Speaker
Description
Since the operational period OP 2.1 the stellarator Wendelstein 7-X (W7 X) operates with a CFC divertor, graphite tiles, and a mostly stainless-steel outer wall. Boronizations are used for wall conditioning in order to reduce the amounts of oxygen and water.
The divertor strike line is a net erosion area [1, 2]; the amount of eroded carbon depends on the concentration of oxygen in the plasma. Some amount of carbon eroded at the strike line is redeposited in the divertor in the vicinity of the strike line, but this redeposited amount is considerably smaller than the eroded amount. The inner wall shows either erosion or deposition, but both effects are relatively small. Erosion at the position of the outer wall was studied by exposing samples to individual discharges using the multi-purpose manipulator (MPM). Erosion was observed, but was also small.
Carbon containing redeposits are observed on a number of first wall components. These were analysed using elastic backscattering spectrometry (EBS) with incident protons at MeV energies, with scanning electron microscopy (SEM), and with focused ion beam cross sectioning (FIB) after the operational periods OP 2.1 and OP 2.3. Thick deposits were found on baffle tiles and adjacent inner wall tiles, the thickest observed deposits exceeded thicknesses of 10 µm. Moderately thick deposits were found on toroidal closure tiles. The surface of deposits can be smooth, but can also show conical structures parallel to the surface. Deposits consist mostly of carbon with 20-40 at.% of hydrogen, 10-20 at.% oxygen, and < 10 at.% of boron. Some deposits flake off. The rupture line can be between deposit and substrate, but can also be inside the layered structure of the deposit. The appearance of flaking could not be directly connected to layer thickness or layer composition.
The erosion rate of boronization layers was considerable higher by about one order of magnitude than that of hydrocarbon layers at the position of the MPM [3]. The expected lifetime of boronization layers on surfaces close to the plasma is small, probably only 1-2 days of plasma operation. However, the effect of boronizations lasts much longer than this expected lifetime. This means, that the gettering effect of boronization layers lasts much longer than the initial lifetime of these layers.
[1] M. Mayer et al., Phys. Scr. T171 (2020) 014035
[2] M. Mayer et al., Nucl. Fusion 62 (2022) 126049
[3] F. Maragkos et al., this conference