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 ∼1600K for W and ∼1100K 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 800-1600K 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 ¹⁹-10²⁰ 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.