Speaker
Description
Following ITER re-baseline and the replacement of beryllium by tungsten (W), there is a need for assessing the efficacy of diborane glow discharge boronization systems [1,2]. Such systems are regularly used in full-W tokamaks such as ASDEX upgrade and WEST as standard conditioning method.
To support ITER research into fuel retention and lifetime of boron layers, a non-uniform boronization was performed in WEST in 2025, using all six anodes and only three of the six B₂D₆ gas inlets, leading to a toroidally non-uniform distribution of the ionization rate of the glow discharge. Two mobile probes on the upper part of the main chamber were used to expose samples during the boronization. The probes were located at two toroidal positions: on sector Q3A near an active gas inlet and on sector Q4B near an inactive gas inlet. Each probe was carrying 21 samples distributed over 5 vertical positions (level#1 to #5, level#1 being the upper part of the probe and level#5 the bottom) on its four faces.
Here, we present the physical and chemical characterization of boron layers deposited on W samples located on level#2 and level#5 of the Q3A and Q4B probes. First, RBS/NRA measurements were performed to quantify the boron (B), deuterium (D) and oxygen (O) contents. Results show that B amount is higher at level#5 than at level#2 and that the D/B ratio is larger at the Q3A location, and that the O/B ratio is higher at Q4B.
Confocal microscopy and electron microscopy techniques on FIB-cut cross sections and lamellae were then performed to analyse the roughness, thickness and nanostructure of the boron layers. The latter are found to be amorphous and dense with no crack or large porosities. The boron layers at the bottom of the probes are thicker than the ones on the upper part (110-170 nm at level#5 vs 50nm at level#2 for Q3A). Moreover, the layers at Q4B#5 are thicker (200-250 nm) than those on Q3B#5, which is located near an active gas inlet. A 10 nm native oxide (WO₃) layer was detected on the W surface, affecting O/B quantification in thin boron layers themselves. By combining RBS/NRA data with the structural characterisation, the density and chemical content of the boron layers will be discussed in relation with their distance and orientation on the probe faces with respect to the anode and the active/inactive gas inlets.
[1] https://doi.org/10.1016/j.nme.2024.101854
[2] https://doi.org/10.1016/j.nme.2025.101891