Search

Tungsten samples were heat-treated to achieve partial recrystallization and exposed to high ion flux deuterium plasma at different temperatures and fluences. Continuous stiffness nanoindentation measurements of near-surface hardness were performed in the grains of specific annealing states and of specific crystallographic orientation, determined by electron backscatter diffraction (EBSD); indentation pile-up was investigated using surface profilometry. Bulk hardness of unexposed tungsten does not strongly depend on grain orientation, but depends on the annealing state of the grain, with values between ∼4.3 GPa for recrystallized grains and ∼5.5 for non-recrystallized ones. Grains with surface normal orientation feature the least pile-up, while grains with orientation the most; pile-up also depends on the annealing state, being generally lower in recrystallized grains. Plasma exposure leads to the increase of hardness, most significantly near the surface. The width of plasma-affected zone increases with the increase of exposure temperature and fluence, as well in recrystallized grains, correlating with the increase of diffusion depth. Plasma exposure does not lead to the emergence of orientation-dependence of hardness. Both indentation pile-up and near-surface indentation pop-ins are generally suppressed by plasma exposure.

A collective Thomson scattering system has been developed for measuring ion temperature, plasma velocity and impurity concentration in the high density magnetized Magnum-PSI plasma beam, allowing for measurements at low temperature (4 × 1020 m−3, while avoiding laser plasma heating caused by inverse Bremsstrahlung. The collective Thomson scattering system is based on the fundamental mode of a seeded Nd:YAG laser and equipped with an LIVAR M506 camera (EBABS technology). The first collective Thomson scattering measurements are taken at the linear plasma generator Pilot-PSI, 40 mm downstream of the cascaded arc source. At this location, the ion temperature is about equal to the electron temperature in the bulk of the plasma beam.

Diamond is a promising candidate for enhancing the negative-ion surface production in the ion sources for neutral injection in fusion reactors; hence evaluation of its reactivity towards hydrogen plasma is of high importance. Single crystal and polycrystalline diamond samples were exposed in Pilot-PSI with the D+ flux of (4‒7)·1024 m−2s−1 and the impact energy of 7–9 eV per deuteron at different surface temperatures; under such conditions physical sputtering is negligible, however chemical sputtering is important. Net chemical sputtering yield Y = 9.7·10−3 at/ion at 800 °C was precisely measured ex-situ using a protective platinum mask (5 × 10 × 2 μm) deposited beforehand on a single crystal followed by the post-mortem analysis using Transmission Electron Microscopy (TEM). The structural properties of the exposed diamond surface were analyzed by Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS). Gross chemical sputtering yields were determined in-situ by means of optical emission spectroscopy of the molecular CH A-X band for several surface temperatures. A bell-shaped dependence of the erosion yield versus temperature between 400 °C and 1200 °C was observed, with a maximum yield of ∼1.5·10−2 at/ion attained at 900 °C. The yields obtained for diamond are relatively high (0.5–1.5)·10−2 at/ion, comparable with those of graphite. XPS analysis shows amorphization of diamond surface within 1 nm depth, in a good agreement with molecular dynamics (MD) simulation. MD was also applied to study the hydrogen impact energy threshold for erosion of [100] diamond surface at different temperatures.

International audience; This study demonstrates a fast one-step synthesis method for nanostructured carbon microparticles on graphite samples using high-flux plasma exposure. These structures are considered as potential candidates for energy applications such as Li-ion batteries and supercapacitors. The samples were exposed to plasmas in the linear plasma generator Pilot-PSI with an average hydrogen ion-flux of ~10 24 m-2 s-1. The parameter window was mapped by varying the ion energy and flux, and surface temperature. The particle growth depended mainly on the sample gross-erosion and the resulting hydrocarbon concentration in the plasma. A minimum concentration was necessary to initiate particle formation. The surface of the sample was covered with microparticles with an average growth rate of 0.2 μm/s, which is significantly faster than most chemical methods. The particles were initially volumetrically grown in in the gas-phase by a multi-phase process and after deposition on the sample their growth proceeded. Scanning and transmission electron microscopy reveal that the core of these microparticles can be made of an * Corresponding author. E-mail: d.aussems@differ.nl (D.U.B. Aussems) 2 agglomeration of nanoparticles, surrounded by crumpled layers of carbon nanowalls. Gas sorption analysis shows sufficient meso-and macropores for fast mass transport. In conclusion, this processing technique could be a novel synthesis route to nanostructure surfaces for electrochemical applications.

