Showing papers on "Ion published in 1981"

Neoclassical transport of impurities in tokamak plasmas

Abstract: Tokamak plasmas are inherently comprised of multiple ion species. This is due to wall-bred impurities and, in future reactors, will result from fusion-born alpha particles. Relatively small densities nI, of highly charged non-hydrogenic impurities can strongly influence plasma transport properties whenever . The determination of the complete neoclassical Onsager matrix for a toroidally confined multispecies plasma, which provides the linear relation between the surface averaged radial fluxes and the thermodynamic forces (i.e. gradients of density and temperature, and the parallel electric field), is reviewed. A closed set of one-dimensional moment equations is presented for the time evolution of thermodynamic and magnetic field quantities which results from collisional transport of the plasma and two-dimensional motion of the magnetic flux surface geometry. The effects of neutral-beam injection on the equilibrium and transport properties of a toroidal plasma are consistently included.

Extracellular ion concentrations during spreading depression and ischemia in the rat brain cortex

Abstract: We compared interstitial ion concentrations in rat brain cortex during two conditions where pronounced changes are observed: spreading depression and ischemia. Initially, during the two phenomena, an increase of [K+]e from 3 to approximately 10 mM were observed, but only small changes of the other ion concentrations. Hereafter, [K+]e exhibited a rapid increase (2–3 s) to 55 mM, whereas [Na+]e rapidly decreased to 60 mM, [Cl-]e to 75 mM, and [Ca++]e to 0.08 mM. The changes were accompanied by a rapid negative shift in the local electrical potential. However, there were differences in the ionic events during the two phenomena. In spreading depression, the initial [K+]e increase took place in 5–10 s, but in ischemia it lasted 1–2 min. The ionic perturbations were spontaneously reverted in SD, but in ischemia they proceeded further and reached after 5 min (mM): [K+]e 75, [Na+]e 50, [Cl-]e 72, and [Ca++]e 0.06. The similar chain of ionic events during spreading depression and ischemia suggests a common mechanism for the ionic changes, probably involving changes of ionic permeability of brain cells.

Cation and Water Diffusion in Nafion Ion Exchange Membranes: Influence of Polymer Structure

Abstract: Membrane self‐diffusion coefficients for sodium ion, cesium ion, and water have been measured for Nafion® 120 perfluorosulfonate ion exchange membranes. Values have been determined as a function of temperature, and as a function of membrane water content by studying samples in heteroionic forms. The diffusional properties of this polymer are found to differ from those of conventional polystyrenesulfonates in several respects, and the free volume theory which describes ionic diffusion in the latter is inappropriate to treat Nafion. Results indicate that cations may exist in two distinct regions in the polymer; the proportion of total cations in each region may depend on the ions size and charge density. A structural model of Nafion, which correlates the membrane's spectroscopic and diffusional properties, is proposed to explain the results.

Wave‐particle interactions near ΩHe+ observed on GEOS 1 and 2 1. Propagation of ion cyclotron waves in He+‐rich plasma

Abstract: The GEOS 1 and 2 spacecraft contain a set of particle and wave detectors which allow for a very comprehensive study of wave-particle interactions occurring within the equatorial region of the magnetosphere. This paper is devoted to interactions involving protons in the energy range 20 keV to 300 keV and ULF waves with frequencies below the proton gyrofrequency. It is shown that mose of the ion cyclotron waves (ICW's) detected in this frequency range have spectra whose charcteristic frequencies are organized in the vicinity of the He/sup +/ gyrofrequency. Simultaneous measurements of the ion composition in the thermal energy range (E< or approx. =110 eV) show these waves to be clearly associated with the abundance of cold He/sup +/ as well as the anisotropy of ions above 20 keV. The general characteristics of these helium-associated ULF events are presented in case studies of four events. The interpretation of this phenomenon is given in the present paper in terms of the propagation of ICW's in a He/sup +/ -rich plasma. It is shown that the shape of the cold plasma dispersion curve (for both parallel and non-parallel propgation) can adequately explain the main characteristics of the observed waves (frequency spectrum, polarization)more » as well as the differences between observations made onboard GEOS 1 and GEOS 2. The generation conditions of ion cylotron waves in such a multi-component plasma, as well as their quasi-linear effects on both the cold He/sup +/ ions and the hot protons, are discussed in a companion paper.« less

Atomic coordination and the distribution of electric field gradients in amorphous solids

