Atomic Density Measurements Research Articles

Depopulation mechanisms of atomic hydrogen in the n = 3 level following two-photon excitation by a picosecond laser

One of the major constraints of measurements of atomic hydrogen densities using two-photon absorption laser induced fluorescence in most plasma and combustion environments is the determination of fluorescence decay times ( τfluo H ), especially when using nanosecond-lasers or slow acquisition systems. Therefore, it is necessary to identify the depopulation processes of the laser excited level in order to correctly estimate τfluo H . In this study, depopulation mechanisms of atomic hydrogen excited by two-photon absorption to the n = 3 level (H(n = 3)) have been investigated using a picosecond-laser excitation and acquisition of fluorescence by a streak camera, which allowed for direct measurement of τfluo H and hence, the atomic hydrogen densities, in an H2 microwave plasma operating in the pressure range 20–300 Pa. By combining these measurements with a detailed H(n = 3) collisional radiative depopulation model, it was found that full mixing amongst the H(n = 3) sub-levels occurs in our discharge conditions, even at a pressure as low as 20 Pa. Moreover, it is also seen that the Lyman β line is only partially trapped, as its escape factor Λ31 decreases from 0.94–0.98 down to 0.58–0.86 while the measured atomic hydrogen density rises from 8±5×1019m−3 to 9±6×1020m−3 . This means that the radiative decay rate of H(n = 3) atoms varies with pressure and the classical Stern–Volmer method used to determine the quenching cross-section of excited H(n = 3) in collisions with H2 molecules, σQHn=3/H2 , is not valid for our measurements. We used two different physics-based approaches, and show that the quenching cross-section σQHn=3/H2 lies in the range 90– 106×10−20m2 , which can be averaged as 98±8×10−20m2 . This substantially improved estimation of σQHn=3/H2 obtained in this work will be useful for the accurate estimation of H(n = 3) fluorescence decay times and therefore the atomic hydrogen densities.

A new 2D fluid-MC hybrid approach for simulating nonequilibrium atmospheric pressure plasmas: density distribution of atomic oxygen in radio-frequency plasma jets in He/O2 mixtures

A spatially two dimensional fluid-MC hybrid (fluid-kinetic) simulation method is developed and applied to the COST reference microplasma jet operated in helium with an oxygen admixture of 0.5%, excited by a single frequency voltage waveform with f = 13.56 MHz and ϕrms=275 V. The simulation approach is based on a fluid model augmented by a Monte Carlo module that generates electron impact rates for the continuity equations solved by the fluid module. This method is capable of providing the same level of accuracy as PIC/MCC simulations with an agreement within 5%–10% at atmospheric pressure, while being significantly faster (with a speedup factor of 30 for serial to 50 for parallel implementation). The simulation results are compared to previous measurements of atomic oxygen densities (Steuer et al 2021 J. Phys. D: Appl. Phys. 54 355204), and show a very good agreement. It is found that the buildup and saturation of the atomic oxygen density distribution along the jet are due to the interplay of chemical and electron impact reactions as well as of the gas flow. Comparing the simulation results to that of Liu et al 2021 J. Phys. D: Appl. Phys. 54 275204, it is inferred that fluid models where a 2-term BE solver is used, fail to describe the COST jet in an accurate manner due to the underestimation of the electron impact rates.

Publisher's Note: “Validation of THz absorption spectroscopy by a comparison with ps-TALIF measurements of atomic oxygen densities” [Appl. Phys. Lett. 123, 081107 (2023)

Publisher's Note: “Validation of THz absorption spectroscopy by a comparison with ps-TALIF measurements of atomic oxygen densities” [Appl. Phys. Lett. <b>123</b>, 081107 (2023)

Validation of THz absorption spectroscopy by a comparison with ps-TALIF measurements of atomic oxygen densities

