Spatial Distribution Of Ions Research Articles

On the accuracy and reliability of different fluid models of the direct current glow discharge

We developed and tested 2D “extended fluid model” of a dc glow discharge using COMSOL MULTIPHYSICS software and implemented two different approaches. First, assembling the model from COMSOL’s general form pde’s and, second, using COMSOL’s built-in Plasma Module. The discharge models are based on the fluid description of ions and excited neutral species and use drift-diffusion approximation for the particle fluxes. The electron transport as well as the rates of electron-induced plasma-chemical reactions are calculated using the Boltzmann equation for the EEDF and corresponding collision cross-sections. The self-consistent electric field is calculated from the Poisson equation. Basic discharge plasma properties such as current-voltage characteristics and electron and ion spatial density distributions as well as electron temperature and electric field profiles were studied. While the solutions obtained by two different COMSOL models are essentially identical, the discrepancy between COMSOL and CFD-ACE+ model solutions is about several percents and caused by the difference in the models due to undocumented details in the software packages. We also studied spatial distributions of particle fluxes in discharge plasma and identified the existence of vortex component of the discharge current.

Spatial distributions of ions and electrons from the plasma sheet to the inner magnetosphere: Comparisons between THEMIS-Geotail statistical results and the Rice convection model

[1] To understand the processes responsible for the formation and structure of plasma sheet and ring current particles, we have used THEMIS and Geotail data to investigate statistically the distributions of ions and electrons from the midtail to the inner magnetosphere and compared them with results from the Rice convection model (RCM). The observed distributions show clear magnetic local time (MLT) asymmetries in the thermal energy and energy fluxes of plasma sheet particles but many more MLT symmetric ring current particles. Our RCM runs include both self-consistent electric and magnetic fields and realistic MLT-dependent outer particle sources. Starting with no initial particles, particles released from the RCM outer sources move along electric and magnetic drift paths and change energy adiabatically. Comparison of the observation with the simulation indicates that the particles along the open drift paths can account for the observed plasma sheet populations and that the observed significant MLT variations are a combined result of species- and energy-dependent drift and location-dependent source strength. The simulated energy and spatial distributions of the particles within closed drift paths are found to be consistent with the observed ring current particles. These ring current particles are originally plasma sheet particles which became trapped along closed paths due to temporal variations of drift paths. The good agreement in key features of the spatial distributions of thermal energy and energy fluxes between the RCM and observations clearly indicates that electric and magnetic drift transport and the associated energization play dominant roles in plasma sheet and ring current dynamics.

Dependence of fast-ion transport on the nature of the turbulence in the Large Plasma Device

Strong turbulent waves (δn/n ∼0.5, f ∼5-40 kHz) are observed in the upgraded Large Plasma Device [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)] on density gradients produced by an annular obstacle. Energetic lithium ions (Efast/Ti≥300, ρfast/ρs~10) orbit through the turbulent region. Scans with a collimated analyzer and with probes give detailed profiles of the fast ion spatial distribution and of the fluctuating wave fields. The characteristics of the fluctuations are modified by changing the plasma species from helium to neon and by modifying the bias on the obstacle. Different spatial structure sizes (Ls) and correlation lengths (Lcorr) of the wave potential fields alter the fast ion transport. The effects of electrostatic fluctuations are reduced due to gyro-averaging, which explains the difference in the fast-ion transport. A transition from super-diffusive to sub-diffusive transport is observed when the fast ion interacts with the waves for most of a wave period, which agrees with theoretical predictions.

Investigation of Spatial Distribution of Hydrogen and Argon Ions and Effects of them on Aluminum Samples in a 2.5 kJ Mater Type Plasma Focus Device

In this work the effects of Hydrogen and Argon ions on Aluminum samples and the spatial distribution of the ions in 2.5 kJ SBUPF1 Mather type plasma focus device in different distance from the anode top have been investigated. For each gas the pressure and operation voltage have been determined to have maximum ion density. Aluminum samples were placed at different distances above the anode. Irradiated samples have been analyzed with SEM technique for morphological and surface study of samples. Melting and surface evaporation effects and generation of cracks were seen in these samples. The result of ions patterns on these samples can be used to determine ions spatial distribution on top of the anode region. The EDX spectroscopy and the SRIM calculations have been done to determine the composition of samples and the penetration depth of the ions in the samples respectively.

