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Description
Boronization is an established techniques to condition the first wall of fusion devices. Boronization systems are designed to deposit a ~10-100 nm thin boron (B) layer on PFCs by injecting diborane (B₂H₆/B₂D₆) mixed with a carrier gas (He, H₂, D₂) into a cleaning Glow Discharge (GD). For optimal boronization performance, thus, achieving a homogeneous layer is desirable in order to getter the oxygen (O) and cover recessed areas of the first wall. This ensures a long lifetime of the boronization effect, thus reduction of the impurity influx and controlled fuel recycling resulting in good plasma performance. The burden to use boronization is the need for specific gas inlets for toxic diborane and the need of a homogenous, stable GD. The B distribution can become highly non-uniform, depending strongly on the spatial arrangement of anodes and gas-feed locations. The regions near the anodes and gas inlets tend to develop substantially thicker B layers, whereas more remote areas exhibit much thinner deposition. Subsequent plasma operation can further redistribute the deposited B, making the dynamic behaviour of B layers and their influence on plasma performance..
In the non-axis symmetric stellarator W7-X, the challenge to achieve homogenous boron coverage is even higher than in tokamaks. The success of boronization in W7-X was demonstrated and the lifetime determined empirically, material samples in the multipurpose manipulator revealed thin B layers of less than ten nm, whereas migrated B of µm thickness was found campaign-integrated in the divertor. No systematic study of B deposition on the anodes was yet performed.
The boronization in W7-X uses a GDC with a 90:10-mixture of He and B₂H₆, typically employing 8-10 electrodes at 310-490 V and 4-11×10⁻³ mbar for 2-5 hours. After three boronizations in OP1.2b and five in OP2.1, five toroidally distributed GD anodes were retrieved, providing an opportunity to examine the spatial distribution of B deposition. ps-LIBS was applied to characterize mixed B deposition on multiple anodes. The results reveal that variations in operational duration, anode position, and local discharge conditions produce pronounced differences in B-layer thickness from nm-to-µm. Significant variation is observed among different anodes located toroidally and different distance to the inlets, and individual anodes exhibit strong radial and circumferential asymmetry. The deposits consist primarily of B-O with minor H incorporation. These findings provide essential experimental benchmark to validate models of B deposition during boronization and subsequently input to B migration studies in W7-X and beyond.