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Nuclear fusion is a potential source for producing unlimited environment-friendly energy. Tungsten (W) is selected as the primary candidate material for plasma facing component in nuclear fusion reactors due to its high melting temperature (3695 K), low sputtering erosion yield and strong mechanical properties. However, recent investigations on W have confirmed that it undergoes severe surface morphology changes during low energy He plasma and/or ion irradiation similar to a harsh fusion environment. Additionally, our previous studies indicate that tantalum (Ta) may show better resistance to the harsh radiation environment and is therefore worthy of investigation. Hydrogen retention properties, specifically deuterium (D) retention in Ta, are not well documented and are extremely important safety issue for fusion reactors. Consequently, we exposed Ta to a fusion-like environment of low-energy D ions and performed thermal desorption spectroscopy (TDS). Then, the samples were rapidly loaded (to avoid possible surface oxidation) into another vacuum chamber where we performed the thermal desorption spectroscopy measurements. Our investigations indicate two binding energy values for D in Ta, i.e. 1.8 eV and 2.1 eV. We observe a higher binding energy and a higher retention rate for D in Ta than W. We also observed that our results are consistent with theoretical predictions based on the absorption of D in Ta. Our preliminary results indicate that Ta shows better resistance to nanostructure formation (fuzz) than W in a fusion environment. However, this is still an open question and part of ongoing investigations.

Nuclear fusion is the most promising renewable energy source for the near future. It can provide a large amount of energy using a very small amount of fuel, as compared with that of the coal, oil, or nuclear fission. The chain reaction in nuclear fusion produces the energy and fuel, from hydrogen isotopes available in see water. Tungsten (W) is a leading candidate material for the plasma-facing component (PFC) in nuclear fusion reactors such as ITER (international thermonuclear experimental reactor), because of its high melting point, high yield strength, low erosion and low hydrogen isotope retention. Recent studies showed deeply convoluted fiber-form nanostructures (fuzz) formation on W surface under high-flux low-energy He+ irradiation relevant to fusion conditions. The fuzz greatly degrades the mechanical, thermal, and optical properties of W. The significant enhanced surface area, and fragility of such fuzz, raise several serious concerns for its usefulness as PFC materials in fusion reactors. The fuzz can also be easily eroded and is a major concern for plasma contamination and short lifetime. In this study, we report on the effect of helium ion irradiation on the surface morphology evolution of W exposed to low energy He+ ions at constant elevated temperature. Submicron thickness W films were deposited on Silicon (100) at room temperature using RF sputtering deposition technique. Several samples were cut and were exposed to 100 eV He+ ions having a constant flux of 1.2 × 1021 ions m−2 s−1 and sample temperature (1173K). The fluence was varied in the range of 4.3 × 1024 – 1.7 × 1025 ions m−2. Post ion-irradiation samples (including pristine) were characterized using field emission scanning electron microscopy (FE-SEM), X-ray photoelectric spectroscopy (XPS), and optical reflectivity measurements for monitoring the changes in surface morphology, chemical composition, and surface roughness/optical properties, respectively. We observed a sequential enhancement in the W fuzz density, sharpness, and protrusions from the film surface, with increasing helium ion fluence. Ex-situ XPS study shows the evidence of W2O3 phase formation due to natural oxidation of W fuzz in the open atmosphere. The study is also relevant to potential applications in solar power technology and in water splitting for hydrogen production.

