Search

Abstract Material testing for plasma facing components (PFCs) in future fusion devices reveals important damage mechanisms that impact lifetime and performance. During operation, PFCs in the divertor region will be subjected to high particle fluxes and heat loads. Tungsten (W) is currently the leading candidate material for divertor PFCs, due to its high melting point, high thermal conductivity, and low tritium retention. Laboratory experiments have effectively characterized mechanisms of damage on W due to different forms of radiation. However, understanding how different species of incident plasma particles interact with one another to affect the resultant strength of the material remains in development. In this study, W samples were exposed to simultaneous ELM-like heat loading and high-flux He+ and D+ ion irradiation at different ELM intensities to further elucidate the complex synergistic effects inherent in a fusion environment. ELM-like heat loading was replicated using a 1064 nm Nd:YAG pulsed millisecond laser at different energy densities. Exposures produced a shale-like microstructure with or without concurrent He+ and D+ ion irradiation. However, adding D+ to a simultaneous laser + He+ irradiation reduced pore formation and inhibited early-stage nanostructure formation. Changes in surface morphology with the addition of D+ could be attributed to super-saturation in the near-surface layer. While the addition of He+ and D+ clearly increased W erosion, the laser energy density did not have as clear of an effect. Increasing the ELM intensity reduced the number of pores, but increased the pore size. Future studies need to explore whether near-surface D impacts pore formation and total He desorption. Continued research on the combined effect of high-flux particle irradiation and transient heat loading will help refine predictions of material performance for ITER and beyond. [ABSTRACT FROM AUTHOR]

Mitigation of transient plasma events in a future fusion device is necessary to minimize thermal damage on surrounding plasma facing components (PFCs). Intense heat loading caused by an edge-localized mode (ELM) can lead to melting and splashing of the PFC surface. ELM mitigation involves triggering these events at higher frequencies, which lowers the effective heat flux imparted on the material. Unfortunately, concurrent, high-flux He + ion irradiation on tungsten (W), which is the leading candidate material for future PFCs, has been shown to drive the development of an arborescent nanostructure, known as fuzz. Large drops in thermal conductivity and mechanical strength with fuzz formation necessitate a reconsideration of the impact of ELM mitigation on material damage and erosion. Three different types of experiments designed to characterize the synergistic effect between He + ion irradiation and pulsed heat loading (replicated with a pulsed millisecond laser) were conducted in the UHFI-II facility at CMUXE. W samples exposed to pulsed heat loading underwent roughening in the form of a shale-like surface morphology, regardless of frequency. The addition of simultaneous He + ion irradiation with pulsed heat loading promoted splashing of molten material and caused significant pore formation. Increasing the frequency to 10 Hz produced a smoother W surface. However, when the total average laser power was held constant, increasing the frequency decreased the laser energy density, which significantly decreased surface melting and enhanced early stage fuzz formation. Results obtained raise an important question of whether ELM mitigation parameters can be tuned to anneal out any He-induced surface structuring without causing unsustainable splashing of molten material. [ABSTRACT FROM AUTHOR]

Currently, tungsten remains the best candidate for plasma-facing components (PFCs) for future fusion devices because of its high melting point, low erosion, and strong mechanical properties. However, continued investigation has shown tungsten to undergo severe morphology changes under fusion-like conditions. These results motivate the study of innovative PFC materials which are resistant to surface morphology evolution. The goal of this work is to examine tungsten-tantalum (W-Ta) alloys, a potential PFC material, and their response to low energy helium ion irradiation. Specifically, W-Ta samples are exposed to 100 eV helium irradiations with a flux of 1.15 × 10 21 ions m −2 s −1 , at 873 K, 1023 K, and 1173 K for 1 h duration. Scanning electron microscopy (SEM) reveals significant changes in surface deterioration due to helium ion irradiation as a function of both temperature and tantalum concentration in W-Ta samples. X-Ray Diffraction (XRD) studies show a slight lattice parameter expansion in W-Ta alloy samples compared to pure W samples. The observed lattice parameter expansion in W-Ta alloy samples (proportional to increasing Ta wt.% concentrations) reflect significant differences observed in the evolution of surface morphology, i.e., fuzz development processes for both increasing Ta wt.% concentration and target temperature. These results suggest a correlation between the observed morphology differences and the induced crystal structure change caused by the presence of tantalum. Shifts in the XRD peaks before and after 100 eV helium irradiation with a flux of 1.15 × 10 21 ions m −2 s −1 , 1023 K, for 1 h showed a significant difference in the magnitude of the shift. This has suggested a possible link between the atomic spacing of the material and the accumulated damage. Ongoing research is needed on W-Ta alloys and other innovative materials for their application as irradiation resistant materials in future fusion or irradiation environments. [ABSTRACT FROM AUTHOR]

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 (∼10 20 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 × 10 23 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 × 10 23 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 (10 20 m −2 –10 21 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. [ABSTRACT FROM AUTHOR]

Lithium conditioning of plasma-facing surfaces has been implemented in National Spherical Torus Experiment (NSTX) leading to improvements in plasma performance such as reduced D recycling and a reduction in edge localized modes. Analysis of postmortem tiles and offline experiments along with atomistic modeling has identified interactions between Li-O-D and Li-C-D as chemical channels for deuterium retention in ATJ graphite. However, previous surface chemistry analysis of NSTX tiles were conducted postmortem (i.e., after a completed annual campaign), and it was not possible to correlate the performance of particular discharges with the state of the material surface at the time. Materials Analysis Particle Probe (MAPP) is the first in-vacuo surface analysis diagnostic directly integrated into a tokamak and capable of chemical surface analysis of plasma facing samples retrieved from the vessel in between discharges. It uses X-ray photoelectron spectroscopy, direct recoil spectroscopy, low energy ion surface spectroscopy, and thermal desorption spectroscopy to investigate the chemical functionalities between D and lithiated graphite at both the near surface (5–10 nm) and top surface layer (0.3–0.6 nm), respectively. MAPP will correlate plasma facing component surface chemistry with plasma performance and lead the way to improved understanding of plasma-surface interactions and their effect on global plasma performance. Remote operation and data acquisition, integrated into NSTX diagnostic and interlocks, make MAPP an advanced PMI diagnostic with stringent engineering constraints. [ABSTRACT FROM PUBLISHER]

