Synergistic effects of hydrogen plasma exposure, pulsed laser heating and temperature on rhodium surfaces

Journal of Nuclear Materials 432 (2013) 388–394

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Synergistic effects of hydrogen plasma exposure, pulsed laser heating and temperature on rhodium surfaces L. Marot a,⇑, G. De Temmerman b, R.P. Doerner c, K. Umstadter c,1, R.S. Wagner a,d, D. Mathys e, M. Duggelin e, E. Meyer a a

Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland FOM Institute DIFFER, Ducth Institute For Fundamental Energy Research, Association EURATOM-FOM, Trilateral Euregio Cluster, P.O. Box 1207, 3430 BE Nieuwegein, The Netherlands Center for Energy Research, University of California at San Diego, 9500 Gilman Dr, La Jolla, CA 92093-0417, USA d Biozentrum and the Swiss Nanoscience Institute, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland e Centre of Microscopy, University of Basel, Klingelbergstrasse 50/70, CH-4056 Basel, Switzerland b c

a r t i c l e

i n f o

Article history: Received 6 April 2012 Accepted 10 August 2012 Available online 23 August 2012

a b s t r a c t The combined effect of hydrogen plasma exposure and surface heating, either continuous or by short laser pulses (5 ns), on the surface morphology of rhodium layers has been studied. Investigations were performed by reflectivity measurements, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). While surfaces exposed at room temperature exhibit little modifications, strong surface changes are observed for surface temperatures higher than 250 °C. At 500 °C, the plasma exposed surface exhibits a nanoscale structure (50–100 nm) with a high level of porosity and a low reflectivity. Additional laser irradiation of the surface strongly enhances the observed surface damage. Localized surface melting is observed with craters extending deep into the substrate together with a dense network of voids. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In current tokamaks, the plasma is viewed and controlled through optical windows or fibers, which will not be possible in ITER due to the high level of neutron radiation expected. Instead, metallic mirrors will be used as light transmitters through labyrinths. The first mirrors (FMs) are thus some of the most critical elements in ITER, since they must maintain a good reflectivity in a harsh environment of particle fluxes due to charge exchange neutrals, UV and X-ray radiation [1,2]. Molybdenum and rhodium are two important candidate materials for first mirrors. Molybdenum, due to its low sputtering yield, is more advantageous under erosion conditions [3]. Rhodium, on the other hand, provides a better reflectivity in the visible range [4] and a low chemical reactivity towards oxide and carbide formation [5,6]. The surface of polycrystalline metals has grains with differently oriented crystalline planes. Due to the difference in sputtering yield of grains with different orientation, the initially smooth mirror surface changes into a stepped structure under ion bombardment [7]. To avoid this phenomenon single crystal mirrors could be used [8,9]. Another solution is to use a polycrystalline metal film deposited on polished metallic substrates with nanometric grains [10–12]. In this case,

⇑ Corresponding author. 1

E-mail address: [email protected] (L. Marot). Present address: KLA-Tencor, RAPID, 1 Technology Drive, Milpitas, CA 95035 USA.

0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.08.013

the surface relief of the coated mirror is small compared to the wavelength of the light. Performances of coated mirrors were investigated under erosion conditions in the TEXTOR tokamak [4,13,14]. Exposure of metallic surface to high fluxes of particles with energy below the threshold for damage production is known to lead to strong surface modifications such as blistering [15]. Recently, strong surface modifications were observed on tungsten surfaces exposed to low energy helium ions [16]. Those effects are strongly linked to the surface temperature. In this article, we study the surface modifications of rhodiumcoated mirrors exposed to high-flux hydrogen plasma combined with surface heating. The latter was achieved by either changing the cooling scheme during plasma exposure, providing a stable temperature, or by additionally exposing the surface to short pulse laser irradiation. 2. Experimental Rhodium coatings were deposited on polished stainless steel sample by magnetron sputtering, the complete deposition technique and films characterization were published in our previous papers [4,10]. Plasma exposures were performed in the PISCES-A linear plasma generator [17,18]. The plasma can be operated continuously in steady-state conditions and is generated by an arc discharge initiated with a heated LaB6 cathode. The anode and axial

