Neutral Density Distribution Research Articles

A New Approach for 4-D Exospheric Tomography Based on Optimal Interpolation and Gaussian Markov Random Fields

Exospheric tomography is a computational 3-D imaging technique that provides the estimates of the neutral density distributions of the terrestrial exosphere from space-based ultraviolet (UV) measurements. Variability of neutral densities during geomagnetically active conditions has been previously reported, motivating the development of time-dependent tomographic techniques that can characterize both the spatial and temporal scales of densities during these events. However, solving the dynamic exospheric tomography problem can be challenging owing to its ill-posedness. In this letter, we introduce a novel algorithm for 4-D exospheric tomography based on optimal interpolation (OI) and Gaussian Markov random field (GMRF) theory. The OI analysis enables iterative reconstructions of the exosphere when a statistical background field is provided. Its mean is selected from previous knowledge of the exosphere, and its covariance matrix is estimated using GMRF. To validate the performance, we apply our proposed methodology to six days of UV data acquired by National Aeronautics and Space Administration (NASA) two-wide angle imaging neutral-atom spectrometer (TWINS) mission during the geomagnetic storm that occurred on June 15, 2008.

Importance of stepwise ionization from the metastable state in electron cyclotron resonance ion thrusters

In electron cyclotron resonance (ECR) thrusters, the plasma mode transition is a critical phenomenon because it determines the maximum thrust performance. In ECR ion thrusters, ionization generally occurs in the magnetic confinement region, where electrons are continuously heated by ECR and confined by magnetic mirrors. However, as the flow rate increases, ionization is also observed outside the magnetic confinement region, and this induces the plasma mode transition. In our previous work, two-photon absorption laser-induced fluorescence (TALIF) analysis revealed that the stepwise ionization from the metastable state plays an important role in the ionization process. However, the distribution of the stepwise ionization has not yet been revealed because of the long lifetime of the metastable state. In this study, this distribution was investigated using one experimental and two numerical approaches. First, TALIF was applied to two types of gas injection with clear differences in thrust performance and ground-state neutral density distribution. In the first simulation, the metastable state particle simulation was used to estimate the excitation rate distribution. In the second study, simulations of the electric field of microwaves were used to estimate the contribution of the stepwise ionization to the plasma density. The experimental and numerical results revealed that the stepwise ionization spreads outside the magnetic confinement region because of the diffusion of metastable particles, and this spread induces the plasma mode transition, explaining the difference between the two types of gas injection.

Effect of ion beam extraction on neutral density distribution inside a gridded microwave discharge ion thruster

In gridded ion thrusters, it is generally known that the ion extraction from the grid significantly decreases the neutral density. However, because it is difficult to measure the density inside thruster, it has not been revealed how the local density decreases including the inhomogeneous effects (e.g., gas injection, ionization, and ion recombination). To investigate these inhomogeneous effects, two-photon laser-induced fluorescence (TALIF) and Direct Simulation Monte Carlo (DSMC) applied to a gridded microwave discharge ion thruster. First, the TALIF directly measured the density distribution. Second, the DSMC reproduced the measured density distribution. From these results, the main effect of the ion extraction is the decrease of ion recombination at the grid. This decrease makes the density distribution more sensitive to the ionization distribution and increases the inhomogeneity. Based on this knowledge, we propose improvement against ‘simulated experiment without ion extraction’ and ‘decoupled numerical simulation’ which reproduce the density distribution of ion extraction under the condition without ion extraction by decreasing flow rate. Specifically, we suggest that the grid transparency is expanded and the rest discrepancy is adjusted by the flow rate because the difference of the inhomogeneous effect due to gas injection can be reduced.

Electron and Neutral Density Distributions of Vacuum Arcs Under Anode-Spot Mode

Electron and Neutral Density Distributions of Vacuum Arcs Under Anode-Spot Mode

Neutral density estimation in the ASDEX upgrade divertor from deuterium emissivity measurements during detachment and shoulder formation

