Self-Consistent Photoionization Models of Planetary Nebulae Luminescence

This work reports the measurement of two-dimensional electron properties over a nanosecond scale integration time across a femtosecond laser-induced plasma filament in atmospheric pressure argon. Radial electron properties across the ∼100\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 100$$\\end{document} μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu$$\\end{document}m diameter filament are obtained at discrete axial locations at 2.5 mm steps by one-dimensional high-resolution laser Thomson scattering with a spatial resolution of 10 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu$$\\end{document}m. These measurements reveal plasma structural information in the filament. The Thomson spectral lineshapes exhibit clear spectral sidebands with an α\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha$$\\end{document} parameter ∼1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 1$$\\end{document}, enabling the measurement of both electron temperature and density profiles. These measurements yield electron densities on the order of 1022\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{22}$$\\end{document}/m3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3}$$\\end{document} and electron temperatures of ∼2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim 2$$\\end{document} eV. Heating from the probe laser due to inverse bremsstrahlung is taken into account to correct the Thomson scattering electron temperature measurements. Under these conditions, electron-neutral collision induced bremsstrahlung becomes the dominant laser-induced plasma heating process associated with the probe laser. The measurements reveal structural features of the filament, including an asymmetrically skewed density structure in the axial direction and reversed radial distributions of electron density and temperature.

In a streamer initiated pre-discharge channel in H2 the axial and radial electron density distributions were measured at different times of the channel development. It was found that at any point in the channel where the electron density reaches ≅ 2 - 1017 cm-3 an immediate thermalisation of the plasma at this point takes place in a time of ≅ 10 nsec. Due to the inhomogeneous radial and axial electron density distribution in a streamer initiated discharge channel the thermalisation begins at first in a constricted region of the channel. The further development of the electron density and temperature distributions in the thermalized channel was studied by means of spectroscopic plasma diagnostic methods in channels initiated by different overvoltages. These experiments have shown that the inhomogeneous axial electron density distribution is equalized after a time of ≅ 100 nsec. The maximum electron density of ≅ 1 - 2 - 1018 cm-3 is reached in the channel axis immediately after the thermalisation of the channel whereas the temperature increases during the time of the highest energy input to the maximum of ≅ 5 - 6 • 104 °K

This paper contains results of numerical and experimental investigation of glow discharge plasma created in a chamber of a new-type large-volume coaxial gridded hollow cathode. The discharge is created in argon at 25 Pa by applying time-varying power with frequency 20 kHz on electrodes. A 2-D model of the discharge was built using COMSOL Multiphysics. Self-consistent description of the discharge was obtained using the extended fluid approach, which couples continuity equations for charged particles and electron energy balance with Poisson's equation for electric potential. Electron transport coefficients and rates of electron-impact reactions were calculated using the electron energy distribution function. The spatial and radial distributions of plasma potential (V p ), electron density (n e ), and electron temperature (T e ) were obtained. It is shown that the plasma inside the chamber is similar to the negative glow of a dc glow discharge. Comparison of numerical results with the Langmuir probe measurements of electron density and electron temperature is presented and showed a good agreement.

A general investigation of linear drift-wave phenomena in cylindrically bounded plasmas, immersed in a magnetic field without shear and curvature, is performed within the two-fluid hydrodynamical approximation, taking into account electron-temperature oscillations and inhomogeneous radial distributions of the undisturbed electron density and temperature. For plasmas in which the electron temperature strongly exceeds the ion temperature the problem is reduced to an ordinary complex second-order differential equation describing the radial distribution of the oscillating electric potential. It is shown that the presence of electron-temperature oscillations (which must always exist in order to satisfy electron-energy conservation) and of radial gradients in the undisturbed electron temperature (which must always exist owing to cooling of the plasma at the boundary) leads to an important modification of the theory of drift waves in cylindrical plasmas (with regard to their stability and the radial distribution of the oscillating quantities) compared with previous papers in which these phenomena were disregarded. A numerical program for solving the corresponding complex-eigenvalue problem has been derived that allows a realistic calculation of all the quantities pertaining to drift-wave phenomena. It has been applied, in particular, to the calculation of the radial distribution of the oscillating coherent magnetic fields accompanying the coherent drift waves. The numerical results prove to be in good agreement with experiments performed with a helium plasma.

