Filament-induced ablation represents an attractive scheme for long-range material identification via optical spectroscopy. However, the delivery of laser energy to the target can be severely hindered by the stochastic nature of multiple-filamentation, ionization of ambient gas, and atmospheric turbulence. In order to mitigate some of these adverse effects, we examine the utility of beam shaping for femtosecond filament-induced breakdown spectroscopy with Gaussian and structured (Laguerre-Gaussian, Airy, and Bessel-Gaussian) beams in the nonlinear regime. Interaction of filaments with copper, zinc, and brass targets was studied by recording axially-resolved broadband emission from the filament-induced plasma. The laser-solid coupling efficacy was assessed by inferring thermodynamic parameters such as excitation temperature and electron density. While under our experimental conditions the ablation rate with Gaussian- and Laguerre-Gaussian beams is found to be similar, the Airy and Bessel-Gaussian beams offer the advantage of longitudinally extended working zones. These results provide insights into potential benefits of structuring ultrafast laser beams for standoff sensing applications.

There is significant motivation to develop and deploy novel nuclear reactor designs to deliver improved performance, safety, and economics for nuclear energy. In gas-cooled fast reactors that use helium as the primary coolant, the presence of xenon could indicate the onset of the fuel failure. We performed a feasibility study using single-pulse laser-induced breakdown spectroscopy to assess the sensitivity for trace xenon detection in a helium buffer of the pressure of 1.25 bar at room temperature. Under these experimental conditions, parametric optimization of recording parameters has ultimately led to the xenon detection limit of about 0.2 μmol mol⁻¹ for 10⁴ laser shots. The results show promise for the use of this technique for online monitoring of reactor fuel integrity, and motivate studies for the development of a compact measurement system that could be integrated with the reactor's primary helium-cooling loop.

Spatial segregation of species presents one of the main challenges in quantitative spectroscopy of laser-produced plasmas, as it may lead to overestimation of the concentration of the heavier species. Analytical capabilities can also be affected by excessive Stark broadening at atmospheric pressure, hindering the ability to spectrally resolve closely spaced spectral lines, such as those belonging to isotopes of the same element. We present an experimental and modeling study of the segregation of species and spectral line broadening in D₂O-H₂O plasma produced by single- and double-pulse nanosecond laser ablation in air. The ability to resolve Balmer spectral lines of hydrogen and deuterium is investigated by considering the effects of plume segregation. Transient plasma properties which lead to improvements in spectral line separation are discussed. While the plume segregation is found to be negligible in air regardless of the ablation scheme used, we observe a significant improvement in the separation of isotopic spectral lines by employing the double-pulse excitation. This study may lead to increased reliability of optical emission spectroscopy in deuterium-rich plasma environments and suggests the potential for sensitive detection of tritium in air via laser-induced breakdown spectroscopy.

Optical measurement techniques can address certain important challenges associated with nuclear safety and security. Detection of uranium over long distances presents one such challenge that is difficult to realize with traditional ionizing radiation detection, but may benefit from the use of techniques based on intense femtosecond laser pulses. When a high-power laser pulse propagates in air, it experiences collapse and confinement into filaments over an extended distance even without external focusing. In our experiments, we varied the initial pulse chirp to optimize the emission signal from the laser-produced uranium plasma at an extended distance. While the ablation efficiency of filaments formed by self-focusing is known to be significantly lower when compared to filaments produced by external focusing, we show that filaments formed by self-focusing can still generate luminous spectroscopic signatures of uranium detectable within seconds over a 10-m range. The intensity of uranium emission varies periodically with laser chirp, which is attributed to the interplay among self-focusing, defocusing, and multi-filament fragmentation along the beam propagation axis. Grouping of multi-filaments incident on target is found to be correlated with the uranium emission intensity. The results show promise towards long-range detection, advancing the diagnostics and analytical capabilities in ultrafast laser-based spectroscopy of high-Z elements.