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magnetic field define the cylindrical plasma with a diameter of about 75 mm and axial length greater than 1 m. The plasma conditions are measured by a reciprocating Langmuir probe and were typically ne = 2  4  1018 cm3, Te = 10 eV. The samples are negatively biased with respect to the plasma potential to control the energy of the impinging ions. Pure hydrogen or mixed hydrogen/ argon plasmas were used. The samples were weighed before and after plasma exposure to determine the amount of eroded material. The morphology of the films was investigated by top and cross view SEM (Hitachi S-4800 field emission at 5 kV). The XPS measurements were performed in a ultra-high vacuum chamber. The electron spectrometer was equipped with a hemispherical analyser and an X-ray source (Mg Ka excitation, h = 1253.6 eV) for core level spectroscopy. As reference for the electron binding energy calibration, the Au 4f7/2 line of a gold sample was set to 84.0 eV. A Tencor alpha-stepper 500 was used to measure the roughness, Ra (arithmetic average), of the surface before and after exposure. A scan length of 1 mm was used and five scans were averaged. Surface topography and roughness area (Ra) was measured by atomic force microscope (AFM Digital Instruments, Dimension 3100) for an image of 10  10 lm2. The AFM tip radius is around 10 nm. Total and diffuse reflectivity measurements were carried out with a Varian Cary 5 spectrophotometer equipped with a 110 mm diameter integrating sphere under nearly normal incidence (3°200 ) in the wavelength range of 250–2500 nm. 3. Results and discussion Rhodium coated mirrors with layer thickness higher than 1 lm, were exposed to hydrogen plasma containing 10–20 % of argon. The idea by adding argon in the plasma was to enhance the erosion rate of the rhodium film. The amount of argon was determined from the partial pressures of the different gases. The sample was biased to 100 V. Given the electron temperature (around 10 eV), only singly charged particles hit the sample. The ion energy is then approximately 80 eV. The sputtering threshold for hydrogen ion of rhodium is around 120 eV, adding argon in our plasma aimed to have also physical sputtering for our experiment (the sputtering yield of argon ion of rhodium is 0.15 for 80 eV) [19]. The bulk surface temperature of the sample was maintained constant at 350 °C by controlling flux to the surface. The central part of the mirror was repeatedly heated by laser pulses to simulate laser irradiation and plasma exposure which was a possible design of some ITER’s FM and especially for the LIDAR diagnostic. A zone of about 1 cm2 was irradiated by the laser during 5 ns with a frequency of 1 Hz, the laser wavelength was 1064 nm and the laser energy was 550 mJ (multimode gaussian). More details about the experimental setup can be found in reference [15,20]. From the thermal and optical properties of rhodium [4] and the laser energy delivered to the surface, the peak surface temperature in the laser-exposed area was estimated to be around 1000 °C, well below the rhodium melting temperature (1966 °C). Such a heating scheme produces a steep temperature profile, and 1 lm below the surface, the peak temperature is only 500 °C. The estimation of the surface and depth temperature is described in the reference [21] for tungsten surface. The

Fig. 1. Optical observations of the rhodium coatings exposed to laser heat pulses; (a) without plasma, (b) with plasma. (b2) corresponds to the laser irradiated area.