Abstract: The observation of nuclear quadrupole interactions in amorphous solids provides a unique possibility of obtaining information about the angular distribution of local ionic coordinations, complementary to the information about radial distributions deduced from x-ray and neutron diffraction and from extended x-ray absorption fine structure measurements. In the present paper the relation between ionic coordinations and the distribution of electric field gradients (EFG) is investigated. It is shown that the distribution function $P({V}_{\mathrm{zz}},\ensuremath{\eta})$ of the splitting parameters ${V}_{\mathrm{zz}}$ (the electric field gradient) and $\ensuremath{\eta}$ (the asymmetry parameter) in general yields zero probability both for ${V}_{\mathrm{zz}}=0$ and for $\ensuremath{\eta}=0$. For solids which are isotropic on the average, the distribution function of the components ${V}_{\mathrm{ik}}$ of the EFG tensor depends only on two variables, the invariant functions of the tensor components [$\mathrm{Det}({V}_{\mathrm{ik}}) \mathrm{and} \ensuremath{\Sigma}{V}_{\mathrm{ik}}^{2}$]. Expressions for these quantities in terms of the radial coordinates of the ions causing the EFG and of the bond angles between pairs of ions are given. For amorphous solids with random ionic coordination an analytic approximation for the distribution function $P({V}_{\mathrm{zz}},\ensuremath{\eta})$ is derived. This function is strongly dominated by the distribution of ions in the first coordination shell. The results are applied to the analysis of M\"ossbauer spectra of $^{155}\mathrm{Gd}$ in amorphous Gd-Ni alloys.

The gramicidin a channel: A review of its permeability characteristics with special reference to the single-file aspect of transport

Abstract: Gramicidin A forms univalent cation-selective channels of ≈4 A diameter in phospholipid bilayer membranes. The transport of ions and water throughout most of the channel length is by a singlefile process; that is, cations and water molecules cannot pass each other within the channel. The implications of this single-file mode of transport for ion movement are considered. In particular, we show that there is no significant electrostatic barrier to ion movement between the energy wells at the two ends of the channel. The rate of ion translocation (e.g., Na+ or Cs+) through the channel between these wells is limited by the necessity for an ion to move six water molecules in single file along with it; this also limits the maximum possible value for channel conductance. At all attainable concentrations of NaCl, the gramicidin A channel never contains more than one sodium ion, whereas even at 0.1M CsCl, some channels contain two cesium ions. There is no necessity to postulate more than two ion-binding sites in the channel or occupancy of the channel by more than two ions at any time.

High density cascade effects

Abstract: The objective of this article is to review the effects of high density cascades in solids. Such cascades can be separated into those occurring in which “high density” refers to either the density of atomic collision events or to the density of ionization. Both types of cascade are discussed with reference to their effects in the bulk (i.e. damage production and inert gas detrapping) and on the surface properties (i.e. sputtering, secondary ion and electron emission). The appropriate experimental data are reviewed and discussed in relation to the various proposed spike models; i.e. displacement, thermal, plasticity and ionization spikes.

Sputtering from elastic‐collision spikes in heavy‐ion‐bombarded metals

Abstract: Estimates are presented for the sputtering yield as well as the specturm of sputtered atoms for metals bombarded by heavy atomic or molecular ions, under the assumption of evaporation from an elastic‐collision spike of high initial temperature. The calculational is based on a cylindrical spike and a thermal diffusivity proportional to the square root of the temperature. Good qualitative agreement is obtained between calculated and measured sputter yields for antimony incident on silver.

Interconversion of components of the bacterial proton motive force by electrogenic potassium transport.

Abstract: The influence of K+ ions on the components of the transmembrane proton motive force (delta mu H+) in intact bacteria was investigated. In K+-depleted cells of the glycolytic bacterium STreptococcus faecalis the addition of K+ ions caused a depolarization of the membrane by about 60 mV. However, since the depolarization was compensated for by an increase in the transmembrane pH gradient (delta pH), the total proton motive force remained almost constant at about 120 mV. Half-maximal changes in the potential were observed at K+ concentrations at which the cells accumulated K+ ions extensively. In EDTA-treated, K+-depleted cells of Escherichia coli K-12, the addition of K+ ions to the medium caused similar, although smaller changes in the components of delta mu H+. Experiments with various E. coli K-12 K+ transport mutants showed that for the observed potential changes the cells required either a functional TrkA or Kdp K+ transport system. These data are interpreted to mean that the inward movement of K+ ions via each of these bacterial transport systems is electrogenic. Consequently, it leads to a depolarization of the membrane, which in its turn allows the cell to pump more protons into the medium.

