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Helium Retention of Ion-Irradiated and Annealed Tungsten Foils

N. Hashimoto et al.

Fusion Science and Technology / Volume 47 / Number 4 / May 2005 / Pages 881-885

Technical Paper / Fusion Energy - Fusion Materials

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 × 1019 3He/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.

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