Atomic Lines Research Articles

Spectroscopic investigations of detachment on the MAST Upgrade Super-X divertor

We present the first analysis of the atomic and molecular processes at play during detachment in the MAST-U Super-X divertor using divertor spectroscopy data. Our analysis indicates detachment in the MAST-U Super-X divertor can be separated into four sequential phases: first, the ionisation region detaches from the target at detachment onset leaving a region of increased molecular densities downstream. The plasma interacts with these molecules, resulting in molecular ions ( and/or ) that further react with the plasma leading to molecular activated recombination and dissociation (MAR and MAD), which results in excited atoms and significant Balmer line emission. Second, the MAR region detaches from the target leaving a sub-eV temperature region downstream. Third, an onset of strong emission from electron–ion recombination (EIR) ensues. Finally, the electron density decays near the target, resulting in the bulk of the electron density moving upstream. The analysis in this paper indicates that plasma–molecule interactions have a larger impact than previously reported and play a critical role in the intensity and interpretation of hydrogen atomic line emission characteristics on MAST-U. Furthermore, we find that the Fulcher band emission profile in the divertor can be used as a proxy for the ionisation region and may also be employed as a plasma temperature diagnostic for improving the separation of hydrogenic emission arising from electron-impact excitation and that from plasma–molecular interactions. We provide evidences for the presence of low electron temperatures (≪0.5 eV) during detachment phases III–IV based on quantitative spectroscopy analysis, a Boltzmann relation of the high-n Balmer line transitions together with an analysis of the brightness of high-n Balmer lines.

Deformation Behavior of Crystalline Cr–Ni Multilayer Coatings by Using Molecular Dynamics Simulation

This work shows the atomic scale deformation behavior of crystalline multilayer coating comprising up to five Cr and Ni layers. A molecular dynamics simulation was conducted to visualize the atomic scale behavior of the multilayer during indentation/unloading and scratch. Normal and shear directional forces were recorded to compare repulsion and friction forces between the multilayer models during the indentation/unloading and scratch simulations. Dislocation lines within the layers were quantified to understand the deformation behavior of each model. Atomic scale deformation and dislocation lines after the indentation and scratch were visualized. Generation and movement of a single dislocation line during the indentation simulation were also visualized within a few picoseconds. The repulsion and friction forces of the five-layer model showed the lowest values among the models. The unloading stiffness of the five-layer model was calculated to be the lowest among the models. The amount of plastic deformation and the wear volume of the one-layer model after the indentation and scratch was calculated to be the highest among the models. The number of dislocation lines of the five-layer model showed an increasing trend during the indentation and scratch. The highest dislocation density of the five-layer model might aid in an enhancement of resistance to the plastic deformation to reduce the wear volume when scratched.

Photometric and Spectroscopic Study of the EXor-like Eruptive Young Star Gaia19fct

Gaia19fct is one of the Gaia-alerted eruptive young stars that has undergone several brightening events. We conducted monitoring observations using multifilter optical and near-infrared photometry, as well as near-infrared spectroscopy, to understand the physical properties of Gaia19fct and investigate whether it fits into the historically defined two classes. We present the analyses of light curves, color variations, spectral lines, and CO modeling. The light curves show at least five brightening events since 2015, and the multifilter color evolutions are mostly gray. The gray evolution indicates that bursts are triggered by mechanisms other than extinction. Our near-infrared spectra exhibit both absorption and emission lines and show time variability throughout our observations. We found lower rotational velocity and lower temperature from the near-infrared atomic absorption lines than from the optical lines, suggesting that Gaia19fct has a Keplerian rotating disk. The CO overtone features show a superposition of absorption and emission components, which is unlike other young stellar objects. We modeled the CO lines, and the result suggests that the emission and absorption components are formed in different regions. We found that although Gaia19fct exhibits characteristics of both types of eruptive young stars, FU Orionis–type objects and EX Lupi–type objects, it shows more similarity with EXors in general.