Cavity-ring-down spectroscopy (CRDS) was implemented to measure the re-deposition of liquid tin under a high flux plasma beam in the linear plasma device Pilot-PSI. A capillary porous system (CPS) consisting of a molybdenum cup and tungsten meshes (pores diameters of 0.2 mm and 0.44 mm) was filled with tin and exposed to argon plasma. The absorption of a UV laser-beam at 286.331 nm was used to determine a number of sputtered neutral tin atoms. The incoming flux of argon ions of ∼50 eV was 1.6-2.7 × 1023 m-2 s-1, and the sample temperature measured by pyrometry varied from 850 °C to 1200 °C during exposures. The use of CRDS for measuring absolute number of particles under such plasma exposure was demonstrated for the first time. The number of sputtered tin particles in the cavity region assuming no losses would be expected to be 5.5 × 1011-1.2 × 1012 while CRDS measurements showed only 5.7-9.9 × 108. About 98-99.8% of sputtered particles were therefore found to not reach the CRDS observation volume. Spectroscopic ratios of Sn I to Sn II ions, as well as equilibrium considerations, indicate that fast ionization as well as plasma entrainment of neutrals is responsible for the discrepancy. This would lead to high re-deposition rates, implying a lowered contamination rate of core plasma and lower required replenishment rates at high-flux conditions than would otherwise be expected.

Liquid metal infused capillary porous structures (CPSs) are considered as a potential divertor solution for DEMO due to their potential power handling capability and resilience to long term damage. In this work the power handling and performance of such Sn-based CPS systems is assessed both experimentally and via modelling. A Sn-CPS target was exposed to heat fluxes of up to 18.1 MW m−2 in He plasma in the Pilot-PSI linear device. Post-mortem the target showed no damage to nor any surface exposure of the underlying W-CPS felt. The small pore size (∼40µm) employed resulted in no droplet formation from the target in agreement with calculated Rayleigh-Taylor and Kelvin-Helmoholtz instability thresholds. The temperature response of the Sn-target was used to determine the thermal conductivity of the mixed Sn-CPS material using COMSOL modelling. These values were then used via further finite element analysis to extrapolate to DEMO relevant monoblock designs and estimate the maximum power handling achievable based on estimated temperature windows for all component elements of the design. For an optimized design a heat-load of up to 20 MW m−2 may be received while the use of CPS also offers other potential design advantages such as the removal of interlayer requirements. Keywords: Liquid-metal, DEMO, CPS, Tin, Power-exhaust, Plasma-wall interaction

Samples made of tungsten (W) doped either with titanium carbide (W-1.1TiC) or tantalum carbide (W-3.3TaC) were exposed to a low-energy (40 eV/D), high-flux (1.8–5 × 10 23 D/m 2 s) deuterium (D) plasma at temperatures of about800 K, 1050 K, and 1250 K to a fluence of about1 × 10 27 D/m 2 . The deuterium (D) inventory in the samples was examined by nuclear reaction analysis and thermal desorption spectroscopy. At 800 K the D bulk concentrations and total D inventories in W-1.1TiC and W-3.3TaC were more than one order of magnitude higher compared to that in pure polycrystalline W. At 1050 K and 1250 K the D concentrations in all types of samples were very low (≤10 −5 at. fr.); however the D inventories in W-1.1TiC were significantly higher compared to those in W-3.3TaC and pure W. It is suggested that D trapping inside the carbide precipitates and at their boundaries is essential at high temperatures and high incident fluxes, especially in W-1.1TiC.

