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Description
Helium(He)–tungsten(W) co-deposited layers are expected to form in future magnetic fusion devices as a consequence of simultaneous sputtering and re-deposition of tungsten (W) under mixed hydrogen and He plasma exposure. Understanding their sputtering behavior is essential for predicting long-term material erosion and impurity sources in ITER and DEMO. In this study, He–W co-deposited layers were synthesized in the linear plasma device Co-NAGDIS, and their microstructure and sputtering characteristics were comprehensively investigated. Binary collision approximation simulations were additionally performed to elucidate the microscopic origins of their reduced sputtering yields.
SEM and TEM analyses revealed that the co-deposited layers consist of nanocrystalline W aggregates containing numerous sub-100-nm cavities, with porosities ranging from 30% to 55%. Unlike the well-defined helium bubble structures typically formed in bulk W, these cavities exhibited indistinct boundaries and were uniformly distributed throughout the layer. X-ray diffraction confirmed that the layers retained crystallinity, while peak broadening indicated the presence of nanoscale crystalline domains. Optical emission spectroscopy during Ar ion bombardment showed that the apparent sputtering yield, inferred from the W I / Ar II line ratio, was suppressed to approximately 30% of the bulk W value at the initial stage of irradiation. As the ion fluence increased, the yield gradually recovered and ultimately reached the bulk value once the co-deposited layer was completely removed. The fluence required for full recovery scaled with the layer thickness, demonstrating that the suppressed sputtering yield is an intrinsic property of the co-deposit itself rather than an artifact of transient surface effects.
To interpret these experimental observations, simulations were performed for He–W mixed structures with varying porosities and defect densities. The initial atomic structures were generated through structural optimization using an Embedded-Atom Method (EAM) potential. The simulations revealed that the nanoporous morphology and complex surface topography characteristic of the co-deposited layers strongly dissipate the energy of collision cascades, thereby reducing the effective momentum transfer to surface atoms. The roughened surface was also found to enhance the re-trapping of sputtered W atoms, providing a mechanism consistent with the experimentally observed suppression of net erosion. Moreover, as the co-deposited layer becomes thinner, sputtering induced by backscattered Ar atoms originating from the underlying bulk material becomes increasingly significant.