temperature profile is a solution of a one-dimensional heat diffusion equation with a step-function thermal load using the absorbed laser irradiance, the thermal conductivity, thermal diffusivity, the time and the distance below (into) the surface. The effect of the laser irradiation alone was studied by exposing a similar sample only to the laser heating for the same total duration without plasma. The eroded depth calculated by weight loss measurements is given Table 1 assuming a uniform mass loss from both parts of the sample. The rhodium layer thickness was higher than 1 lm. The ion fluence and the roughness before and after exposure are also shown in the table. In the case of the laser-only exposure, no modification of the sample surface is noticed (Fig. 1a). An SEM image of the top surface (Fig. 2a) is identical to a rhodium layer after deposition. This shows that the laser energy is below the damage threshold for rhodium. On the contrary, considerable surface modifications are observed in the case of the simultaneous plasma/laser irradiation. Strong surface modifications are observed already for the areas not exposed to the laser, which shows a white color after plasma exposure (Fig. 1b). Small crystallites are still visible by SEM (Fig. 2b) although a general increase of the apparent grain size is observed. Similar results were reported in reference [10] for deposition of Rh at elevated temperature, i.e. a size increase observed by SEM. It is important in this case to distinguish between the dimension of the columnar structures (seen in SEM images), and the dimensions of the individual grains that form columns (calculated from XRD), as described by Weibenrieder [22]. The films are formed by columnar structure containing numerous smaller grains. The diameter of the columns, observed in SEM images (Fig. 2b) is about 650 nm, i.e., larger than the diameter of the grains determined by the Scherrer formula [23]. Moreover, the recrystallization temperature of rhodium is around half of the melting temperature, i.e. 983 °C which was not achieved for the entire film. The surface modifications are more drastic in the area exposed to the laser. Large craters are visible throughout the laser-exposed area (Fig. 3a) whose depth can be as high as 20 lm (inset of Fig. 3). This means that those craters actually extend far beyond the rhodium layer and go through the substrate. Around the large craters, accumulation of re-solidified material can be observed whose height is in the 10-lm range. Droplet/

Table 1 Plasma exposure conditions, the temperature, eroded depth, ion fluence and the roughness before and after plasma exposure are given. The roughness is measured by profilometry. Sample

Temperature (°C)

Eroded depth (nm)

Ion fluence (1024 m2)

Roughness (Ra) before exposure (nm)

Roughness (Ra) after exposure (nm)

Laser exposed area Laser unexposed area No plasma

1000 300–350 RT

270 270 0

11.2 11.2 0

9 9 7

177 16 7

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Fig. 2. SEM images of rhodium coatings; (a) corresponds to the mirror in Fig. 1a and (b) is from position (b1) from Fig. 1b (exposed only to the plasma).

Fig. 3. SEM images of rhodium coating (b) from Fig. 1 in the laser exposed area (b2). The inset is the crater profile of figure (b).

particle ejections from the surface following the laser pulse were observed by eye during the experiment. In addition, micrometersize holes are observed throughout the exposed area (Fig. 3c and d). In our previous paper [24] the effect of rhodium/silicon deposition on silicon wafer at elevated temperature was investigated. For a temperature of 900 °C diffusion of rhodium in the silicon wafer occurred leading to hole formation in the film and also in the substrate on the entire surface. The same phenomenon appeared here for a temperature around 1000 °C, i.e. holes formation in the film and also in the stainless steel substrate. Surprisingly those voids appear to be aligned along a given direction (Fig. 3d)). The specular reflectivity (Fig. 4) of the sample exposed to the plasma with laser heat pulses decreases significantly mostly due to a strong increase of the diffuse reflectivity which is more than 30 % at 250 nm (RSpec = RTot  RDiff). XPS measurements of the samples shown in Fig. 1a and b (on both areas) revealed pure rhodium and no rhodium oxide formation (not shown here). Traces from the substrate, i.e. Fe, Ni and Cr are seen on the XPS survey on the laser exposed (Fig. 1b2) area but not in the plasma exposed area (Fig. 1b1). As shown in the inset of Fig. 3 some craters are far deeper than the 1 lm rhodium film. It was already reported that blister formation was observed for tungsten exposed to deuterium plasma [25]. For nanometric rhodium crystalline no reference on blistering effect can be found. Deuterium plasma exposure of rhodium layers in the laboratory (2  1024 m2) or in the TEXTOR tokamak (3.4  1024 m2) did not evidence any blister formation- although

