Physical insights of cavity confinement enhancing effect in laser-induced breakdown spectroscopy

Laser Ablation Plasmas and Spectroscopy for Nuclear Applications

The development of measurement methodologies to detect and monitor nuclear-relevant materials remains a consistent and significant interest across the nuclear energy, nonproliferation, safeguards, and forensics communities. Optical spectroscopy of laser-produced plasmas is becoming an increasingly popular diagnostic technique to measure radiological and nuclear materials in the field without sample preparation, where current capabilities encompass the standoff, isotopically resolved and phase-identifiable (e.g., UO and UO2 ) detection of elements across the periodic table. These methods rely on the process of laser ablation (LA), where a high-powered pulsed laser is used to excite a sample (solid, liquid, or gas) into a luminous microplasma that rapidly undergoes de-excitation through the emission of electromagnetic radiation, which serves as a spectroscopic fingerprint for that sample. This review focuses on LA plasmas and spectroscopy for nuclear applications, covering topics from the wide-area environmental sampling and atmospheric sensing of radionuclides to recent implementations of multivariate machine learning methods that work to enable the real-time analysis of spectrochemical measurements with an emphasis on fundamental research and development activities over the past two decades. Background on the physical breakdown mechanisms and interactions of matter with nanosecond and ultrafast laser pulses that lead to the generation of laser-produced microplasmas is provided, followed by a description of the transient spatiotemporal plasma conditions that control the behavior of spectroscopic signatures recorded by analytical methods in atomic and molecular spectroscopy. High-temperature chemical and thermodynamic processes governing reactive LA plasmas are also examined alongside investigations into the condensation pathways of the plasma, which are believed to serve as chemical surrogates for fallout particles formed in nuclear fireballs. Laser-supported absorption waves and laser-induced shockwaves that accompany LA plasmas are also discussed, which could provide insights into atmospheric ionization phenomena from strong shocks following nuclear detonations. Furthermore, the standoff detection of trace radioactive aerosols and fission gases is reviewed in the context of monitoring atmospheric radiation plumes and off-gas streams of molten salt reactors. Finally, concluding remarks will present future outlooks on the role of LA plasma spectroscopy in the nuclear community.

Influence of surface roughness on nanosecond laser-induced shock wave enhancement effects

In this paper, an effective method is proposed for improving the energy of the shock waves that are generated by plasma expanding outward and colliding with another gas. Silicon targets are used as the response medium with roughness of 2.3 nm, 457.8 nm, 1.1 µm, and 37.1 µm, respectively. A 532-nm-laser with a pulse duration of 8 ns and a repetition rate of 10 Hz is used as the irradiation source. An intensified charge-coupled device (ICCD) is used to photograph the morphology of the shock waves. The time-resolved emission images of silicon plasma plumes are observed between 20-200 ns. As the surface roughness of the target increases, the intensity of the shock wave gradually increases, and the energy of the shock wave reaches up to 39.45 mJ at a roughness of 37.1 µm.

Spatial confinement effects of laser-induced breakdown spectroscopy at reduced air pressures

Spatial confinement is a simple and cost-effective method for enhancing signal intensity and improving the detection sensitivity of laser-induced breakdown spectroscopy (LIBS). However, the spatial confinement effects of LIBS under different pressures remains a question to be studied, because the pressure of the ambient gas has a significant influence on the temporal and spatial evolution of plasma. In this study, spatial confinement effects of LIBS under a series of reduced air pressures were investigated experimentally, and the plasma characteristics under different air pressures were studied. The results show that the reduced air pressure can lead to both earlier onset and weakening of the enhancement effect of the spatial confinement on the LIBS line intensity. When the air pressure drops to 0.1 kPa, the enhancement effect of the emission intensity no longer comes from the compression of the reflected shock wave on the plasma, but from the cavity's restriction of the plasma expansion space. In conclusion, the enhancement effect of spatial confinement technology on the LIBS is still effective when the pressure is reduced, which further expands the research and application field of spatial confinement technology.

