Density Of Neutral Atoms Research Articles

A Community Ionosphere‐Thermosphere Observing System Simulation Experiment (OSSE) Tool: Geospace Dynamics Constellation Example

AbstractObserving System Simulation Experiments (OSSEs) provide an effective way to evaluate the impact of assimilating data from a specific observing system on hindcasting, nowcasting, and forecasting of environmental systems. The NSF NCAR's Data Assimilation (DA) Research Testbed/Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (DART/TIEGCM) tool, to be hosted at the NASA Community Coordinated Modeling Center, serves as a valuable and accessible community resource for quantitatively evaluating the impact of observations from both current and future ionosphere‐thermosphere (IT) observing systems. This study demonstrates the utility of DART/TIEGCM as an IT OSSE tool, using synthetic observations simulated using a currently planned NASA Geospace Dynamics Constellation (GDC) observing system design. Five sets of OSSEs are carried out to compare the effects of assimilating various combinations of prospective GDC observations (e.g., neutral temperature, neutral wind, neutral composition, atomic oxygen ion density, and ion and electron temperature) during a major geomagnetic storm period of the St Patrick's Day Storm on 17 March 2013. The maximum error reduction in neutral temperature and atomic ion oxygen density is 24.6% and 43.3% compared to the control experiment. These OSSEs indicate the benefits of coupled IT DA approaches implemented in DART/TIEGCM to maximize the impact of multi‐parameter IT observations, such as those expected from the GDC mission. Although more work is required to draw any definitive conclusion on the GDC data impact, the study provides an illustrative example of how the DART/TIEGCM community tool can be used to evaluate observational impacts of planned or existing missions for geospace research and applications.

The Role of Pickup Ions in the Interaction of the Solar Wind with the Local Interstellar Medium. I. Importance of Kinetic Processes at the Heliospheric Termination Shock

The role of pickup ions (PUIs) in the solar wind interaction with the local interstellar medium is investigated with 3D, multifluid simulations. The flow of the mixture of all charged particles is described by the ideal MHD equations, with the source terms responsible for charge exchange between ions and neutral atoms. The thermodynamically distinct populations of neutrals are governed by individual sets of gas dynamics Euler equations. PUIs are treated as a separate, comoving fluid. Because the anisotropic behavior of PUIs at the heliospheric termination shocks is not described by the standard conservation laws (a.k.a. the Rankine–Hugoniot relations), we derived boundary conditions for them, which are obtained from the dedicated kinetic simulations of collisionless shocks. It is demonstrated that this approach to treating PUIs makes the computation results more consistent with observational data. In particular, the PUI pressure in the inner heliosheath (IHS) becomes higher by ∼40%–50% in the new model, as compared with the solutions where no special boundary conditions are applied. Hotter PUIs eventually lead to charge-exchange-driven cooling of the IHS plasma, which reduces the IHS width by ∼15% (∼8–10 au) in the upwind direction, and even more in the other directions. The density of secondary neutral atoms born in the IHS decreases by ∼30%, while their temperature increases by ∼60%. Simulation results are validated with New Horizons data at distances between 11 and 47 au.

Investigating the impact of the molecular charge-exchange rate on detached SOLPS-ITER simulations

Plasma-molecular interactions generate molecular ions which react with the plasma and contribute to detachment through molecular activated recombination (MAR), reducing the ion target flux, and molecular activated dissociation (MAD), both of which create excited atoms. Hydrogenic emission from these atoms has been detected experimentally in detached TCV, JET and MAST-U deuterium plasmas. The TCV findings, however, were in disagreement with SOLPS-ITER simulations for deuterium, indicating a molecular ion density () that was insufficient to lead to significant hydrogenic emission, which was attributed to underestimates of the molecular charge exchange rate () for deuterium (obtained by rescaling the hydrogen rates by their isotope mass). In this work, we have performed new SOLPS-ITER simulations with the default rate setup and a modified rate setup where ion isotope mass rescaling was disabled. This increased the content by . By disabling ion isotope mass rescaling: (1) the total ion sinks are more than doubled due to the inclusion of MAR; (2) the additional MAR causes the ion target flux to roll-over during detachment; (3) the total emission in the divertor increases during deep detachment by roughly a factor of four; (4) the neutral atom density in the divertor is doubled due to MAD, leading to a 50% increase in neutral pressure; (5) total hydrogenic power loss is increased by up to 60% due to MAD. These differences result in an improved agreement between the experiment and the simulations in terms of spectroscopic measurements, ion source/sink inferences and the occurrence of an ion target flux roll-over. Extrapolating simplified scalings of divertor molecular densities (TCV & MAST-U) to reactor-relevant densities suggests the underestimation of molecular charge exchange could strongly impact divertor physics (neutral atom density, ions sinks) and hydrogen emission (which has implications for detachment control) in deeply detached conditions, warranting further study.