Samples with tungsten (W) and tungsten-yttrium (Y) coatings on molybdenum were exposed to plasmas simulating the divertor conditions of ITER. The exposed surfaces were studied using in-situ laser induced breakdown spectroscopy diagnostics (LIBS) and the results were compared to those obtained by other surface characterization methods. Our results show that LIBS is a reliable technique for in situ monitoring of erosion, deposition, and fuel retention processes under reactor-relevant conditions. In the regions of the highest gross erosion the thickness of the remaining tungsten layer is thinnest and at the central part of samples both mixing and deposition of components took place. LIBS also allowed reliable recording of D retention. D accumulation is influenced by the surface temperature as well as by the surface irregularities.

In this article, laser induced breakdown spectroscopy (LIBS) has been used to study plasma-induced erosion processes. Samples with ITER-relevant coatings were exposed to controlled plasma fluxes whose parameters were characteristic to those occurring in the reactor walls. After the experiments, erosion of the coatings and changes in their surface structure were determined using LIBS and a set of other surface-diagnostics methods. This way, the applicability of LIBS for in situ erosion/deposition measurements, particularly the influence of plasma exposure on the obtained LIBS spectra could be investigated in detail. Elemental depth profiles, extracted from the LIBS results matched well with those obtained by the other surface analysis methods. However, we observed that plasma-induced changes of the crystalline structure of target materials can significantly alter the laser-ablation rate. This is the main factor which influences the reliability of results obtained by LIBS.

Hydrocarbon injection experiments on molybdenum targets facing high-density plasmas in Pilot-PSI were simulated with the 3D Monte Carlo impurity transport and PSI code ERO. Impurity transport and calculation of redeposition profiles were decoupled by calculating carbon redistribution matrices with ERO. Redeposition was found to be strongly dependent on the electron density: greater densities result in a much more localized redeposition pattern. The calculated average number of recycling events of hydrocarbon molecules on the surface went up from 1.5 for n e = 5 × 10 19 m - 3 to 19.2 for n e = 4 × 10 20 m - 3 ; at the latter density, only 2.4% of the hydrocarbon molecules escapes the simulated plasma beam without returning to the target at least once. Agreement with experimental deposition profiles in argon was fair. The results in hydrogen point towards a strong gradient in chemical erosion yield along the target.

Methane was injected into a high density low temperature hydrogen plasma beam ( n e ⩽ 10 21 m −3 , T e ∼ 1 eV) in Pilot-PSI to study carbon transport and provide a benchmark case for modeling. Gas injection was performed through molybdenum samples that acted as collectors of redeposits. The CH A–X band emission plumes are localized near the injection hole with a typical width of ∼3 mm and a decay length of ∼1.5 mm. The geometry of the emission profile is not influenced by the electron temperature. The redeposition efficiency and codeposit structure depend on the surface temperature. Surface temperatures below 500 K promote growth of a-C:H films across the surface and lead to redeposition of up to 38% of injected carbon. Above 800 K the redeposition efficiency decreases strongly and carbon microparticles are found near the injection hole.

A steady-state high-flux H or He plasma beam was balanced against the pressure of a Sn vapor cloud for the first time, resulting in a self-regulated heat flux intensity near the liquid surface. A temperature response of the liquid surface characterized by a decoupling from the received heating power and significant cooling of the plasma in the neutral Sn cloud were observed. The plasma heat flux impinging on the target was found to be mitigated, as heat was partially dissipated by volumetric processes in the vapor cloud rather than wholly by surface effects. These results motivate further exploration of liquid metal solutions to the critical challenge of heat and particle flux handling in fusion power plants.

