Helium Retention of Ion-Irradiated and Annealed Tungsten Foils

To understand the helium retention characteristics and helium bubble distribution in tungsten, ³He(d,p)⁴He nuclear reaction analysis (NRA) and transmission electron microscopy (TEM) have been performed for two forms of tungsten: single crystal and polycrystalline, implanted up to 1 × 10^{19 3}He/m² at 850°C and annealed at 2000°C. The NRA results indicated that as-implanted single crystal and polycrystalline tungsten exhibited similar helium retention characteristics. In addition, a flash anneal at 2000°C had no effect on the retention of helium. However, when 10¹⁹ He/m² was implanted into single crystal tungsten in 1000 cycles of 10¹⁶ He/m² each followed by a 2000°C flash anneal, the observed helium yield dropped by 95% compared to 70% for polycrystalline tungsten. The microstructure of single crystal tungsten implanted with 1 × 10¹⁹ He/m² and annealed at 2000°C in a single step showed numerous tiny cavities at a depth of ~1.6 m, while no visible cavities were observed in the 1000 step annealed single crystal. However, in the case of polycrystalline tungsten, a single step annealing led to significant cavity growth at grain boundaries. The reduced He retention suggests a preference for inertial fusion energy armor of single crystal over polycrystalline tungsten.