Dynamics of Deuterium Retention in Boron Films on Tungsten During High-Flux Plasma Exposures in DIONISOS

Understanding tritium retention in boron-coated tungsten (B/W) plasma-facing components is essential for next-generation fusion devices such as SPARC and ITER. In this work, we present time-resolved measurements of deuterium (D) uptake and release in B films deposited on polished polycrystalline W during high-flux (< 10²² m^-2s^-1), low-energy (<100eV) D plasma exposure in the DIONISOS linear plasma device. Measurements were performed using in operando 3MeV 3He Nuclear Reaction Analysis (NRA). Identical exposures on pure W served as a control. For pure W, D retention increased during plasma exposure and saturated after plasma termination, with no observable post-exposure outgassing. In contrast, the B/W sample showed a two-phase behavior: the D inventory initially increased, then decreased while the plasma was still on, eventually stabilizing post-exposure. This decrease began as the surface temperature exceeded 640K, consistent with the thermally driven dissociation of B-D and B-D-B bonds. Post-exposure NRA spectra suggest that D remained confined within the B layer, implying that the thin B film acts as a diffusion barrier for D in W. While B was initially eroded, it reached a steady-state surface concentration during the plasma exposure, likely due to the formation of B–W bonds that enhance surface binding energy. No dynamic D release was observed after plasma termination for either sample, which we attribute to beam-induced trapping sites in W and the thermal stability of D-containing bonds in B/W at the relevant temperatures.