The first study of the interaction of metallic dust (tungsten, aluminum) adhered on tungsten substrates with transient plasma heat loads is presented. Experiments were carried out in the Pilot-PSI linear device with transient heat fluxes up to 550 MW m−2 and in the DIII-D divertor tokamak. The central role of the dust-substrate contact area in heat conduction is highlighted and confirmed by heat transfer simulations. The experiments provide evidence of the occurrence of wetting-induced coagulation, a novel growth mechanism where cluster melting accompanied by droplet wetting leads to the formation of larger grains. The physical processes behind this mechanism are elucidated. The remobilization activity of the newly formed dust and the survivability of tungsten dust on hot surfaces are documented and discussed in the light of implications for ITER.

__

For tungsten exposed to low-energy hydrogen-plasmas, it has been thought that grains with 〈 1 1 1 〉 surface normal are most susceptible to blistering while those with 〈 0 0 1 〉 surface normal are virtually impervious to it. Here, we report results showing that non-uniformity of blister distribution depends on the state of the surface due to polishing. In electrochemically polished material blisters appear on the grains with all orientations, while in mechanically polished material blister-free areas associated with particular orientations emerge. On the other hand, blistering is shown to have a strong dependence on the level of deformation within particular grains in partially recrystallized material.

The concentration of helium in tungsten fuzz-like nanostructures has been measured by means of thermal desorption spectroscopy. Fuzz was formed on the W surface under intensive plasma irradiation at 1500 K. The helium content was measured first in the as-irradiated sample, and then in a similar sample with the fuzz mechanically scraped from the sample surface. The difference gave the He content in the fuzz, which was estimated to be He/ W = (13 ± 4)%.

Tungsten is considered as a plasma facing component in the divertor region of the International Thermonuclear Experiment Reactor (ITER). High flux, high fluence helium (He) exposure of tungsten surfaces induces severe morphology changes and nanostructure formation, which may eventually erode tungsten and risk the operation of the reactor. In this study, we investigate the response of ultrafine-grained tungsten under low flux (∼1020 ions m−2 s−1), low fluence, low energy (30–70 eV) He irradiation at different temperatures in order to study the early stage of nanostructure formation. Rod-shape nanostructures formed at low temperatures (600 °C) and a He fluence of 1 × 1023 m−2. High resolution, cross-section TEM images of irradiated grains demonstrated bubble formation not inside the nanostructures but deep inside the grains. At higher temperatures (900 °C) and the same fluence of 1 × 1023 m−2, large tungsten asperities (stone-shape and fiber-form structures), which are attributed to the burst and erosion of the surface grains. Moreover, low fluence (1020 m−2–1021 m−2) and high temperature (900 °C) irradiation demonstrated low density of non-coalesced bubbles inside the TEM samples. The results suggest that morphology changes can exist in He irradiated tungsten even with low bubble densities; thus, an additional factor such as surface stresses may dictate the observed nanostructure formation.

The effect of severe plastic deformation on the deuterium retention in tungsten exposed to high-flux low-energy plasma (flux ∼ 1024 D/m2/s, energy ∼ 50 eV, and fluence up to 3 × 1026 D/m2) at the plasma generator Pilot-PSI was studied by thermal desorption spectroscopy and scanning electron microscopy. The desorption spectra in both reference and plastically deformed samples were deconvolved into three contributions attributed to the detrapping from dislocations, deuterium-vacancy clusters, and pores, respectively. The plastically induced deformation, resulting in high dislocation density, does not change the positions of the three peaks, but alters their amplitudes as compared to the reference material. The appearance of blisters detected by scanning electron microscopy and the desorption peak attributed to the release from pores (i.e., deuterium bubbles) were suppressed in the plastically deformed samples but only up to a certain fluence. Beyond 5 × 1025 D/m2, the release from the bubbles in the deformed material is essentially higher than in the reference material. Based on the presented results, we suggest that a dense dislocation network increases the incubation dose needed for the appearance of blisters, associated with deuterium bubbles, by offering numerous nucleation sites for deuterium clusters eventually transforming into deuterium-vacancy clusters by punching out jogs on dislocation lines.

