Simultaneous LIBS signal and plasma density measurement for quantitative insight into signal instability at elevated pressure

A spectral bias-error stepwise correction method of plasma image-spectrum fusion based on deep learning for improving the performance of LIBS

Poor spectral stability seriously hinders the wide application of laser-induced breakdown spectroscopy (LIBS), so how to improve its stability is the focus, hotspot, and difficulty of current research. In this study, to achieve high precision quantitative analysis under complex detection conditions, utilizing the fusion of multi-dimensional plasma information and the integration of physical models and algorithmic models, a spectral bias-error stepwise correction method of plasma image-spectrum fusion based on deep learning (SBESC-PISF) was proposed. In this method, based on the statistical properties of LIBS spectra, the actual obtained spectra were decomposed into three parts: the ideal spectral intensity related only to the element concentration, and the spectral bias and spectral error caused by the fluctuation of complex high-dimensional plasma parameters. Further, the deep learning methods were used to fully excavate all the effective features in the plasma images and spectra to invert the complex high-dimensional plasma parameters according to the physical models. Finally, the estimation models of spectral bias and spectral error were established based on these features, to realize the high-precision correction of spectral intensity. To verify the feasibility of SBESC-PISF, the spectra of aluminum alloy samples obtained under three complex detection conditions were used for analysis. Under the experimental condition of laser energy fluctuation, after correction by SBESC-PISF, R2 of the three calibration curves was all increased to 0.999, RMSE and STD of the validation set (RMSEV, STDV) were reduced by 55.246 % and 50.167 %, respectively. Under the experimental condition of defocusing amount fluctuation, R2 was also all increased to 0.999, RMSEV and STDV were decreased by 58.201 % and 51.006 %, respectively. When the laser energy and defocusing amount fluctuate simultaneously, R2 was increased to 0.999, 0.996 and 0.988, RMSEV and STDV were reduced by 58.776 % and 54.397 %, respectively. These experimental results demonstrate that the spectral fluctuation correction of SBESC-PISF under complex detection conditions is effective and has wide applicability.

Machine learning in analytical spectroscopy for nuclear diagnostics [Invited]

Analytical spectroscopy methods have shown many possible uses for nuclear material diagnostics and measurements in recent studies. In particular, the application potential for various atomic spectroscopy techniques is uniquely diverse and generates interest across a wide range of nuclear science areas. Over the last decade, techniques such as laser-induced breakdown spectroscopy, Raman spectroscopy, and x-ray fluorescence spectroscopy have yielded considerable improvements in the diagnostic analysis of nuclear materials, especially with machine learning implementations. These techniques have been applied for analytical solutions to problems concerning nuclear forensics, nuclear fuel manufacturing, nuclear fuel quality control, and general diagnostic analysis of nuclear materials. The data yielded from atomic spectroscopy methods provide innovative solutions to problems surrounding the characterization of nuclear materials, particularly for compounds with complex chemistry. Implementing these optical spectroscopy techniques can provide comprehensive new insights into the chemical analysis of nuclear materials. In particular, recent advances coupling machine learning methods to the processing of atomic emission spectra have yielded novel, robust solutions for nuclear material characterization. This review paper will provide a summation of several of these recent advances and will discuss key experimental studies that have advanced the use of analytical atomic spectroscopy techniques as active tools for nuclear diagnostic measurements.

10 kHz laser-induced schliere anemometry for velocity, Mach number, and static temperature measurements in supersonic flows

A new, to the best of our knowledge, technique for measuring velocity and Mach number in freestream flow is discussed and demonstrated. The technique, laser-induced schliere anemometry, uses a laser to write a laser-induced schliere in the flow, which can then be imaged using high-speed schlieren imaging. Here, we use a laser-induced plasma from the focusing of nanosecond-duration laser pulses from a pulse burst laser to write the disturbance. The resulting localized index of refraction gradient left from the plasma is tracked well beyond the plasma emission lifetime using schlieren imaging, and velocity is found from tracking or through a simple correlation analysis. The blast wave is also used to independently determine the Mach number via the Mach cone effect, which provides information about the mean static temperature. This technique shows great potential for use in characterizing freestream flow in supersonic facilities and is demonstrated here in a Mach 2 blowdown facility and a Mach 4 Ludwieg tube.

