He-containing plasma to track the motion of helium atoms in tungsten*

Experiments using ³He-containing plasma have been performed in PISCES-A to allow quantification of the resultant He content in W samples using D ion Nuclear Reaction Analysis (NRA). Measurements at temperatures where fuzz forms on the W surface reveal that only a small amount (~25%) of the He in the samples after the exposure is located in the tendrils themselves, with the majority of the He contained in the W below the fuzz layer. Measurements (again at high temperature) with time varying ³He content in the plasma showed the dynamic behavior of the He bubbles, as ³He injected only early in the discharge was removed from the W by subsequent ⁴He plasma exposure. The ³He content in the W base scales with the ³He fluence when ³He is contained in the plasma at the end of the discharge, demonstrating that fuzz is not completely opaque for helium ions impinging on the sample. The continuous bubble rupturing process is also thought to be a material migration source at high temperature. ³He was also observed to be removed from W during subsequent ⁴He plasma at lower temperature (~500°C), but no material migration was detected. These measurements will help guide MD simulations of He behavior in W and provide data for bench-marking such codes.