Helium retention of ion-irradiated and annealed tungsten foils

To understand the helium retention characteristics and helium bubble distribution in tungsten, He-3(d,p)He-4 nuclear reaction analysis (NRA) and transmission electron microscopy (TEM) have been performed for two forms of tungsten: single crystal and polycrystalline, implanted up to 1x10(19) He-3/m(2) at 850 degrees C and annealed at 2000 degrees 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 degrees C had no effect on the retention of helium. However, when 10(19) He/m(2) was implanted into single crystal tungsten in 1000 cycles of 10(16) He/m(2) each followed by a 2000 degrees C flash anneal, the observed helium yield dropped by 95% compared to 70% for polycrystalline tungsten. The microstructure of single crystal tungsten implanted with 1x10(19) He/m(2) and annealed at 2000 degrees C in a single step showed numerous tiny cavities at a depth of similar to 1.6 mu 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.