Highly depth-resolved characterization of fusion-related tungsten material based on picosecond laser-induced breakdown spectroscopy

The objective of the present study has been to evaluate the potential applications of picosecond laser-induced breakdown spectroscopy (ps-LIBS) in 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 a 35 ps laser with lambda = 355 nm in order to achieve high depth-resolution diagnosis of 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 the spectral emission features of ps-LIBS investigated utilizing CCD and ICCD spectrometers, the intensities of the continuum background and atomic lines emission showed the different rising tendencies with the laser fluence in these three ablation regimes. Three representative craters in the three ablation regimes were characterized by scanning electron microscopy (SEM) to investigate the crater morphologies and microstructures. The results for the ablation rates and spectral emission features as well as the crater morphologies suggest that the first ablation regime was a superior choice for the diagnosis of the W wall by ps-LIBS due to the limited AAR (<40 nm per 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 the ablation crater diameter in the first ablation regime were systematically studied. The results demonstrate that the AAR was weakly dependent on the crater diameter when keeping the same laser fluence, while the spectral intensities of the ps-LIBS increased significantly with the crater diameter increasing. Moreover, a rectangle-like cross-section of the crater could 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.