Absorption tomography of laser induced plasmas

An emission tomography of laser-induced plasmas employed in the laser induced breakdown spectroscopy (LIBS) requires signal integration times in a microsecond range during which the LIBS plasma cannot be considered stationary. Consequently, the use of the data for reconstructing the plasma properties under the assumption that the latter does not change significantly during the integration time leads to inaccurate results. To reduce the integration time, it is proposed to measure a plasma absorption in parallel rays using a scanning rectangular aperture whose dimension Δ along the scanning direction is about a characteristic size of plasma plumes (Δ∼1cm) and the other dimension Δp is of the order of a uniformity length of plasma parameters (Δp∼10μm). The aperture is moved step by step along the scanning direction and the total energy of photons coming through the aperture is measured during time T at each position of the aperture. Owing to the large size of the aperture, the integration time T is reduced by a factor ∼Δp/Δ. A numerical data processing is proposed to restore the spatial resolution of the plasma absorption along the scanning direction. It is determined by the scanning step Δs≤Δp. Another advantage of the proposed procedure is that inexpensive linear CCD or non-discrete (PMT, photodiode) detectors can be used instead of costly 2-dimensional detectors.

► Plasma tomography is proposed in parallel ray absorption using a large aperture. ► The procedure allows for a substantial reduction of the integration time and, hence, is suitable to study transient plasmas. ► The needed spatial resolution is achieved by a novel numerical data processing.