Speaker
Description
The extreme conditions in ITER demand a carefully selected first wall material. Tungsten, which replaced the originally planned beryllium, offers improved resilience and reactor relevance but introduces new challenges in impurity management. A thin boron coating (~100 nm) has been proposed to mitigate these issues [1]. However, during ITER operation, erosion and redeposition processes are expected to lead to the erosion, migration, and redeposition of boron, forming layers several micrometres thick [2]. The behaviour of these layers under plasma loading and during venting is not yet understood and may contribute to unwanted dust generation and tritium retention.
Studies have been carried out using the Magnum-PSI and Upgraded Pilot-PSI linear plasma devices to investigate these issues. In ITER, boron deposition will be carried out via helium glow-discharge with diborane gas. Due to the toxicity of diborane an alternative in-situ deposition to grow thick, fusion relevant boron layers is required. We have conducted scoping studies of several deposition techniques, including hydrogen plasma chemical erosion, powder-injection ablation, laser-assisted evaporation, and plasma-assisted pulsed laser deposition (PA-PLD), where laser-ablated material is entrained in the plasma and deposited onto the substrate. Among the investigated techniques, laser evaporation and PA-PLD demonstrated the highest potential to produce thick ITER-relevant layers suitable for future studies.
Boron, oxygen and deuterium co-deposited layers with thicknesses between 0.5 and 5 μm grown by PA-PLD have been created on a Tungsten substrate kept at 400 K, and subsequently were exposed to ITER first wall and divertor relevant plasma conditions in Upgraded Pilot-PSI by stepwise increase in power density, between 0.5 and 3 MW/m2, until flaking occurred. Layer integrity and flaking behaviour was monitored using infrared and ultra-fast camera imaging, while layer composition and thickness were quantified via ion beam analysis. This integrated methodology enables controlled studies of layer stability and adhesion. Layer stability was found to be higher for rougher substrates and thinner layers. In addition, the effects of different surface temperatures and growth rates are discussed.
[1] R.A. Pitts, et al., J. Nucl. Energy, 42 (2025) 101854
[2] K. Schmid, T. Wauters, J. Nucl. Energy 41 (2024) 101798