We report on the role of carbon (C) impurities, in molybdenum (Mo) fuzz evolutions on Mo surface during 100 eV He + ion irradiations. In this study we considered 0.01, 0.05, and 0.5% C + ion impurities in He + ion irradiations. For introducing such tiny C + ion impurities, gas mixtures of He and CH 4 have been chosen in following ratios; 99.95: 0.05, 99.75: 0.25, and 97.5: 2.5. Apart from these three cases, two additional cases, 100% He + ion (for Mo fuzz growth due to only He + ions) and 100% H + ion (for confirming the significance of tiny 0.04–2.0% H + ions in terms of Mo fuzz evolutions on Mo surface, if any), have also been considered. Ion energy (100 eV), ion fluence (2.6 × 10 24 ions m −2 ), and target temperature (923 K) were kept constant for each experiment and their selections were based on our previous studies [1,2]. Our study shows homogeneously populated and highly dense Mo fuzz evolutions on entire Mo surface for 100% He + ion irradiation case. Enhancement of C + ion impurities in He + ions causes a sequential reduction in Mo fuzz evolutions, leading to almost complete prevention of Mo fuzz evolutions for 0.5% C + ion impurity concentrations. Additionally, no fuzz formation for 100% H + ion irradiation at all, were seen (apart from some tiny nano-structuring, in very limited regions). This indicates that there is no significant role of H + ions in Mo fuzz evolutions (at least for such tiny amount, 0.04–2.0% H + ions). The study is significant to understand the behavior of potential high-Z plasma facing components (PFCs), in the, presence of tiny amount of C impurities, for nuclear fusion relevant applications. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) represents a promising but largely untested plasma-facing material (PFM) for next-step fusion devices. In this work, an analytic mixed-material erosion model is developed by calculating the physical (via SDTrimSP) and chemical (via empirical scalings) sputtering yield from SiC, Si, and C. The Si content in the near-surface SiC layer is predicted to increase during D plasma bombardment due to more efficient physical and chemical sputtering of C relative to Si. Silicon erosion from SiC thereby occurs primarily from sputtering of the enriched Si layer, rather than directly from the SiC itself. SiC coatings on ATJ graphite, manufactured via chemical vapor deposition, were exposed to repeated H-mode plasma discharges in the DIII-D tokamak to test this model. The qualitative trends from analytic modeling are reproduced by the experimental measurements, obtained via spectroscopic inference using the S/XB method. Quantitatively the model slightly under-predicts measured erosion rates, which is attributed to uncertainties in the ion impact angle distribution, as well as the effect of edge-localized modes. After exposure, minimal changes to the macroscopic or microscopic surface morphology of the SiC coatings were observed. Compositional analysis reveals Si enrichment of about 10%, in line with expectations from the erosion model. Extrapolating to a DEMO-type device, an order-of-magnitude decrease in impurity sourcing, and up to a factor of 2 decrease in impurity radiation, is expected with SiC walls, relative to graphite, if low C plasma impurity content can be achieved. These favorable erosion properties motivate further investigations of SiC as a low-Z, non-metallic PFM. [ABSTRACT FROM AUTHOR]

Compared to tungsten (W), tantalum (Ta) has shown superior resistance to helium (He)-induced surface morphology changes under fusion-relevant irradiation conditions. However, Ta is also expected to have a stronger interaction with hydrogen isotopes, potentially limiting its use as a plasma-facing material. Despite these concerns, detailed investigations on hydrogen irradiation effects on Ta are scarce. In this study, pristine and fuzzy (He + ion-irradiated) Ta samples are irradiated with 120 eV deuterium (D) ions at various temperatures and examined with a combination of thermal desorption spectroscopy (TDS), scanning electron microscopy (SEM), and optical reflectivity. TDS reveals discrete D desorption temperatures at 660 and 760 K, corresponding to trapping energies of 1.82 and 2.11 eV, respectively. Although D is retained in Ta both in higher quantities and at higher temperatures compared to W, extreme surface temperatures expected in tokamak divertors may exceed these desorption temperatures and counteract retention. Furthermore, this study indicates that Ta is relatively resistant to adverse surface structuring under D + ion irradiation. In fact, D + is shown to prevent and suppress Ta fuzz formation in sequential D + /He + ion irradiation experiments. While further investigations are needed to elucidate this behavior, these initial investigations show a strong potential for the use of Ta as a PFC material. [ABSTRACT FROM AUTHOR]

The objective of the materials analysis particle probe (MAPP) in NSTX is to enable prompt and direct analysis of plasma-facing components exposed to plasma discharges. MAPP allows multiple samples to be introduced to the level of the plasma-facing surface without breaking vacuum and analyzed using X-ray photoelectron spectroscopy (XPS), ion-scattering and direct recoil spectroscopy, and thermal desorption spectroscopy (TDS) immediately following the plasma discharge. MAPP is designed to operate as a diagnostic within the ∼12 min NSTX minimum between-shot time window to reveal fundamental plasma-surface interactions. Initial calibration demonstrates MAPP's XPS and TDS capabilities. [ABSTRACT FROM AUTHOR]