A Localized Kinematic Structure Detected in Atomic Carbon Emission Spatially Coincident with a Proposed Protoplanet in the HD 163296 Disk

Over the past 5 yr, studies of the kinematics in protoplanetary disks have led to the discovery of new protoplanet candidates and several structures linked to possible planet−disk interactions. We detect a localized kinematic bipolar structure in the HD 163296 disk present inside the deepest dust gap at 48 au from atomic carbon line emission. HD 163296's stellar jet and molecular winds have been described in detail in the literature; however, the kinematic anomaly in C i emission is not associated with either of them. Further, the velocity of the kinematic structure points indicates a component fast enough to differentiate it from the Keplerian profile of the disk, and its atomic nature hints at a localized UV source strong enough to dissociate CO and launch a C i outflow or a strong polar flow from the upper layers of the disk. By discarding the stellar jet and previously observed molecular winds, we explore different sources for this kinematic feature in C i emission that could be associated with a protoplanet inflow/outflow or disk winds.

Interpreting molecular hydrogen and atomic oxygen line emission of T Tauri disks with photoevaporative disk-wind models

Context. Winds in protoplanetary disks play an important role in their evolution and dispersal. However, the physical process that is actually driving the winds is still unclear (i.e. magnetically versus thermally driven), and can only be understood by directly confronting theoretical models with observational data. Aims. We aim to interpret observational data for molecular hydrogen and atomic oxygen lines that show kinematic disk-wind signatures in order to investigate whether or not purely thermally driven winds are consistent with the data. Methods. We use hydrodynamic photoevaporative disk-wind models and post-process them with a thermochemical model to produce synthetic observables for the spectral lines o–H2 1–0 S(1) at 2.12 µm and [OI] 1D2–3P2 at 0.63 µm and directly compare the results to a sample of observations. Results. We find that our photoevaporative disk-wind model is consistent with the observed signatures of the blueshifted narrow low-velocity component (NLVC) – which is usually associated with slow disk winds – for both tracers. Only for one out of seven targets that show blueshifted NLVCs does the photoevaporative model fail to explain the observed line kinematics. Our results also indicate that interpreting spectral line profiles using simple methods, such as the thin-disk approximation, to determine the line emitting region is not appropriate for the majority of cases and can yield misleading conclusions. This is due to the complexity of the line excitation, wind dynamics, and the impact of the actual physical location of the line-emitting regions on the line profiles. Conclusions. The photoevaporative disk-wind models are largely consistent with the studied observational data set, but it is not possible to clearly discriminate between different wind-driving mechanisms. Further improvements to the models are necessary, such as consistent modelling of the dynamics and chemistry, and detailed modelling of individual targets (i.e. disk structure) would be beneficial. Furthermore, a direct comparison of magnetically driven disk-wind models to the observational data set is necessary in order to determine whether or not spatially unresolved observations of multiple wind tracers are sufficient to discriminate between theoretical models.

The Role of the Spectral Lines of Atoms and Ions in Radiative Heating of the Surfaces of Space Capsules

The Role of the Spectral Lines of Atoms and Ions in Radiative Heating of the Surfaces of Space Capsules

Time-evolution characteristics of spectrum and temperature of lightning discharge plasma

Temperature is one of the crucial parameters reflecting the energy and current transfer characteristics in lightning discharge plasma channels. According to the spectra of six lightning return strokes discharges recorded simultaneously by two high-speed slit-less spectrographs with different time resolutions, the spectral intensity and temperature evolution of the plasma channels over time was quantitatively analyzed. The spectral characteristics show that the ionic line intensity decayed rapidly with time as the current declined, while the atomic line intensity decreased more slowly. Additionally, it is found that the ionic lines existed for a much longer time than previously reported values, up to hundreds of microseconds in the spectra of continuing current process. It further indicates that the ionic line intensities are associated with the discharge currents and that their radiation mechanisms are closely related to the collision excitation under the action of strong currents. The temperature calculated by the ionic lines can reflect the thermodynamic properties of the current-carrying channel. The temperature calculated using atomic lines is significantly lower than that calculated by the ionic lines in the same spectrum. The radiation mechanism for a majority of the atomic lines differs from that of the ionic lines. During the continuing current, the channel temperatures calculated by both ionic and atomic lines showed a similar evolution feature which declined slowly or even remained basically unchanged. This property reflects the persistent heating effect of the current.

Utilization of laser-induced breakdown spectroscopy, with principal component analysis and artificial neural networks in revealing adulteration of similarly looking fish fillets.

