Nuclear fusion has enormous potential to greatly affect global energy production. The next-generation tokamak ITER, which is aimed at demonstrating the feasibility of energy production from fusion on a commercial scale, is under construction. Wall erosion, material transport, and fuel retention are known factors that shorten the lifetime of ITER during tokamak operation and give rise to safety issues. These factors, which must be understood and solved early in the process of fusion reactor design and development, are among the most important concerns for the community of plasma–wall interaction researchers. To date, laser techniques are among the most promising methods that can solve these open ITER issues, and laser-induced breakdown spectroscopy (LIBS) is an ideal candidate for online monitoring of the walls of current and next-generation (such as ITER) fusion devices. LIBS is a widely used technique for various applications. It has been considered recently as a promising tool for analyzing plasma-facing components in fusion devices in situ. This article reviews the experiments that have been performed by many research groups to assess the feasibility of LIBS for this purpose.

The diagnosis of the fuel retention and impurity deposition on the plasma facing components (PFCs) is very important for monitoring plasma-wall interactions and improving the performance of long-pulse operation for tokamak devices. In this study, a remote in situ laser-induced breakdown spectroscopic (RIS-LIBS) system has been developed to be an effective and routine method for the diagnosis of the composition of the PFCs on Experimental Advanced Superconducting Tokamak (EAST). The RIS-LIBS system can be operated between EAST discharges via a remote network control system. This allows a flexible diagnosis for the PFCs at a specific EAST discharge operation or under planned plasma scenarios according to the experimental requirement. Measurements on the fuel retention and impurity deposition of the PFCs have been performed for the test of the RIS-LIBS system, and the depth resolution and the lateral resolution of the RIS-LIBS system have been achieved to be ∼100 nm and ∼3.0 mm, respectively. For the test of detectable elements, the fuel (deuterium) and impurities have been detected and identified clearly. In addition, the measurement of fuel abundance on the first wall as a function of the days of EAST deuterium plasma discharges has been carried out for the first time. These results well manifest a significant prospect of the RIS-LIBS for the diagnosis of the PFCs in the upcoming fusion devices like China Fusion Engineering Test Reactor (CFETR) and ITER.

The onset of self-absorption of laser-induced plasma poses a problem for converting emission line intensities to concentrations, which is one of the main bottlenecks in quantitative laser-induced breakdown spectroscopy (LIBS) measurements. In this paper, the effects of atmosphere and laser fluence on self-absorption reduction of the plasma induced on tungsten-copper alloy target were investigated with nanosecond infrared (1064 nm) laser pulse over a range of 2.9 to 18.2 J/cm². The time-resolved features of self-absorption, and temperature and electron density of the plasma were characterized in atmospheric air and argon, respectively. The experimental results show the effect of self-absorption can be significantly reduced by increasing the laser pulse energy. The argon atmosphere is more helpful for self-absorption reduction. The time-resolved diagnostics of emission spectra in the early stage of the plasma formation are very effective to prevent self-absorption to improve the LIBS analytical performance.

Laser-induced breakdown spectroscopy (LIBS) is a promising method for the in situ online diagnosis of plasma–wall interaction processes in tokamaks. The effect of gas pressure on the plume expansion and spectral emission of nanosecond laser-induced plasma of the tungsten wall material was systematically investigated using a coaxial optical configuration. A nanosecond pulsed laser (5 ns, 1064 nm) was used to ablate a tungsten target surface to produce plasma at various air pressure levels from 10⁻³ Pa to 10⁵ Pa. It was found that the plasma size was strongly dependent on the ambient gas pressure, and the plume presented complex behaviors such as free expansion, splitting, sharpening, confinement, etc. The spectral intensity, the signal to background ratio, and signal to noise ratio were also affected significantly by the ambient pressure. The ambient pressure influence on the plasma plume expansion and spectral emission can be ignored when the gas pressure is lower than ∼1 Pa. The pressure of 10–100 Pa and delay time of 200–500 ns were the optimum conditions for the LIBS application in tungsten impurity of tokamak wall diagnosis. Moreover, the ablation-depth results indicated that the depth-resolution of fusion-related material analysis can be controlled by the ambient gas pressure. Temporally-resolved emission measurements were carried out to further investigate the continuum radiation and ionic and atomic emission behaviors. The temporal evolution of the electron density and electron temperature at different pressures was also determined. The plasma parameter change and laser ablation depth reduction were observed which are attributed to the ambient gas confinement and enhanced plasma shielding.

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 (T_s) 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 T_s has been found. An increase in T_s 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 T_s. 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.