Characteristics and suppression methods of low-frequency oscillation in Hall thruster

Low frequency oscillation in the discharge channel of Hall thruster is an important physical phenomenon that affects its performance and stability. In this paper, the characteristics of low-frequency oscillation of the discharge current of Hall thruster and its suppression method are numerically studied by using a one-dimensional fluid model. Assuming that the discharge channel satisfies the quasi-neutral condition, the effects of electron-neutral collision, electron anomalous transport and electron-wall collision on conductivity are considered. The changes of plasma parameters and the effects of magnetic field, preionization rate, and atomic velocity on the amplitude and frequency of discharge current oscillation are also studied. Research results show that the variation of electron temperature in the discharge channel is closely related to the ionization process, and the electron temperature increases as the ionization intensity increases. The fluctuations in neutral gas flow rate and atomic density in the discharge process cause the ionization region to move forward and backward and the ionization intensity to change, which are the main driving forces for the low-frequency oscillation of discharge current in the channel. The magnetic field intensity in the discharge channel affects the axial current by influencing the electron mobility. With the increase of field strength, the oscillation frequency of current decreases, and under different magnetic field strengths, the current amplitude drops as the discharge voltage decreases. When the preionization rate of the working gas increases to above 4%, the amplitude of the discharge current oscillation gradually decreases. When the preionization rate is greater than 3% and the atomic velocity is less than 160 m/s, the discharge current oscillation in the channel exhibits damping attenuation, achieving a stabilizing effect which conduces to stabilizing the discharge of the Hall thruster.

Simultaneous measurements of axial motion and azimuthal rotation of non-uniformities (“spokes”) in a Hall thruster

Low-frequency instabilities are often present in Hall thrusters and are associated with axial and/or azimuthal oscillations of plasma non-uniformities. The axial oscillations are related to the so-called breathing mode (periodic depletion of the neutral atom density due to ionization) and are associated with large amplitude current oscillations. The low-frequency azimuthal instabilities are characterized by local non-uniformities of the light emission rotating in the azimuthal direction and are generally called “rotating spokes.” The possibility of coexistence of these two modes has been discussed in the literature but without clear experimental evidence of their correlation. In this paper, we present for the first time simultaneous measurements of the axial and azimuthal positions of the spoke. These measurements have been obtained with a high-speed camera using a triangulation method based on parallax. This method has proven to be sufficiently sensitive to track the 3D position (r–θ–z) of local non-uniformities of the light emission. The optical method has been synchronized with measurements of the current on a segmented anode. The results show that under some conditions, breathing oscillations and spoke rotation in the E×B direction are coupled. During the current rise, the spoke moves from the anode region toward the channel exhaust region while rotating in the E×B direction and seems to follow the movement of the front of neutral atoms progressively filling the channel. About 60% ± 20% of the total anode current is carried by the spoke.

A diagnostic to measure neutral-atom density in fusion-research plasmas.

A femtosecond two-photon-absorption laser-induced-fluorescence (TALIF) diagnostic was designed, installed, and operated on the Princeton-Field-Reversed Configuration-2 device to provide non-invasive measurements of the time and spatially resolved neutral-atom densities in its plasmas. Calibration of the Ho density was accomplished by comparison with Kr TALIF. Measurements on plasmas formed of either H2 or Kr fill gases allowed examination of nominally long and short ionization mean-free-path regimes. With multi-kW plasma heating and H2 fill gas, a spatially uniform Ho density of order 1017m-3 was measured with better than ±2mm and 10µs resolution. Under similar plasma conditions but with Kr fill gas, a 3-fold decrease in the in-plasma Kr density was observed.

A spectroscopic inference and SOLPS-ITER comparison of flux-resolved edge plasma parameters in detachment experiments on TCV

This work presents the use of a collisional-radiative model to infer plasma parameters from 2D emissivities of several deuterium Balmer lines. The emissivities were obtained by MANTIS, an absolutely calibrated, ten-camera imaging polychromator with spatial resolution, up to 800 Hz frame rate, viewing the lower divertor tangentially. Our analysis of those image frames generates 2D maps of plasma parameters such as electron density, temperature, neutral atomic density and the reaction rates for ionisation, recombination and charge exchange as a function of time. The analysis is compared and validated against a SOLPS-ITER simulation accounting for drifts. The results are compared against the simulation in 2D, radial and poloidal profiles to probe the numerous effects of particle transport in the scrape-off layer, in particular in the approach to detached divertor leg conditions. The inferred inner divertor leg radial profiles of the electron density and temperature were consistent with the SOLPS-ITER predictions. A significant transport of particles to the private flux region is found experimentally, that is not captured in the simulation. The simulation diverges from the experiment at the outer divertor target, where the plasma emission appears to be consistent with the emission driven by plasma–molecule interactions. Our analysis also shows prospects for aiding the power exhaust control efforts by potentially providing an optical tracking of the particle balance in the divertor.

