•D plasma ions impact on BMG leads to D absorption and modification of some properties.•At D ion energy< 120 eV, the amount of D absorbed increases linearly with ion fluence.•At D ion energy> 450 eV, ...the sputtering process dominates.•Plane BMG samples exposed in D plasma are bent due to D trapping in near-surface layer.
Samples of two kinds of bulk metallic glasses (BMG) with different compositions, Zr48Cu36Al8Ag8 and Zr57Cu15.4Al10Ni12.6Nb5, were polished to mirror-like quality and, after measuring the initial reflectance at normal light incidence, were subjected to impact of ions of deuterium or argon plasma with fixed energy. The main attention was paid to phenomenon on deuterium absorption that reveals as the weight gain after every exposure of the samples in deuterium plasma. For a fixed ion energy (in the range<100 eV), the amount of absorbed deuterium increased linearly with increasing of the ion fluence. Bending of the samples was observed when one side of sample was bombarded by deuterium plasma ions. At that, the radius of curvature of samples was decreasing with increasing the amount of trapped deuterium in such a way that its curvature was increasing proportionally to the amount of trapped deuterium. This is an evidence of near-surface distribution of the trapped deuterium. The results demonstrate that D ions within a fluence of ∼2·1025 ions/m2 are initially accumulated in a thin near-surface layer of the samples. Further, they are noted effects of stress relaxation and of deuterium redistribution.
The effect of heavy sputtering and of neutron irradiation simulated by displacement damaging with of 20MeV W6+ ions on the optical properties of tungsten mirrors was studied. Ar+ ions with 600eV of ...energy were used as imitation of charge exchange atoms ejected from fusion plasma. The ion fluence dependence of the surface topography and the optical properties of polycrystalline, recrystallized tungsten (grain size 20–100μm) were studied by optical microscopy, interferometry, reflectometry and ellipsometry. Furthermore, after sputtering in total a layer of 3.9μm in thickness, the orientation and the thickness of the eroded layer of many individual grains was determined by electron backscattering diffraction and confocal laser scanning microscopy. Concluding from the obtained data the neutron irradiation, at least at the damage level would be achieved in ITER, has not to make an additional contribution in the processes developing under impact of charge exchange atoms only.
Mirrors will be used in all optical and laser-based diagnostic systems of ITER. In the severe environment, the optical characteristics of mirrors will be degraded, hampering the entire performance of ...the respective diagnostics. A minute impurity deposition of 20
nm of carbon on the mirror is sufficient to decrease the mirror reflectivity by tens of percent outlining the necessity of the mirror cleaning in ITER. The results of R&D on plasma cleaning of molybdenum diagnostic mirrors are reported. The mirrors contaminated with amorphous carbon films in the laboratory conditions and in the tokamaks were cleaned in steady-state hydrogenic plasmas. The maximum cleaning efficiency of 4.2
nm/min was reached for the laboratory and soft tokamak hydrocarbon films, whereas for the hard tokamak films the carbidization of mirrors drastically decreased the cleaning efficiency down to 0.016
nm/min. This implies the necessity of sputtering cleaning of contaminated mirrors as the only reliable tool to remove the deposits by plasma cleaning. An overview of R&D program on mirror cleaning is provided along with plans for further studies and the recommendations for ITER mirror-based diagnostics.
Because tungsten (W) is used in present fusion devices and it is a reference material for ITER divertor and possible plasma-facing material for DEMO, we strive to understand the response of different ...W grades to ion bombardment. In this study, we investigated the behavior of mirrors made of four polycrystalline W grades under long-term ion sputtering. Argon (Ar) and deuterium (D) ions extracted from a plasma were used to investigate the effect of projectile mass on surface modification. Depending on the ion fluence, the reflectance measured at normal incidence was very different for different W grades. The lowest degradation rate of the reflectance was measured for the mirror made of recrystallized W. The highest degradation rate was found for one of the ITER-grade W samples. Pre-irradiation of a mirror with 20-MeV W6+ ions, as simulation of neutron irradiation in ITER, had no noticeable influence on reflectance degradation under sputtering with either Ar or D ions.
