Speaker
Description
Boron (B) layers deposited on the PFCs erode quickly off of surfaces that receive high incident ion fluxes such as divertor targets, RF antenna protection limiters, and startup limiters. However, main wall surfaces away from the ion flux areas or in areas shadowed by other components may sustain their layers for significantly longer. To study the short time scale erosion and migration of B around WEST, samples were installed on the reciprocating collector probe (RCP) and exposed in the far-SOL during two dedicated experiments in the spring of 2024 and spring of 2025. The samples collected from the RCP after exposures with and without the use of isotopically enriched B-10 powder in the impurity powder dropper (IPD) both have substantial B layers deposited on them suggesting that transport through the SOL is a major contributor to material buildup in addition to local erosion and redeposition.
To determine campaign integrated deposit thickness and infer layer growth rates in net deposition zones, two sets of samples were installed around the main chamber of WEST on the central solenoid and the outboard side main wall during the same campaigns as the RCP experiments. One set was exposed to only glow discharge boronization (GDB) while the other set was exposed to GDBs plus IPD injections to compare the migration of B from the two techniques. In the samples collected from the GDB-only campaign a factor of two asymmetry in the total B density, a proxy for layer thickness, was observed on samples installed roughly 150° toroidally apart. Analysis of the deposited layer’s structure shows discrete layers where undisturbed B from GDB and mixed plasma modified layers from deposition in the main chamber can be differentiated. Samples collected from the IPD experiment show a similar long-range asymmetry in total B and an enhanced isotope ratio near the lower inner target, as expected based on prior modelling of B transport from the IPD [1]. It is proposed that far from the divertor the modification of B layers by ion bombardment is low and the primary evolution occurs from charge exchange neutrals that either erode or deposit onto and cover the B layers depending on their energy. Interpretive modelling supporting the hypothesis that undisturbed boronization layers are being capped with redeposits during operation is also presented.
Work supported by U.S.D.O.E. under grant number DE-SC0020414
[1] K. Afonin et al., Nuclear Fusion, vol. 63, no. 12, Dec 2023.