Panorama of “fuzz” growth on tungsten surface under He irradiation

Highlights

• Morphology of the W surface was investigated to determine essential factors resulting in nanostructure on the surface.

• The surface protrusions preferentially accommodate He bubbles.

• The protrusions further contribute to the heave of the surface after the rupture of He bubbles.

• He diffusion plus clustering in the protrusions and bubble bursting process constitute critical elements for the recursion event of the fiber-like structure growth.

Abstract

Nanostructure formation on tungsten (W) surfaces under helium (He) irradiation is a unique and interesting phenomenon that directly impacts the performance and lifetime of W-based materials as plasma-facing materials (PFMs) in nuclear fusion reactors. The evolution of He bubbles beneath the W surface and its effect on surface morphology are essential for understanding the deterioration of the surface properties of PFMs. In this work, the morphology of the W surface under different irradiation conditions was investigated using molecular dynamics in order to extract essential factors that determine the nanostructure of the W surface. The design concerned with investigating the W surface morphology encompassed: (1) the morphology of the W-free surface without He bubbles vs. recoil energy; (2) the morphology of the W surface in the presence of He bubbles vs. recoil energy; and (3) sputtering vs. surface roughness in the absence of He bubbles. A mass of adatoms generated primarily by loop punching of sub-surface He bubbles slightly roughens the surface. Bubble bursting further heightens surface protrusions. In this case, the fiber-like structure of the surface is attributable to two processes involved in He bubble evolution: He diffusion plus aggregation and bubble expansion plus bursting. A high temperature window expedites He diffusion and aggregation beneath the protruded surface. Once the He bubbles burst regardless of spontaneous bursting (when the bubble pressure is beyond the critical value), or bursting due to external irradiation, the protrusions will be further heightened, thus serving as precursors of the fiber-like structure. Continuous aggregation, followed by bursting, should constitute the recursive process of protrusion upgrowth. The panoramic view of the fiber-like structure growth described here is expected to provide a clue for designing tungsten-based materials with high radiation-resistance performance.

Introduction

Magnetic confinement fusion reactors, expected to successfully produce electricity in the future, are still facing significant scientific and engineering challenges; one of these issues involves the safe operation of the walls of the divertor (LWD) and plasma-facing materials (PFMs) for long durations while exposed to large heat and particle flux (>10²³ m⁻² s⁻¹) at high temperatures (≧1000 K). Tungsten (W) is a promising candidate material for PFMs and divertor-liner materials because of its high melting temperature and high sputtering threshold (low sputtering yield) [[1], [2], [3]]. Notably, the plasma contains a mixture of ionized and energetic neutral species of hydrogen (H) isotopes and helium (He). Experimental studies on exposing the W surface to energetic He have been performed intensively [4,5]. It has been reported that the morphology of the W surface is severely modified under plasma irradiation (even below the sputter threshold of incident-ion energies) [6,7], which will affect hydrogen retention, and the consequential performance of plasma in nuclear fusion reactors. Studies have demonstrated that the evolution of morphology is dependent on the surface temperature [8]. A low density of small pits is usually generated below 1000 K [9], nanostructural “fuzz” (also known as “coral”) appears in the temperature range between 1000 and 2000 K [[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]], and micron-sized holes are observed on further increasing the temperature above 2000 K [8,21]. The nanostructured layers are fragile and can be easily removed from the surface. This may cause fatal degradation of reactor performance because the easily exfoliated “high-Z” W atoms may enter the core plasma [22]. In addition, the morphology modification may have important implications for the surface thermal properties, optical reflectivity, hardness, embrittlement, fuel retention, and erosion properties of PFMs and LWD in future devices [[23], [24], [25], [26]]. To date, intensive studies have focused on the growth mechanism of the nanostructured layers both experimentally and theoretically. It was found that there is a square root of time dependence for the kinetics of the layer growth, which implies that something within the “fuzz” layers may diffuse following Fick's law [7]. The growth of nanostructural layers in thickness has been reported to occur in thermally activated processes [6]. The phenomena occurring is expected to involve the formation of forming helium bubbles [27] beneath the surface since experimental results have revealed that helium atoms diffuse easily in the matrix and are easily trapped by grain boundaries [[28], [29], [30], [31], [32], [33]], vacancies, and defects to form large He bubbles, and the nanometer-sized helium bubbles indeed exist inside the nanostructure [34]. Meanwhile, the process is accompanied by sputtering and dust [15] under plasma bombardment. It has observed using transmission electron microscope that “fuzz” emerges when the helium bubbles burst [16]. This is evidenced by the fact most bubbles will be near the surface given that He atoms are prone to moving toward the surface due to an elastic interaction [68], and He as a source from the plasma continually implanting into the surface only penetrates a few nanometers before reaching thermal velocity [76], which benefits the formation of overpressurized He bubbles on the surface. This means that the dynamic process definitely plays a crucial role in the nanostructure growth.

