Neutral Species Densities Research Articles

Changes in Thermospheric Neutral and Ionic Species Densities during Global (2018) and Regional (2016) Scale Martian Dust Storms

Changes in Thermospheric Neutral and Ionic Species Densities during Global (2018) and Regional (2016) Scale Martian Dust Storms

Low-pressure inductively coupled plasmas in hydrogen: impact of gas heating on the spatial distribution of atomic hydrogen and vibrationally excited states

Non-equilibrium inductively coupled plasmas (ICPs) operating in hydrogen are of significant interest for applications including large-area materials processing. Increasing control of spatial gas heating, which drives the formation of neutral species density gradients and the rate of gas-temperature-dependent reactions, is critical. In this study, we use 2D fluid-kinetic simulations with the Hybrid Plasma Equipment Model to investigate the spatially resolved production of atomic hydrogen in a low-pressure planar ICP operating in pure hydrogen (10–20 Pa or 0.075–0.15 Torr, 300 W). The reaction set incorporates self-consistent calculation of the spatially resolved gas temperature and 14 vibrationally excited states. We find that the formation of neutral-gas density gradients, which result from spatially non-uniform electrical power deposition at constant pressure, can drive significant variations in the vibrational distribution function and density of atomic hydrogen when gas heating is spatially resolved. This highlights the significance of spatial gas heating on the production of reactive species in relatively high-power-density plasma processing sources.

Measurement of the main neutral species densities and temperatures in iodine plasmas using optical absorption techniques

Iodine is a promising propellant for future plasma thrusters used in space propulsion. It is therefore important to understand the basic physics and chemistry of low-pressure iodine plasmas. In the present work, optical absorption methods are used to measure the densities of iodine molecules, I2, and iodine atoms, I, the translational temperature of the atoms and the dissociation fraction. The plasma is generated in a long quartz tube by a capacitively coupled RF discharge, and the pressure is varied between a few Pa and a few tens of Pa. The translational temperature of the atom vapour increases both with RF power and with pressure and reaches 1000 K at 50 W and 25 Pa. The molecules appear to be efficiently dissociated, with a dissociation fraction found above 65%, on average along the line-of-sight, at 120 W and 5 Pa. The population of the upper, 2P1/2∘ , fine-structure level of the atomic ground term is found to be negligible, which confirms the existence of a high quenching rate, due to collisions with molecules and/or atoms. These measurements can be helpful for chemistry models of iodine plasmas.

The Baryonic Content of Galaxies Mapped by MaNGA and the Gas Around Them

We analyze the cool gas in and around 14 nearby galaxies (at z < 0.1) mapped with the Sloan Digital Sky Survey IV MaNGA survey by measuring absorption lines produced by gas in spectra of background quasars/active galactic nuclei at impact parameters of 0–25 effective radii from the galactic centers. Using Hubble Space Telescope/Cosmic Origins Spectrograph, we detect absorption at the galactic redshift and measure or constrain column densities of neutral (H i, N i, O i, and Ar i), low-ionization (Si ii, S ii, C ii, N ii, and Fe ii), and high-ionization (Si iii, Fe iii, N v, and O vi) species for 11 galaxies. We derive the ionization parameter and ionization-corrected metallicity using cloudy photoionization models. The H i column density ranges from ∼1013 to ∼1020 cm−2 and decreases with impact parameter for r ≳ R e . Galaxies with higher stellar mass have weaker H i absorption. Comparing absorption velocities with MaNGA radial velocity maps of ionized gas line emissions in galactic disks, we find that the neutral gas seen in absorption corotates with the disk out to ∼10 R e . Sight lines with lower elevation angles show lower metallicities, consistent with the metallicity gradient in the disk derived from MaNGA maps. Higher-elevation angle sight lines show higher ionization, lower H i column density, supersolar metallicity, and velocities consistent with the direction of galactic outflow. Our data offer the first detailed comparisons of circumgalactic medium (CGM) properties (kinematics and metallicity) with extrapolations of detailed galaxy maps from integral field spectroscopy; similar studies for larger samples are needed to more fully understand how galaxies interact with their CGM.

