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) similar to 10(20) m(-3), T-e similar to 2 eV) in the linear plasma device Pilot-PSI. The peak surface temperatures of the targets are similar to 1600 K for W and similar to 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 = 10(19)-10(20) Dm(-2)) despite exposure to high plasma flux densities (similar to 10(24) Dm(-2) s(-1)). This is likely due to the elevated surface temperature due to plasma thermal loading during exposure.