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- LIBS (2) (entfernen)
The Radon transform is tested as a method for reconstruction of the emissivity distribution of asymmetric laser induced plasmas. Two types of experiments were carried out. First, the plasma asymmetry is introduced via focusing the laser by a cylindrical lens to create plasma plumes elongated along the symmetry axis of the lens. Second, an asymmetric power distribution across the laser beam is created by reflecting the latter from a damaged mirror. Various effects on the quality of the plasma emissivity reconstructed by the Radon tomography method are investigated. The understanding of these effects appears to be essential to design a proper experimental setup to study LIBS plasmas by the Radon tomography method. It is demonstrated that the Radon tomography can successfully be used for experimental studies of asymmetric LIBS plasmas.
The Radon transform tomography is used for reconstruction of the emissivity distribution in single- (SP) and double-pulse (DP) laser induced plasmas in orthogonal geometry. The orthogonal DP plasma is intrinsically asymmetric and thus suitable for the Radon reconstruction. The DP plasma consists of two plasmas separated by a short time interval of ~ 1 µs. The first plasma is created in air near the surface of a Si wafer and is followed (pre-ablation mode) or preceded (post-ablation mode) by the second plasma induced on this surface. A spectrometer moves in a semi-circular path around the plasma keeping the plasma in the rotation center. The optical detection is arranged so that a thin plasma layer parallel to the target surface is monitored. The axial symmetry of the SP plasma is investigated by comparing data from the Abel inversion taken at different angles and Radon reconstruction. The multi-angle measurements are used to estimate errors of the Abel reconstruction due to asymmetries of the plasma. Time-resolved Radon reconstruction in white light is performed for the DP plasma in both pre- and post-ablation modes. In the former case, the effect of ablated aerosol on the formation of the air plasma is monitored. In the latter case, a formation of an asymmetric compression shock created by the target plasma inside the air plasma is visualized. This observation is supported by computer simulations. An interaction of the two plasmas is studied by spectrally resolved Radon reconstruction revealing a complex distribution of target and ambient species inside the plasma at all studied delay times. Overall, it is demonstrated that Radon-based tomography is an informative tool to study transient asymmetric laser induced plasmas.