Shin Kajita

Helium plasma irradiation is a unique surface modification technique that can induce various surface textures, including self-organized nanostructures, porosity, defects, and compositional changes, on a wide variety of materials. Initially investigated on tungsten mainly in the context of plasma-material interaction for fusion applications, helium-plasma-induced nanostructuring has recently attracted increasing attention as a versatile route for functional surface engineering. In particular, the resulting nanostructured and porous surfaces exhibit strongly altered optical, physicochemical, and electrochemical properties, enabling emerging applications in broadband optical absorbers, light diffusers, photocatalysis, photoelectrochemical water splitting, electrocatalysis, gas sensing, and energy-storage electrodes. This topical review summarizes the fundamentals of helium plasma treatment, including irradiation conditions, morphology evolution, and associated changes in material properties, and then surveys recent progress in application-oriented studies.

Transient heat loads like Edge Localized Modes (ELMs) can degrade detached plasmas that are essential for reducing divertor heat fluxes in future fusion reactors. To investigate the response of detached plasmas to transient electron energy deposition, a pulsed electron beam was injected into a detached helium plasma in the linear plasma device NAGDIS-II. Spatiotemporal evolutions of electron temperature and density were measured using laser Thomson scattering synchronized with the beam injection with high-temporal resolution. Despite the small beam source diameter (2 mm), significant changes in plasma parameters extended over a wide radial range (> 15 mm). The electron beam rapidly increased the electron temperature near the plasma center, resulting in a local transition from a recombining plasma to an ionizing plasma. In contrast, the electron density exhibited a slower temporal evolution, suggesting different underlying mechanisms governing temperature and density responses. The measured spatiotemporal plasma parameters were further analyzed using a collisional-radiative model and reproduced previously observed line-integrated emission dynamics. These results demonstrate that localized electron energy deposition can induce a global response in detached plasmas and provide new insight into the stability and dynamic behavior of detached plasmas subjected to transient energy loads.

We developed an advanced diagnostic system by adding an image intensifier and wavelength filters to the conventional high-speed camera setup to analyze rotating plasma ejection events in detached plasma which can be realized in the liner magnetized plasma device NAGDIS-II. The gating capability of the image intensifier achieved temporal resolution exceeding the camera frame rate, while conditional averaging and tomography processing revealed detailed structures of high-speed phenomena previously undetectable. Multi-wavelength measurements enabled spatial mapping of electron temperature distribution through intensity ratio analysis. This enhanced technique provides the first quantitative correlation between electron temperature evolution and spatial development of rotating ejection events.

Using pulsed laser deposition (PLD) at relatively high pressure environment, porous tungsten thin films were successfully fabricated for photoelectrochemical (PEC) water splitting. In this study, porous tungsten films were prepared on fluorine-doped tin oxide substrates via relatively high pressure PLD followed by post-annealing to form tungsten oxide thin film. The film morphology, crystallinity, chemical states, optical band gap, and electrochemically active surface area were systematically characterized, and the effects of these factors on PEC performance were discussed. Because no AM 1.5 filter was used, the photocurrent values are discussed only for relative comparison under identical internal illumination conditions.

Tungsten (W) is considered the primary material for the divertor and first wall in future fusion reactors. Under the irradiation of helium (He) plasma, a porous layer named ‘fuzz’ forms on W surface, accompanied by changes in its physical properties. In this paper, the early stage of fuzz is constructed using a molecular dynamics (MD) and Monte Carlo (MC) hybrid method. It provides a more physically realistic structure by allowing the nanostructure to emerge naturally from the interplay of atomic-scale processes. The generated fuzz structures are subsequently employed to evaluate the changes in surface properties, including both sputtering and hydrogen (H) permeation/retention. Simulation results show that the fuzz layer ceases to grow once it reaches a certain thickness. Further analysis indicates that the formation of fuzz structure can suppress the diffusion and accumulation of He from the surface to bulk material. The simulation results also indicate that higher ion energy results in greater He retention, which consequently leads to an increase in the porosity of the fuzz layer. Simulations of He atoms cascades show that, although the energy directly transferred from He atoms (<100 eV) to W atoms is lower than the sputtering threshold energy (∼8.9 eV), W surface modifications and sputtering can still occur via adatoms formation. Compared with flat W surface, fuzzy surface tends to recapture the sputtering W atoms, exhibiting lower net sputtering yield under the same He irradiation conditions. The bombardment by He atoms can also promote the curling of fuzz structures, which may subsequently influence W surface diffusion. Finally, the He–H hybrid simulations indicate that the formation of fuzz structure significantly suppresses the H permeation and retention in the W bulk area and enhances H retention on W surface.

Tungsten (W) fuzz formed on divertor targets under helium plasma irradiation significantly alters the thermal conduction properties of the underlying W substrate. Hence, it is essential to conduct detailed studies on its thermal conductivity. Modeling heat transfer in the media characterized by complex rough morphology and porous structure is a formidable challenge. To address this, a novel computational framework named Lattice Boltzmann Simulation (LBS), leveraging the lattice Boltzmann method (LBM), has been developed to simulate three-dimensional heat conduction within W fuzz and calculate its effective thermal conductivity (ETC). The modeled ETCs of fuzzy layers with different thicknesses by LBS code show a good agreement with the experimental values measured in the NAGDIS-II device. Furthermore, an empirical fitting formula that describes the temperature-dependent ETC of W fuzz can be derived based on the LBS simulations, enabling rapid thermal conductivity calculations. Leveraging this relationship, a 1D heat transfer model has been utilized to investigate the impacts of W fuzz on thermal conduction for divertor targets with fuzzy surfaces. The results reveal that the formation of W fuzz reduces the critical heat flux required for surface melting, thereby exacerbating the risk of material damage under high heat loads.