Fish is an essential source of many nutrients necessary for human health. However, the deliberate mislabeling of similar fish fillet types is common in markets to make use of the relatively high price difference. This is a type of explicit food adulteration. In the present work, spectrochemical analysis and chemometric methods are adopted to disclose this type of fish species cheating. Laser-induced breakdown spectroscopy (LIBS) was utilized to differentiate between the fillets of the low-priced tilapia and the expensive Nile perch. Furthermore, the acquired spectroscopic data were analyzed statistically using principal component analysis (PCA) and artificial neural network (ANN) showing good discrimination in the PCA score plot and a 99% classification accuracy rate of the implemented ANN model. The recorded spectra of the two fish indicated that tilapia has a higher fat content than Nile perch, as evidenced by higher CN and C2 bands and an atomic line at 247.8nm in its spectrum. The obtained results demonstrated the potential of using LIBS as a simple, fast, and cost-effective analytical technique, combined with statistical analysis for the decisive discrimination between fish fillet species.

Absolute frequency measurement of the 6s21S0 → 6s6p3P1F = 3/2 → F' = 5/2 201Hg transition with background-free saturation spectroscopy.

We report the development of a method for eliminating background-induced systematic shifts affecting precise measurements of saturation absorption signals. With this technique, we measured the absolute frequency of the 6s21S0 → 6s6p3P1 transition in 201aHg (F = 3/2 → F' = 5/2) to be 1181541111051(83) kHz. The measurement was referenced with an optical frequency comb synchronized to the frequency of the local representation of the UTC. This specific atomic line is situated on the steep slope of the Doppler background at room temperature, which results in a frequency systematic shift. We determined the dependence of this shift on the properties of both the spectral line and the background of the measured signal.

Study of spectral intensity of the laser ablated tungsten plasma and ablation mass at various laser spot sizes and laser fluence in vacuum environment

The ablation features such as spot size and crater volume are essential parameters for the in-situ laser-induced breakdown spectroscopy (LIBS) technique to be well applied in the quantitative diagnosis of the plasma-wall interaction (PWI) processes on the first wall or divertor in tokamak devices. The spectroscopic characteristics and the ablation features as well as the plume dynamics of laser-induced tungsten plasma in vacuum environment at various spot sizes were investigated in this work. A nano-second laser pulse (5 ns, 1064 nm) with a maximum energy of 200 mJ was focused on the tungsten target with different spot sizes (from 0.42 mm to 2.71 mm) by adjusting the distance from focal point to sample surface. A paraxially collective configuration (15°) and time-integrated spectral collection mode were applied, similar to the present in-situ LIBS system on Experimental Advanced Superconducting Tokamak (EAST). It was observed that the spot size affects the spectral intensity significantly, and the optimal spot size is 1.16 mm both for the observed tungsten ionic and atomic lines at the laser energy of 200 mJ. The electron temperature (Te), electron density (ne) and ablation mass also reach maxima at the optimal spot size. Temporally spectral dynamic investigation shows the tungsten line emission time at the optimal spot size lasts longer and decays more slowly than other sizes, and the fast photography investigation also verified this. It was found that the central intensity profile of the plasma plume image from the target surface along with the opposite direction of the laser incident at the early stage (< 8 ns, during the laser pulse) is stronger at the smaller spot size while it decays faster. This was attributed to the stronger plasma shielding effect leading to the relatively less total ablation mass. Moreover, the optimal spot size was found to increase with the laser energy in the energy range from 65 mJ to 200 mJ, and the corresponding optimal laser fluence almost keeps the constant value of ∼18 J/cm2, which provides valuable information for optimizing the current in-situ LIBS for quantitative diagnosis of the PWI processes such as fuel retention and impurity deposition in EAST.