International audience; Dust trajectories have been recorded with an unprecedented, under fusion-relevant plasma conditions, spatial resolution of 9 mu m/pixel in Pilot-PSI. The optical setup allowed the use of fast cameras as a basic microscope. It is demonstrated that such a resolution is essential for the correct interpretation of experiments on several aspects of dust-surface interactions. Highly resolved tungsten dust dynamics measurements are presented from dedicated experiments on dust collisions with plasma facing components, motion in the vicinity of castellated samples and remobilization from planar samples.

Dust-surface collisions impose size selectivity on the ability of dust grains to migrate in scrape-off layer and divertor plasmas and to adhere to plasma-facing components. Here, we report first experimental evidence of dust impact phenomena in plasma environments concerning low-speed collisions of tungsten dust with tungsten surfaces: re-bouncing, adhesion, sliding and rolling. The results comply with the predictions of the model of elastic-perfectly plastic adhesive spheres employed in the dust dynamics code MIGRAINe for sub- to several meters per second impacts of micrometer-range metal dust.

Taking the example of tungsten, we demonstrate that high-flux plasma exposure of recrystallized and plastically deformed samples leads to principal differences in the gas trapping and associated surface modification. Surface of the exposed pre-deformed samples exhibits ruptured µ m-sized blisters, a signature of bubbles nucleated close to the surface on the plastically induced dislocation network. Contrary to the recrystallized samples, no stage attributable to gas bubbles appeared in the desorption spectrum of the deformed samples demonstrating the strong impact of dislocations on hydrogen retention.

Dust remobilization plays a key role in diverse plasma-wall interaction issues. The most important aspect of remobilization is connected with the identification of in-vessel dust accumulation sites and consequently the development of dust removal techniques. In ITER, PFCs will be castellated, and shaped, and gaps have been assumed to be a preferred dust accumulation site. Despite its importance, remobilization of dust grains in normal operating conditions has only been recently addressed. A novel experimental technique for its study in fusion plasmas has been developed and its main theoretical aspects have been analyzed [1]. The technique is based on controlled adhesion of dust grains on tungsten surfaces combined with detailed mapping of the dust deposition profile prior to and post plasma exposure. Experiments with planar tungsten surfaces have been carried out in Pilot-PSI [1]. Here we report on the recent experiments carried out in Pilot-PSI with the aim to study the effect of ELM-like pulses on remobilization under normal and glancing B field, as well as remobilization from castellated PFCs under steady-state plasma conditions. The observations by two fast cameras with a resolution down to ten micrometres per pixel allowed us for the first time to resolve the details of dust motion in the sheath. Discussion on the difference of the dust remobilization activity and release velocity under normal and ELM-like conditions is provided along with the first results of remobilization of W dust from ITER like castellated structures. We also present evidence of a particular phenomenon taking place under interaction of ELM-like plasma with W dust particles residing on the W surface - multiple molten grains tend to coagulate into a single nearly spherical grain.

Tungsten coatings were exposed to He/D2 plasma in the linear plasma machine Pilot PSI. Different He contents were used in the plasma. The sample surface temperature and plasma ion energy were varied. Changes in the surface morphology were studied using scanning electron microscopy (SEM) and x-ray diffraction (XRD). SEM images revealed that compared with the unexposed samples, there were significant changes in the surface morphology. The structures formed were strongly dependent on the experimental conditions. XRD spectra showed that exposure to plasma leads to an increase in the crystallite size and to a decrease in the lattice parameter.