Nuclear fusion is a great potential energy source that can provide a relatively safe and clean limitless supply of energy using hydrogen isotopes as fuel material. ITER (international thermonuclear experimental reactor) is the world first fusion reactor currently being built in France. Tungsten (W) is a prime candidate material as plasma facing component (PFC) due to its excellent mechanical properties, high melting point, and low erosion rate. However, W undergoes a severe surface morphology change when exposed to helium ion (He+) bombardment under fusion conditions. It forms nanoscopic fiber-form structures, i.e., fuzz on the surface. Fuzz is brittle and can easily contaminate the plasma, and therefore preventing the fusion chain reaction. In this study, we report on the effect of temperature on the surface morphology evolution of W coatings under low energy He+ ion irradiation, relevant to fusion conditions. Submicron thickness W films have been deposited on Si (100) at room temperature using RF sputtering deposition technique. Several samples were cut from the same wafer and exposed to 100 eV He+ ions having a constant flux of 1.2 × 1021 ions m−2 s−1 (total fluence of 4.3 × 1024 ions m−2) at several temperatures in the range of 1073 – 1273 K. During each ion irradiation experiments the applied sample temperature were constant throughout that experiment. Post ion-irradiation samples (including pristine) were characterized using field emission scanning electron microscopy (FE-SEM), X-ray photoelectric spectroscopy (XPS), and optical reflectivity measurements for monitoring the changes in surface morphology, chemical composition, and surface roughness/optical properties, respectively. Our analysis shows a sequential enhancement in W fuzz density, sharpness, and protrusions from the film surface, with increasing sample temperature, during helium ion irradiation. Ex-situ XPS study shows the evidence of W2O3 phase formation due to natural oxidation of W fuzz in the open atmosphere, for all irradiated samples. The study is significant in the understanding processes of fuzz formation on high-Z refractory metals for fusion applications. In addition, the observed W2O3 fuzz structure may have potential applications in solar power concentration technology and in water splitting for hydrogen production.

Plasma facing components (PFCs) in fusion devices must be able to withstand high temperatures and erosion due to incident energetic ion radiations. Tungsten has become the material of choice for PFCs due to its high strength, thermal conductivity, and low erosion rate. However, its surface deteriorates significantly under helium ion irradiation in fusion-like conditions and forms nanoscopic fiber-like structures, or fuzz. Fuzz is brittle in nature and has relatively lower thermal conductivity than that of the bulk material. Small amounts of fuzz may lead to excessive contamination of the plasma, preventing the fusion reaction from taking place. Despite recent efforts, the physical mechanism of the surface deterioration is still not clear. This necessitates finding alternative materials for PFCs. In this report, the effect of helium ion energy on molybdenum surfaces is investigated. Helium ion irradiations on mirror finished polished molybdenum samples are performed as a function of helium ion energy from 100-1600eV with fixed values of ion-flux (7.2 x 1020 ions m-2 s-1), ion-fluence (2.6 x 1024 ions m-2), and temperature (923K). The surface modifications were studied using scanning electron and atomic force microscopy along with X-ray photoelectron spectroscopy and optical-reflectivity measurements. Reduction in the “protrusion” of fuzz from the surface and fuzz density at increased energy have been seen from microscopy and optical reflectivity studies. These findings further the understanding of fuzz formation on high-Z refractory metals for fusion applications. KEYWORDS

Ion sputtering is the removal of surface atoms or molecules in a solid under energetic ion irradiation. This technique is promising for its applications in material modification and characterization. Sputtering yield, the average number of atoms removed from a sample per incident ion, is a crucial parameter in material modification. In the present study, a quartz crystal microbalance was used within an ultra-high vacuum chamber (10E-8 torr) to measure the sputtering yield of gold. An NTI-1401 ion gun was used to bombard argon and helium ions onto a gold sample. The argon and helium ions used ranged in energy from 100 – 5000 eV. Theoretical sputtering yield values were determined through simulation with Stopping and Range of Ions in Matter (SRIM) software and compared with experimentally measured values. The experimental values show a significantly higher sputtering yield for gold than that of the SRIM values; however they followed the same trend. Another observation was that for a constant ion energy bombardment the sputtering yield decreased with time. An atomic force microscopy (AFM) was performed before and after sputtering. The AFM showed that the surface of the sample became smoother after a period of ion bombardment. These observations suggest that surface topography significantly affects the sputtering yield values. The reasoning behind the discrepancy between experimental and SRIM sputtering yield values will be discussed.