[en] Eurofer97, P92 and Fe samples were exposed simultaneously in the linear plasma device PSI-2 to deuterium plasma and with addition of He, Ar or Kr at elevated temperature in the range of 800–950 K. Samples were exposed to plasma with an incident ion energy of 60–80 eV and an incident ion fluence of 5 × 10²⁵ m⁻². Surface morphology investigation of exposed samples reveals formation of fuzz-like structures on the surface steels when exposed to D and D+He plasma. Tungsten enrichment on the surface of the steels was observed after all exposures, but the average tungsten concentration varied from 4 to 18 at.%, depending on plasma composition. The highest tungsten enrichment on the surface was observed for samples exposed to D+He plasma. Addition of He to the plasma increased D retention, which was attributed to near-surface bubble formation, confirmed by transmission electron microscopy studies. (topical issue article)

[en] Highlights: • The deuterium permeation flux through Eurofer97 and 316L(N)-IG was successfully measured and compared. • Oxidation or roughening of the surface leads to a small reduction of the permeation flux compared to polished surfaces. • A technical surface will reduce the deuterium permeation flux by less than an order of magnitude. -- Abstract: The development and application of tritium permeation barriers (TPB) are crucial for safe and economical fusion reactor operation. In order to specify the requirements and important characteristics of TPB, the deuterium permeation flux through two different fusion relevant steels, namely Eurofer97 and 316L(N)-IG, were measured and compared. Furthermore, the influence of oxidized and rough surfaces on the deuterium permeation flux was investigated. With this study, the influence of technical or plasma roughened surfaces on the permeation behavior can be estimated.

[en] EUROFER, P92 steel and iron samples were exposed in the linear plasma generator PSI-2 at a sample temperature of about 470–500 K with an incident ion flux of about 3–5 × 10²¹ m⁻² s⁻¹, an incident ion fluence of 1 × 10²⁶ m⁻² and an incident ion energy of 60–70 eV. Samples were exposed to deuterium plasma and with additional seeding impurities of He, Ar, Ne, Kr or Kr + He. Laterally averaged surface W enrichment varied between 0.6 and 6 at.%, depending on the exposure conditions, measured by energy dispersive x-ray spectroscopy with low energy electron beam and Rutherford backscattered spectroscopy. Microstructure observation revealed a complex morphology depending on the plasma composition. W enrichment was mostly located in the spike nano structures. Addition of He to the plasma rounded and enlarged the spikes on the surface whereas addition of heavier species to the plasma resulted in smoothing the steels surface. In case of steel samples exposed to D + He plasma, fine nano-bubbles with sizes below 3 nm were found near the sample surface. Sputtering rate increases by one order of magnitude by Ar and Ne seeding and by two orders of magnitude by Kr seeding for both types of steels. Measured D retention increases with He addition and decreases with higher-Z species seeding. (paper)

[en] The erosion from plasma-facing components has to be monitored in many kind of laboratory and fusion plasmas. For this purpose, spectroscopy is an essential tool. Under certain conditions the particle flux can be calculated from the absolute line intensities of the sputtered material using so-called S/XB values. The impact of light reflection on the emission induced by sputtered particles at the mirror-grade polished surface of tungsten (W) and aluminum (Al) was investigated in a low-density (n _e ≈ 2 × 10¹² cm⁻³) and low-temperature (T _e ≈ $3\mathrm{e}\mathrm{V}$) argon (Ar) plasma in the linear plasma device PSI-2 using high-resolution spectroscopy. Using the line shape affected by Doppler shift we show that the light reflection has a considerable impact on the number of measured photons and has to be taken into account for calculating particle fluxes. The Al target was sputtered by Ar ions at the incident ion energy of $120\mathrm{e}\mathrm{V}$. The measured profile of the Al I line ($3961.52\stackrel{˚<!--\; ??\; -->}{\mathrm{A}}$) was compared with a Doppler-shifted emission model based on the Thompson energy distribution function. In this new model, the instrumental broadening and the impact of the Zeeman effect were also taken into account. The parameter for the high-energy fall-off n of the energy distribution function ($\propto <!--\; ???\; -->1/E^{n+1}$), the surface binding energy E _b and the surface reflectance were derived by comparing the experimental and the synthetic spectrum. The W target was sputtered by Ar ions at incident ion energies in the range of $30$–$160\mathrm{e}\mathrm{V}$. The influence of the ion impact energy on the energy distribution of the sputtered particles was demonstrated. (topical issue article)

[en] Highlights: • Deuterium retention in RAFM steel EUROFER’97, steel P92, pure iron and tungsten was measured after exposure to deuterium/helium plasma at fluences ≥ 10²⁶ D/m². • Exposure resulted in D retention for steels in the 10¹⁹ D/m² range predominantly at depths ≥8.6 µm. • The steels studied exhibited similarities in D detrapping behavior and dependence on He admixture. • Surface composition and morphology are assumed not to play a critical role in D retention, which is mostly attributed to carbide precipitates and grain boundaries in steel bulk material. - Abstract: Deuterium retention and detrapping behavior in the ferritic-martensitic steels EUROFER’97 and P92 after exposure to plasma at high fluences ≥ 10²⁶ D/m² was studied using thermal desorption spectroscopy (TDS), supported by nuclear reaction analysis. Low-temperature irradiation at 450 K and fluences ≥ 10²⁶ D/m² with low impact energy D⁺ / D⁺+He⁺ ions of 40 eV at PSI-2 resulted in a deuterium inventory of 7–18 × 10¹⁹ D/m² predominantly at depths ≥8.6 µm. Helium admixture led to a reduction of total D retention in both steels, irrespective of surface erosion and composition. The deuterium spectra of both steels displayed one D₂ desorption peak at ∼ 540–570 K and HD maxima at 540–590, 700–730 and 900–930 K. It is suggested that deuterium is mostly retained in the bulk of steel material on interfaces of carbide precipitates and on grain boundaries.