This work discusses the response of ultrafine-grained tungsten materials to high-flux, high-fluence, low energy pure He irradiation. Ultrafine-grained tungsten samples were exposed in the Pilot-PSI (Westerhout et al 2007 Phys. Scr. T128 18) linear plasma device at the Dutch Institute for Fundamental Energy Research (DIFFER) in Nieuwegein, the Netherlands. The He flux on the tungsten samples ranged from 1.0 x 10(23)-2.0 x 10(24) ions m(-2) s(-1), the sample bias ranged from a negative (20-65) V, and the sample temperatures ranged from 600-1500 degrees C. SEM analysis of the exposed samples clearly shows that ultrafine-grained tungsten materials have a greater fluence threshold to the formation of fuzz by an order or magnitude or more, supporting the conjecture that grain boundaries play a major role in the mechanisms of radiation damage. Pre-fuzz damage analysis is addressed, as in the role of grain orientation on structure formation. Grains of (1 1 0) and (1 1 1) orientation showed only pore formation, while (0 0 1) oriented grains showed ripples (higher structures) decorated with pores. Blistering at the grain boundaries is also observed in this case. In situ TEM analysis during irradiation revealed facetted bubble formation at the grain boundaries likely responsible for blistering at this location. The results could have significant implications for future plasma-burning fusion devices given the He-induced damage could lead to macroscopic dust emission into the fusion plasma.

We have investigated the formation of various carbon nanostructures using extreme plasma fluxes up to four orders of magnitude larger than in conventional plasma-enhanced chemical vapor deposition processing. Carbon nanowalls, multi-wall nanotubes, spherical nanoparticles and nanotips are among the structures detected with electron microscopy methods. Precursor injection or surface pretreatment were not required for the synthesis of the nanostructures. Preliminary experiments with varied plasma composition, sample bias and surface temperature have demonstrated the potential for optimizing the growth of the nanostructures in the current experimental set-up.

The plasma-surface interactions expected in the divertor of a future fusion reactor are characterized by extreme heat and particle fluxes interacting with the plasma-facing surfaces. Powerful linear plasma generators are used to reproduce the expected plasma conditions and allow plasma-surface interactions studies under those very harsh conditions. While the ion energies on the divertor surfaces of a fusion device are comparable to those used in various plasma-assited deposition and etching techniques, the ion (and energy) fluxes are up to four orders of magnitude higher. This large upscale in particle flux maintains the surface under highly non-equilibrium conditions and bring new effects to light, some of which will be described in this paper.

We report measurements of chemical sputtering yields of graphite exposed to low temperature nitrogen plasmas. The influence of surface temperature and incoming ion energy on the sputtering yields has been investigated in two distinct ion flux density regimes. Sputtering yields grow consistently with increasing temperatures in experiments with low flux density (Gamma(i) approximate to 10(20) m(-2)s(-1) - 10(21) m(-2)s(-1)) and high flux density (Gamma(i) approximate to 10(23) m(-2)s(-1)). Moreover, empirical fitting of the data suggests that the temperature of 670 degrees C is optimal for chemical sputtering at high flux density. Negative biasing of the samples was used to vary the ion energy in the low flux density regime. The sputtering yield in this case increases from 0.07 atoms/ion for E-i = 1.5eV to 0.19 atoms/ion for E-i = 35 eV. After taking into account the dependence of the yields on temperature and ion energy, we evidenced a flux dependence of sputtering, similar to that found for chemical sputtering of carbon by hydrogen.

Fine-grain graphite samples were exposed to high density low temperature ( ⁠ n e ∼ 10 20 m − 3 , T e ∼ 1 eV ⁠ ) hydrogen plasmas in the Pilot-PSI linear plasma generator. Redeposition of eroded carbon is so strong that no external precursor gas injection is necessary for deposits to form on the exposed surface during the bombardment. In fact, up to 90% of carbon is redeposited, most noticeably in the region of the highest particle flux. The redeposits appear in the form of carbon microparticles of various sizes and structures. Discharge parameters influence the efficiency of the redeposition processes and the particle growth rate. Under favorable conditions, the growth rate reaches 0.15 μ m/s. The authors used high resolution scanning electron microscopy and transmission electron microscopy to study the particle growth mode. The columnar structure of some of the large particles points toward surface growth, while observation of the spherical carbon nanoparticles indicates growth in the plasma phase. Multiple nanoparticles can agglomerate and form bigger particles. The spherical shape of the agglomerates suggests that nanoparticles coalesce in the gas phase. The erosion and redeposition patterns on the samples are likely determined by the gradients in plasma flux density and surface temperature across the surface.