Fig. 4. Specular reflectivity of the sample exposed to laser heat pulses with plasma (Fig. 1b).

the ion fluence is admittedly relatively low [4,13]. For higher deuterium fluence, even without blistering, a saturated Rh surface forms microscopic bubbles in the near-surface region, which then are heated and ruptured during laser heating. This then causes the release of the trapped gas, rhodium clusters and particles from the surface, which is subsequently ionized in the plasma and can interact elsewhere on the surface. If these ions have enough incident energy, they can also cause additional material damage. The

L. Marot et al. / Journal of Nuclear Materials 432 (2013) 388–394 Table 2 Plasma exposure conditions, the temperature, eroded depth, ion fluence and calculated sputtering yield are given. Temperature (°C)

Eroded depth(nm)

Ion fluence (1024 m2)

Sputtering yield

55 250–300 500

185 348 377

162 162 162

8.3  105 1.6  104 1.7  104

net result is a synergistic effect between heat pulse and deuterium plasma causing greater surface roughening and material removal due to formation of clusters and microparticles from rupturing of near-surface bubbles [20]. In the current design of the LIDAR diagnostic system in ITER, the laser beam is injected through a hole in the FM. This has the key advantage of separating the requirements for high power handling of the laser mirror from wide spectral bandwidth for the collection mirror [26–28]. This also avoids the synergistic effect presented in this paper. As described, a strong modification of the surface morphology was observed in the area of the sample exposed only to the plasma. To clarify the role of the substrate temperature on the observed changes, three rhodium coatings were exposed to a high flux

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(4 – 5  1018 m2 s1) hydrogen plasma for different temperature. The sample was biased to 150 V for 1 h exposure. The surface temperature was varied by changing the flux to and cooling efficiency of the target while keeping the plasma parameters identical. The conditions are presented in Table 2. The sputtering yield calculated, from the mass loss measurements, increases with the temperature but is lower than the calculated value (1.3  103) from the TRIDYN code [29]. The specular reflectivity measured using a spectrophotometer is plotted in Fig. 5. A strong correlation between the surface temperature and the surface reflectivity is observed, the higher the temperature the lower the reflectivity (Fig. 5). For temperatures below 300 °C, the decrease of reflectivity is mainly observed in the visible range, while the reflectivity for higher wavelengths remains unaltered. In this case the light is diffusely scattered by the surface (RDiff > 40% for 250 nm). In contrast, at 500 °C, a decreased surface reflectivity is observed in the whole wavelength range, and a blackening of the surface is apparent from the pictures. In this case, the decrease is not due to the increased diffuse reflectivity (RDiff < 16% for 250 nm) but is caused by a strong decrease of the specular reflectivity. Hence, the light is absorbed in the nanostructured surface. A similar effect has been described for porous beryllium layers [30], where a high level of porosity blackens the surface and significantly decreases the specular reflectivity. A smoothing of the grain structure is observed for the sample

Fig. 5. Specular reflectivity of hydrogen plasma exposed rhodium mirrors for three temperatures; pictures of the corresponding samples are also presented.

Fig. 6. SEM images of rhodium coatings, after deposition (Top left) and after plasma exposures at 55, 250 and 500 °C.

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Fig. 7. AFM topographies of the same films presented in Fig. 6. The surface roughness Ra indicated is measured by AFM. Scan area 10  10 lm2.