Terahertz (THz) absorption spectroscopy has recently been developed as a diagnostic technique for measuring absolute ground-state atomic oxygen densities in plasmas. To demonstrate the validity of this approach, we present in this Letter a benchmark against a more established method. Atomic oxygen densities were measured with THz absorption spectroscopy and compared to those obtained from picosecond (ps) two-photon absorption laser induced fluorescence (TALIF) measurements on the same capacitively coupled radio frequency oxygen discharge. Similar changes in the atomic oxygen density were observed with both diagnostics when varying the applied power (20–100 W) and the gas pressure (0.7–1.3 mbar). Quantitatively, the results are in good agreement as well, especially when considering the total margin of error of the two diagnostics. For example, for a gas pressure of 1.3 mbar and an applied power of 30 W, atomic oxygen densities measured with THz absorption spectroscopy and TALIF were (7.0 ± 1.7)×1014 cm−3 and (5.3 ± 3.2)×1014 cm−3, respectively. This shows that THz absorption spectroscopy is an accurate technique that can be reliably used for real-world applications to determine atomic oxygen densities in plasmas.

Accurate measurement of effective atomic number and electron density with X-ray attenuation coefficient spectrum

Energy spectra of X-ray attenuation coefficients, μ(E)s, are measured using a system comprising a laboratory X-ray source and a photon-counting-type detector with an accuracy of 3% or less. We propose a method to derive the electron density (ρe) and effective atomic number (Zeff) of an object from measured μ(E) values. The method is based on a simple approximation in which an atomic cross-section of attenuation is expressed by a linear equation of Z4 and experimental calibration using standard materials. The μ(E) values of aqueous ethanol and acetic acid aqueous solutions are measured to get the ρes and Zeffs. The proposed method yielded a ρe value with an accuracy of 1.4% despite the use of standard materials, whereas it yielded a Zeff value with an accuracy of 1.1% for standard materials whose values of Z are similar to that of the sample.

The temporal behavior of O atom of a nonequilibrium atmospheric pressure plasma driven by kHz nanosecond voltage pulses

AbstractThis work investigates the temporal dynamics of O atoms in nonequilibrium atmospheric pressure plasma (NAPP) generated by kHz nanosecond pulsed discharge. Two‐photon laser‐induced fluorescence (TALIF) method is used to measure the time resolution of O atom density from the first discharge pulse in two gas mixtures, He + 0.4%O2 and He + 0.4%air. The discharge frequencies of 1 and 10 kHz are considered in the experiment. The results show that the O atom density does not accumulate with increasing number of pulses in both gas environments at 1 kHz. However, at 10 kHz, a cumulative effect of O atom density with the number of pulses is observed in both gas mixtures. After 10 and 300 discharge pulses in He + 0.4%O2 and He + 0.4%air, respectively, the O atom density saturates at the same moment after each discharge cycle. It was found that even after a long operating period of discharge, the decay of O atom density during each discharge cycle is not negligible. The O atom density in He + 0.4%O2 varies in the range of 2.85 × 1021 m−3–4.29 × 1021 m−3 while the O atom density in He + 0.4%air varies in the range of 2.60 × 1021 m−3–3.19 × 1021 m−3. This indicates that the choice of diagnostic time points is important for the O atom density measurements when using TALIF to diagnose kHz pulsed NAPP. In addition, 0D plasma chemical kinetics models are developed for the two gas mixtures to investigate O atoms' production and consumption processes. The causes of the cumulative effect of O atom density at 10 kHz, the saturation effect, and the formation of the periodic variation trend are also investigated. The simulation results show that the consumption rate of O atoms and the O atom density are directly correlated. As the number of pulses increases, the O atom consumption rate increases, which gradually counteracts the number of O atoms generated during the pulse discharge. This leads to delay and saturation of the cumulative effect of O atoms.

In-situ measurement and close-loop control of atomic number density in an optically pumped magnetometer based on light absorption

For optically pumped atomic magnetometers, the attenuation of the pumping light through the alkali-metal vapor due to light absorption is related to the number density of alkali-metal atoms. In this study, we propose an in-situ measurement and control method of atomic number density based on light absorption in the temperature range of 60 °C–160 °C, which is a much wider temperature range than considered in previous reports. A light absorption-density model is proposed to accurately describe the relationship between the light transmittance and the atomic number density. The influence of static and oscillating magnetic fields on the atomic number density measurement is also analyzed. Based on this model, a close-loop system is constructed to control the atomic number density using an electric heater. The experimental results exhibit that the proposed method can limit the fluctuation of the atomic number density in the range of 1.4%.