Plasma dynamics in a capacitively coupled discharge driven by a combination of a single high frequency and a tailored low frequency rectangular voltage waveform

Spatiotemporal dynamics in a capacitively coupled plasma discharge generated using a combination of 13.56 MHz sinusoidal voltage and 271.2 kHz tailored rectangular voltage is examined both experimentally and computationally. In the experiments, a fast-gated camera is used to measure the space and time-resolved emission at a wavelength of 750.39 nm from the Ar 2p1 1s2 transition. A particle-in-cell model is used to simulate the Ar plasma. The rectangular waveform is formed using 20 consecutive harmonics of 271.2 kHz, and the waveform duty cycle (DC) is varied between 5%–50%. The experiments and simulation show that excitation and argon metastable (Ar) production are primarily caused by electrons accelerated by the expanding sheath. Species generation occurs asymmetrically with more production happening adjacent to the powered electrode when the low frequency (LF) voltage is positive and vice versa. Densities of charged and excited-state neutral species decrease with increasing LF voltage due to the thinning of the plasma region and enhanced charged species loss at surfaces. At DC = 10%, the plasma responds strongly when the LF rectangular voltage switches from a small negative to a large positive voltage. Emission from the plasma and Ar production decrease considerably during this phase. When the LF voltage becomes negative again, species production and excitation remain suppressed for some time before returning to the pre-positive-pulse conditions. This reduction in plasma production is linked to the spike in electron current to the powered electrode during the positive LF voltage period, which depletes the electrons in the plasma bulk and adjacent to the grounded electrode and also raises the mid-chamber plasma potential. Plasma production suppression after the LF positive negative voltage transition lasts longer at higher LF voltage and lower high frequency voltage due to lower plasma density.

Characterization of neutral species in an NH3-Ar plasma generated by a capacitively coupled plasma source at various radio frequencies

Plasma-enhanced atomic layer deposition of SiNx is promising for many device applications, but a lack of growth and material property conformality on three-dimensional device surfaces limits its application. Changing the neutral composition of the nitridation plasma can have an impact on side wall nitridation. In plasma deposition experiments, the role of frequency of the delivered power on neutral species formation is underexplored. We measure the densities of various neutral species generated in a capacitively coupled, 300 mTorr ammonia/argon plasma as a function of driving frequency between 13.56 and 200 MHz using line-of-sight threshold ionization mass spectroscopy. High-density species, which are on the order of 1015 molecules/cm3, such as NH3, H2, H, and N2, as well as lower density species at densities on the order of 1012 molecules/cm3, including N2H2, N2H4, and N2 metastables, were measured and their densities varied with drive frequency.

FEATURES OF PLASMA COMPOSITION AND FLUORINE ATOM KINETICS IN CHF3 + O2 GAS MIXTURE

The effects of initial composition of CHF3 + O2 gas mixture on electro-physical plasma parameters, steady-state densities of active species and fluorine atom kinetics were investigated under the condition of constant gas pressure and input power. The combination of plasma diagnostics by Langmuir probes and model-based analysis of plasma chemistry confirmed known from previous work features of plasma composition in the absence of oxygen (in particular, the domination of HF molecules in a gas phase) as well as demonstrated how the oxygen influences steady-state densities of neutral species through kinetics of both electron-impact and atom-molecular reactions. It was shown that the addition of O2 with a proportional decrease in the content of CHF3 a) results in noticeable changes in electrons- and ions-related plasma parameters (electron temperature, electron density, ion flux); b) provides the effective conversion of fluorocarbon radicals (CHFx, CFx) into such compounds as CFxO, FO and COx; and b) causes an increase in the fluorine atom density up to 50% O2. The last phenomenon contradicts with the change in the total F atom formation rate, but reflects a decrease of their loss frequency in the reaction family of CHFx + F → CFx + HF. The model-based analysis of heterogeneous process kinetics was carried out using the set of tracing parameters based on gas-phase plasma characteristics. It was found that the addition of oxygen lowers the plasma polymerizing ability through both decreasing flux of polymerizing species and accelerating the oxidative destruction of deposited polymer film.