[en] Highlights: • Kr and Ar seeding of D plasma did not result in the formation of new D trapping sites in RAFM steel Eurofer’97 and commercial steel P92. • The majority of the retained D was trapped in the steel bulk, irrespective of the substantial surface damage by ion impact and effective removal by Kr and Ar sputtering. • D desorption from the bulk material of the ferritic-martensitic steels after Kr/Ar-seeded plasma exposures exhibits lower, but similarly positioned desorption peaks as after pure D exposures. • Kr causes more intensive steel sputtering than Ar, and therefore results in much less D retention. - Abstract: Total deuterium (D) retention from the bulk material of reduced-activation ferritic-martensitic (RAFM) steel Eurofer’97 (EU’97) and commercial ferritic-martensitic grade P92 was traced experimentally by means of thermal desorption spectroscopy (TDS) and linked to the role of krypton (Kr) and argon (Ar) seeding during high-fluence plasma exposure. The influence of impurity seeding on the steel microstructure was determined using scanning electron microscopy (SEM) and extended with focused ion beam (FIB) cross-sectioning. D capture at depths in the µm range was measured by nuclear reaction analysis (NRA). Plasma exposure of the steel samples occurred at 470 K with ion energy of 30–40 eV in the linear plasma device PSI-2 with up to 10% simultaneous impurity admixture in plasma. D inventory achieved values in the 10²⁰ D/m² range after plasma exposure with high fluences of up to 1 × 10²⁶ D⁺/m². In pure and mixed plasmas, the majority of D was trapped in the steel bulk. The Kr and Ar seeding of D plasma resulted in the population of multiple already existing D trapping sites. A similarity of D desorption spectra suggests that D trapping in P92 follows the same mechanism as in EU’97. Kr and Ar seeding mostly contributed to the surface sputtering of the steel samples, yielding less D retention due to the material loss.

[en] Fusions reactors have to handle numerous specifications before being able to show viable commercial operation, one of which is to find a proper Plasma Facing Material (PFM) which can withstand the high heat loads of several tens of megawatts per square meters combined with the pulse operation of a tokamak and many other problematics (Brezinsek et al 2017 Nucl. Fusion 57 116041). Nowadays, only tungsten is considered as a PFM for high heat flux areas of a tokamak divertor. Tungsten has been selected due to its favorable physical properties, but tungsten has a major drawback: it is brittle under temperatures typically used for water-cooled plasma-facing components (PFC). Under these temperatures the damage threshold due to thermal fatigue induced by ELM is very low, which will dramatically reduce the life-time of the tungsten PFC. The ANSYS simulations and experiments with a millisecond pulsed laser demonstrate a strongly improved ability to withstand thermal fatigue by micro-structuring of the tungsten surface with the help of 150–240 μm diameter tungsten fibres. (topical issue article)

[en] Polycrystalline, recrystallized W targets were subjected to implantation with 20 MeV W⁶⁺ ions in order to simulate radiation damage caused by fusion neutrons. Three samples with cumulative damage of 0.01, 0.1 and 0.89 dpa were produced. The near-surface zone of each sample has been analyzed by transmission electron microscopy (TEM). To this end, lamellae oriented perpendicularly to the targets implanted surface were milled out using focused ion beam (FIB). A reference lamella from non-irradiated, recrystallized W target was also prepared to estimate the damage introduced during FIB processing. TEM studies revealed a complex microstructure of the damaged zones as well as its evolution with cumulative damage level. The experimentally observed damage depth agrees very well with the one calculated using the Stopping and Range of Ions in Matter (SRIM) software

[en] Highlights: • In-situ LEIS is utilised to detect W surface enrichment by preferential sputtering. • EUROFER steel is irradiated with 250 eV D atoms at various temperatures up to 800 K. • W surface enrichment is detected at room temperature but vanishes at 800 K. • An upper limit for the diffusion activation energy of W in EUROFER is 1.6 eV. - Abstract: Tungsten surface enrichment of EUROFER steel by 250 eV deuterium sputtering is in-situ measured using low energy He${}^{+}$ ion scattering spectroscopy. The samples are irradiated at various temperatures between 300 K and 800 K with a deuterium atom flux of 2 × 10¹⁸ m${}^{-2}$s${}^{-1}$ and maximum fluence up to 1.1 × 10²³ m${}^{-2}$. The measurements at room temperature show a clear increase of tungsten surface density, but already at 520 K the observed enrichment is only half as large. At a temperature of 800 K no tungsten surface enrichment is detectable. The obtained data yield an upper limit of 1.6 eV for the diffusion activation energy of tungsten in EUROFER.