Terahertz absorption spectroscopy for measuring atomic oxygen densities in plasmas

This paper describes the first implementation of terahertz (THz) quantum cascade lasers for high-resolution absorption spectroscopy on plasmas. Absolute densities of ground state atomic oxygen were directly obtained by using the fine structure transition at approximately 4.75 THz. Measurements were performed on a low-pressure capacitively coupled radio frequency oxygen discharge. The detection limit in this arrangement was found to be cm−3, while the measurement accuracy was within 5%, as demonstrated by reference measurements of a well-defined ammonia transition. The results show that the presented method is well suited to measure atomic oxygen densities, and it closes the THz gap for quantitative atomic density measurements in harsh environments such as plasmas.

Differentiation of Benign Solid Renal Lesions from Malignant Lesions Using Dual-Energy CT

Objective: To evaluate the quantitative measurement of effective atomic number (Zeff) and iodine density reconstructed from dual-energy computed tomography (CT) in the discrimination of malignant renal lesions from benign renal lesions. Materials and Methods: The present study was a retrospective study that included patients with renal lesions who underwent dual-energy CT between January 2017 and June 2020. All subjects had a pathological diagnosis or follow-up imaging of at least 24 months. Two observers independently measured the Zeff, iodine density, and conventional CT attenuation of the renal lesions. Associations between the different tumor types and the quantitative CT measurements reconstructed from dual-energy CT were assessed, and interobserver agreement was evaluated. Results: Sixty-one renal lesions from 48 patients were included in the present study. No statistical differences were found in mean Zeff, iodine density, or CT attenuation between the benign renal lesions and the malignant renal lesions. However, significant differences were observed in Zeff, iodine density, and CT attenuation between the renal cell carcinoma (RCC) and the other malignant tumors, and between the clear cell RCC and the papillary RCC. The optimal thresholds for Zeff, iodine density, and CT attenuation for distinguishing RCC from other malignant tumors were 8.22, 1.71 mg/mL, and 79.50 HU, respectively, which provided sensitivity of 76%, 76%, and 80%, respectively. Conclusion: Benign solid renal lesions cannot be differentiated from malignant renal tumors by Zeff and iodine density measurements alone. However, dual-energy Zeff and iodine density have potential for the non-invasive differentiation of RCC from the other malignant renal tumors, and for the differentiation of RCC subtypes. Keywords: Dual-energy CT; Spectral CT; Renal cell carcinoma; Renal mass; Iodine density; Effective atomic number

Two-photon absorption laser induced fluorescence measurements of absolute neutral deuterium density, temperature, and bulk flow in Proto-MPEX

Neutral particle control is critical for fusion fueling and confinement. Neutral diagnostics for fusion-relevant plasmas are commonly restricted to line-integrated or ex situ methods. A non-perturbative, two-photon absorption laser induced fluorescence (TALIF) diagnostic is implemented on the Prototype Material Plasma Exposure eXperiment (Proto-MPEX) to probe neutral atomic deuterium in a fusion-relevant plasma at 1 cm intervals along the radius of the vacuum vessel. The diagnostic is situated ∼20 m from the vacuum vessel, and a signal is collected along the laser injection axis, requiring only one line-of-sight. TALIF measurements are absolutely calibrated using xenon and krypton. Absolute atomic densities derived from xenon calibration are compared to absolute atomic densities derived from krypton calibration. Here, preliminary measurements of absolute atomic deuterium density, temperature, and local bulk flow dependence on radial location and input power in Proto-MPEX are presented. Neutral atomic deuterium velocity distribution functions are measured throughout a one-second plasma pulse with a time resolution of 250 ms.

Accuracy and reproducibility of effective atomic number and electron density measurements from sequential dual energy CT.