Modeling spark-plug discharge in humid air

Detailed knowledge of the species involved in the oxidation steps during combustion is of interest for technological applications. Using a formerly developed numerical model for a spark-plug discharge in dry air at atmospheric pressure, we studied here the influence of air humidity in the evolution of the densities of neutral and charged species and the gas temperature. The reduced electric field, electronic density, and temperature previously obtained from experimental measurements are parametrically introduced in the model. The effect of relative humidity with the values of 25%, 50%, 75%, and 100% is studied considering the water–air mixture formed by 63 species and coupled by 738 physical and chemical processes. The source term of the chemical reactions is calculated with the ZDPlaskin tool coupled to the numerical model. The analysis of the predominant pathways in the production and consumption of selected species is also carried out. Highly reactive species originated from the inclusion of water in the plasmochemical cycle reach relatively large density values and might play a significant role when considering an air–fuel mixture.

A Comparative DFT Study on the Antioxidant Activity of Some Novel 3-Hydroxypyridine-4-One Derivatives.

The current study on the antioxidant activity of kojic acid and 3-hydroxypyridine-4-one derivatives was performed by implementation of density functional theory calculations with the B3LYP hybrid functional and the 6-311++G** basis set in Polarizable Continuum Model of solvation. Compounds under evaluation were previously synthesized by our research group. The DPPH scavenging effect and the IC50 values in mM concentrations were evaluated. Subsequently, various electronic and energetic descriptors such as HOMO and LUMO energy gaps, bonding dissociation enthalpy of an OH bond, ionization potential, electron affinity, hardness, softness, NBOs and spin density of radical and neutral species were used to study antioxidant properties of investigated compounds. The computations detected two compounds, HP3 and HP4, with significant antioxidant activity. Energetic descriptors indicated that the SPLET mechanism is preferred over the other antioxidation mechanism and computational results were in accordance with the experimental results.

Electron heating mode transitions in radio-frequency driven micro atmospheric pressure plasma jets in He/O2: a fluid dynamics approach

A two-dimensional fluid model is used to investigate the electron heating dynamics and the production of neutral species in a capacitively coupled radio-frequency micro atmospheric pressure helium plasma jet—specifically the COST jet—with a small oxygen admixture. Electron heating mode transitions are found to be induced by varying the driving voltage amplitude and the O2 concentration numerically and experimentally. The helium metastable density, and the charged species densities are highly relevant to the electron heating dynamics. By analyzing the creation and destruction mechanisms of the negative ions, we find that the generation of negative ions strongly depends on the O2 concentration. The increase of the electronegativity with the increasing O2 concentration leads to an enhancement of the bulk drift electric field. The distributions of the different neutral species densities along the direction of the gas flow inside the jet, as well as in the effluent differ a lot due to the relevant chemical reaction rates and the effect of the gas flow. The simulated results show that a fluid model can be an effective tool for qualitative investigations of micro atmospheric pressure plasma jets.

NRLMSIS 2.0: A Whole‐Atmosphere Empirical Model of Temperature and Neutral Species Densities

