Micro-trench measurement of selective boron deposition during boron powder dropper operation in DIII-D

The selective boron (B) deposition rate is reported as measured using the micro-trench method in the DIII-D divertor during deuterium (D) H-mode outer strike point (OSP) sweeping plasma discharges with B powder injection via the impurity powder dropper (IPD). The selective deposition is defined as a result of B deposition and impurity sputtering (deposited B sputtered by B ions or other impurity ion species) in a micro-scale region geometrically shadowed from direct fuel/D+ ion bombardment. To perform this work, micro-trenches 32 µm × 32 µm × 4 µm deep were fabricated on a single-crystal silicon sample. The sample was exposed to DIII-D H-mode “cleaning” plasma discharges with D fueling and OSP sweeping via the Divertor Material Evaluation System (DiMES). The IPD injected B powder at rates of 5-20 mg/s during 2-5 s of H-mode plasmas in multiple discharges, adding up to 0.4 g injected in total. Injected B powders were ablated, ionized, and transported to PFC surfaces. Post-mortem analysis revealed that impurity deposition concentrated in a D+ ion shadowed area on the micro-trench floor, while no B deposition was observed outside of the micro-trench, which implied that net impurity deposition can be significantly enhanced by rough micro-structures on PFC surfaces, i.e., selective deposition. An energy-dispersive X-ray spectroscopy (EDS) analysis detected impurity deposition containing B, carbon, oxygen, and silicon. Combining the impurity deposition height profile measured by atomic force microscopy and the relative B concentration profiles measured by EDS, we determined the selective B deposition thickness of ~25 nm in the diverter region. The measured B deposition thickness agreed with a mass balance analysis to within half the magnitude when the B impurity flux was assumed to be concentrated near the OSP, as predicted by a real-time wall conditioning model developed to optimize IPD operations of advanced fusion reactors.