Improving Electrostatic Descriptions of Ions Around Double-Stranded DNA

Ions play an essential role in governing the structure and function of nucleic acids, due to the large negative charge associated with the nucleic acid backbone. The addition of even small numbers of multivalent, positively charged ions induces intra-strand attraction in DNA and thus efficiently packages the extended polymer into compact toroids. Anomalous small angle X-ray scattering (ASAXS) has emerged as a powerful technique to report the spatial distribution of ions associated to nucleic acids with unprecedented levels of detail and resolution. To determine more detailed information about these highly mobile ions relative to the underlying nucleic acid surface requires tight coordination with theoretical or computational tools. Presently, very few robust theoretical or computational tools exist for understanding ion-nucleic interactions. Current atomically-detailed computational approaches that represent the solvent environment explicitly, as discrete water molecules and ions, are prohibitively expensive for systematic studies of biologically relevant structures on time scales needed to fully understand these interactions. The alternative approach of “implicit solvent” models, that represent solvent implicitly as a continuum, could potentially overcome this difficulty. However, most “implicit solvent” models reduce computational effort at the expense of simplifying assumptions that preclude their application to highly charged systems in the presence of concentrated salt solutions or multivalent ions. We have been examining the ability of traditional and “size-modified” Poisson-Boltzmann models to predict the distribution of multivalent ions at low concentration around B-form DNA. These predictions are tested through direct comparison with ASAXS data on similar systems. The goal of this work is to examine the role of size-exclusion in the well-known failure of traditional Poisson-Boltzmann approaches for describing multivalent ions in highly charged systems. This work lays the foundation for a systematic approach to improve implicit solvent models for nucleic acid systems.

Characterization of DNA and RNA Ion Atmospheres Using Multiple-Energy Asaxs

The number and spatial distribution of small positively-charged ions around highly negatively charged DNA or RNA contribute to the free energy of binding in vitro and in vivo. However, the majority of charge compensating ions around nucleic acids forms a diffuse counterion “cloud” that is not amenable to investigation by traditional methods that rely on rigid structural interactions. With 2 x-ray energies, one near and the other 100 eV away from the ion absorption edge, we have successfully used Anomalous Small-Angle X-ray Scattering (ASAXS) to compare and differentiate the ion spatial distribution around comparably sequenced short DNA and RNA helices. Here, we present further information gained when using multiple x-ray energies (up to 5) in an ASAXS experiment. We describe proper treatment of multiple-energy SAXS data including absolute SAXS intensity calibration and measurement of scattering factors from x-ray fluorescence. We discuss the strengths and limitations of this approach and derive useful parameters in depicting the nucleic acid ion atmosphere.

Kinetic energy and spatial distribution of ions in high irradiance laser ionization source

Characteristics of laser ionization in vacuum and low pressure background gas (He) have been investigated through the measurement of kinetic energy and spatial distributions of copper and tungsten ions. A Q-switched Nd:YAG laser with 532 nm wavelength was utilized and the laser irradiance was fixed at 9 × 109 W cm−2. A plume splitting was observed in the low pressure regime investigated (from 100 Pa to 2000 Pa). The plume propagation translates from a free expansion in vacuum to shockwave-like expansion at relative low pressure and finally diffusion into background gas at relative high pressure environment. A measurement of ion spatial distribution in the ion source has also been carried out for characterizing ions at different pressures and the behaviors of doubly charged, singly charged, and polyatomic ions to reveal the effect of plume–background gas interaction.