The study of discharge performance was performed for argon, nitrogen, and the H2–C2H5OH gas mixture in a power range from 250 to 1000 W, and a pressure range from 10 to 1000 Pa. With argon, a surface wave at the plasma-quartz interface can be excited above 250 W. The surface wave modes can be identified as m=4 and m=6 with pressures of 10 and 230 Pa. With nitrogen, surface waves propagate in two or more modes superposed with a different value of m below the pressure of 133 Pa. In contrast to the above gases, the H2–C2H5OH gas mixture showed no surface wave generation for power levels up to 1000 W; only a hemisphere plasma with a diameter of 8 cm was observed just above the substrate. The plasma, having a diameter of 160 mm, is uniform below the pressure of 133 Pa in nitrogen discharges, but the plasma density is lower than in argon discharges. The radial distributions of electron density in the quartz tube also prove the existence of surface waves for argon and nitrogen. In addition, the axial distributions of electron density prove that the traveling and standing surface waves can be excited by argon discharges in each case: with or without the short plunger at the top of the setup.

The paper deals with the measurement of the radial distribution of electron densityN(r) in a plasma cylinder made with the aid of a multibeam microwave interferometer. It analyses the properties of focused beams and the demands on the interferometer, and describes a method by which one can compute the radial distribution of electron densityN(r) from the measured values. A curve ofN(r) experimentally determined with the aid of a four-beam 8 mm interferometer is compared with that obtained by Langmuir probe measurements. The shape of the curve ascertained with the microwave technique, is in very good agreement with that of probe measurements. The difference in absolute values of the measured densities is less than 50%.

Inductively coupled plasmais widely used in semiconductor and display process because of its desirable characteristics such as high plasma density, simple structure and independently controllable ion energy. The driving frequency is a significant parameter that generates and maintains the plasma. However, the effects of different driving frequencies on the radial distribution of the plasma parameters are hardly investigated. So a large area cylindrical inductively coupled plasma source driven separately by 2 MHz and 13.56 MHz is investigated. In order to perform a comprehensive investigation about the effect of driving frequency, the radially resolved measurements of electron density, electron temperature and density of metastable state atoms for the argon discharge are systematically analyzed by Langmuir double probe and optical emission spectroscopy at various power values and gas pressures. It is found that input power values at high frequency (13.56 MHz) and low frequency (2 MHz) have different effects on plasma parameters. When discharge is driven at high frequency, the electron density increases obviously with the increase of power. However, when discharge is driven at low frequency, the electron temperature increases evidently with the increase of power. This can be explained by calculating the skin depths in high and low frequency discharge. When the discharge is driven at high frequency, the induced electromagnetic field is higher than that at low frequency, and the single electron obtains more energy. It is easier to ionize, so the energy is mainly used to increase the electron density. When the discharge is driven at low frequency, the skin layer is thicker, the number of heated electrons is larger, and the average energy of electrons is increased, so the energy is mainly used to raise the electron temperature. At a gas pressure of 10 Pa, the electron density shows a ‘convex’ distribution and increases with the increase of input power for both the high-frequency and low-frequency discharge. While the distributions of electron temperature are obviously different. When the discharge is driven at high frequency, the electron temperature is relatively flat in the center of the chamber and slightly increases on the edge. When the discharge is driven at low frequency, the electron temperature gradually decreases along the radial position. This is due to the one-step ionization in the high-frequency discharge and the two-step ionization in the low-frequency discharge. In order to prove that the low-frequency discharge is dominated by two-step ionization, the spectral intensities of the argon plasma under the same discharge conditions are diagnosed by optical emission spectroscopy. The number density of metastable states is calculated by the branch ratio method. The results are consistent with the analyses. At a gas pressure of 100 Pa, the electron density increases and then decreases with the increase of radial distance, and the overall distribution shows a " saddle shape” for high frequency and also for low frequency discharge. Although the uniformity of electron density improves with the gas pressure, the uniformity at low frequency is better than that at high frequency. The reason can be attributed to the fact that the skin layer of low frequency is thicker and the heating area is wider.

We propose that the fast low-ionization features (FLIERS) observed in some planetary nebulae are recombination zones behind stationary supersonic recombination fronts produced in mass-loaded jets. The main physical characteristics can be reproduced by jets with opening angles of a few degrees and momentum fluxes of ∼ 1028 g cm s-2, which mass-load from embedded clumps in the nebular envelope. Very simple considerations based on inferred clump masses imply that the nebular envelope contains ∼ 600-60, 000 clumps. Although this range is wide, it is not incompatible with clump numbers estimated from observational data.