This study assesses the accuracy of effective atomic number (Zeff ) and electron density measurements acquired from dual energy CT and characterizes the response to clinically relevant variables representative of challenges in patient imaging, including: phantom size, material position within the phantom, variation over time, off-center positioning, and large cone beam angle. The Gammex Multi-Energy CT head and body phantoms were used to measure Zeff and electron density from 35 rod inserts that mimic tissues and varying concentrations of iodine and calcium. Scans were performed on a Canon Aquilion ONE Genesis CT scanner over a period of 6months using default dual energy protocols appropriate for each phantom size. Theoretical Zeff and electron density values were calculated using data provided by the phantom manufacturer and compared to the measurements. Sources of variance were separated and quantified to identify the influences of random photon statistics, ROI placement, and variation over time. A subset of measurements were repeated with the phantom shifted in the vertical and horizontal directions, and over all slices in the volumetric scan. All measurements showed strong correlation (r>0.98) with their corresponding theoretical values; however, the system did demonstrate a bias of -0.58 atomic units in the body phantom and 0.28 atomic units in the head phantom for Zeff measurements. The mean absolute percent error (MAPE) was 6.3% for the body phantom and 3.2% for the head phantom. Electron density measurements of the body and head phantoms gave MAPE values of 4.6% and 1.0%, respectively. Zeff and electron density measurements significantly varied within the solid water background, showing a positional dependence within the phantom that dominated the total standard deviation in measurements. Zeff values dropped by 0.2 atomic units when the phantom was off-center; electron density measurements were less affected by phantom position. Along the z-axis, the accuracy drops off markedly at more than 50-60mm from the central slice. The Canon dual energy system offers an accurate way of measuring the Zeff and electron density of clinically relevant materials. Accuracy could be improved further by calibration to remove bias, careful attention to centering within the FOV, and avoiding measurements at the edges of the cone beam.

Neutral atom density measurements of xenon plasma inside a μ10 microwave ion thruster using two-photon laser-induced fluorescence spectroscopy

This paper reports measurements of the xenon ground state and excited state densities inside a μ10 microwave ion thruster. This thruster exhibits a 40% thrust enhancement upon changing from the waveguide to the discharge chamber propellant injection mode. In the present work, the associated mechanism was quantitatively evaluated using two-photon laser induced fluorescence (TALIF) spectroscopy to monitor the thruster waveguide. The 834.7 nm emission from excited state xenon was investigated with a 224.3 nm dye laser to excite the Xe I 5p61 S0 6pʹ [3/2]2 state, compared with the emission without the laser. The resulting data confirm that the neutral density exhibits a linear relationship with the propellant flow rate in the cold gas and ionized state, while the ion acceleration decreases the neutral density by the same order of magnitude as the propellant utilization efficiency is changed. As the propellant flow rate increases, the collisions of neutrals that generate excited states occur in the waveguide and, when this process plateaus, the ground state emission suddenly increases. Propellant injection from the discharge chamber is evidently effective at suppressing collisions with electrons in the waveguide that generate excited states and that potentially interfere with microwave propagation.

Atomic nitrogen density measurements by actinometry method in the toroidal device NAGDIS-T

Nitrogen (N) atomic density is measured by an actinometry method in the toroidal shaped plasma device NAGDIS-T, where the plasma produced in a DC arc discharge is cooled as spiraling down inside the vacuum vessel. The plasma was characterized with an electrostatic probe, and we applied the actinometry method to obtain the N density using the deduced electron density and temperature. The actinometry method used the Ar line emissions at 750.4 or 811.5 nm and N emission at 746.8 nm. The measured N densities from the actinometry was compared with that from the vacuum ultraviolet absorption spectroscopy.

Absolute atomic hydrogen density measurements in an atmospheric pressure plasma jet: generation, transport and recombination from the active discharge region to the effluent

Atomic hydrogen (H) generated by an atmospheric pressure radio frequency (RF) plasma jet is investigated by two-photon absorption laser induced fluorescence in a He–H2 mixture. The reported time and spatially resolved H density profiles enable a detailed analysis of the species generation, transport and decay along the axis of symmetry of the jet. The H density distribution for a continuous RF plasma jet corresponds well with the outcome of a Pseudo-1D plug flow model. In addition, RF power modulation has a strong effect on the H density in the active discharge region but has a negligible effect on the H density in the far effluent.

