium plasma exposed tungsten surface at a fluence of 1 × 10 26 m −2 .The top image shows the whole image before making the cross-sectional sample.The white line indicates the cutting line for fabricating the cross-sectional sample.The bottom three images are enlarged ones of each grain and projected crystal orientations, which are measured by EBSD.(b) Cross-sectional EBSD map and measured crystal orientation in each grain.

Figure 5 .
Figure 5. Incident energy dependence of the erosion amount at a fluence of 1 × 10 26 m −2 .The circle, square, and triangle symbols denote the measured erosion amount with hydrogen, helium, and their mixed plasmas.The thick lines denote calculated global erosion, and the thin lines denote partial erosion by plasma components, i.e. hydrogen, helium, and oxygen ions.