AbstractNRLMSIS® 2.0 is an empirical atmospheric model that extends from the ground to the exobase and describes the average observed behavior of temperature, eight species densities, and mass density via a parametric analytic formulation. The model inputs are location, day of year, time of day, solar activity, and geomagnetic activity. NRLMSIS 2.0 is a major, reformulated upgrade of the previous version, NRLMSISE‐00. The model now couples thermospheric species densities to the entire column, via an effective mass profile that transitions each species from the fully mixed region below ~70 km altitude to the diffusively separated region above ~200 km. Other changes include the extension of atomic oxygen down to 50 km and the use of geopotential height as the internal vertical coordinate. We assimilated extensive new lower and middle atmosphere temperature, O, and H data, along with global average thermospheric mass density derived from satellite orbits, and we validated the model against independent samples of these data. In the mesosphere and below, residual biases and standard deviations are considerably lower than NRLMSISE‐00. The new model is warmer in the upper troposphere and cooler in the stratosphere and mesosphere. In the thermosphere, N2 and O densities are lower in NRLMSIS 2.0; otherwise, the NRLMSISE‐00 thermosphere is largely retained. Future advances in thermospheric specification will likely require new in situ mass spectrometer measurements, new techniques for species density measurement between 100 and 200 km, and the reconciliation of systematic biases among thermospheric temperature and composition data sets, including biases attributable to long‐term changes.

EFFECTS OF ARGON ATOMS IN EXCITED STATES ON PROPERTIES OF ARGON-ACETYLENE DUSTY PLASMA

It is studied how dissociation and ionization of acetylene molecules in their collisions with argon atoms in excited states Ar* may affect properties of argon-acetylene plasma with growing inside of plasma volume dust particles. The study is carried out using a volume-averaged model. To analyze the effects of Ar* atoms on the electron and ion densities, the effective electron temperature and the densities of radical and nonradical neutral species, the values of ionization and dissociation rates for collisions of acetylene molecules with Ar* atoms are varied in numerical calculations. It is found that the collisions of Ar* atoms with acetylene molecules affect essentially the argon-acetylene dusty plasma.

Numerical investigation of the effect of variation of gas mixture ratio on density distribution of etchant species (Br, Br+, Cl, Cl+, and H) in HBr/Cl2/Ar plasma discharge

A self-sustained fluid model is used to study the effect of variation of noble gas fraction (Ar) and variation of gas mixture ratio (HBr/Cl2/Ar) on density distribution of important etchant species (Br, Br+, Cl, Cl+, and H) in HBr/Cl2/Ar capacitive coupled plasma discharge that is being vastly used for dry etching of Si, GaAs, GaSb and other group III and IV semiconductor materials. A comprehensive set of 80 reactions in this plasma discharge is presented which contains electron impact reactions, neutral–neutral reactions, ion–ion reactions, and charge transfer reactions. Based on simulation results, it is found that densities of neutral species have normal distribution curve; density is higher at center of the reactor and decreases as we move towards electrode surface, while densities of charged species follow bimodal distribution in which peaks occur near electrodes. Furthermore, the addition of Ar in HBr/Cl2 causes electron density to increase which promotes ionization and dissociation. It was found that at constant supply of Cl2 gas, by increasing Ar fraction (HBr/Cl2/Ar from 80/10/10 to 10/10/80) densities of Br, Br+ and H will go down and at constant supply of Ar gas, raising Cl2 fraction in feedback gases (HBr/Cl2/Ar from 70/20/10 to 20/70/10) promoted the production of Cl and Cl+ while densities of Br, Br+, and H are dropped-off. Hence, densities and fluxes of important etchant species – for chemical vs. physical etching – can be controlled by tuning of mixture ratio.

A New Detailed Plasma‐Chemistry Model for the Potential Impact of Blue Jet Streamers on Atmospheric Chemistry

AbstractA detailed ion‐neutral chemistry model named MiPO‐Streamer model has been developed to simulate the impact of blue jet streamers on stratospheric chemistry. It is based on the MiPLaSMO model (Microphysical and Photochemical Lagrangian Stratospheric Model of Ozone) widely used over the last 20 years to interpret balloon and satellite measurements associated with stratospheric ozone. In this study, instead of using a reduced electric field with constant value during blue jet streamer discharge process, we used a time‐dependent reduced electric field coming from an explicit streamer model. The simulations are performed on daytime/nighttime and in 2‐day long‐term duration at 27 km where the catalytic cycle of NOx is the most efficient to destroy ozone. Among 117 species considered, we put the focus on nitrogen, oxygen, chlorine, and bromine species, and ozone perturbation. The model results indicate that the densities of neutral species through an electrodynamic streamer simulation show an impact of a factor 2 as compared to the pulse‐based simulation at 27 km.