Comparison of heating mechanisms of argon helicon plasma in different wave modes with and without blue core

In this work, we investigated the discharge characteristics and heating mechanisms of argon helicon plasma in different wave coupled modes with and without blue core. Spatially resolved spectroscopy and emission intensity of argon atom and ion lines were measured via local optical emission spectroscopy, and electron density was measured experimentally by an RF-compensated Langmuir probe. The relation between the emission intensity and the electron density was obtained and the wavenumbers of helicon and ‘Trivelpiece-Gould’ (TG) waves were calculated by solving the dispersion relation in wave modes. The results show that at least two distinct wave coupled modes appear in argon helicon plasma at increasing RF power, i.e. blue core (or BC) mode with a significant bright core of blue lights and a normal wave (NW) mode without blue core. The emission intensity of atom line 750.5 nm (I ArI750.5nm) is related to the electron density and tends to be saturated in wave coupled modes due to the neutral depletion, while the intensity of ion line 480.6 nm (I ArII480.6nm) is a function of the electron density and temperature, and increases dramatically as the RF power is increased. Theoretical analysis shows that TG waves are strongly damped at the plasma edge in NW and/or BC modes, while helicon waves are the dominant mechanism of power deposition or central heating of electrons in both modes. The formation of BC column mainly depends on the enhanced central electron heating by helicon waves rather than TG waves since the excitation of TG waves would be suppressed in this special anti-resonance region.

High-resolution Linear Spectropolarimetry of Red Supergiant VY CMa: Identification of Atomic and Molecular Features

We present a high-resolution optical spectropolarimetric observation of the red supergiant VY CMa. The optical spectrum of VY CMa contains numerous emission features that are mainly due to molecular bands of TiO, ScO, and VO. In addition, emission lines of Ba ii, Ca i, Na i, K i, and Rb i are identified in the spectrum. Our observation shows that all the molecular bands exhibit a very large and abrupt linear polarization reduction, i.e., depolarization, across the band heads of up to 10% or higher with respect to the continuum polarization level. The high-resolution observations demonstrate clearly that the molecular band heads are associated with the largest depolarization. Using this property together with a simulated emission spectrum of TiO we are able to identify most of the broad emission features in the optical spectrum of VY CMa as the TiO band heads in emission. Our observations also reveal a previously unknown phenomenon that the atomic lines either in emission or absoprtion also exhibit large depolarization across the line profile, with the K i doublet showing almost complete depolarization. The observed polarization pattern is very similar among either absoprtion or emission lines, even though the spectral lines belong to different atomic species. Together with recent high spectral resolution observations, we discuss the origin of this phenomenon and suggest that the observed large depolarization across molecular bands and atomic lines is due to absorption and scattering in the complex circumstellar environment of VY CMa, especially in the dense dusty clumps located almost on the plane of the sky.

Enhancement of spectral line intensity and lifetime of femtosecond laser-induced Cu plasma in a cylindrical cavity

The spatial confinement effects in the femtosecond laser-induced Cu plasma contained within a cylindrical cavity are studied by optical emission spectroscopy. The Cu atomic emission intensity and spectral line lifetime are enhanced significantly when a cylindrical cavity with an inner diameter of 6 mm confined the femtosecond laser-induced Cu plasma. The enhancement factor of the emission intensity under the spatial confinement depends on the upper level energy of the spectral line, and the maximum enhancement factors is about 2.48 for the Cu I 529.3 nm spectral line. The spatial confinement can effectively prolong the lifetime of the femtosecond-induced Cu and the persistence time of the plasma is near to 7 μs. The enhancements of the spectral line intensity and lifetime are attributed to the increased electron temperature and density owing to the spatial confinement, and the mechanism for the enhancement effects with the cylindrical cavity are discussed in terms of the shockwave interactions.

Simultaneous analysis of long-pulse laser irradiated plasma-facing materials (PFMs) microstructure and hardness by in-situ laser Opto-ultrasonic dual detection (LOUD)