Influence of the Molecular Polyimide Brush on the Gas Separation Properties of Polyphenylene Oxide

A new hybrid gas separation membrane was prepared from poly(2,6-dimethyl-1,4-phenylene oxide) modified with graft copolyimide with side poly(methyl methacrylate) chains. The changes in the membrane structure on introducing up to 15 wt % modifier were evaluated by atomic force microscopy and density measurements. The microphase separation in modified polyphenylene oxide films was demonstrated. Introduction of graft copolyimide leads to an increase in the density of the hybrid films. The gas transport properties of the membranes were evaluated for H2, CO2, O2, O4, and N2. Introduction of up to 10 wt % modifier does not noticeably alter the permeability of the hybrid membranes to all the gases but increases the selectivity in gas separation.

Two-Photon Laser-Induced Fluorescence Measurements of Atomic Oxygen Density in Hypersonic Plasma Flow

An experimental investigation of high-enthalpy air flow generated by L2K arc-jet facility of DLR is carried out by means of two-photon absorption laser-induced fluorescence (TALIF) measurements of atomic oxygen at transition. The experiment has been conducted with an arc current of 600 A, a mass flow rate of , and a total pressure of , resulting in both mass-specific enthalpy of and a reservoir temperature of 4200 K. Atomic oxygen has been characterized in terms of translational temperature and absolute number density either in the freestream or behind a shock wave upstream a blunt body. A calibration method based on TALIF measurements of xenon at transition has been used. The results highlight that in the freestream the fluorescence signal exhibits a peak activity lower with respect to that inspected in the shock layer. The measured lifetimes of and indicate that, in the shock layer, the depletion of the atomic oxygen excited state occurs mainly via quenching. The analysis of the line broadening of the spectral profile of atomic oxygen reveals an increment of the translational temperature in the shock layer with respect to that measured in the freestream, resulting in and , respectively. It is found that in the freestream the absolute number density attains and the presence of the shock wave induces an increment of the number density of , that is, 4.85 times higher than that detected in the freestream.

Kinetics of highly vibrationally excited O2(X) molecules in inductively-coupled oxygen plasmas

The high degree of vibrational excitation of O2 ground state molecules recently observed in inductively coupled plasma discharges is investigated experimentally in more detail and interpreted using a detailed self-consistent 0D global kinetic model for oxygen plasmas. Additional experimental results are presented and used to validate the model. The vibrational kinetics considers vibrational levels up to v = 41 and accounts for electron impact excitation and de-excitation (e-V), vibration-to-translation relaxation (V-T) in collisions with O2 molecules and O atoms, vibration-to-vibration energy exchanges (V-V), excitation of electronically excited states, dissociative electron attachment, and electron impact dissociation. Measurements were performed at pressures of 10–80 mTorr (1.33 and 10.67 Pa) and radio frequency (13.56 MHz) powers up to 500 W. The simulation results are compared with the absolute densities in each O2 vibrational level obtained by high sensitivity absorption spectroscopy measurements of the Schumann–Runge bands for O2(X, v = 4–18), O(3P) atom density measurements by two-photon absorption laser induced fluorescence (TALIF) calibrated against Xe, and laser photodetachment measurements of the O− negative ions. The highly excited O2(X, v) distribution exhibits a shape similar to a Treanor-Gordiets distribution, but its origin lies in electron impact e-V collisions and not in V-V up-pumping, in contrast to what happens in all other molecular gases known to date. The relaxation of vibrational quanta is mainly due to V-T energy-transfer collisions with O atoms and to electron impact dissociation of vibrationally excited molecules, e+O2(X, v)→O(3P)+O(3P).

Blending PPO‐based molecules with Pebax MH 1657 in membranes for gas separation

ABSTRACTThis work explores the possibilities to blend block copolymers, i.e., Pebax MH 1657, with a variety of cheap poly(propylene oxide)‐rich molecules which could potentially play a double role in the resulting membranes as dispersing/stabilizing agents in multi‐component casting solutions and as a gas transport medium in the final membrane. These membranes were prepared by solution casting and were characterized by differential scanning calorimetry, scanning electron microscopy, atomic force microscopy, X‐ray diffraction, density measurements, and Fourier transform infrared‐attenuated total reflection, while additive incorporation was also studied with theoretical calculations. Gas permeation measurements showed that this approach resulted in increased permeabilities at the expense of mixed‐gas selectivity. An interpretation of the blend structure was finally made using gas transport models. The compatibility of these additives with the synthesis of selective gas separation membranes may enable a potential double role in membrane synthesis, i.e., as stabilizing agents in membrane synthesis and as a gas transport medium in the final membrane. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46433.