•The reflectance of W mirror specimens pre-damaged with W ions to a dose of 0.45 and 1.45 dpa behave just as the undamaged specimens when being subjected to sputtering with ions of Ar or D plasmas.•Behavior of the reflectance depends significantly on the W grade, namely, on the microstructure of W, such as distributions of grains on their size and orientation relative to the surface of the sample.•The dominant factor that defines the reflectance and surface roughness is the difference between the heights of grains with different sputtering rates of faces differently orientated relatively to the sample surface.
Simulation of neutron irradiation and sputtering on ITER-grade tungsten was studied. The effects of neutron-induced displacement damage have been simulated by irradiation of tungsten target with W6+ ...ions of 20MeV energy. Bombardment by Ar+ ions with energy 600eV was used as imitation of impact of charge exchange atoms in ITER. The sputtering process was interrupted to perform in between measurements of the optical properties of the eroded surface and the mass loss. After sputtering was finished, the surface was thoroughly investigated by different methods for characterizing the surface relief developed due to sputtering. The damaging to, at least, the level that would be achieved in ITER does not lead to a decisive additional contribution to the processes under impact of charge exchange atoms only.
The surface topography and optical properties of recrystallized tungsten exposed to a low-energy (38
eV/D), high flux (10
22
D/m
2
s) deuterium plasma with an ion fluence of 10
26
D/m
2 at various ...temperatures was investigated. It was found that the surface morphology weakly depends on the exposure temperature in the range 320–695
K with the exception of the narrow temperature region around 535
K, where large changes to all optical characteristics occurs. After plasma exposure at this temperature, the surface topography of the W sample is characterized by active blistering as has already been indicated in previous publications. The reflectance found in direct measurements at normal incidence drops in the wavelength interval 220–650
nm, whereas the estimations of reflectance using the ellipsometry data demonstrate some increase.
Almost all optical diagnostics systems on ITER will be based on in-vessel metallic first mirrors. The possible deterioration of their surface reflectivity as a result of erosion by charge-exchange ...neutrals and re-deposition of material eroded from the plasma-facing components represents a serious concern for the reliability of spectroscopic and laser signals. A concerted effort within the tokamak community has been initiated to characterize these effects and seek mitigation methods. To date, the different damaging effects (erosion and deposition) have always been considered independently, neglecting any role that may be played by the substrate. In this contribution, we attempt to assess the influence of substrate material using different approaches: laboratory simulation experiments, mirror exposures in the divertor region of the TCV tokamak and numerical simulations using the Monte-Carlo code TRIDYN. Our results have potentially important implications for the choice of ITER first mirror materials.
The reflectance of Be mirrors due to impact by ions from a deuterium plasma has been studied under several bombardment conditions. Analysis of the resulting surface films has been performed using ...various diagnostic techniques, with the conclusion that the primary factor leading to the decrease in reflectance following bombardment with energetic ions is the conversion of the surface oxide layer, composed of BeO, to the hydroxide, Be(OD)
2, with a corresponding increase in the optical extinction coefficient. The increase in the thickness of the layer is also important. Modifications to the surface layer are thought to involve a balance between the ion-induced diffusion of Be atoms to the surface where they may react with incident D and O atoms, and physical and chemical sputtering processes. For incident ion energies less than ∼50
eV, chemical reactions leading to disoxidation of the oxide–hydroxide film dominate, while keV-range ions (primarily D, but with some O impurities) lead to the formation of hydroxide, and an increase in the surface layer thickness.
Metallic mirrors are foreseen in ITER diagnostic systems as optical elements directly viewing the plasma radiation. In the frame of an EFDA contract, metallic mirror samples have been exposed for ...long pulse plasma discharges in Tore Supra (TS) in order to investigate surface modifications caused by erosion and re-deposition processes. Three different materials have been selected: mono-crystalline molybdenum (mc-Mo), polycrystalline stainless steel (SS) and copper (Cu). The mc-Mo samples showed after TS exposure almost no surface roughness modifications and the lowest net-erosion. A slight reflectivity reduction, most pronounced in the near UV, is attributed to light absorption in a thin carbon deposit. Cu mirrors showed by far the highest surface roughness, erosion and diffusive reflectivity. Comparative laboratory glow discharge experiments with virgin reference samples and numerical simulations of erosion/deposition confirm the dominant contribution of conditioning procedures to erosion of mirrors exposed (without shutter protection) in Tore Supra.
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