Deuterium retention and removal in liquid lithium determined by in situ NRA in Magnum-PSI

Abstract

In this work, Li-filled 3D-printed porous tungsten samples were exposed to deuterium (D) plasma in Magnum-PSI with a wide ion flux from 4 × 1022 to 1.5 × 1024 m−2 s−1 and with a corresponding wide temperature range from below Li melting point (180.5 °C) to above Li deuteride (LiD) melting point (∼690 °C). The formation, decomposition and melting of LiD have been directly observed in the experiment via infra-red thermometry and visually post-mortem while still in vacuo, and correlated to the D retained content. The LiD formation was characterized by a solid precipitate layer formed on the surface with high emissivity (0.6–0.9) characterized by a blue or dark blue color after exposure. The melting of Li–LiD layer was found to occur close to the temperature predicted by Li–LiD phase diagram. In situ nuclear reaction analysis (NRA) was applied to perform the measurement of D retained in Li samples immediately after exposure without breaking the vacuum. D depth profiles were determined by NRA, in which the highest D concentration (15–45 at.%) was found in the top several micrometers and decreases with depth to low levels (<5%) within 5–30 μm. No pure LiD layer was found on the sample surfaces, however a D concentration close to 50 at.% was observed on a Li-D co-deposited layer on the clamping ring in some cases. The experiments also indicate that the D retained increases with increasing temperature until ∼500 °C. At temperatures beyond ∼500 °C the dissociation of LiD starts to dominate and the deuterium retention started to decrease. Overall, D retained fraction for all cases was found to be below ∼2%, which is significantly different from literatures where full uptake has been suggested. A 1D reaction–diffusion (RD) model based on D diffusion and chemical reactions with Li has been built. D depth profiles from the RD modelling can roughly match that from NRA measurement and a low D retained fraction below ∼2% was also indicated by the model. The model can also help explain the relationship between D retained and the surface temperature and fluence. After D plasma exposure, either helium or H plasma was utilized to remove the retained D in Li and both were proved to be effective and the removal efficiency can be as high as 96% above 420 °C.

Bubble formation in liquid Sn under different plasma loading conditions leading to droplet ejection

“…A possible solution is to add other elements, like lithium, which have good affinity with H into liquid tin to form stable hydride and deplete these dissolved H atoms in tin. We do not observe lithium droplets under hydrogen isotope plasmas even at very high flux up to ∼10 24 m −2 s −1 in our lithium experiments [63], while droplet ejection under high helium ion flux plasma loadings still exists. In the view of surface instability, lithium-tin alloys could be a better choice.…”

Plasma rotation and axial flow velocities in Magnum-PSI from cross-correlation measurements

“…Magnum-PSI is a linear magnetized plasma generator designed to study the plasma-surface interaction (PSI) with relevance for the divertor region of the future ITER [13] and DEMO [14] fusion reactors: ion flux densities greater than 10 25 m −2 s −1 and energy flux densities greater than 50 MW m −2 [15]. Besides the knowledge of plasma parameters at the target, plasma flow is an important factor for understanding the physics of the PSI studies conducted in Magnum-PSI, such as: ITER divertor monoblock performance [16], plasma detachment [17], target surface modifications (erosion/redeposition) [18], impurity seeding effects [18,19], fuel retention [18,20] etc. Moreover, sheared plasma rotation in poloidal/azimuthal direction, which is mainly driven by E × B and diamagnetic drifts, is related to the achievement of the magnetic confinement in fusion reactors [21] and the transport of particles and heat in the divertor region [22].…”

Deuterium retention in co-deposition with lithium in Magnum-PSI: experimental analysis and comparison with SOLPS-ITER

“…An additively manufactured, top-hat shaped tungsten target, identical to those used in [5,19] was used as the lithium reservoir. This target has a 24.5 mm diameter and a thickness of 17 mm.…”

“…Given the divergent reports on the role of temperature in D retention levels of LiD, one of the goals of this study is to evaluate the temperature dependence of D retention in LiD co-deposits up to 400 • C. Moreover, since the previous studies were conducted with both D gas and D plasma, we aimed to study the influence of the D species' state. In order to perform such studies, lithium-filled additively manufactured CPSs [5,19] were exposed to deuterium plasma in Magnum-PSI close to ITER relevant conditions and six stainless steel witness plates (WPs) were placed next to each CPS on a heater so that the temperature of the set of WPs was set to be varied between 200 • C and 400 • C while the CPS was loaded with identical D + plasma. The WPs were placed at radial distances between 25 and 85 mm to the edge of CPS.…”