Highly depth-resolved characterization of the fusion-related tungsten material based on picosecond Laser-Induced Breakdown Spectroscopy
The objective of the present study is to evaluate the potential applications of picosecond Laser-Induced Breakdown Spectroscopy (ps-LIBS) in the nuclear fusion devices. The laser ablation behaviors and the spectral emission features of ps-LIBS at broad laser fluences have been investigated under a high vacuum using 35 ps laser with λ= 355 nm in order to achieve the high depth-resolution diagnosis for the key fusion-related material, tungsten (W). For the ablation behaviors, three ablation regimes in laser fluences have been clearly identified both in terms of average ablation rate (AAR) changes and surface morphology variations. For spectral emission features of ps-LIBS investigated utilizing the CCD and ICCD spectrometers, the intensities of continuum background and atomic lines emission show the different rising tendencies with laser fluence at these three ablation regimes. Three representative craters in three ablation regimes are characterized by Scanning Electron Microscopy (SEM) to investigate the crater morphologies and microstructures. The results of ablation rates and spectral emission features as well as crater morphologies suggest that the first ablation regime is a superior choice for the diagnosis of W wall by ps-LIBS due to the limited AAR (<40 nm/pulse) , the small thermal effect and thus the potentially high depth-resolution capacity. Further, the AARs and spectral emission features of ps-LIBS as a function of ablation crater diameter in the first ablation regime have been studied, systematically. The results demonstrate that the AARs is weakly dependent on the crater diameter when keep the same laser fluence, while the spectral intensities of the ps-LIBS increase significantly with the crater diameter increasing. Moreover, a rectangle-like cross-section of crater can be achieved in the first ablation regime. This has a unique benefit for depth analysis of the deposited layer on the first wall of nuclear fusion devices by ps-LIBS.