Neutral density distribution in the divertor region of ASDEX Upgrade is estimated in different discharge conditions. The 2D divertor emission of Dα and Dγ is obtained with a tomographic inversion of their brightness measured by two cameras. From these emissions, 2D maps of neutrals density n0 can be obtained by a simple model that takes into account both excitation and recombination processes populating deuterium n = 3 and n = 5 levels. In attached discharges, the neutral density n0 is larger in the inner divertor, and when increasing core ne its maximum moves toward the dome and the outer divertor region. In particular, the modification of the n0 distribution is studied in plasmas where the mean electron density in the core increases, moving from an attached to a detached phase. In these discharges, the outboard midplane scrape off layer electron density increases and its profile flattens, with the formation of the so called density shoulder. The modification of the neutral density is followed during this evolution, showing that both the shape and the absolute value of the n0 distribution change in time, suggesting that they play a role in the two phenomena of shoulder formation and detachment. Moreover, in the detached phase, also the recombination contributes to the neutral deuterium emission.

Metastable argon atoms under significant neutral depletion in helicon plasmas by laser-induced fluorescence

We study the spatial distribution of the metastable-state argon atoms in high density helicon plasmas by means of laser-induced fluorescence. It is observed that the neutral argon in metastable-state has an anomalous radial distribution in density; it has a caldera-like shape radially, which is rare in typical low-temperature plasmas such as inductively coupled plasmas wherein the density increases toward the discharge center, as previously reported. The formation of the distribution can be explained as it forms by the combined effects of significant neutral depletion in high plasma density, off-axis electron-density distribution, and increasing diffusive loss toward the wall. To establish the assertion with the underlying physics, we calculate a simple global model and obtain the neutral density distribution in metastable-state under various conditions. The calculated results qualitatively agree with the experimental results.

High resolution 3D gas-jet characterization

We present a tomographic characterization of gas jets employed for high-intensity laser-plasma interaction experiments where the shape can be non-symmetrically. With a Mach-Zehnder interferometer we measured the phase shift for different directions through the neutral density distribution of the gas jet. From the recorded interferograms it is possible to retrieve 3-dimensional neutral density distributions by tomographic reconstruction based on the filtered back projections. We report on criteria for the smallest number of recorded interferograms as well as a comparison with the widely used phase retrieval based on an Abel inversion. As an example for the performance of our approach, we present the characterization of nozzles with rectangular openings or gas jets with shock waves. With our setup we obtained a spatial resolution of less than 60 μm for an Argon density as low as 2 × 10(17) cm(-3).

Propagation of ultrashort laser pulses in optically ionized gases

Propagation of 800 nm, 120 fs laser pulses with intensities of 4×1016 W/cm2 in supersonic gas jets of N2 and H2 is studied using a shear-type interferometer. The plasma density distribution resulting from photoionization is resolved in space and time with simultaneously measured initial neutral density distribution. A distinct difference in laser beam propagation distance is observed when comparing propagation in jets of H2 and N2. This is interpreted in terms of ionization induced refraction, which is stronger when electrons are produced from states of higher ionization potential. Three dimensional particle-in-cell simulations, based on directly solving the Maxwell–Lorentz system of equations, show the roles played by the forward Raman and ionization scattering instabilities, which further affect the propagation distance.

Comparison between experiment and simulation for argon inductively coupled plasma

In order to include the nonlocal characteristics of electrons and investigate the inductively coupled plasma (ICP) resources more completely, we have developed a hybrid Monte Carlo (MC)/fluid hybrid model and calculated the axial and radial distributions of electron density, electron temperature, plasma potential, and electron energy distribution functions (EEDFs) of Ar discharge in a planar ICP. Furthermore, to make the model more practical, we still incorporate the effects of metastable atoms, whose sets of rate coefficients and density are, respectively, calculated through the electron MC part and fluid module. Besides, the corresponding Langmuir probe measurements are used to compare these data to validate the simulated results. Under all the selected discharge powers and pressures, the theoretically simulated and experimentally measured quantity profiles agree reasonably with each other, embodied in the generally identical magnitude ranges and spatial distributions. Furthermore, the interpretations about their detailed differences are given, which are based on the designs of both experimental schematic and model configuration. The analysis implements that the inclusions of electron-electron collision and a neutral density distribution into the hybrid model are likely to improve the comparison between the model predictions and experiment diagnostics. Furthermore, the evolution of plasma parameters and EEDFs with discharge conditions is discussed.