Compact multichannel imaging camera for wide-field imaging of diffused sources

We describe a multichannel camera operating in the visible to near-infrared wavelengths (450 to 900 nm) with an etendue of 0.08 cm2 sr that can be used to image extended sources across multiple optical bands, capable of 3σ detection of 20 Rayleigh signal in 120 s. It has an angular resolution of 0.1 deg and a 35 deg × 25 deg field of view, employing a charge-coupled device detector in a compact 1U CubeSat compatible form factor (10 × 10 × 8 cm3). The design uses commercial off-the-shelf components while offering versatility using a mosaic of bandpass filters to select different spectral channels to address a wide range of remote sensing needs. The specific implementation described here covers the application of the instrument to explore the neutral sodium and potassium atom density distribution in the atmospheres of the Earth or the Moon.

Numerical simulations of gas puff imaging using a multi-component model of the neutral–plasma interaction in the tokamak boundary

A three-dimensional simulation of gas puff imaging (GPI) diagnostics is carried out by using a self-consistent multi-component model of the neutral–plasma interaction. The simulation, based on the drift-reduced Braginskii model for the plasma and a kinetic model for the neutrals, is performed in a toroidally limited plasma with gas puff sources located at the low field side equatorial midplane. In addition to electrons, the simulation evolves the turbulent dynamics of D+ and D2+ ions as well as D and D2 neutral species. The Dα emission arising from the excitation of D atoms and the contributions from dissociation of D2 molecules and D2+ ions are considered. The simulation points out the importance of considering Dα emission due to molecular dissociation in the far scrape-off layer (SOL), since it is the dominant source of Dα emission at distances from the gas puff considerably smaller than the mean free path of D2 molecules. The correlation functions between the Dα emission rate and the plasma and neutral quantities, namely, the electron density, ne, electron temperature, Te, and density of neutral atoms, nD, are evaluated considering each contribution to Dα emission and analyzing the correlation functions between these quantities. The correlation functions strongly depend on the location considered within the edge and SOL with an important impact on the interpretation of GPI measurements. The statistical moments and the turbulence properties computed for different components of the Dα emission as well as for the relevant plasma and neutral quantities are also investigated. While neglecting neutral density fluctuations is a reasonable approximation that is widely used in the analysis of GPI measurements, this work reveals a 20%–30% influence of neutral fluctuations on most of the quantities measured through the GPI diagnostics with a possibly larger impact for some quantities in specific regions. These results, therefore, suggest the importance of considering neutral fluctuations for the accurate quantitative interpretations of GPI measurements.

The propagation of femtosecond laser filaments in air with continuously varying pressures

The characteristics of the filaments generated by propagating a femtosecond Gaussian beam in a 2 m long gas cell with continuously varying pressures at different focal distances are numerically investigated. The simulated results indicate that the onset distance and the length of the filaments depend sensitively on the variation of pressure. These effects are enhanced with the increase of the focal length for the initial power Pin=5Pcr. The main reason is that some parameters (the group velocity dispersion, the nonlinear refractive index, multiphoton ionization rate and the neutral atom density) are variational with the gaseous pressure. And the stability of the filament is determined by the combined effect of the pressure and the focal distance. In addition, we discuss the differences of the filament characteristics between the fixed pressure and continuously varying pressures. Through the analyses of the temporal dynamics and supercontinuum spectra, we find that maintaining a large pressure (1 atm) and changing to a large pressure (such as 0.3∼1 atm) are beneficial to the propagation of the filament and the broadening of the spectra. It is interesting that, for fixing the initial energy Ein=1 mJ, although the critical power of laser pulse is higher at the low pressure, the filament length at the continuously varying pressure p=0.3∼1 atm is still longer than that of p=1∼0.3 atm as the focal distance increases. This research is of great significance to remotely detect atmospheric pollutant components.

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.