Experiments were performed in the Pilot-PSI linear plasma device to study the formation of helium-induced nanostructures under high heat fluxes (>10 MW m−2), the erosion of a tungsten surface at the elevated temperatures where those structures typically form and most importantly the behavior of a helium-induced fibreform structure during ELM-like pulses. A strong correlation between the characteristic size of the helium-induced morphology and the size of the voids observed in the near-surface region has been found, and both increase in size with increasing temperatures. Temperature-dependent erosion of tungsten surfaces was observed despite the ion energy being below the sputtering threshold, with an exponential increase of the erosion for temperatures higher than 2400 °C. The same effect was observed during ELM-like plasma pulses. Finally, a complete disappearance of the tungsten fibreform structure was observed after only one plasma pulse for energy densities higher than 0.5 MJ m−2.

Accidental melting of metallic plasma-facing materials in future fusion devices poses serious issues regarding the material lifetime and power-handling capabilities as well as core plasma performances. The behaviour of aluminium (as a proxy for beryllium) and tungsten materials was investigated in the Pilot-PSI linear plasma device to study the melt-layer motion and droplet ejection under ITER-relevant plasma conditions. Heat fluxes of up to 50 MW m(-2) raised the surface temperature to values up to 5000 K. The melt-layer rotation was found to depend on the magnetic field (up to 1.6 T) strength and target potential and is attributed to J x B forces caused by radial currents in the plasma. The amount of droplets ejected from the molten surface depends on the material-more droplets ejected from aluminium than from tungsten-and the heat flux to the target. The average droplet velocity was determined to be around 60 m s(-1) for both materials with droplets being ejected mainly in the axial direction. Droplet ejection is only observed during helium discharges, no ejection can be observed with hydrogen plasmas despite similar heat fluxes. Bubble boiling appears to be the main mechanism contributing to the observed droplet ejection.

Growth of tungsten nano-tendrils (“fuzz”) has been observed for the first time in the divertor region of a high-power density tokamak experiment. After 14 consecutive helium L-mode discharges in Alcator C-Mod, the tip of a tungsten Langmuir probe at the outer strike point was fully covered with a layer of nano-tendrils. The depth of the W fuzz layer (600 ± 150 nm) is consistent with an empirical growth formula from the PISCES experiment. Re-creating the C-Mod exposures as closely as possible in Pilot-PSI experiment can produce nearly-identical nano-tendril morphology and layer thickness at surface temperatures that agree with uncertainties with the C-Mod W probe temperature data. Helium concentrations in W fuzz layers are measured at 1–4 at.%, which is lower than expected for the observed sub-surface voids to be filled with several GPa of helium pressure. This possibly indicates that the void formation is not pressure driven., United States. Dept. of Energy (Award DE-SC00-02060)

Experiments were performed in the Pilot-PSI linear plasma device, to determine the sheath heat transmission coefficients in a high recycling regime under various conditions of density (1–20 × 1020 m−3) and plasma composition (H2, Ar, N2) relevant for the ITER divertor plasma. The 2D surface temperature profile on a tungsten surface was measured with high spatial (0.33 mm) and temporal (200 Hz) resolution using an infrared camera. The target heat flux is calculated using a 2D axis-symmetric Ansys model, the heat transfer is determined from target calorimetry. The plasma parameters are measured with a high resolution Thomson scattering system located 17 mm away from the target surface. Radial profiles of the sheath heat transmission factors can thus be determined. Preliminary results show that γ varies between 4 and 40 depending on the plasma conditions and composition. The value derived from the heat flux calculated with Ansys is significantly lower than theory predicts.