Lower hybrid acceleration and ion evolution in the suprauroral region

Abstract: It is shown that ions can be accelerated perpendicularly to the magnetic field lines by resonant interactions with lower hybrid modes. Taking into account the effects of the magnetic field inhomogeneity, we demonstrate that the accelerated portion of the ions of ionospheric origin in the suprauroral region of the magnetosphere can evolve into conic distributions and propagate upwards along the field lines. Thus these ions can reach the region where they can be strongly energized along the field lines by the electrostatic shocks. We can argue that the ion distribution resulting from the combination of these accelerated ions with the background ions can depart significantly from a thermal distribution and lead to the excitation of electrostatic ion cyclotron modes.

Toroidal drift modes driven by ion pressure gradients

Abstract: Ion pressure gradient‐driven drift modes are analyzed for their parametric dependence on the shear, the toroidal aspect ratio, and the pressure gradient using the ballooning toroidal mode theory. An approximate formula for the anomalous ion thermal conductivity is derived for the turbulent regime.

Fast atom bombardment mass spectrometry: a powerful technique for the study of polar molecules

Abstract: A study of a range of polar organic molecules by fast atom bombardment (FAB) mass spectrometry is reported Compounds studied include organic salts, polar antibiotics, nucleoside phosphates, and underivatized peptides In these classes of compounds, molecular weights in the range 300 to 2000 daltons have been routinely determined, operating in both positive and negative ion modes Molecular weights of peptides are readily obtained on less than 1 nmol of material, and sequence information is conveniently deduced from sample sizes in the range 2 to 50 nmol

Optical properties of Nd3+ in tellurite and phosphotellurite glasses

Abstract: Optical‐absorption and emission spectra and fluorescence lifetimes were measured for Nd3+ in tellurite glasses containing various alkali and higher valence state cations and in a series of new phosphotellurite glasses. Judd‐Ofelt intensity parameters were determined and used to calculate radiative lifetimes and stimulated emission cross sections for 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions. Cross sections for several of the glasses are the largest obtained for any pure oxide glass. The dependence of the spectroscopic properties on composition and the application of tellurite glasses for lasers are discussed.

A finite material temperature model for ion energy deposition in ion‐driven inertial confinement fusion targets

Abstract: We have developed a model for use in ion‐driven inertial confinement fusion (ICF) target design to describe the deposition of energy by an arbitrary ion traversing a material of arbitrary composition, density, and temperature. This model particularly emphasizes the deposition physics of light ions having specific energies of 3 MeV/amu or less. However, the model is also applicable to heavy ion fusion problems where there are specific energies in excess of 10 MeV/amu. We have found that an accurate description of the cold material stopping power must include both shell corrections to the Bethe theory as well as the alternative LSS (Linhard‐Scharff‐Schio/tt) model at low energies. We have incorporated finite temperature effects by scaling the relevant bound electron parameters with the degree of material ionization as well as by including the free‐electron stopping power. We discuss both the phenomenon of range shortening and range relengthening in heated material. Our preliminary calculations indicate that...

Energetic ion beams at the edge of the plasma sheet: ISEE 1 observations plus a simple explanatory model

Abstract: It has been found that energetic ions with energies in the range from 10 keV to 2 MeV are an important part of the geomagnetic tail particle population. A description is presented of energetic ion, three-dimensional distribution observations from ISEE 1 during 0400-2000 UT, May 3, 1978, a time when several encounters with the geomagnetic tail plasma sheet occurred. Pronounced energetic ion streaming is observed at each entry to and exit from the plasma sheet. Azimuthal gradients observed during these times indicate that this streaming occurs within two gyroradii of the plasma sheet edge. Streaming characteristics show both narrow and broad angular flows at arbitrary angles to B. These flows are observed simultaneously and individually. The time, energy, and pitch angle evolution of the ion fluxes show that ISEE 1 is encountering a dynamic and not a static equilibrium situation at the edge of the plasma sheet.