Spatially resolved LAAS diagnostics of a free-burning ar arc: Analysis of line broadening

Spatially resolved laser atomic absorption spectroscopy (LAAS) diagnostics are applied to a free-burning DC arc in argon at atmospheric pressure. The novel LAAS apparatus is capable of mode-hop free wavelength tuning over the larger range than in conventional systems (up to 300 GHz in the present work). It allows for the measurements of both the absorption coefficient and the spectral line profile continuously from the hot arc center to the non-equilibrium arc wings. The results of such measurements are presented in this work with the emphasize on the combined analysis of laser absorption, which gives densities of excited atoms, and line broadening, which delivers information on either electron density (in case of Stark broadening) or neutral atom density (in case of van der Waals broadening). Contributions by different broadening mechanisms (Stark, Doppler, van der Waals, Resonance) are separated and analyzed. Experimentally retrieved density and temperature profiles of electrons and neutral atoms are compared with the predictions of a self-consisted nonequilibrium arc model. Stark widths of Ar I lines 750.38, 751.46, and 763.51 nm are obtained in the temperature range of 11000–18000 K with the aim to extend the rather scarce data for those lines. The measured widths are compared with existing values from literature and theoretical predictions.

Inactivation of Staphylococcus aureus by the synergistic action of charged and reactive plasma particles

In this paper, a low-pressure capacitively coupled plasma discharge sustained in an argon-oxygen mixture was studied in order to evaluate its properties in terms of inactivation of Staphylococcus aureus. The plasma parameters as electron temperature and plasma density were measured by the Langmuir probe (Ne ≈ 1015 m−3, Te ≈ 1.5 eV), while the neutral atom density was in the range of 1021 m−3. In the plasma phase, oxygen radicals were taken as reference of the reactive species with antimicrobial activity, and oxygen spectral lines, over a range of plasma process parameters, were investigated by the optical emission spectroscopy. Optimal plasma conditions were found, and a count reduction of 4 log in a few minutes of the bacterium proves the potentiality of an industrial grade plasma reactor as a sterilization agent.

Numerical mathematical model for calculating ion density in the gas-discharge chamber of a radio-frequency ion thruster

The mathematical model for calculation of the ion density in the gas-discharge chamber of a radio-frequency ion thruster is developed. The model is based on the assumption of higher potential of the central plasma volume comparing to the chamber walls due to higher mobility of electrons. Firstly, the initial approximation for the plasma potential is set and parameters of the ion trajectories in quasistationary electric field are computed. Then, from the assumption of keeping the total ion flow for each trajectory, the ion density is computed as well as the next approximation for plasma potential. The computation is performed iteratively until the change in the ion density is less than the predefined small value. The model was verified by comparing the results of numerical simulation with the analytical solution for spherical volume. The high accuracy of the numerical model is shown. Also, the ion density in the discharge chamber of a radio-frequency ion thruster was calculated with the uniform distribution of neutral atom density and coefficient of ionization rate. It was shown that at such conditions the ion density has a maximum in central part of the discharge chamber and it decreases towards the walls.

Simulation of the multi-channel motional Stark effect diagnostic on EAST Tokamak

A multi-channel motional Stark effect (MSE) diagnostic is under development for providing accurate information of the internal magnetic field on EAST tokamak. To assist the ongoing design and implementation of the diagnostic, an optimization study is performed using ALCBEAM and NBASS codes. The distribution of the attenuated neutral beam atom density is obtained by ALCBEAM for plasma parameters of Te=2.2keV, neo=1.6×1013cm-3.The MSE spectrum, which is emitted from the beam-plasma interaction and collected by a double-column fiber array with a total number of 19 fibers, is simulated by NBASS on the condition that BT = 2.256 T. The intensity of Dα beam emission for 29 channels is obtained, and the radial spatial resolution ranges from 0.9 cm to 2.3 cm (from edge to core). The influences of the filter’s bandwidth on polarization and transmission are also considered and it is found that 50 % of the polarization and transmission can be achieved when the filter’s bandwidth is around 0.48 nm. For EAST’s MSE system, a 0.48 nm filter is used, which is predicted to have sufficiently high polarization and transmission efficiencies to obtain high-quality MSE signals. When the high voltage of the neutral beam changes, the center wavelength of the spectra shifts. The corresponding central wavelengths for different high voltages of 50 kV, 55 kV and 60 kV are calculated as a function of the major radius. This can serve as a reference for the deployment of the MSE diagnostic.

