Speaker
Description
In future tokamak reactors, tungsten (W) is expected to serve as the main plasma-facing material. When subjected to helium (He) plasma—a byproduct of fusion reactions—and heated above 900 K, W surfaces can develop nanofiber-like structures known as "fuzz," which significantly degrade thermal conductivity [1]. This phenomenon has been linked to the dynamics of helium bubble formation and rupture, but the detailed mechanisms, particularly those governing growth from tens of nanometers to micrometers, are still under active study.
Previous studies have consistently shown that the lowest temperature required for tungsten fuzz formation is approximately 900 K [2]. However, in our work, we found that when the initial surface already contains nanoscale features, fuzz can develop at temperatures significantly below 900 K. This observation first emerged from He plasma and W co-deposition experiments performed on Si nanocone substrates. At a substrate temperature of 790 K, W fuzz was observed to grow on top of the Si nanocones [3]. Motivated by this result, we directly irradiated nanostructured W substrates with He plasma. Fuzz was first produced at 1000 K under He plasma, which can be considered as an initial, after which the temperature was lowered while irradiation continued. Remarkably, fuzz growth persisted even at ~750 K [4]. To exclude the possibility that high-temperature He-plasma pretreatment was responsible for the reduced fuzz-growth temperature, we pre-textured W surfaces using femtosecond-pulse laser irradiation to generate periodic ripple structures (LIPSS). At ~750 K, the typical fuzz morphology did not form; instead, the resulting surface resembled that of untreated W under the same conditions. However, at the edges of the LIPSS region, where the laser energy was excessive, nodule-like features transformed fully into fuzz at 750 K. This indicates the importance of the sharp structure to reduce the temperature threshold.
The growth rate of fuzz with an initially nanostructured surface at low temperature shows different comparing to the conventional high temperature irradiation. Thermal desorption spectroscopy (TDS) was performed to investigate the role of He on the above phenomenon. This study provides a new perspective on understanding the further growth of fuzz and lowers the practical barrier for applying metal fuzz structures.
[1] S. Kajita et al., Nucl. Fusion 49 (2009) 095005.
[2] G. De Temmerman et al., Plasma Phys. Control. Fusion 60 (2018) 044018.
[3] Q. Shi et al., Nucl. Mate. Energy 39 (2024) 101668.
[4] Q. Shi et al., J. Nucl. Mate. 617 (2025) 156134.