Chemical sputtering of fluorinated silicon

Abstract: Sputtering can be defined as the process whereby particles leave the surface as a direct consequence of the presence of incident radiation. When particles leave the surface as a result of receiving momentum from the collision cascade induced by the incident radiation, the process is called "physical sputtering." If the incoming radiation (ions, electrons, or photons) induces a chemical reaction which leads to the subsequent desorption of particles, the process could be classified as "chemical sputtering." There are a number of molecules such as C${\mathrm{H}}_{4}$, C${\mathrm{F}}_{4}$, C${\mathrm{F}}_{3}$H, C${\mathrm{F}}_{3}$Cl, etc., whose binding energy to a large variety of surfaces is believed to be only a few kcal/mole. Therefore, these molecules will not remain adsorbed at room temperature. Consequently, if they are generated from surface atoms by radiation-induced processes, they will almost immediately desorb into the gas phase. This process is one type of chemical sputtering. Recent data obtained in plasma environments suggest that this type of reaction is a widely occurring phenomena; however, few systematic quantitative investigations of the subject have been completed. As a prototype system the chemical sputtering of silicon and Si${\mathrm{O}}_{2}$ under argon-ion bombardment in the presence of a molecular beam of Xe${\mathrm{F}}_{2}$ has been investigated. Under these conditions, 25 or more silicon atoms can leave the surface per incident argon ion. About 75% of the silicon is emitted as Si${\mathrm{F}}_{4}$ (gas) and the rest leaves as silicon atoms or $\mathrm{Si}{\mathrm{F}}_{x}$ radicals. The total yield (silicon plus fluorine) is greater than 100 atoms/ion. The measured yields are a strong function of Xe${\mathrm{F}}_{2}$ flux and a much weaker function of ion energy in the range 500-5000 eV. The chemical-sputtering yield for Si${\mathrm{O}}_{2}$ is smaller than that of silicon by about an order of magnitude, but it is still larger than the physical-sputtering yield. Moreover, Si${\mathrm{O}}_{2}$ is also sputtered by electrons. These results indicate that the incident radiation induces a chemical reaction between silicon and adsorbed fluorine which produces Si${\mathrm{F}}_{4}$, and the Si${\mathrm{F}}_{4}$ is subsequently desorbed into the gas phase. We define this process as chemical sputtering. The large yields are probably a consequence of weak binding between the surface and the Si${\mathrm{F}}_{4}$ molecule.

Monte Carlo studies of the structure of dilute aqueous sclutions of Li+, Na+, K+, F−, and Cl−

Abstract: Monte Carlo–Metropolis statistical thermodynamic computer simulations are reported for dilute aqueous solutions of Li+, Na+, K+, F−, and Cl−. The calculations are carried out on systems consisting of one ion and 215 water molecules at 25 °C and experimental densities. The condensed phase environment is modeled using periodic boundary conditions. The configurational energies are developed under the assumption of pairwise additivity by means of potential functions representative of nonempirical quantum mechanical calculations of the ion–water and water–water energies. The internal energies, radial distribution functions, and related thermodynamic properties are calculated for each system. The structure of the local solution environment around each dissolved ion is analyzed in terms of quasicomponent distribution functions. The results are compared with analogous calculations on a smaller system to estimate the effect of long‐range forces in the ion–water potential function on the calculated results.

Mixtures of hard ions and dipoles against a charged wall: The Ornstein–Zernike equation, some exact results, and the mean spherical approximation

Abstract: The Ornstein–Zernike equation for a mixture of ions and dipoles near a hard charged wall is obtained. It is shown that the same exact contact and monotonicity theorems, previously derived for the primitive (continuum dielectric) case, also are valid for this model. Rather simple expressions for the contact density, potential difference, capacitance, and distribution functions are obtained in the mean spherical approximation (MSA). These expressions reduce to previously known results in the limits of low and high concentrations of ions. It is found that cooperative alignment of the dipoles near the wall results in an increased potential difference and reduced capacitance of the double layer compared to that calculated when the solvent is represented by a continuum dielectric.

Ion streams in the magnetotail

Abstract: Ion mass spectrometer observations of low-temperature streaming plasmas in the earth's magnetotail are reported. Measurements in the energy per charge range 0 to 17 keV/e were made at geocentric radial distances less than 23 earth radii from the ISEE 1 spacecraft. Ion streams of solar wind origin in the magnetotail boundary layer and of ionospheric origin in the tail lobes and plasma sheet are described. A statistical study of some of the characteristics of the streams allows the inference that the central tail lobe plasmas are primarily constituted of ion streams of ionospheric origin and that the ionosphere is a significant contributor to the hot plasmas that form the plasma sheet.

The magnetic properties of the oxygen-hole aluminum centres in crystalline SiO2. IV. [AlO4/Na]+

Abstract: The centre [AlO4/Na]+, formed in α-quartz by X irradiation at 77 K, contains an aluminum ion substituted for a silicon ion, with an electron hole on a nearest-neighbor oxygen ion, and an interstiti...