LIBS measurements inside the FTU vessel mock-up by using a robotic arm

The analytical capabilities of laser-induced breakdown spectroscopy (LIBS) for in situ elemental analysis of nuclear materials at elevated temperatures were estimated. A certified molybdenum–tungsten alloy (Mo 70 wt% and W 30 wt%) was chosen as a replacement for the high Z impurities deposited on plasma facing components (PFCs) in tokamak. The influence of sample temperature (Ts) on the optical emission of the laser-induced molybdenum–tungsten plasma was investigated with a nanosecond infrared (1064 nm) laser pulse. LIBS experiments were carried out in a wide temperature range from 20 to 410 °C. Surface morphology and depth profiling measurements of the ablated crater for the different temperature conditions were performed. A noticeable dependence of signal emission intensity on Ts has been found. An increase in Ts leads to an increase in the spectral line emissions. In the laser ablation process, the ablation rate, plasma temperature and electron density increase significantly with Ts. In addition, the reflectivity of the alloy surface drops dramatically once oxidation occurs. The decrease of surface reflectivity leads to a greater laser energy coupled to the target and a higher LIBS signal. These results will help improve the LIBS quantitative analysis of high Z impurities in the tokamak device.

Influence of sample temperature on the laser-induced breakdown spectroscopy of a molybdenum–tungsten alloy

Laser-produced plasmas (LPPs) engulf exotic and complex conditions ranging in temperature, density, pressure, magnetic and electric fields, charge states, charged particle kinetics, and gas-phase reactions, based on the irradiation conditions, target geometries, and the background cover gas. The application potential of the LPP is so diverse that it generates considerable interest for both basic and applied research areas. Although most of the traditional characterization techniques developed for other plasma sources can be used to characterize the LPPs, care must be taken to interpret the results because of their small size, transient nature, and inhomogeneities. The existence of the large spatiotemporal density and temperature gradients often necessitates non-uniform weighted averaging over distance and time. Among the various plasma characterization tools, optical-based diagnostic tools play a key role in the accurate measurements of LPP parameters. The optical toolbox contains optical probing methods (Thomson scattering, shadowgraphy, Schlieren, interferometry, velocimetry, and deflectometry), optical spectroscopy (emission, absorption, and fluorescence), and passive and active imaging. Each technique is useful for measuring a specific property, and its use is limited to a certain time span during the LPP evolution because of the sensitivity issues related to the selected measuring tool. Therefore, multiple diagnostic tools are essential for a comprehensive insight into the entire plasma behavior. In recent times, the improvements in performance in the lasers and detector systems expanded the capability of the aforementioned passive and active diagnostics tools. This review provides an overview of optical diagnostic tools frequently employed for the characterization of the LPPs and emphasizes techniques, associated assumptions, and challenges. 

Optical diagnostics of laser-produced plasmas

Surface hardness and microstructural properties are important parameters of plasma-facing material and play a key role in long pulse operation of fusion reactor. Nowadays, nuclear reactors are facing the problem of change in physical properties especially surface hardness due to dominant extreme conditions. Consequently, it is important to monitor these changes. Laser-induced breakdown spectroscopy (LIBS) has a potential diagnostic ability to monitor in situ surface hardness and their correlation with plasma wall interaction. In this work, the hardness of different tungsten heavy alloy grades is measured by stand-off approach using LIBS. The difference in hardness was attributed to grain size, crystal size, dislocations density and energy band gap (Eg) of materials. These microstructural and electronic structure properties have direct impact on electron temperature in laser-ablated plasma. Plasma electron temperature has been determined using Boltzmann plot method in the range from 1.76 ± 0.01 to 1.90 ± 0.02 eV, while electron density has been derived using Stark broadening spectral profile of (W-I) 429.47 nm line. The obtained direct relation between the ionic to atomic species of (WII/WI) and the material hardness are associated to increase in the value of plasma electron temperature (Te). The energy band gap of these tungsten heavy alloy targets has been observed from 3.24 to 3.59 eV as hardness increases from 314 ± 2.2 to 354 ± 1.1. The results showed that the energy band gap of these targets increases with hardness and have direct relation with plasma electron temperature. Ablation efficiency was also measured as a function of laser irradiance from crater depth analysis. The results showed that average ablation rate is decreased from soft to hard material.

An approach of stand-off measuring hardness of tungsten heavy alloys using LIBS

In this review, we report and critically discuss the application of LIBS for the determination of plasma-emission fundamental parameters, such as transition probabilities, oscillator strengths, Stark broadening and shifts, of the emission lines in the spectrum. The knowledge of these parameters is of paramount importance for plasma diagnostics or for quantitative analysis using calibration-free LIBS methods. In the first part, the theoretical basis of the analysis is laid down; in the second part, the main experimental and analytical approaches for the determination by LIBS of the spectral line spectroscopic parameters are presented. In the conclusion, the future perspectives of this kind of analysis are discussed.

https://www.mdpi.com/2076-3417/10/14/4973/pdf

Laser-Induced Breakdown Spectroscopy for Determination of Spectral Fundamental Parameters

We report the first measurements of Stark broadening widths of W I lines (426.9?nm, 429.4?nm, and 430.2?nm) as a function of electron density, . The electron density is obtained from Stark broadening of a C II line at 426.7?nm in nanosecond laser-induced tungsten carbide plasmas. A linear relation between the W I Stark widths and is confirmed. The electron temperature, , is evaluated from Boltzmann plots of W I transitions with an oscillator strength , since systematically lower population densities are observed for W I transitions with , indicating that absorption occurs. This is consistent with an overestimated derived from Stark broadening of the 429.4?nm line () at a high ambient gas pressure.

D Nishijima

Papers

Stark width measurements and Boltzmann plots of W I in nanosecond laser-induced plasmas