He-ion induced surface morphology change and nanofuzz growth on hot tungsten surfaces

Abstract

Tungsten has been recently chosen as the divertor material in ITER and is a leading candidate for use in DEMO and future fusion reactors. Linear plasma device and laboratory experiments have shown substantial nanostructured surface modification in tungsten, of low-density ‘fuzz’-like appearance, when exposed to helium ions with impact energies that are below the displacement damage and sputtering thresholds for tungsten, and under temperature and ion flux conditions likely to be encountered in such fusion devices. Such surface features affect a number of material surface properties, such as heat transfer, embrittlement, and deuterium/tritium retention, and can increase the likelihood of high-Z contamination of the fusion plasma by dust formation. Despite its potential detrimental effects on fusion plasmas, and extensive experimental and theoretical research effort, a complete and comprehensive understanding of fuzz formation is not yet in hand. In this topical review article, the current status of experiment and theory are presented, and some of the remaining issues and open questions discussed.

A review of late-stage tungsten fuzz growth

“…It would also be useful to see if there is a different thickness-fluence relationship at very high fuzz thicknesses if the mechanism changes. Meyer [8] speculates whether adatoms would be generated midway upon tendril sides instead of at the base. To avoid the long times needed for this, tungsten deposition (see Sect.…”

“…4, a large concentration of atoms from the bulk would not be seen at the edges of tendrils. During late-stage growth, bubbles within the fuzz could also lead to adatom formation on the side of tendrils by the same mechanism [8].…”

“…2.3. The fuzz layer could reduce the divertor's thermal conductivity [7] and contaminate the plasma [8]. Understanding the fuzz formation mechanism will be key to preventing these problems, and could allow fuzz to be used for beneficial purposes, such as catalysing water splitting [9] or organic compound breakdown [10].…”

“…2.2. However, it has not been possible for simulations to reproduce a complete fuzz structure [8] and outstanding questions remain.…”

Space Weathering Effects in Troilite by Simulated Solar‐Wind Hydrogen and Helium Ion Irradiation

“…Although a number of additional mechanisms have been proposed to play a role in formation of space weathering textures, including micrometeoroid impacts (Loeffler et al., 2016; Sasaki et al., 2001) and other sources of heating (Thompson et al., 2017), irradiation of surfaces by solar‐wind ions is considered the principal weathering mechanism on asteroids (Schläppi et al., 2008), directly linked to the formation of npFe and iron‐cladding on exposed surfaces (Matsumoto et al., 2020; Noguchi et al., 2014). Moreover, research on metals and oxides has identified other damage modes associated with light‐ion irradiation at energies similar to solar‐wind ions, such as phase‐change, pitting, and blistering (Auciello, 1984; Kajita et al., 2016; Meyer, 2018; Tanyeli et al., 2015).…”