Studies of the boron erosion and deposition in shadowed areas in EAST

Boronization is a widely employed technique for oxygen gettering and impurity suppression. It is expected to be an initial routine wall conditioning method for tungsten (W) plasma-facing components (PFCs) in ITER. To assess boron (B) performance under metal wall conditions, experimental campaigns with boronization were conducted in the Experimental Advanced Superconducting Tokamak. A quartz crystal microbalance installed at the mid-plane of port C (C-QMB), positioned 0.5 m behind the limiter, enabled in-situ monitoring of material erosion and deposition in magnetic shadowed areas during the wall conditioning processes and subsequent plasma discharges. Material erosion was detected in the majority (>50%) of discharges, regardless of whether they were normal plasma operations or terminated by disruptions. Transitions from erosion to deposition during normal discharges at the C-QMB have been shown to provide critical insights for estimating the lifetime of B-based coatings on nearby PFCs. Erosion rates were also found to be significantly influenced by the heating configuration. Electron cyclotron resonance heating (ECRH) discharges induced erosion rates 1.95 times higher than those in combined lower hybrid wave and ECRH discharges. Following a single boronization using 10 g of carborane, the B-based coating on C-QMB exhibited a lifetime of ∼104 s under plasma exposure. Post-mortem analyses revealed that about 30 nm of a boron-carbon film remained on the C-QMB, demonstrating strong oxygen gettering capability and minor iron and copper contamination. This residual film exhibited a deuterium retention at a level of 2.12 × 1020 m−2, more than eight times higher than that of pure W, highlighting the pronounced trapping capacity of B-containing films in low-flux regions. These results provide valuable insights into the application of boron in next-step devices such as ITER.