Rapid quantitative analysis of trace elements in plutonium alloys using a handheld laser-induced breakdown spectroscopy (LIBS) device coupled with chemometrics and machine learning

We present the first reported quantification of trace elements in plutonium via a portable laser-induced breakdown spectroscopy (LIBS) device and demonstrate the use of chemometric analysis to enhance the handheld device's sensitivity and precision. Quantification of trace elements such as iron and nickel in plutonium metal via LIBS is a challenging problem due to the complex nature of the plutonium optical emission spectra. While rapid analysis of plutonium alloys has been demonstrated using portable LIBS devices, such as the SciAps Z300, their detection limits for trace elements are severely constrained by their achievable pulse power and length, light collection optics, and detectors. In this paper, analytical methods are evaluated as a means to circumvent the detection constraints. Three chemometric methods often used in analytical spectroscopy are evaluated; principal component regression, partial least-squares regression, and artificial neural networks. These models are evaluated based on goodness-of-fit metrics, root mean-squared error, and their achievable limits of detection (LoDs). Partial least squares proved superior for determining content of iron and nickel in plutonium metal, yielding LoDs of 15 and 20 ppm, respectively. These results of identifying the undesirable trace elements in plutonium components are critical for applications such as fabricating radioisotope thermoelectric generators or nuclear fuel.

Emissions in short-gated ns/ps/fs-LIBS for fuel-to-air ratio measurements in methane-air flames

A study of short-gated 10 nanosecond (ns), 100 picosecond (ps), and 100 femtosecond (fs) laser induced breakdown spectroscopy (LIBS) was conducted for fuel-to-air ratio (FAR) measurements in an atmospheric Hencken flame. The intent of the work is to understand which emission lines are available near the optical range in each pulse width regime and which emission ratios may be favorable for generating equivalence ratio calibration curves. The emission spectra in the range of 550-800 nm for ns-LIBS and ps-LIBS are mostly similar with slightly elevated atomic oxygen lines by ps-LIBS. Spectra from fs-LIBS show the lowest continuum background and prominent individual atomic lines, though have significantly weaker ionic emission from nitrogen. A qualitative explanation based on assumed local thermodynamic equilibrium and electron temperatures calculated by the ${{\rm{N}}_{\rm{II}}}({{565}}\;{\rm{nm}})$ and ${{\rm{N}}_{\rm{II}}}({{594}}\;{\rm{nm}})$ emissions is presented. In studying line emission ratios for FAR calculation, it is found that ${{\rm{H}}_\alpha}({{656}}\;{\rm{nm}})/{{\rm{N}}_{\rm{II}}}({{568}}\;{\rm{nm}})$ is best for FAR measurements with ns-LIBS and remains viable for ps-LIBS, while ${{\rm{H}}_\alpha}({{656}}\;{\rm{nm}})/{{\rm{O}}_{\rm I}}({{777}}\;{\rm{nm}})$ is optimal for the ps-LIBS and fs-LIBS cases. Due to low continuum background and short time delay for spectra collection, fs-LIBS is very promising for high-speed FAR measurements using short-gated LIBS.

Time-Gated Single-Shot Picosecond Laser-Induced Breakdown Spectroscopy (ps-LIBS) for Equivalence-Ratio Measurements

Time-gated picosecond laser-induced breakdown spectroscopy (ps-LIBS) for the determination of local equivalence ratios in atmospheric-pressure adiabatic methane-air flames is demonstrated. Traditional LIBS for equivalence-ratio measurements employ nanosecond (ns)-laser pulses, which generate excessive amounts of continuum, reducing measurement accuracy and precision. Shorter pulse durations reduce the continuum emission by limiting avalanche ionization. Furthermore, by contrast the use of femtosecond lasers, plasma emission using picosecond-laser excitation has a high signal-to-noise ratio (S/N), allowing single-shot measurements suitable for equivalence-ratio determination in turbulent reacting flows. We carried out an analysis of the dependence of the plasma emission ratio H_α (656 nm)/N_II (568 nm) on laser energy and time-delay for optimization of S/N and minimization of measurement uncertainties in the equivalence ratios. Our finding shows that higher laser energy and shorter time delay reduces measurement uncertainty while maintaining high S/N. In addition to atmospheric-pressure flame studies, we also examine the stability of the ps-LIBS signal in a high-pressure nitrogen cell. The results indicate that the plasma emission and spatial position could be stable, shot-to-shot, at elevated pressure (up to 40 bar) using a lower excitation energy. Our work shows the potential of using ps-duration pulses to improve LIBS-based equivalence-ratio measurements, both in atmospheric and high-pressure combustion environments.

