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 the divertor in future fusion reactors. However, lithium’s strong affinity for hydrogen isotopes raises concerns regarding tritium inventory requirements and tritium breeding ratio. In particular, tritium-lithium co-deposits studies are critical, as co-deposits may contribute significantly to tritium retention in inaccessible areas, complicating tritium recovery. In this work, we investigate Li-D co-deposits formed under high-flux deuterium plasmas in the linear plasma device Magnum-PSI to form several μm thick co-deposits. Tungsten Capillary Porous Structures (CPSs) 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 to 520 ◦C, followed by one-hour vacuum outgassing (0.04 Pa) at 200-500◦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 Nuclear Reaction Analysis (NRA) and Elastic Backscattering Spectroscopy (EBS) were employed to characterize the co-deposits. Results show that the D:Li ratio in the co-deposits remains at 40:60 close to the maximum of 50:50 and is independent of substrate temperature up to 450◦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 ◦C, the 100-120 nm thick co-deposits were fully converted to Li2O before ion beam analysis, preventing conclusive retention measurements at this temperature. 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◦C. Furthermore, water vapor plays a more influential role in retention and release processes than previously believed.