Improvement of LIBS signal stability for NaCl solution using femtosecond laser-induced water film

This paper studies the analysis of Na element concentration in NaCl aqueous solution using laser-induced breakdown spectroscopy (LIBS). The NaCl solution is transformed to a thin water film. The water film can provide a stable liquid surface, and overcome the disadvantage that laser focusing position cannot be fixed due to liquid level fluctuation (when nanosecond laser is used as the excitation light source, there is serious liquid splash phenomenon, which affects the signal stability). And, femtosecond pulse laser is used to excite the water film to produce the plasma, avoiding liquid splashing. The measured emission lines are Na (I) at 589.0 nm and 589.6 nm. The calibration curves of sodium are plotted by measuring different concentrations of NaCl solution. The linear correlation coefficients of Na (I) lines at 589.0 nm and 589.6 nm are 0.9928 and 0.9914, respectively. In addition, the relative standard deviation is also calculated; its range is from 1.5% to 4.5%. The results indicate that the combination of femtosecond laser and water film can significantly improve the signal stability for liquid analysis in LIBS.

The parameter optimization of lasers' energy ratio of the double-pulse laser induced breakdown spectrometry for heavy metal elements in the soil

Laser-induced breakdown spectroscopy (LIBS) is a rapid, no-sample preparation, remote detection method that has been applied widely in the area of heavy metal detection in the soil. However, the promotion of LIBS is limited by its disadvantages, such as low precision analysis, a high detection limit, and so on. In recent years, many studies have been conducted to improve the LIBS spectral intensity. The double-pulse LIBS (DP-LIBS) is a representative technology in this area. Most of the research work focuses on the analytical methods of DP-LIBS, including the spatial configuration, the inter-pulse time, and the effect of signal enhancement of the DP-LIBS. However, there are few reports about the effect of the energy proportion of the two lasers and the contribution of different laser energies on the signal enhancement, and the inter-pulse time under the conditions of different laser energies. Moreover, DP-LIBS is mostly evaluated by the enhancement factor of the spectral signal, and there are few reports on the quantitative analysis of double-pulse LIBS. This study, which mainly detects Cu, Ni, and Pb in the soil, focuses on the contribution of the signal enhancement by adjusting the energy ratio of the two lasers and the best inter-pulse time under the conditions of different laser energies. Then, quantitative analysis of spectral signals obtained by single-pulse LIBS (SP-LIBS) and DP-LIBS are performed based on the random forest (RF) model. The results demonstrate that DP-LIBS shows better analytical performance than SP-LIBS, the coefficients of determination (R²) of the test have great improvement, the root-mean-squared error (RMSE) is much decreased and the relative error is much improved. Thus, this study shows that DP-LIBS is an effective method for the quantitative analysis of heavy metals in the soil.

Detection of lead in soil implying sample heating and laser-induced breakdown spectroscopy

The emission line intensities enhancement and sensitivity of laser-induced breakdown spectroscopy (LIBS) has been a subject of great interest for the last several years to improve the detection of the trace elements in soil and other environmental samples. Among several other methodologies, LIBS of the heated targets is emerging as one of the effective techniques to achieve the objective. We have investigated the effect of target heating (room temperature, 100°C, and 200°C) on the emission enhancement and plasma parameters of the laser-produced plasma on the soil sample containing 80 ppm lead. In addition, the limit of detection (LOD) of lead in soil has been determined at a fixed target temperature (200°C) and with varying lead concentration (20 ppm, 80 ppm, and 100 ppm) in the soil samples. With increasing the target temperature, not only do the emission line intensities, the excitation temperature, and electron number density increase, but also the spectral lines of Pb emerge, which were absent in the soil spectra recorded at room temperature. The limit of detection of lead in LIBS of the heated soil target has been determined as 3.8 ppm. This study reveals the potential application of the LIBS of a pre-heated target for the detection of lead with an improved LOD in the environmental sample.

Experimental investigation of the jet-on-jet physical phenomenon in laser-induced forward transfer (LIFT)

Understanding the physics behind the ejection dynamics in laser-induced forward transfer (LIFT) is of key importance in order to develop new printing techniques and overcome their limitations. In this work, a new jet-on-jet ejection phenomenon is presented and its physical origin is discussed. Time-resolved shadowgraphy imaging was employed to capture the ejection dynamics and is complemented with the photodiode intensity measurements in order to capture the light emitted by laser-induced plasma. A focus scan was conducted, which confirmed that the secondary jet is ejected due to laser-induced plasma generated at the center of the laser spot, where intensity is the highest. Five characteristic regions of the focus scan, with regards to laser fluence level and laser spot size, were distinguished. The study provides new insights in laser-induced jet dynamics and shows the possibility of overcoming the trade-off between the printing resolution and printing distance.

