Surface Pore Formation in Helium Implanted Fine-Grain Tungsten and Tungsten Needles as Engineered First Wall and Divertor Plate Materials

Abstract

Surface morphology changes of sub-micron tipped tungsten needles (W.N.) and an engineered fine-grain tungsten (FGW) were studied after implantation with He ions at reactor relevant conditions. Surface and subsurface pore formation was observed on all of the samples by using scanning electron microscopy (SEM) and focused ion beam (FIB) milling. Additionally, helium retention analysis was performed on the FGW and compared to several previously studied W materials.

Three samples of FGW were irradiated with 30 keV ³He ions to 3×10¹⁷ He⁺/cm² at 700 °C, 9×10¹⁷ He⁺/cm² at 850 °C, and 1×10¹⁹ He⁺/cm² at 1050 °C. SEM analysis revealed that the threshold for visible pore formation was below ˜10¹⁸ He⁺/cm². The sample irradiated to the highest fluence showed “coral-like” morphology on the surface, and FIB analysis showed that the sub-surface semi-porous layer extended almost one micron below the surface. The percentage of implanted helium retained in the samples ranged from 4.5-40%.

Two W.N. were implanted with 100 keV ⁴He ions to conditions of 3×10¹⁸ He⁺/cm² at 700 °C and 1.3x10¹⁹ He⁺/cm² at 1000 °C. Extensive pore formation was observed on both specimens. FIB analysis revealed that a sub-surface semi-porous layer developed after ion implantation that extended ˜300 nm in the W.N. implanted to the lower dose, and over 1500 nm in the needle implanted to the higher dose. This second needle also exhibited a drastic morphology change, which appears to be a result of the unraveling of the grains at the tip.