Characterization of electrostatic discharge induced plasmas in dielectrics irradiated by multi-MeV electron beam

This work is devoted to experimental analyses of plasmas induced by electrostatic discharges in dielectric materials. Electrostatic discharges are produced in polytetrafluoroethylene (Teflon) and polyethylene samples irradiated by a 6-MeV electron beam generated by a linear accelerator facility. The time and space evolution of the conductivity of the plasma is determined by microwave transmission measurements across a cavity followed by comparison with 3D Maxwell calculations. Furthermore, plasma parameters such as average electron energies and densities of electrons and neutral background species are determined from a 0D collisional radiative model. This analysis infers a plasma expansion velocity of about 150 km/s and a maximum electric conductivity of about 40 S/m. The electron density is estimated to be about 1012 cm−3, and electron average energies are up to 60 eV, while neutral species densities do not exceed 1018 cm−3.

Modeling of C4F8 inductively coupled plasmas: effects of high RF power on the plasma electrical properties

Part of the electrical parameters of C4F8 inductively coupled plasmas is analyzed experimentally using a global kinetic model. To perform a relevant study of C4F8 plasmas, we performed Langmuir probe measurements. The latter is based on solving the mass balance equations of all the considered neutral and charged species coupled to the power balance equation and the charge neutrality equation. The model allows the calculation of the densities of neutral and charged species as well as the electron temperature versus the operating conditions. Our study is focused on the analysis of the evolution of C4F8 plasmas by varying the operating conditions, especially for a very large range of RF power. The simulation reveals an unusual phenomenon characterized by an abrupt transition from an electronegative plasma to an electropositive plasma. Such an abrupt transition is translated into high power by increasing the pressure. This transition phenomenon is explained by the fact that for a power lower than the power transition the electron collision processes are mainly done with the primary molecule, C4F8 and the heavy molecules such as C3F6 and C2F4. Under these conditions, excitation, dissociation, and attachment processes are more important than those of ionization. However, beyond the transition power values, the electron collisions appear to favor the ionization process of atomic neutrals, F and C, because of the high dissociation of secondary molecules of C4F8 plasmas. In this case, the light neutrals such as CF2 and CF are also highly dissociated. So, the ionization process to create much electrons becomes the dominant process because of the strong dissociation processes that occur during the transition phase. Thus, the RF power is essentially absorbed through ionization processes leading to the efficient creation of electrons. Under these conditions, the electron energy losses by excitation, attachment, and dissociation processes become lower than the losses due to ionization processes.

Characteristics of a radio-frequency cold atmospheric plasma jet produced with a hybrid cross-linear-field electrode configuration

Cold atmospheric plasma (CAP) jet has wide applications in various fields including advanced materials synthesis and modifications, biomedicine, environmental protection and energy saving, etc. Appropriate control on the volume, temperature and chemically reactive species concentrations of the CAP jet is of great importance in actual applications. In this paper, an radio-frequency atmospheric-pressure glow discharge (RF-APGD) plasma generator with a hybrid cross-linear-field electrode configuration is proposed. The experimental results show that, with the aid of the copper mesh located at the downstream of the traditional co-axial-type plasma generator with a cross-field electrode configuration, a linear field between the inner powered electrode of the traditional plasma generator and the copper mesh can be established. This liner-field can, to some extent, enhance the discharges at the upstream of the copper mesh, resulting in small increments (all less than 12.5%) of the species emission intensities, electron excitation temperatures and gas temperatures by keeping other parameters being unchanged. And due to the intrinsic transparent and conducting features of the grounded copper mesh to the gas flowing, electric current and heat flux of the plasma plumes, a plasma region with higher concentrations of chemically reactive species and larger plasma plume diameters is obtained at the downstream of the grounded copper mesh on the same level of the gas temperature and electron excitation temperature compared to those of the plasma free jet. In addition, the charged particle number densities at the same downstream axial location of the grounded copper mesh decrease significantly compared to those of the plasma free jet. This means that the copper mesh is also, to some extent, helpful for separating the chemically reactive neutral species from the charged particles inside a plasma environment. The preceding results indicate that the cross-linear-field electrode configuration of the plasma generator is an effective approach for tuning the characteristics of the RF-APGD plasma jet in order to obtain an appropriate combination of the plasma jet properties with higher chemically reactive species concentrations, especially relative higher number densities of neutral species, larger plasma volumes and lower gas temperatures.