Unstable modes of oscillations of a tangential discontinuity in partly ionized plasma with charge exchange: long waves

An analytical study of linear unstable modes of oscillations of a tangential velocity discontinuity in partly ionized space plasma with charge exchange is carried out.The long-wave approximation is considered. In this approximation, the scale of perturbations is much larger than a certain characteristic size, which is determined by the charge-exchange cross-section and number density of neutral atoms. It is shown that along with the classical Kelvin – Helmholtz instability, there exists an unstable mode whose increment does not depend explicitly on the relative velocity of the two media separated by the discontinuity. The presence of this mode is entirely determined by the presence of the charge-exchange process.

Quench Dynamics of Neutral Atoms in Out-Equilibrium One-Dimensional Optical Lattices**Supported by Universidad Santiago de Cali Project No. 935-621115-N23, Conciencias Project Nos. 123365842816, FP44842-014-2015

A quantum simulator is proposed for nucleation and growth dynamics using an out-of equilibrium optical lattice. We calculate the density of neutral atoms in the lattice and we establish the connection with the Kolmogorov-Mehl-Johnson-Avrami model. Here we show that an Avrami equation can describe most of the evolution in time of the population growth in the lattice, coherence between neutral atoms leads a complex growth rate.

Influence of molecular dissociation on blob-induced atom density perturbations

The effects of enhanced electron and ion pressure perturbations mediated in filamentary structures (blobs) on the densities of neutral atoms and molecules are investigated through a self-consistent dynamical fluid model for plasma and neutral fields. The electron and ion densities and pressures, and the generalized vorticity, are simulated by a 2D drift-fluid model in an edge and scrape-off layer slab domain of a toroidally magnetically confined plasma. The plasma dynamics are coupled with a diffusion model for densities of neutral atoms and molecules. The combined model allows for determining the response of the density of neutrals with various temperatures to blobs. It is found that blobs locally deplete densities of molecules and atoms that do not originate from dissociation of molecules, whereas the density of atoms created by dissociation may increase during blob events. The neutral species, their temperature, and origin should thus be taken into consideration when estimating the effect of blobs on neutral density perturbations when calculating emission rates, e.g. for gas puff imaging.

Synthetic NPA diagnostic for energetic particles in JET plasmas

Neutral particle analysis (NPA) is one of the few methods for diagnosing fast ions inside a plasma by measuring neutral atom fluxes emitted due to charge exchange reactions. The JET tokamak features an NPA diagnostic which measures neutral atom fluxes and energy spectra simultaneously for hydrogen, deuterium and tritium species. A synthetic NPA diagnostic has been developed and used to interpret these measurements to diagnose energetic particles in JET plasmas with neutral beam injection (NBI) heating. The synthetic NPA diagnostic performs a Monte Carlo calculation of the neutral atom fluxes in a realistic geometry. The 4D fast ion distributions, representing NBI ions, were simulated using the Monte Carlo orbit-following code ASCOT. Neutral atom density profiles were calculated using the FRANTIC neutral code in the JINTRAC modelling suite. Additionally, for rapid analysis, a scan of neutral profiles was precalculated with FRANTIC for a range of typical plasma parameters. These were taken from the JETPEAK database, which includes a comprehensive set of data from the flat-top phases of nearly all discharges in recent JET campaigns. The synthetic diagnostic was applied to various JET plasmas in the recent hydrogen campaign where different hydrogen/deuterium mixtures and NBI configurations were used. The simulated neutral fluxes from the fast ion distributions were found to agree with the measured fluxes, reproducing the slowing-down profiles for different beam isotopes and energies and quantitatively estimating the fraction of hydrogen and deuterium fast ions.

Plasma effects on the bacteria Escherichia coli via two evaluation methods

The degradation of Escherichia coli bacteria by treatment with cold, weakly ionised, highly dissociated oxygen plasma, with an electron temperature of 3 eV, a plasma density of 8 × 1015 m−3 and a neutral oxygen atom density of 3.5 × 1021 m−3 was studied. To determine the ‘real’ plasma effects, two methods were used for evaluation and determination, as well as a comparison of the number of bacteria that had survived: the standard plate count technique (PCT) and advanced fluorescence-activated cell sorting (FACS). Bacteria were deposited onto glass substrates and kept below 50 °C during the experiments with oxygen plasma. The results showed that the bacteria had fully degraded after about 2 min of plasma treatment, depending slightly on the amount of bacteria that had been deposited on the substrates. The very precise determination of the O flux on the substrates and the two-method comparison allowed for the determination of the critical dose of oxygen atoms required for the destruction of a bacterial cell wall—about 6 × 1024 m−2—as well as deactivation of the substrates—about 8 × 1025 m−2. These results were taken in order to discuss other results obtained by comparable studies and scientific method evaluations in the determination of plasma effects on bacteria.