To date, molecular dynamics (MD) is still an irreplaceable and available simulation method for probing the evolution of surface morphology from an atomic point of view, by considering both kinetic and thermodynamic processes of surface evolution using large-scale of simulation cells. MD simulation have identified point defects, stacking faults, and craters on the surface of FCC gold nanowires under irradiation [35]; they have also vividly described the characteristics of helium bubbles in the W matrix [[36], [37], [38], [39]] and the deformation behavior of the surface morphology caused by helium bubbles [[40], [41], [42]]. The primary factors determining the surface variation involve the interactions between He bubbles and dislocation [43], the diffusion and coalescence of sub-surface He bubbles [4,44], and the self-trapping of He, among other factors. MD was also employed to study the sputtering yield of the W surface irradiated by He [45]. It showed that there is no significant influence of the sub-surface helium bubbles on the sputtering yield of tungsten. Other studies indicated that the sputtering yield increased gradually with the increase in temperature and energy [46]. Sputtering roughens the surface, which in turn affects the sputtering. Moreover, MD simulations [47] proved that due to more surviving vacancies and sputtered atoms, stable He bubbles can easily burst from the (1 0 0) and (1 1 0) surfaces after neutron irradiation. There were three states for the He bubbles, termed “standing,” “expanding,” and “bursting,” on the W surface [48]. Even under low-energy incidence of He plasma, self-interstitials were produced, which evolved into bundles of 〈1 1 1〉 crowdions moving toward the surface and forming into adatoms [49,50]. Evidently, adatoms, craters, and pinholes appearing under He incidence roughened the surface [51]. These were regarded as the initial stage of nanoscale “fuzz” on the W surface [52]. Studies further suggested that the development of pinholes, which might contribute to the formation of the fiber-like structure, is related to the coalescence of helium bubbles around the bottom of the holes [15]. In these cases, ruptured bubbles may serve as pathways deeper into the material and potentially roughen the surface inward. However, it was experimentally observed that the nanostructured layers grew away from the original W surface [7]. Therefore, further exploration of the underlying mechanism for the fuzz growth is required with subsequent attainment of a panorama of “fuzz” growth. An in-depth understanding of the evolution of the surface morphology impacted by He bubbles by comprehensively considering thermodynamic and kinetic processes, including the resulting variation of surface stress, is necessary; this may assist in designing new radiation-resistant PFMs.

In this work, we designed a series of simulation schemes to elaborate on the formation of a nanostructural surface using the MD method: the schemes involved simulating bombardment on the (0 0 1) surface of W by choosing a surface W atom as a primary knock-on atom (PKA) to carry out the recoil process, where PKA energy is in the range of 100–5000 eV, and particularly paying close attention to the relationship between surface topography and atomic stress caused by He bubbles at different depths. The simulations revealed the mechanism of the formation of adatoms by tracing the transfer of PKA energy, and bubble bursting by monitoring the pressure of the He bubbles and the stress distribution of the surface as a function of depth. When the He bubble was located at a significant depth, the pressure of the He bubble was not sufficient to break through the surface; thus, loop punching occurred to release pressure. The simulations quantified the windows of PKA energy and the depth of the He bubble for forming adatoms. The surface roughness increased with the PKA energy, and the He bubbles roughened the surface. The rough surface facilitated a high sputtering yield. Combining two distinct processes involving He bubbles - thermally activated process and bubble bursting process, we illustrated the growth of the nanostructured layers that grow away from the W surface, as observed in experiments [7]. The present research examines several ways of roughening the surface, which may contribute to the complete understanding of the topology change of the W surface under He irradiation.