Ultrafine tungsten as a plasma-facing component in fusion devices: effect of high flux, high fluence low energy helium irradiation

Abstract

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 × 1023–2.0 × 1024 ions m−2 s−1, the sample bias ranged from a negative (20–65) V, and the sample temperatures ranged from 600–1500 °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.

Temperature-dependent surface modification of Ta due to high-flux, low-energy He+ ion irradiation

“…The resulting surface morphology is shown in Figure 3. Irradiation at this higher fluence produces features that may resemble early-stage formation of nanotendrils (fuzz) on Ta surface, but the features are overall relatively less pronounced than those observed in W [12,14]. While the thermal conductivity of bulk Ta is initially lower than the other two materials, the relatively less pronounced appearance and lower aspect ratio of these fine nanostructures may suggest that this property is less likely to degrade in Ta under large particle fluxes.…”

“…On the other hand, a simultaneous sequential enhancement in the large pore density, i.e., 6.1 (923K) to 8.2 (1023K) to 61.2 (1123K) to 73.5 pores µm -2 (1223K) was also observed (Fig 1). A summary of both small and large pore density variation as a function of target temperature is shown in Figure 1 high-Z metals such as W [14] and Mo [15][16][17] where clear temperature windows and lower ion fluence thresholds for nanoscopic filaments and/or fibers, commonly known as fuzz growth, were observed at similar fluxes and fluences of low-energy He + ion radiation. Therefore, it seems that under high-flux, low-energy He + irradiation conditions studied in this work, Ta is free from the formation of fine nano-tendril structures that are observed in Mo irradiated under similar conditions [15][16].…”

Grain size threshold for enhanced irradiation resistance in nanocrystalline and ultrafine tungsten

“…density and thus to an enhancement of the irradiation resistance of the microstructure. Tungsten (W), which is an important material for the major components of several nuclear fusion systems including the divertor region in ITER [5], is an example of a material in which grain size refinement can significantly enhance its performance [6]. When exposed to He irradiation at high temperatures, W has been shown to suffer from high He bubble densities in the grain matrices [7].…”

“…However, as the grain size in coarse-grained W is above the threshold grain size identified here, it is reasonable to assume that it would behave similarly to the ultrafine-grained W. Assuming that the hypothesis that fuzz formation is correlated with high bubble densities in the matrix as proposed by Kajita et al [8] is correct and neglecting other possible effects (such as defect and bubble-induced stresses [27,28]), it may be conjectured that a W sample of 35 nm grains may be able to resist fuzz formation to an ion fluence at least one order of magnitude greater than a sample of ultrafine or fine W based on the results reported here. Indeed, recently a sample of nanocrystalline and ultrafine W was demonstrated to have an order of magnitude greater threshold for fuzz formation when compared to coarse-grained W [6].…”

Evidence of a temperature transition for denuded zone formation in nanocrystalline Fe under He irradiation

“…Clustering of these defects can lead to the formation of more complex defect structures, such as vacancy loops, interstitial loops, voids, and bubbles, which all can potentially degrade the mechanical properties [3,4] of the irradiated material and ultimately cause failure [1]. To mitigate these detrimental effects, recent studies have investigated novel engineering materials with high densities of defect sinks, such as grain boundaries [5][6][7] or structural interfaces [8]. Grain boundaries are thought to act as defect sinks and, in particular, to enhance point defect annihilation by absorbing interstitials and then reemitting them back to recombine with nearby vacancies [9].…”