Fig. 8. SEM images of rhodium coatings after plasma exposure at 500 °C. Images (a) and (b) are top views and (c) and (d) are cross sectional views.

exposed at 55 °C (Fig. 6), where the individual grains are hardly noticeable contrary to the non-exposed surface. The morphology of the sample exposed at 250 °C, is quite similar to that of the sample exposed at 300 °C in a hydrogen/argon plasma. In this case however, surface pitting is evident in some areas of the surface. For the highest temperature (500 °C), the surface has re-organized into a porous finger-like nanostructure with typical size in the range 20–100 nm (Fig. 6). AFM topographies of the same samples are shown in Fig. 7. The surface roughness (Ra) are indicated on the pictures. The increasing surface roughness is seen to correlate with the higher surface temperature during the plasma exposure. XPS measurements were carried out after plasma exposure. For the exposure at 250 and 500 °C rhodium was partially oxidized. After fitting of the XPS data another doublet is presents at a

binding of 308.2 eV corresponding to Rh2O3 [31]. The percentage of Rh metal is 42.5 and 16.7 at.% and Rh2O3 is 14.8 and 12.6 at.% for 250 and 500 °C exposures, respectively. Traces of molybdenum (<5 at.%) are present on the surface possibly due to the sputtering of the sample holder in PISCES-A; the rest is carbon and oxygen. No traces from stainless steel substrate (Fe, Ni, and Cr) are revealed. The specific structure of the sample exposed at 500 °C is also shown in Fig. 8. Views are from the top for images (a) and (b); cross sectional views are (c) and (d). This structure presents some similarities with that of tungsten surfaces exposed to helium plasma at elevated surface temperature, i.e. so-called tungsten fuzz [16,32,33]. W fuzz is a commonly seen micrometers-scale tendril formation of plasma exposed surfaces due to the action of >10 eV He ions at elevated surface temperature. Experiments indicate a

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Fig. 9. Normalized reflectivity change for rhodium (1 lm) coatings on tungsten (left graph) and on molybdenum (right graph) during vacuum annealing. The corresponding surface temperature is also plotted.

lower limit of about 800 °C for the nanostructure formation [33]. However, in our case the surface temperature was lower (500 °C) and our plasma was a pure hydrogen plasma. The phenomenon is new and not explained for the moment. However, a synergistic effect of the temperature and the plasma exposure is obvious. Similar results have been shown for rhodium mirror deposited by the same technique and exposed to deuterium plasma in TEXTOR for a temperature around 650 °C [34]. To discriminate the effect of the temperature, an annealing was performed in vacuum (106 mbar). Rhodium coatings (>1 m) deposited on polished molybdenum or tungsten were used. During the annealing the change of the reflectivity was recorded using 2 lasers (532 and 670 nm). The change of the normalized reflectivity is plotted in Fig. 9 as a function of the time. The corresponding temperature measured by a thermocouple welded on the surface is also plotted. For the molybdenum and tungsten substrate no change appeared before 725 and 835 °C, respectively. XPS measurements were performed without breaking the vacuum after this experiment, for both sample less than 15 at.% of the metal substrate is on the surface, rhodium remained in metallic state. An interdiffusion process occurs for this temperature affecting the reflectivity. As W, Mo and Fe have a body centered cubic structure, as below 620 °C for all 3 materials we are in a solid solution [34] and as the self-diffusion coefficient are not too different for this range of temperature, we can assume the same effect shown before for a Rh film on a stainless steel substrate. This experiment, performed in another chamber, clearly shows that for PISCES-A exposure the synergistic effect is responsible of the structural change and not only the temperature.

4. Conclusion Rhodium mirrors exposed to hydrogen plasma and laser heat pulses in PISCES-A shows an important damage of the surface leading to a reflectivity decrease and even crater formation on the surface. This experiment revealed a synergistic effect between the laser irradiation and the plasma exposure, as previously observed for tungsten [15]. In the current design of the LIDAR diagnostic system in ITER, the laser beam is injected through a hole in the FM. This has the key advantage of separating the requirements for high