Measurements and kinetic modeling of atomic species in fuel-oxidizer mixtures excited by a repetitive nanosecond pulse discharge

This work presents the results of number density measurements of metastable Ar atoms and ground state H atoms in diluted mixtures of H2 and O2 with Ar, as well as ground state O atoms in diluted H2–O2–Ar, CH4–O2–Ar, C3H8–O2–Ar, and C2H4–O2–Ar mixtures excited by a repetitive nanosecond pulse discharge. The measurements have been made in a nanosecond pulse, double dielectric barrier discharge plasma sustained in a flow reactor between two plane electrodes encapsulated within dielectric material, at an initial temperature of 500 K and pressures ranging from 300 Torr to 700 Torr. Metastable Ar atom number density distribution in the afterglow is measured by tunable diode laser absorption spectroscopy, and used to characterize plasma uniformity. Temperature rise in the reacting flow is measured by Rayleigh scattering. H atom and O atom number densities are measured by two-photon absorption laser induced fluorescence. The results are compared with kinetic model predictions, showing good agreement, with the exception of extremely lean mixtures. O atoms and H atoms in the plasma are produced mainly during quenching of electronically excited Ar atoms generated by electron impact. In H2–Ar and O2–Ar mixtures, the atoms decay by three-body recombination. In H2–O2–Ar, CH4–O2–Ar, and C3H8–O2–Ar mixtures, O atoms decay in a reaction with OH, generated during H atom reaction with HO2, with the latter produced by three-body H atom recombination with O2. The net process of O atom decay is O + H → OH, such that the decay rate is controlled by the amount of H atoms produced in the discharge. In extra lean mixtures of propane and ethylene with O2–Ar the model underpredicts the O atom decay rate. At these conditions, when fuel is completely oxidized by the end of the discharge burst, the net process of O atom decay, O + O → O2, becomes nearly independent of H atom number density. Lack of agreement with the data at these conditions is likely due to diffusion of H atoms from the partially oxidized regions near the side walls of the reactor into the plasma. Although significant fractions of hydrogen and hydrocarbon fuels are oxidized by O atoms produced in the plasma, chain branching remains a minor effect at these relatively low temperature conditions.

Measurements of Absolute Atomic Nitrogen Density by Two-Photon Absorption Laser-Induced Fluorescence Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

For the first time, absolute densities of atomic nitrogen in its ground state (N4S) have been measured in hot dry and humid air plasma columns under post-discharge regime. The determination of space-resolved absolute densities leads to obtain the dissociation degrees of molecular nitrogen in the plasma. The hot plasmas are generated inside an upstream gas-conditioning cell at 600 mbar when the air gas flow is directly injected at 10 slm in a microwave resonant cavity (2.45 GHz, 1 kW) placed in the downstream side. Density measurements based on laser induced fluorescence spectroscopy with two-photon excitation (TALIF), are more particularly performed along the radial and axial positions of the plasma column. Calibration of TALIF signals is performed in situ (i.e. in the same gas-conditioning cell but without plasma) using an air gas mixture containing krypton. Optical emission spectroscopy is considered to estimate the rotational gas temperature by adding a small amount of H2 in dry air to better detect OH (A-X) spectra. The rotational temperatures in humid air plasma column (50% of humidity) are larger than those of dry air plasma column by practically 30% near the nozzle of resonant cavity on the axis of the plasma column. This is partly due to attachment heating processes initiated by water vapor. A maximum of the measured absolute nitrogen density is also observed near the nozzle which is also larger for humid air plasma column. The obtained dissociation degrees of molecular nitrogen in both dry and humid air plasma along the air plasma column are lower than the cases where only thermodynamic equilibrium is assumed. This is characteristic of the absence of chemical and energetic equilibria not yet reached in the air plasma column dominated by recombination processes.