Laser-induced transient heat load events such as edge localized mode (ELM) could induce recrystallization, grain growth, surface degradation, and hardness changes. In this regard, we used the long-pulse laser beams with power densities ranging from 0.165 to 1.909 GW m−2 to simulate transient heat load events for pure tungsten and tungsten heavy alloy (WHA) materials. The investigated samples showed that pure tungsten exhibited a higher recrystallization threshold, steady grain growth, and less surface hardness deterioration than WHA. However, the real-time simultaneous analysis of structural and mechanical properties such as matrix elements, grain size, surface degradation, and hardness changes is challenging in this domain. Therefore, in this study, we utilized a novel laser Opto-ultrasonic dual detection (LOUD) multimodal technique that integrates the benefits of laser-induced breakdown spectroscopy (LIBS) and laser ultrasonic testing (LUT) for simultaneous and in-situ detection of elemental information, grain size and hardness. The preliminary results showed that the variation in hardness is well correlated with the LIBS calibration curve formed by the ionic to atomic line intensity ratio and plasma electron temperature (Te). Despite the different matrix compositions of both samples, the observed increase in the value of Te with hardness justifies the Saha-Eggert relation. The coefficient of determination (R2 ≥ 0.970) value in the calibration curve and Te for hardness evaluation showed the goodness of the fitted model. Furthermore, the correlation of grain size with ultrasonic acoustic attenuation (α) was analyzed by the pulse-echo technique of the ultrasonic waveform for investigated samples. The results showed that grain size is also directly associated with acoustic attenuation with R2 > 0.989. The findings showed that the regression coefficient has a good approximation, which demonstrates the preciseness and potential ability of the LOUD technique for in-situ grain size and hardness analysis.

Enhancement by Spark Discharge in LIBS Detection of Copper Particle Contamination in Oil-Immersed Transformer

The detection of copper particles in transformer oil based on laser-induced breakdown spectroscopy (LIBS) technology has been preliminarily studied, but its limit of detection (LOD) needs to be further strengthened. Therefore, spark-discharge assistant method was used to solve the problem, and its enhancement mechanism has been studied in the research. The study of the correlation between spark-discharge-assisted LIBS (SD-LIBS) spectral intensity and Cu concentration shows that spark discharge has a significant increase in plasma size and spectral intensity; for Cu atomic lines at 324.75 and 327.39 nm, the determination coefficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}^{{2}}$ </tex-math></inline-formula> between their internal standard correction intensity and Cu concentration is promoted to 0.996 and 0.997, respectively; spark-discharge assistant method improves the LOD of LIBS from 0.775 to 0.112 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{g}$ </tex-math></inline-formula> /g. Due to the great improvement in detection performance, the application prospect of LIBS as fine particles detection method in transformer oil has become practical.

Discharge Characteristics of Atmospheric Pressure Ar/N₂/O₂ a Plasma Jet Under a Square Wave Pulse

Plasma jets have broad application in the fields of sterilization and material processing. In recent years, studies have shown that adding a small amount of active gas into a single inert gas can change the distribution of active particles in the plasma. This not only produces the desired active species but also changes the length of the jet to make it more suitable for plasma applications. In this article, the characteristics of plasma jets were studied under an argon atmosphere with a small amount of nitrogen and oxygen. By measuring the voltage and current waveforms, luminescence images, and emission spectra under the three mixed gases, the main parameters such as discharge power, the three main particle emission spectrum intensities (the OH spectral line, the second positive band of nitrogen molecules and its diffraction band, argon atomic line), electron excitation temperature, molecular vibration temperature, and molecular rotation temperature are calculated. The results show two discharges in one cycle under the square wave pulse voltage. The addition of nitrogen and oxygen reduces the peak current of the Ar plasma jet but increases the power. The addition of N2 increases and stabilizes the length of the jet, while oxygen decreases the length. The spectral line intensities of OH and Ar atoms decrease with the increase in N2 doping concentration, while the spectral line intensity of nitrogen gas increases. The addition of oxygen reduces the intensity of all the three emission spectra. The electron excitation temperature is the highest in the mixed gas (Ar/N2/O2) and the lowest in pure argon. In the mixed gas (Ar/N2), the vibration temperature and rotation temperature are basically unchanged. After the addition of oxygen, the vibration temperature remained unchanged, but the rotation temperature increased slightly.

A study of the sign reversal from a transmission peak to an absorption dip in a V-type system using the D1 and D2 lines of 87Rb in the presence of a buffer gas

In this work, we have investigated the transmission peak and absorption dip in the Doppler broadened V-type system using the D1 and D2 lines of 87Rb atoms in the presence of nitrogen buffer gas. The effects of the probe power variations at a fixed pump power on these resonances are examined both experimentally and theoretically. We have explored the fact that the transmission peak essentially turns into an absorption dip when the probe power is stronger than the pump power. Moreover, the theoretical model of a five-level V-type system is found to be sufficient to support the experimental outcomes when the effect of collision due to the presence of buffer gas is included in the theoretical model. We have also shown the variation of the resonance amplitude and population distribution in the upper level (|5〉) with increasing probe power.

