Evaluation of silicon carbide as a divertor armor material in DIII-D H-mode discharges

Abstract

Silicon carbide (SiC) represents a promising but largely untested plasma-facing material (PFM) for next-step fusion devices. In this work, an analytic mixed-material erosion model is developed by calculating the physical (via SDTrimSP) and chemical (via empirical scalings) sputtering yield from SiC, Si, and C. The Si content in the near-surface SiC layer is predicted to increase during D plasma bombardment due to more efficient physical and chemical sputtering of C relative to Si. Silicon erosion from SiC thereby occurs primarily from sputtering of the enriched Si layer, rather than directly from the SiC itself. SiC coatings on ATJ graphite, manufactured via chemical vapor deposition, were exposed to repeated H-mode plasma discharges in the DIII-D tokamak to test this model. The qualitative trends from analytic modeling are reproduced by the experimental measurements, obtained via spectroscopic inference using the S/XB method. Quantitatively the model slightly underpredicts measured erosion rates, which is attributed to uncertainties in the ion impact angle distribution, as well as the effect of edge-localized modes. After exposure, minimal changes to the macroscopic or microscopic surface morphology of the SiC coatings were observed. Compositional analysis reveals Si enrichment of about 10%, in line with expectations from the erosion model. Extrapolating to a DEMO-type device, an order-of-magnitude decrease in impurity sourcing, and up to a factor of 2 decrease in impurity radiation, is expected with SiC walls, relative to graphite, if low C plasma impurity content can be achieved. These favorable erosion properties motivate further investigations of SiC as a low-Z, non-metallic PFM.

In-situ coating of silicon-rich films on tokamak plasma-facing components with real-time Si material injection

“…The RMS roughness values demonstrate that the Si deposition and erosion are in the nanometer range and have only minimal impact on µm structures (table 1). While the presence of pore structures or dust particles of larger sizes may augment the RMS roughness, the base surface could concurrently exhibit flattening due to the interplay of erosion and deposition, as suggested in [24]. It is important to note that the main (micro-)roughness observed is most likely primarily due to the manufacturing and polishing process conducted during sample preparation.…”

Developing solid-surface plasma facing components for pilot plants and reactors with replenishable wall claddings and continuous surface conditioning. Part A: concepts and questions

“…Silicon carbide has long been identified as a promising low-Z refractory material for fusion reactors as a structural material since it is highly resistant to neutrondamage [43][44][45][46][47][48][49][50][67][68][69][70][71][72]. Although it has been less assessed with regard to its use as a plasma-facing material compatible with good plasma performance, SiC is considered to be promising for this application as well [27,28,47,[73][74][75]:…”

Developing solid-surface plasma facing components for pilot plants and reactors with replenishable wall claddings and continuous surface conditioning. Part B: required research in present tokamaks

“…Carbon, by contrast, is known to experience 'fire polishing' or plasma-machining, eroding away misaligned features which stand proud of the surface and filling in misalignment recesses by deposition. Less is known about plasmamachining for other non-metallic refractories; however, tests have recently been performed in DIII-D on divertor graphite tiles that had been SiC-coated using chemical vapor deposition (CVD) [52]. After a campaign-integrated exposure the SiC coatings showed no macroscopic flaking, cracking or other damage upon visual inspection.…”