Shock Waves in Laser-Induced Plasmas

The production of a plasma by a pulsed laser beam in solids, liquids or gas is often associated with the generation of a strong shock wave, which can be studied and interpreted in the framework of the theory of strong explosion. In this review, we will briefly present a theoretical interpretation of the physical mechanisms of laser-generated shock waves. After that, we will discuss how the study of the dynamics of the laser-induced shock wave can be used for obtaining useful information about the laser–target interaction (for example, the energy delivered by the laser on the target material) or on the physical properties of the target itself (hardness). Finally, we will focus the discussion on how the laser-induced shock wave can be exploited in analytical applications of Laser-Induced Plasmas as, for example, in Double-Pulse Laser-Induced Breakdown Spectroscopy experiments.

Impacts of a collection system on laser-induced breakdown spectroscopy signal detection

Collecting strong enough and repeatable signals from laser-induced plasmas is the primary goal of laser-induced breakdown spectroscopy optical detection systems. Typically, the light emitted from the plasma is refracted by the lens, collected by the fiber, and measured by the spectrometer. In the present work, we established a three-dimensional model to systematically evaluate the overall emission collected from different positions of the plasma for a typical optical collection system composed of a focus lens and a collection fiber, and sensitivity analyses were further performed. In addition, experiments were conducted and partially validated the model. Results showed that for the collection system with an optical fiber located on the focal point of the collection lens, the collection efficiency distribution is almost constant within a large cylindrical-shaped area, while for that located off the focal point, there is a rhombus-shaped area with higher collection efficiency than other areas. This much higher collection efficiency area is small in size but has a large impact on the detected spectral intensity. The spatially distributed collection efficiency on the lens parameters, such as size and position, was further discussed to clarify the impacts of the collection system. Furthermore, sensitivity analyses were performed to evaluate the impact of the collection system on the signal repeatability. Based on these calculations, recommendations for the design of the collection for optimized spectral intensity and stability were proposed.

Accuracy enhancement of laser induced breakdown spectra using permittivity and size optimized plasma confinement rings

The inevitable problems in laser induced breakdown spectroscopy are matrix effect and statistical fluctuation of the spectral signal, which can be partly avoided by utilizing a proper confined unit. The dependences of spectral signal enhancement on relative permittivity were studied by varying materials to confine the plasma, which include polytetrafluoroethylene(PTFE), nylon/dacron, silicagel, and nitrile-butadiene rubber (NBR) with the relative permittivity 2.2, ~3.3, 3.6, 8~13, 15~22. We found that higher relative permittivity rings induce stronger enhancement ability, which restricts the energy dissipation of plasma better and due to the reflected electromagnetic wave from the wall of different materials, the electromagnetic field of plasma can be well confined and makes the distribution of plasma more orderly. The spectral intensities of the characteristic lines Si I 243.5 nm and Si I 263.1 nm increased approximately 2 times with relative permittivity values from 2.2 to ~20. The size dependent enhancement of PTFE was further checked and the maximum gain was realized by using a confinement ring with a diameter size of 5 mm and a height of 3 mm (D5mmH3mm), and the rings with D2mmH1mm and D3mmH2mm also show higher enhancement factor. In view of peak shift, peak lost and accidental peaks in the obtained spectra were properly treated in data progressing; the spectral fluctuation decreased drastically for various materials with different relative permittivities as confined units, which means the core of plasma is stabilized, attributing to the confinement effect. Furthermore, the quantitative analysis in coal shows wonderful results-the prediction fitting coefficient R² reaches 0.98 for ash and 0.99 for both volatile and carbon.

Two-dimensional fluorescence spectroscopy of laser-produced plasmas

We use a two-dimensional laser-induced fluorescence spectroscopy technique to measure the coupled absorption and emission properties of atomic species in plasmas produced via laser ablation of a solid aluminum target at atmospheric pressure. Emission spectra from the Al I 394.4 nm and Al I 396.15 nm transitions are measured while a frequency-doubled, continuous wave (cw) Ti:sapphire laser is tuned across the Al I 396.15 nm transition. The resulting two-dimensional spectra show the energy coupling between the two transitions via increased emission intensity for both transitions during resonant absorption of the cw laser at one transition. Time-delayed, gated detection of the emission spectrum is used to isolate resonantly excited fluorescence emission from thermally excited emission from the plasma. In addition, the tunable cw laser measures the absorption spectrum of the Al transition with ultrahigh resolution after the plasma has cooled, resulting in narrower spectral linewidths than observed in emission spectra. Our results highlight that fluorescence spectroscopy employing cw laser re-excitation after pulsed laser ablation combines benefits of both traditional emission and absorption spectroscopic methods.