Color and spectral characteristics of long-lived meteor trail formed by the Tunka bolide

The paper addresses color characteristics and possible spectral composition of emission of a long-lived (~40 min) meteor trail of uncommon geometry, which was formed due to the bolide passage in the Tunka Valley on November 17, 2017. Analysis of dynamics of RGB channels of the meteor trail colored image shows that during the first ~8 minutes the meteor trail emission might have been contributed by the ionization trail. The ionization trail was formed by particles of the meteor matter neutral and ionized components that were heated to high temperatures on the surface of the main meteoroid and separated from it. We also examine the discussed mechanism of heterogeneous chemical reactions occurring on the surface of meteoric dust (FeS, FeO, etc.) with participation of atoms and molecules of atmospheric gases. The yellowish color of the Tunka bolide meteor trail was assumed to be determined, first of all, by the emission of molecular nitrogen N₂ band within the 570–750 nm spectral range (the first positive system) and/or enhancement of NO*₂ continuum in heterogeneous chemical reactions. The meteor trail emission spectrum should also include relatively bright atomic lines and molecular bands of the meteoric matter and atmospheric gases FeI, MgI, CaI, SiI, NaI, FeO and SO₂, OI, OH, etc.

Цветовые и спектральные характеристики долгоживущего метеорного следа, образованного Тункинским болидом

The paper addresses color characteristics and possible spectral composition of emission of a long-lived (~40 min) meteor trail of uncommon geometry, which was formed due to the bolide passage in the Tunka Valley on November 17, 2017. Analysis of dynamics of RGB channels of the meteor trail colored image shows that during the first ~8 minutes the meteor trail emission might have been contributed by the ionization trail. The ionization trail was formed by particles of the meteor matter neutral and ionized components that were heated to high temperatures on the surface of the main meteoroid and separated from it. We also examine the discussed mechanism of heterogeneous chemical reactions occurring on the surface of meteoric dust (FeS, FeO, etc.) with participation of atoms and molecules of atmospheric gases. The yellowish color of the Tunka bolide meteor trail was assumed to be determined, first of all, by the emission of molecular nitrogen N₂ band within the 570–750 nm spectral range (the first positive system) and/or enhancement of NO*₂ continuum in heterogeneous chemical reactions. The meteor trail emission spectrum should also include relatively bright atomic lines and molecular bands of the meteoric matter and atmospheric gases FeI, MgI, CaI, SiI, NaI, FeO and SO₂, OI, OH, etc.

Survival of Terrestrial N2–O2 Atmospheres in Violent XUV Environments through Efficient Atomic Line Radiative Cooling

Atmospheres play a crucial role in planetary habitability. Around M dwarfs and young Sun-like stars, planets receiving the same insolation as the present-day Earth are exposed to intense stellar X-rays and extreme-ultraviolet (XUV) radiation. This study explores the fundamental question of whether the atmosphere of present-day Earth could survive in such harsh XUV environments. Previous theoretical studies suggest that stellar XUV irradiation is sufficiently intense to remove such atmospheres completely on short timescales. In this study, we develop a new upper-atmospheric model and re-examine the thermal and hydrodynamic responses of the thermospheric structure of an Earth-like N2–O2 atmosphere, on an Earth-mass planet, to an increase in the XUV irradiation. Our model includes the effects of radiative cooling via electronic transitions of atoms and ions, known as atomic line cooling, in addition to the processes accounted for by previous models. We demonstrate that atomic line cooling dominates over the hydrodynamic effect at XUV irradiation levels greater than several times the present level of the Earth. Consequentially, the atmosphere’s structure is kept almost hydrostatic, and its escape remains sluggish even at XUV irradiation levels up to a thousand times that of the Earth at present. Our estimates for the Jeans escape rates of N2–O2 atmospheres suggest that these 1 bar atmospheres survive in early active phases of Sun-like stars. Even around active late M dwarfs, N2–O2 atmospheres could escape significant thermal loss on timescales of gigayears. These results give new insights into the habitability of terrestrial exoplanets and the Earth’s climate history.