We have synthesized well-defined multiarmed star polystyrenes, with 6, 9, 17, 33, and 57 arms, and studied their molecular shapes and structural characteristics in a good solvent (tetrahydrofuran at 25 degrees C) and in a theta (Theta) solvent (cyclohexane at 35 degrees C) by small-angle X-ray scattering (SAXS) using a synchrotron radiation source. Analysis of the SAXS data provided a detailed characterization of the molecular shapes, including the contributions of the blob morphology of the arms, the radius of gyration, the paired distance distribution, the radial electron density distribution, and the Zimm-Stockmayer and Roovers g-factor, for the multiarmed star polystyrenes. In particular, the molecular shapes of the star polystyrenes were found to change from a fuzzy ellipsoid, for the 6-armed polystyrene, to a fuzzy sphere, for the 57-armed polystyrene, with an increasing number of arms. The ellipsoidal character of the star polystyrenes with fewer arms may originate from the extended anisotropically branched architecture at the center of the molecule. The arms of the star polystyrenes were found to be more extended than those of the linear polystyrenes. Furthermore, the degree of chain extension in the arms increased with the number of arms.

A simplified approach in the use of focused microwave systems for the investigation of radially varying plasmas is assessed by comparison to electrostatic probes on a dc arc argon jet. The approach consists in the mathematical inversion into a radial electron density distribution, of a fit to the phase shift profile obtained by scanning a focused interferometer across a cylindrical plasma. A close fit given by Bockasten's coefficient method, and a loose fit utilizing a modified Gaussian, are considered. The assessment covers a wide range of radial refractive index gradients (as large as 0.4/cm) with electron density up to critical on axis. The modified Gaussian fit approach shows a maximum disagreement of 25% relative to the probe results for an impact parameter smaller than 0.5 and of 350% across the plasma jet. A simple modification to the method would apparently permit the reduction of the overall discrepancy to less than 30%. The close fit is found to yield discrepancies which are worse by a factor of 2–3.

A novel laser interferometric technique involving the use of an unstable external resonator is presented. The unstable resonator greatly facilitates electron density measurements in the presence of plasma density gradients transverse to the direction of beam propagation. The technique is used to obtain time-resolved radial electron density distributions in the implosion stage of a linear z-pinch discharge in argon at filling pressures of 0.1 and 1.0 Torr.

From data on size and chemical composition, low density lipoprotein (LDL) can be described as a spherical particle having cholesteryl esters and triglycerides contained in a spherical core covered by the closely packed hydrophobic ends of phospholipids and unesterified cholesterol, while the head groups of the phospholipids, together with protein, occupy the surface. Such a model is compatible with early small angle X-ray and neutron scattering studies which, by prostulating spherical symmetry, assigned the LDL constituents to locations predicted from the radial electron density distribution. However, the concept of spherical symmetry, as applied to LDL structure, was recently challenged by results obtained from freeze-etching electron microscopy and small angle X-ray scattering experiments. Novel interpretations of these data suggest that the surface of LDL contains 4 electron-dense globules, located at tetrahedral positions, which have a capacity for structural remodeling at least as a function of the 2 temperatures studied (21C and 41C). It is reasonable to presume that the LDL protein (apo LDL) plays a role in the organization of the surface and overall LDL structure. However, until the chemical properties of apoLDL, and its behavior in solution and at the water-lipid interface are better understood, the validity of the proposed models cannot be assessed.

Solutions of the wave equation are obtained for TE0mn and TMlm0 modes in cylindrical cavities containing a coaxial, annular plasma column. The radial electron density distribution is assumed to be of the form ne=n0[1−γ1 (r/a)2−γ2(a/r)2], where n0, γ1, and γ2 are constants and a is the outer radius of the plasma. The results can be used to determine the effects of density gradients on measurements of electron densities and collision frequencies by the cavity method. Some illustrative examples are presented.

Radio maps of the free-free radio continuum flux (angular resolution ≅ 1.3 arcseconds) from NGC 7027 were made with the VLA operating at 20-cm, 6-cm, and 2-cm wavelengths which are near and straddle unit optical depth. Mean line-of-sight electron temperature and emission measure distributions were calculated by pairing the 2-cm and 6-cm maps, and the electron temperature distribution on the near side of the nebula was then obtained from the 20-cm map. The results suggest that the energy balance is complex in this planetary. For example, mean line-of-sight temperatures are higher than average in the direction of one of the bright lobes but not in the direction of the other. Especially noteworthy is an apparent “hot spot” on the near side of the nebula which has no apparent relation to either of the bright lobes, but it is approximately coincident with the brightest portion of the optical image.

Correlation of Amino Acid Sequence and Conformation in Tobacco Mosaic Virus