Hydrogenic retention of high-Z refractory metals exposed to ITER divertor-relevant plasma conditions

Tungsten (W) and molybdenum (Mo) targets are exposed to the plasma conditions expected at the strike point of a detached ITER divertor (n e ∼ 1020 m−3, T e ∼ 2 eV) in the linear plasma device Pilot-PSI. The peak surface temperatures of the targets are ∼1600 K for W and ∼1100 K for Mo. The surface temperatures and plasma flux densities decrease radially towards the edges of the target due to the Gaussian distribution of electron density (n e) and temperature (T e) in the plasma column. A 2D spatial scan of the W and Mo targets using nuclear reaction analysis (NRA) shows D retention is strongly influenced by surface temperature in the range 800–1600 K and this dependence dominates over any plasma flux dependence. NRA and thermal desorption spectroscopy (TDS) show no clear dependence of retention on incident plasma fluence for the W targets with retained fractions ranging from 10−8–10−5 D retained/D incident. NRA and TDS for the Mo targets show retention rates a factor of 4–5 higher than the W targets and this is likely due to the lower surface temperatures for the Mo plasma exposures. NRA also reveals a thin boron layer on the Mo targets but the presence of boron does not correspond to a significant increase in D retention. Overall hydrogenic retention in W and Mo is shown to be low (D retained = 1019–1020 D m−2) despite exposure to high plasma flux densities (∼1024 D m−2 s−1). This is likely due to the elevated surface temperature due to plasma thermal loading during exposure.