power handling of the laser mirror from wide spectral bandwidth for the collection mirror. For heated mirrors exposed to plasma, a significant drop of the reflectivity appeared at 250 °C and nanostructured rhodium are formed up to 500 °C. However, only the heating of the mirror without plasma is not responsible for this change. In ITER, for rhodium FM in erosion condition, it will be important to maintain the mirror at low temperature and one possibility is to cool it. ITER FM mock-ups have been realised with a rhodium coated film of 5 lm thickness on a water cooled substrate to investigate all these points [35]. Acknowledgements The financial support of the Swiss Federal Office of Energy and of the Federal Office for Education and Science is gratefully acknowledged. Mrs Verena Thommen and Mrs Monica Schönenberger are thanked for the AFM measurements. References [1] A. Litnovsky, V. Voitsenya, T. Sugie, G. De Temmerman, A.E. Costley, A.J.H. Donné, K.Yu. Vukolov, I. Orlovskiy, J.N. Brooks, J.P. Allain, V. Kotov, A. Semerok, P.-Y. Thro, T. Akiyama, N. Yoshida, T. Tokunaga, K. Kawahata, Nucl. Fusion 49 (2009) 075014. [2] V. Voitsenya, A. Costley, V. Bandourko, A. Bardamid, V. Bondarenko, Y. Hirooka, S. Kasai, V. Konovalov, M. Nagatsu, K. Nakamura, D. Orlinskij, F. Orsitto, L. Poperenko, S. Solodovchenko, A. Stan, T. Sugie, M. Taniguchi, M. Vinnichenko, K. Vukolov, S. Zvonkov, Rev. Sci. Instrum. 72 (2001) 475. [3] J.N. Brooks, J.P. Allain, J. Nucl. Mater. 48 (2008) 045003. [4] L. Marot, G. De Temmerman, G. Covarel, A. Litnovsky, P. Oelhafen, Rev. Sci. Instrum. 78 (2007) 103507. [5] L. Marot, D. Mathys, G. De Temmerman, P. Oelhafen, Surf. Sci. 602 (2008) 3375. [6] L. Marot, R. Steiner, G. De Temmerman, P. Oelhafen, J. Nucl. Mater. 390–391 (2009) 1135. [7] M. Balden, A.F. Bardamid, A.I. Belyaeva, K.A. Slatin, J.W. Davis, A.A. Haasz, M. Poon, V.G. Konovalov, I.V. Ryzhkov, A.N. Shapoval, V.S. Voitsenya, J. Nucl. Mater. 329–333 (2004) 1515. [8] A. Litnovsky, P. Wienhold, V. Philipps, G. Sergienko, O. Schmitz, A. Kirschner, A. Kreter, S. Droste, U. Samm, Ph. Mertens, A.H. Donné, J. Nucl. Mater. 363–365 (2007) 1395. [9] A. Litnovsky, G. De Temmerman, K. Vukolov, P. Wienhold, V. Philipps, O. Schmitz, U. Samm, G. Sergienko, P. Oelhafen, M. Buttner, I. Orlovskiy, A. Yastrebkov, U. Breuer, A. Scholl, Fusion Eng. Des. 82 (2007) (2007) 123. [10] L. Marot, G. De Temmerman, V. Thommen, D. Mathys, P. Oelhafen, Surf. Coat. Tech. 202 (2008) 2837. [11] B. Eren, L. Marot, A. Litnovsky, M. Matveeva, R. Steiner, V. Emberger, M. Wisse, D. Mathys, G. Covarel, E. Meyer, Fusion Eng. Des. 86 (2011) 2593.