Fine-grain graphite targets have been exposed to ITER divertor relevant plasmas in Pilot-PSI to address material migration issues in fusion devices. Optical emission spectroscopy and mass loss measurements have been employed to quantify gross chemical erosion and net erosion yields, respectively. Effects of the ion impact energy and target geometry on carbon erosion yields have been studied. It is concluded that temporal evolution of gross chemical erosion is strongly connected with changes in morphology of plasma exposed surfaces. The net carbon erosion yield is increased when the targets are partly covered by insulating boron-nitride rings., Comment: 14 pages, 4 figures, Contribution to the 19th International Conference on Plasma Surface Interactions

The formation of metallic nanostructures by exposure of molybdenum and tungsten surfaces to high fluxes of low energy helium ions is studied as a function of the ion energy, plasma exposure time, and surface temperature. Helium plasma exposure leads to the formation of nanoscopic filaments on the surface of both metals. The size of the helium-induced nanostructure increases with increasing surface temperature while the thickness of the modified layer increases with time. In addition, the growth rate of the nanostructured layer also depends on the surface temperature. The size of the nanostructure appears linked with the size of the near-surface voids induced by the low energy ions. The results presented here thus demonstrate that surface processing by low-energy helium ions provides an efficient route for the formation of porous metallic nanostructures. (C) 2012 American Vacuum Society.

For understanding carbon erosion and redeposition in nuclear fusion devices, it is important to understand the transport and chemical break-up of hydrocarbon molecules in edge plasmas, often diagnosed by emission of the CH A(2)Delta-X-2 Pi Gero band around 430 nm. The CH A-level can be excited either by electron-impact (EI) or by dissociative recombination (DR) of hydrocarbon ions. These processes were included in the 3D Monte Carlo impurity transport code ERO. A series of methane injection experiments was performed in the high-density, low-temperature linear plasma generator Pilot-PSI, and simulated emission intensity profiles were benchmarked against these experiments. It was confirmed that excitation by DR dominates at T-e < 1.5 eV. The results indicate that the fraction of DR events that lead to a CH radical in the A-level and consequent photon emission is at least 10%. Additionally, quenching of the excited CH radicals by EI de-excitation was included in the modeling. This quenching is shown to be significant: depending on the electron density, it reduces the effective CH emission by a factor of 1.4 at n(e) = 1.3 x 10(20) m(-3), to 2.8 at n(e) = 9.3 x 10(20) m(-3). Its inclusion significantly improved agreement between experiment and modeling.

Transport of hydrocarbon impurities in a high-density (>10 20 m−3), low-temperature (

Samples of EUFROFER, a reduced activation ferritic martensitic steel, were exposed in the linear plasma device Pilot-PSI to a deuterium (D) plasma with incident ion energy of similar to 40 eV and incident D flux of 2-6 x10(23) D/m(2) s to fluences up to 10 27 D/m(2) at surface temperatures ranging from 400 K to 950 K. The main focus of the study lays on the surface morphology changes dependent on the surface temperature and the surface composition evolution, e.g., the enrichment in tungsten; but also the erosion and the D retention are studied. The created surface morphology varies strongly with surface temperature from needle-like to corral-like structures. The visible lateral length scale of the formed structures is in the range of tens of nanometres to above 1 mu m and exhibits two thermal activated regimes below and above similar to 770 K with activation energies of 0.2 eV and 1.3 eV, respectively. The lateral variation of the enrichment of heavy elements on the surface is correlated to this surface morphology at least in the high temperature regime, independent of the origin of the enrichment (intrinsic from the sample or deposited by the plasma). Also the erosion exhibits temperature dependence at least above similar to 770 K as well as a fluence dependence. The amount of deuterium retained in the top 500 nm is almost independent of the exposure temperature and is of the order of 10(18) D/m(2), which would correspond to a sub-monolayer D coverage on the surface. The retained D in the volume summing up over the complete samples exceeds the D retained close to the surface by one order of magnitude. (C) 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.