Characterization of impurity deposition on carbon fiber composite movable limiter for EAST by fiber-optic LIBS system

In tokamak devices, impurities generated by plasma–wall interactions (PWI) can cause erosion of plasma-facing components (PFCs), reduce their operational lifetime, and adversely affect fusion plasma performance. As a critical component directly exposed to edge plasma, the movable limiter was subjected to intense PWI during discharges, resulting in substantial impurity accumulation on its surface. In this study, an integrated fiber-optic LIBS system was employed to characterize the impurity deposition on the surface of the carbon fiber composite (CFC) movable limiter from the EAST tokamak. Depth-resolved LIBS spectra combined with Focused Ion Beam (FIB) and Scanning Electron Microscopy (SEM) cross-sectional imaging were used to estimate the thickness of the impurity layer. The average laser ablation rate was determined to be approximately 250 nm/pulse. Spatial distribution analysis of impurity elements reveals that impurity accumulation is more pronounced on the electron-side regions compared to the ion-side. An abnormally high concentration of copper (Cu) was observed in the bottom region, which is likely associated with local damage to the limiter and subsequent migration of the Cu bonding layer under intense plasma exposure. Calibration-free LIBS (CF-LIBS) was employed to quantitatively analyze two samples collected from the upper and bottom regions to verify the accuracy of the spatial distribution measurements. The calculated Cu content in the bottom sample reached 87.28%, significantly higher than the 33.27% in the upper sample, which is consistent with the results obtained from the spatial distribution measurements. These results provide valuable insights for understanding PWI and optimizing impurity control strategies in fusion devices and verify the feasibility of in situ measurements.