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[12] M. Passoni, D. Dellasega, G. Grosso, C. Conti, M.C. Ubaldi, C.E. Bottani, J. Nucl. Mater. 404 (2010) 1. [13] M. Matveeva, A. Litnovsky, L. Marot, B. Eren, E. Meyer, V. Philipps, A. Pospieszczyk, H. Stoschus, D. Matveev, and U. Samm, Material choice for first ITER mirrors under erosion conditions, in: 37th EPS Conf. on Plasma Physics, Dublin Ireland, 2010. http://ocs.ciemat.es/EPS2010PAP/pdf/ P2.105.pdf. [14] M. Matveeva, A. Litnovsky, L. Marot, B. Eren, V. Kotov, V. Philipps, H. Stoschus, D. Matveev, A. Pospieszczyk, P. Wienhold, U. Samm, and E. Meyer, Mirrors for ITER diagnostics: material choice and performance under erosion conditions, Journal of Nuclear Materials (submitted for publication). [15] K. Umstadter, R. Doerner, G. Tynan, Nucl. Fusion 51 (2011) 053014. [16] M. Baldwin, R. Doerner, D. Nishijima, K. Tokunaga, Y. Ueda, J. Nucl. Mater. 390– 391 (2009) 886. [17] D.M. Goebel, G. Campbell, R.W. Conn, J. Nucl. Mater. 121 (1984) 277. [18] L. Schmitz, R. Lehmer, G. Chevalier, G. Tynan, P. Chia, R. Doerner, R.W. Conn, J. Nucl. Mater. 176–177 (1990) 522. [19] Y. Yamamura, H. Tawara, At. Data Nucl. Data Tables 62 (1996) 149. [20] K.R. Umstadter, R. Doerner, G. Tynan, J. Nucl. Mater. 386–388 (2009) 751. [21] Y. Ueda, M. Toda, M. Nishikawa, K. Kondo, K.A. Tanaka, Fusion Eng. Des. 82 (2007) 1904. [22] K.S. Weißenrieder, J. Müller, Thin Solid Films 30 (1997) 300. [23] G.W. Brindley, G. Brown, Crystal Structures of Clay Minerals and their X-ray Identification, Mineralogical Society, London, 1980. p. 131.

[24] L. Marot, R. Schoch, R. Steiner, V. Thommen, D. Mathys, E. Meyer, Nanotechnology 21 (2010) 365707. [25] N. Ohno, S. Kajita, D. Nishijima, S. Takamura, J. Nucl. Mater. 363–365 (2007) 1153. [26] M.J. Walsh, M. Beurskens, P.G. Carolan, M. Gilbert, R.B. Huxford, M. Loughlin, A.W. Morris, V. Riccardo, C. Walker, Y. Xue, Rev. Sci. Instrum. 77 (2006) 10E525. [27] G.A. Naylor, R. Scannell, M. Beurskens, M.J. Walsh, I. Pastor, A.J.H. Donné, B. Snijders, W. Biel, B. Meszaros, L. Giudicotti, R. Pasqualotto, L. Marot, J. Instrum. 7 (2012) C03043. [28] A.E. Costley, IEEE Trans. Plas. Sci. 38 (2010) 2934. [29] W. Möller, W. Eckstein, J.P. Biersack, Comput. Phys. Commun. 51 (1988) 355. [30] G. De Temmerman, M.J. Baldwin, R.P. Doerner, D. Nishijima, R. Seraydarian, K. Schmid, F. Kost, C. Linsmeier, L. Marot, J. Appl. Phys. 102 (2007) 083302. [31] Y. Abe, K. Kato, M. Kawamura, K. Sasaki, Surf. Sci. Spectra 8 (2001) 117. [32] M.J. Baldwin, R.P. Doerner, Nucl. Fusion 48 (2008) 035001. [33] N. Yoshida, Irradiation effects in tungsten plasma facing materials for ITER and beyond, . [34] L. Marot, P. Oelhafen, G. De Temmerman, A. Litnovsky, and V. Voitsenya Plasma exposure of rhodium mirrors, 14 ITPA of TG on Diagnostics, Lausanne April 15th 2008. http://www.rijnhuizen.nl/ITPA/itpa-14.htm. [35] M. Joanny, J.M. Travere, S. Salasca, L. Marot, E. Meyer, C. Thellier, C. Cammarata, G. Gallay, J. J Ferme, IEEE T. Plasma. Sci. 40 (2012) 692.