Theoretical simulation of hydrogen glow discharge in the positive column of plasma sources

Direct current (DC) glow discharge positive column has played an important role in the field of plasma material surface modification. In this kinetic study of hydrogen glow discharge in the positive column, it is found that electron energy distribution function (EEDF) obviously departs from Maxwell distribution. The EEDF tail moves to higher energy when the ratio between electric field E and density of neutral species N (E/N) increases and the tail moves toward higher energy if hydrogen atom proportion increases at the same E/N. It is also shown that electron temperature augments with the increase of E/N. Additionally, the dissociation ratio percentage of H2 rises with the increase of E/N.

Self-consistent plasma chemistry model for surface microdischarge in humid air including effects of ohmic heating and gas flow

A numerical model is presented for surface microdischarges (SMDs) in flowing humid air at atmospheric pressure, to investigate the effects of the direct ohmic heating of gases in the discharge layer, and the transports of heat and particles by gas flow. Using a simplified configuration of heat transfer and gas flow, the proposed model calculated not only the densities of neutral species but also the temperatures of gases as time dependent variables. The calculated dynamics for various reactive oxygen and nitrogen species showed reasonable agreement with the experimental results obtained by Fourier transformed infrared absorption spectroscopy, while the calculated dynamics without ohmic heating of gases in the discharge layer showed significant disagreement. These results imply that local ohmic heating of the thin discharge layer by the microdischarge itself considerably affected the rate constants of the temperature dependent chemical reactions. The dynamics of the neutral species were also affected by gas flow, both directly through particle transport, and indirectly through cooling. Accordingly, to properly simulate the dynamics of reactive neutral species in SMDs, plasma chemistry models should treat plasmas as sources of both particles and heat which can be deliberately transported by gas flow.

Multiscale approach for simulation of silicon etching using SF6/C4F8 Bosch process

This study is dedicated to the development of a multiscale approach for the simulation of silicon etching using the Bosch process. The etching simulator is composed of three modules: plasma kinetic model, sheath model, and surface model. The top down connection of these three modules allows us to track the etch profile evolution through the mask versus the reactor parameters. Complex reaction schemes are used for both SF6 and C4F8 plasmas to quantify the neutral and ion species densities and fluxes. For the SF6 plasma used in the etching cycle and in our pressure and power ranges, the fluorine flux is still preponderant compared to the positive ion flux, indicating that the silicon etching is mainly ensured by the chemical processes. For the C4F8 plasma used in the deposition cycle, the simulation results reveal that CFx species are preponderant in comparison with atomic fluorine, confirming that, in our operating conditions, CFx species promote the deposition of a polymer layer instead of the chemical etching by fluorine F. Plasma parameters in terms of the reactive neutral and ion fluxes and sheath parameters in terms of ion energy distribution functions are used in the surface model to track the etch profile evolution. The duty cycle corresponding to the SF6 to C4F8 plasma pulse ratio is a crucial parameter in controlling the Bosch process performances in terms of the etch rate, anisotropy, and scalloping defect development. A parametric study is performed to show the role of the duty cycle in the etch anisotropy and the scalloping propagation along the sidewall. While the scalloping defect is more important when the duty cycle is higher, the diminution of this parameter may reduce the global etch rate of silicon.