• Energy Research

This work reports on the investigation of Eurofer-97 erosion behaviour when exposed to the deuterium plasma of the linear device GyM. The erosion dependence of Eurofer-97 on the deuterium ion fluence, Φ≤2.3×1025 m−2, and temperature of the samples, T = 600 K and 990 K, was addressed. A bias voltage of −200 V was applied to GyM sample holder during the experiments. Samples were deeply characterised by: profilometry, scanning electron microscopy, atomic force microscopy, energy-dispersive X-ray spectroscopy, time-of-flight secondary ion mass spectrometry, Rutherford backscattering spectroscopy and particle-induced X-ray emission.The behaviour of Eurofer-97 erosion rate with the ion fluence strictly depends upon temperature. At 600 K, it was ∼0.14 nm/s after 4.7 × 1024 m−2, then decreased, reaching a steady state value of ∼0.01 nm/s from 8.0 × 1024 m−2. At 990 K instead, the erosion rate was roughly constant around 0.019 nm/s for Φ≤1.24×1025 m−2. The value at 2.35 × 1025 m−2 was slightly lower. The erosion rate at 990 K was greater than that at 600 K for every ion fluence.Microscopy and surface analysis techniques showed that Eurofer-97 erosion rate dependence on Φ at 600 K was primarily determined by the preferential sputtering of iron and other mid-Z elements of the alloy, leading to a surface rich in W and Ta difficult to be sputtered. The erosion behaviour at 990 K was dominated by the morphology dynamics, instead. The different properties of the morphology developed at the two temperatures can explain the higher erosion rate at 990 K for all the ion fluences.

Net erosion of plasma-facing materials was investigated at the low-field-side (outer) strike-point area of the ASDEX Upgrade (AUG) divertor during H-mode discharges with small and frequent ELMs. To this end, Au andMo marker samples with different surface morphologies and geometries were exposed to plasmas using the DIMIIdivertor manipulator. The results were compared to existing erosion and deposition patterns from various LandH-mode experiments, in the latter case the main difference was the size and frequency of the ELMs.We noticed that increasing surface roughness reduces net erosion but less than what is the case in L-mode. Onthe other hand, net-erosion rates in H-mode are generally 2–5 times higher than the corresponding L-modevalues, in addition to which exposure in H-mode conditions results in strong local variations in the poloidal andtoroidal erosion/deposition profiles. The latter observation we associate with the large migration length, on theorder of several cm, of the eroded material, resulting in strong competition between erosion and re-depositionprocesses especially at poloidal distances > 50 mm from the strike point. Considerable net erosion wasmeasured throughout the analysed poloidal region unlike in L-mode where the main erosion peak occurs in thevicinity of the strike point. We attribute this qualitative difference to the slow decay lengths of the plasma fluxand electron temperature in the applied H-mode scenario.Both erosion and deposition require detailed analyses at the microscopic scale and the deposition patterns maybe drastically different for heavy and light impurities. Generally, the rougher the surface the more material willaccumulate on locally shadowed regions behind protruding surface features. However, rough surfaces alsoexhibit more non-uniformities in the quality or even integrity of marker coatings produced on them, thuscomplicating the analyses of the experimental data.We conclude that local plasma parameters have a huge impact on the PFC erosion rates and, besides incidentplasma flux, surface morphology and its temporal evolution have to be taken into account for quantitative estimatesof erosion rates and PFC lifetime under reactor-relevant conditions.

The use of tin (Sn) as a liquid metal for plasma facing components has been recently proposed as a solution to the high heat load issue on the divertor target plates in nuclear fusion reactors. Due to its low vapor pressure, low reactivity with hydrogen and good resilience to neutron impact, tin is a good candidate as plasma facing component. However its high atomic number poses concerns about plasma contamination. In this paper two fundamental aspects have been investigated: deuterium retention and erosion fluxes from the Sn surface towards the plasma. The samples were exposed to plasma inside the linear machine GyM in magnetic cusp configuration. This setup permits to expose free liquid specimens without the need for the Capillary Porous System. Moreover it permits to lower the magnetic field in order to increase Sn Larmor radius and consequently limit Sn re-deposition in erosion experiments. Ex-situ analyses by ion beam diagnostics on solid samples exposed to deuterium plasma have proved that the amount of retained atomic deuterium is very low, approximately 0.18 at% estimated by Nuclear Reaction Analysis and 0.25 at% estimated by Elastic Recoil Detection Analysis. In the framework of erosion studies, the spectroscopic parameter S/XB was evaluated in Ar plasma for the SnI line at 380.1 nm by Optical Emission Spectroscopy and mass loss measurements in the 5-11 eV Te range, at a density ne ~ 1.5×1011 cm-3. An average value of 150 ± 23 was obtained.