Laser-induced breakdown spectroscopy of ammonia gas with resonant vibrational excitation

In this work, laser-induced breakdown spectroscopy (LIBS) of gaseous ammonia (NH₃) molecules on- and off-resonant vibrational excitation was studied in open air. A wavelength-tunable, continuous wave (CW), carbon dioxide (CO₂) laser tuned at a resonant absorption peak (9.219 µm) within the infrared radiation (IR) range was used to resonantly excite the vibration of the N-H wagging mode of ammonia molecules. A pulsed Nd:YAG laser (1064 nm, 15 ns) was used to break down the ammonia gas for plasma imaging and spectral measurements. In this study, plasmas generated with the ammonia molecules without additional CO₂ laser beam irradiation and with additional CO₂ laser beam irradiation with the wavelengths on- and off-resonant vibrational excitation of ammonia molecules were investigated and referred as LIBS, LIBS-RE-ON and LIBS-RE-OFF, respectively. The experimental results showed that the temporal and spatial evolution as well as electron temperature and density of plasmas induced with LIBS and LIBS-RE-OFF were consistent but differed from LIBS-RE-ON. Compared with LIBS and LIBS-RE-OFF, plasmas in LIBS-RE-ON showed larger spatial expansion and enhanced emission after a delay time of 1 µs in this study, as well as significantly enhanced electron temperature by ∼ 64%. Time-resolved electron temperatures and densities showed that the emission signal enhancement in LIBS-RE-ON can be primarily attributed to the electron temperature enhancement. Signal enhancement in LIBS indicated improved detection sensitivity. This study could inspire future works on LIBS for gas detection with improved sensitivity and selectivity probably by using ultrafast/intense laser-induced molecular breakdown/ionization with resonant vibrational excitation of molecules.

Measurement of electron density and temperature from laser-induced nitrogen plasma at elevated pressure (1-6 bar)

Laser-induced plasmas experience Stark broadening and shifts of spectral lines carrying spectral signatures of plasma properties. In this paper, we report time-resolved Stark broadening measurements of a nitrogen triplet emission line at 1-6 bar ambient pressure in a pure nitrogen cell. Electron densities are calculated using the Stark broadening for different pressure conditions, which are shown to linearly increase with pressure. Additionally, using a Boltzmann fit for the triplet, the electron temperature is calculated and shown to decrease with increasing pressure. The rate of plasma cooling is observed to increase with pressure. The reported Stark broadening based plasma diagnostics in nitrogen at high pressure conditions will be significantly useful for future studies on high-pressure combustion and detonation applications.

Laser pulse width control using inverse-Bremsstrahlung photon absorption

Nanosecond laser pulses (6 ns FWHM, produced by a Q-switched, frequency-doubled Nd:YAG laser) are chopped using the inverse-Bremsstrahlung (IB) photon absorption process in a cell with variable pressure. The IB process that quickly absorbs the majority of the laser pulse energy is triggered by focusing the pulse in the cell. Prior to the initiation of the IB process, the gaseous medium in the cell is transparent, while it suddenly becomes opaque with the IB process activated; therefore, the pressure cell can be used as a virtual optical shutter. The shutter "closing time" depends strongly on the pressure of the cell and the laser pulse energy and thus can be controlled. Dependence of the "closing time" on these two parameters is experimentally investigated.

Comparison of existing laser-induced breakdown thermometry techniques along with a time-resolved breakdown approach

Temperature is an important parameter for characterizing chemical, physical, and flow processes occurring in combustion environments. Laser-induced breakdown is a process widely used to determine a material's elemental components and its composition, known as laser-induced breakdown spectroscopy (LIBS). The breakdown event, or more specifically the breakdown threshold, for a low-pressure gas strongly depends on density effects emanating in the likelihood for multiphoton and avalanche ionization. In this work, a comparison of thermometry techniques using laser-induced breakdown is made and an approach to perform simultaneous gas-phase thermometry on a shot-to-shot basis and spectroscopy is demonstrated by monitoring the moment in time the thermal plasma develops along the intensity gradient of a laser pulse. Breakdown thresholds are profiled along the height of a lean methane-air and partially combusting rich propane-air McKenna flame, and correlated to radiation and convection-corrected thermocouple readings.

A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics

The applications of femtosecond lasers to the diagnostics of combustion and flow field have recently attracted increasing interest. Many novel spectroscopic methods have been developed in obtaining non-intrusive measurements of temperature, velocity, and species concentrations with unprecedented possibilities. In this paper, several applications of femtosecond-laser-based incoherent techniques in the field of combustion diagnostics were reviewed, including two-photon femtosecond laser-induced fluorescence (fs-TPLIF), femtosecond laser-induced breakdown spectroscopy (fs-LIBS), filament-induced nonlinear spectroscopy (FINS), femtosecond laser-induced plasma spectroscopy (FLIPS), femtosecond laser electronic excitation tagging velocimetry (FLEET), femtosecond laser-induced cyano chemiluminescence (FLICC), and filamentary anemometry using femtosecond laser-extended electric discharge (FALED). Furthermore, prospects of the femtosecond-laser-based combustion diagnostic techniques in the future were analyzed and discussed to provide a reference for the relevant researchers.