The role of temperature, deuterium pressure and residual gases in deuterium retention and outgassing of lithium deuterium co-deposits

Liquid lithium (Li) divertor concepts offer a promising solution for managing the extreme heat fluxes expected in future fusion reactors. However, lithium’s strong affinity for hydrogen isotopes raises concerns regarding tritium inventory requirements and tritium breeding ratio. In particular, the lack of studies on tritium-lithium co-deposits is critical, as such co-deposits may contribute significantly to tritium retention in inaccessible areas, complicating tritium recovery. High-temperature retention measurements in co-deposits and in-situ outgassing studies from fully saturated LiD samples are scarce. In this work, we investigate Li-D co-deposits formed under high-flux deuterium plasmas (flux: ∼ 7x10 24 m-2 s-1) in the linear plasma device Magnum-PSI to form several µm thick co-deposits. selective laser melting-based tungsten capillary porous structures filled with Li were exposed to D plasmas, while stainless steel witness plates placed nearby were used to collect the deposits. The Li:D ratio was analyzed across a temperature range of 160 degrees C–520 degrees C, followed by one-hour vacuum outgassing (0.04 Pa) at 200 degrees C–500 degrees C. Additional experiments studied the influence of D puffing (20 Pa D2) during outgassing and the role of residual gases in freshly deposited films. in-situ ion beam analysis was employed to characterize the co-deposits: nuclear reaction analysis quantified the Li and D areal densities, while elastic backscattering spectroscopy measured oxygen content. Results show that the D:Li ratio in the co-deposits remains at 40:60 close to the theoretical maximum of 50:50 and is largely independent of substrate temperature up to 450 degrees C. However, residual water vapor present in the vacuum vessel was found to chemically react with LiD, forming Li2O and releasing D. This surface-mediated process primarily affects thinner films, leading to substantial D loss and explaining variations in D:Li ratio across samples from the same plasma exposure but with different thicknesses. At 520 degrees C, the 100 nm and 120 nm thick co-deposits were fully converted to Li2O before ion beam analysis, preventing conclusive retention measurements at this temperature. Notably, Li2O formation was found to increase D desorption at low temperatures but inhibit it at higher temperatures, modifying the expected outgassing behavior based on thermal release alone. These findings highlight that, to avoid significant tritium retention in Li-T co-deposits, tokamak surfaces may need to be maintained above 450 degrees C. Furthermore, water vapor plays a more influential role in retention and release processes than previously believed.