Integral descriptors of the vertical structure of the ocean

We propose a simple polynomial expression for neutral density and nutrients as a function of potential temperature, pressure and salinity. The expression is applied to the 1988 North Atlantic A16N WOCE meridional section and the polynomial coefficients are calculated using an inverse technique. The resulting polynomials show good skill in reproducing the nutrients and density structure, as verified through an analysis of variance (ANOVA) test. The large-scale changes in the polynomial coefficients occur between equatorial (E), tropical (T), subtropical (ST), and subpolar (SP) waters. The temperature and pressure coefficients experience substantial changes at all transitions (E-T, T-ST, and ST-SP), while the salinity ones only have major variations at the T-ST transition. Mesoscale-like oscillations occur all along the section but are relatively small, except between about 40 and 50°N, in a region of rough bottom topography. The density field is reconstructed using individual and group coefficients, and the contribution of each coefficient is identified. The method is also applied to analyze the neutral density distribution in a nearly identical 2003 section, removing near-surface density values that may be related to different warming/cooling of the surface layer. The results show close resemblances but also some significant variations, which are discussed in terms of interdecadal variability. Hence, we argue that the set of calculated coefficients provides good integral descriptors for the vertical structure of the ocean.

Measurement and Analysis of Visible Line Spectra with Inhomogeneous Spatial Distribution in LHD

Spatially resolved spectra have been measured with visible spectroscopy in Large Helical Device (LHD). Strong line emissions of H I, He I and C III have been observed at a limited area near the inboard-side X-point. Such a poloidally inhomogeneous radiation structure is drastically changed when the radius of the magnetic axis is shifted from Rax = 3.6 m to 3.9 m: another strong emission emerges near the outboard-side X-point. In order to understand the physical mechanism the electron temperature and density are evaluated from intensity ratios of neutral hydrogen and helium lines at poloidal locations near both the X-points and at the bottom O-point edge. It is found that the changes of the electron temperature and density along the poloidal direction are too moderate to explain the inhomogeneity of line emissions for both the configurations. The present result suggests that the poloidal inhomogeneity of neutral emissions arises owing to the inhomogeneous neutral density distribution rather than the difference of the electron density or temperature.

Enhancement of H− extraction from a compact source by streaming neutral gas injection

A new negative ion extraction geometry with streaming neutral gas injector (SNGI) was tested in its performance to enhance negative hydrogen ion (H−) at low operational pressure. The experiments were performed using a test ion source equipped with a SNGI having the wall perpendicular to the gas emission holes. The results showed that the SNGI was capable of reducing the operating pressure of the ion source from 0.14to0.07Pa. At 0.14Pa, the operation using the SNGI produced 20% more negative ions than the operation without SNGI. A compact ion source was constructed with a smaller SNGI and having a tapered wall for the gas injection nozzles. The neutral density distribution within the central region of the SNGI within the ion source was simulated using direct simulation Monte Carlo (DSMC) method. It was realized that the neutral density distribution produced by the SNGI with the tapered wall was at most 35% lower than a SNGI structure without the taper.

A model for the azimuthal plasma velocity in Saturn's magnetosphere

We present a model for Saturn's magnetospheric azimuthal plasma velocities measured by Voyager 1 and 2. The observed velocity profiles deviate from full corotation and show two dips slightly outside the orbits of Dione and Rhea. Our velocity model includes as sources for the deviation: radial mass transport, friction between magnetospheric ions and neutrals, and ion pickup. We combine the latter two processes in a quantity called magnetospheric conductance ΣM. We find that inside of 12 Saturn radii (RS) the magnetospheric conductance dominates the slowing of magnetospheric plasma as compared to the effect of radial mass transport. The magnetospheric conductance peaks twice, i.e., slightly outside the orbit of Dione (6.4 RS) and Rhea (8.8 RS), respectively. These two peaks result from a combination of the radially varying magnetic field magnitude, the radial plasma density distribution, and the neutral density distribution. A key finding of our work is that the two magnetospheric conductance maxima can explain the two dips in the observed azimuthal velocity profile that occur at roughly similar locations. Mismatches in the dip locations can be used to retrieve neutral gas profiles. We achieve reasonable agreement with the observations for very small effective Pedersen conductances, ΣI ∼ 0.014 S (Voyager 1) and ΣI ∼ 0.035 S (Voyager 2) in Saturn's ionosphere, for a total magnetospheric mass source of 40 kg s−1. Larger mass sources require appropriately higher ionospheric conductances. We also find that mass cannot leave, on average, the ≤10 RS regions as plasma. It must be mostly lost via conversion to neutrals.