Nitrogen used to reduce the power load onto the divertor in tokamak fusion devices (JET, AUG) has the drawback of ammonia formation. Non-negligible quantity of tritiated ammonia could be a serious concern for ITER, since it cannot be reduced through the presently designed fuel cycle loop. In this paper we report a study of the production of ND 3 as a function of the electron temperature (T e ) and neutral pressure in a N 2 /D 2 plasma mixture in the linear machine GyM. The nitrogenized compounds were monitored by Optical Emission Spectroscopy (OES) and Mass Spectrometry (MS). Measurements were performed at different values of T e ranging from 3 eV to 6 eV by varying the microwave power (2.45 GHz, up to 0.6 kW cw) that sustains the plasma, and for different neutral pressure at a constant ratio of nitrogen and deuterium partial pressures. The effect of introduction of He or Ar in the N 2 /D 2 mixtures has been also investigated. The ND 3 produced during plasma experiments has been quantified with a dedicated setup based on an in-line LN 2 trap and Liquid Ion Chromatography (LIC). Mass-spectrometry results showed that ND 3 is formed only during the plasma phase of the experi- ment while LIC showed that ammonia production increases with T e and with the total neutral pressure. Optical Emission Spectroscopy confirms the presence of ND species in all the cases studied. The addition of He and Ar in the N 2 /D 2 plasma, not modifing T e and n e , causes a reduction of ammonia formation, that is negligible in the case of Ar addition, while in the case of He reaches 80%. This suggests that He modifies the physical chemical process occurring at the wall, where the adsorbed He inhibits the reac- tions leading to ammonia formation at the metallic surface of the vessel. These results confirm the active role of a metallic surface in ammonia production and indicate He injection as a promising solution to limit the formation of the tritiated ammonia in the N seeded plasma of ITER. ©2017 The Authors. Published by Elsevier Ltd.

Nitrogen (N) seeding is routinely applied in tokamaks with tungsten (W) walls to control the power exhaust toward the divertor. Open questions, concerning the interaction of N with W, are the influence of ion energy and W temperature on retention of implanted N and the erosion by deuterium (D) of the tungsten nitride being formed. Moreover, the extremely high particle fluxes in ITER and DEMO will erode the W tiles and the sputtered atoms will re-deposit forming W-based layers with a different behaviour toward the interaction with N seeded D plasmas. In this work, W films with different morphology and structure were exposed to the N seeded D plasma of the linear device GyM, in order to address all these issues. The experiments were performed at the fixed N2/D2 partial pressure ratio of ~4% keeping the total pressure constant at 5.3×10-4 mbar. The exposure conditions were: (i) sample temperature of ~850 K, (ii) particle fluxes of 2-2.2×1020 ions?m-2?s-1 and (iii) particle energies up to ~320 eV. W columnar films (c-W) with properties close to those of virgin W coatings deposited on the tiles of JET Iter-Like Wall and ASDEX Upgrade and W amorphous films (a-W) resembling nanostructured W-based deposits found in present-day tokamaks and expected in ITER and DEMO, were considered. W columnar and amorphous coatings were produced by means of magnetron sputtering and pulsed laser deposition, respectively. The specimens were characterised by profilometry, X-ray depth-profiling photoelectron spectroscopy, optical microscopy, scanning electron microscopy, atomic force microscopy and X-ray diffraction. The main evidence is that the behaviour of the W films upon D+N plasma exposure in GyM strictly depends on their morphology and nanostructure. For all the films, a surface N-enriched layer, which is thermally stable and does not decompose at least up to ~850 K, is observed. Moreover, blisters are not present on the surface of the samples. The c-W coatings erode faster than the a-W ones and have a higher nitrogen retention and diffusivity. The mechanisms behind these results are here discussed together with their possible implications from the point of view of the topic of plasma-wall interaction in tokamaks.

In this work, we discuss the exposures of polycrystalline tungsten (W) and nanostructured W coatings to high energy (~30 300eV ÷ ) medium-flux D (~10 m s 20 2 1) deuterium plasmas of the linear machine GyM at an overall fluence of 10 m24 2. The energy and particle flux are tailored to that of charge-exchange neutrals at the vessel main chamber of present-day tokamaks and expected in ITER. Scanning Electron Microscopy measurements show the formation of surface nanostructures that are found to strongly depend on the W morphology and crystallinity, as well as on the energy of the ion species. The obtained nanostructures are few nanometres deep and similar to the one obtained after exposure of bulk W to divertor-like conditions. Dedicated annealing experiments are performed on selected samples, showing that the nanostructures developed on W coatings are thermally stable up to their recrystallisation temperature. The development of micrometric-sized blisters is also observed and found to strongly depend on the features of the W coatings, the substrate and the applied bias voltage.

Iron-tungsten (Fe-W) mixed films were exposed to the low flux deuterium plasma of GyM in order to study the behavior of the sputtering yield with the ion fluence and temperature of the samples. From literature, it is known that an increase of the former lowers the Fe-W layers' sputtering yield as a consequence of the preferential sputtering of Fe leading to an enrichment in W of the outermost layers. An opposite trend was instead found for the latter probably due to the inter-diffusion of Fe and W (effective from 200 degrees C) resulting in the suppression of the W enrichment. Moreover, from 500 degrees C, also W segregation to the surface occurs. What is missing from literature is a systematic investigation of the role of temperature on W enrichment. In this work, dedicated experiments in GyM were carried out to fill this gap. After exposure, W enrichment was evaluated by Rutherford Backscattering Spectrometry (RBS) and inferred from measuring the eroded thickness of the samples using RBS and profilometer. Concerning the Fe-W sputtering yield as a function of fluence, it decreases by a factor of similar to 3 between the lowest (3.0 x 10(22) D+ m(-2)) and the highest fluence (9.0 x 10(23) D+ m(-2)) values considered. The other main result is that, at the lowest fluence, the exposure at room temperature leads to an erosion of the Fe-W samples more pronounced than that associated to the exposure at 500 degrees C. (C) 2017 The Authors. Published by Elsevier Ltd.