Global density and magnetic field structures in spiral galaxies: spiral structure and bisymmetric magnetic field

Spiral galaxies exhibit global structure both in their neutral density distribution as well as in magnetic field distribution. The former is now known to be related to the spiral structure of these galaxies. However, some problems relating to the nature of the spiral patterns remain, namely, what determines the nature of the patterns, "leading" vs. "trailing" patterns. For the magnetic field structure, on the other hand, there is as yet no satisfactory theory to explain its growth and maintenance, the "kinematic dynamo theory" having been shown recently to have failed.We give here theories for both the density wave in the neutral disk as well as for the bisymmetric magnetic field. We show that the temperature (velocity dispersion) of the disk plays a crucial role in determining the (observed) "trailing" pattern. For the magnetic field bisymmetric form we invoke the inverted density distribution of the HII regions (found in most galaxy types close to the disk centre) so that the Rayleigh-Taylor instability of the inverted distribution supplies the free energy required for the growth of the bisymmetric magnetic field spirals.

Plasma scrape-off effect in a magnetized hollow-cathode arc discharge

Simultaneous measure of Thomson and Rayleigh light scattering were performed in order to determine electron density, electron temperature, and neutral density distributions in a hollow-cathode arc. These measurements were performed in argon, nitrogen, helium, and hydrogen at approximately 1 Pa. The hollow-cathode arc has also been studied in nitrogen and argon at high pressures ranging from 1–550 Pa. In this paper we present only high-pressure data for nitrogen, although comparison with some results obtained in argon is provided. A plasma scrape-off effect leads to a reduction in electron density in front of the anode and consequently to an enhancement of the anode fall. This enhancement was detected through an increase in electron temperature.

“Alfvén wing” models of the induced electrical current system at Io: A probe of the ionosphere of Io

Models are presented of the electrodynamic interaction between corotating Jovian magnetospheric plasma and the ionosphere of Io. The interaction is treated as a purely transverse Alfvén wave which is controlled by the electrical conductivity distribution of Io's ionosphere. By quantifying the connection between the ionospheric conductivity and the magnetic field and velocity perturbations observed during the Voyager 1 flyby, the observations serve as a probe of the ionosphere, under the assumption that the ionosphere constitutes the principal cross‐field conductance near Io. If the electron density is assumed to be that measured earlier by Pioneer 10, the ionospheric resistivity is controlled by the electron‐neutral collision rate. Parameterized approximations for the neutral density distribution are varied to produce a fit between the calculated magnetic field and observations made by Voyager 1. The best‐fitting ionospheric models assume a neutral density distribution containing a significant global component (of the order of 109 to 1010 cm−3). However, the cold‐trap model (according to which Io has a very low‐density atmosphere) can be accommodated with the assumption of a significant SO2 coverage of the trailing hemisphere resulting from outgassing of the major plumes there. After taking account of field line curvature and Alfvén speed variation over the distance between Io and Voyager 1, the Io plasma torus charged‐mass density required for a proper match to the data is larger than earlier estimates based on the width of the Alfvén wing signature.

An analytic solution for the distribution of neutral particles in a Maxwellian plasma using the method of singular eigenfunctions

The method of singular eigenfunctions is used to obtain a solution to the problem of penetration of neutrals into a dense plasma with a realistic Maxwellian ion distribution. A general solution is obtained and explicit results given for models of the reflection processes at the wall. These include the neutral density distribution, the emergent neutral velocity distribution and the neutral particle and energy fluxes to the wall. The analytic results indicate the validity of the diffusion approximation and provide a boundary condition for such a treatment.

Density in a planetary exosphere

Recently Öpik and Singer [1959] have developed a theory that gives the ballistic component of the neutral density distribution in a planetary exosphere. Their theory assumes that above the base of the exosphere, collisions may be entirely neglected, at least as far as an approximate calculation of the neutral density profile is concerned. The particles at the base of the exosphere are assumed to be in a truncated Maxwellian distribution, with incoming particles of velocities greater than escape velocity absent. The absence of such incoming particles implies a sink, located at some distance from the planet, that prevents the eventual build-up of a full Maxwellian distribution above the escape level, and hence an extension of the barometric law beyond the base of the exosphere.