Ion and Solvent Density Distributions around Canonical B-DNA from Integral Equations

We calculate the water and ion spatial distributions around charged oligonucleotides using a renormalized three-dimensional reference interaction site theory coupled with the HNC closure. Our goal is to understand the balance between inter-DNA strand forces and solvation forces as a function of oligonucleotide length in the short strand limit. The DNA is considered in aqueous electrolyte solutions of 1 M KCl, 0.1 M KCl, or 0.1 M NaCl. The current theoretical results are compared to molecular dynamics (MD) simulations and experiments. It is found that the integral equation (IE) theory replicates the MD and the experimental results for the base-specific hydration patterns in both the major and the minor grooves. We are also able to discern characteristic structural pattern differences between Na(+) and K(+) ions. When compared to Poisson-Boltzmann methods, the IE theory, like simulation, predicts a richly structured ion environment, which is better described as multilayer rather than double layer.

Counting Ions around DNA with Anomalous Small-Angle X-ray Scattering

The majority of charge-compensating ions around nucleic acids form a diffuse counterion "cloud" that is not amenable to investigation by traditional methods that rely on rigid structural interactions. Although various techniques have been employed to characterize the ion atmosphere around nucleic acids, only anomalous small-angle X-ray scattering (ASAXS) provides information about the spatial distribution of ions. Here we present an experimentally straightforward extension of ASAXS that can be used to count the number of ions around nucleic acids.

Turbulent transport of fast ions in the Large Plasma Device

Strong drift wave turbulence is observed in the Large Plasma Device [H. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] on density gradients produced by a plate limiter. Energetic lithium ions orbit through the turbulent region. Scans with a collimated ion analyzer and with Langmuir probes give detailed profiles of the fast ion spatial distribution and the fluctuating fields. The fast ion transport decreases rapidly with increasing fast ion gyroradius. Unlike the diffusive transport caused by Coulomb collisions, in this case the turbulent transport is nondiffusive. Analysis and simulation suggest that such nondiffusive transport is due to the interaction of the fast ions with stationary two-dimensional electrostatic turbulence.

Fast, High Resolution Mass Spectrometry Imaging Using a Medipix Pixelated Detector

In mass spectrometry imaging, spatial resolution is pushed to its limits with the use of ion microscope mass spectrometric imaging systems. An ion microscope magnifies and then projects the original spatial distribution of ions from a sample surface onto a position-sensitive detector, while retaining time-of-flight mass separation capabilities. Here, a new type of position-sensitive detector based on a chevron microchannel plate stack in combination with a 512 × 512 complementary metal-oxide-semiconductor based pixel detector is coupled to an ion microscope. Spatial resolving power better than 6 μm is demonstrated by secondary ion mass spectrometry and 8-10 μm spatial resolving power is achieved with laser desorption ionization. A detailed evaluation of key performance criteria such as spatial resolution, acquisition speed, and data handling is presented.

Numerical Simulation of Ion Extraction Through Ion Thruster Optics

Based on the particle-in-cell (PIC) method, a two-dimensional numerical scheme was developed to investigate the ion beam extraction phenomena through the ion thruster optics. According to the calculated results, the plasma sheath upstream of the screen grid, the electric field in the calculation domain, and the ion and electron spatial distributions are obtained for different accelerator grid voltages. The results indicate that the accelerator grid voltage affects the plasma sheath upstream of the screen grid significantly. It is found that a moderate accelerator grid voltage results in an ion optical performance better than either a higher or lower voltage, from a point of ion extraction from the discharge chamber and erosion mitigation of the accelerator grid due to the direct ion impingement.

Sunflower Plants as Bioindicators of Environmental Pollution with Lead (II) Ions

In this study, the influence of lead (II) ions on sunflower growth and biochemistry was investigated from various points of view. Sunflower plants were treated with 0, 10, 50, 100 and/or 500 μM Pb-EDTA for eight days. We observed alterations in growth in all experimental groups compared with non-treated control plants. Further we determined total content of proteins by a Bradford protein assay. By the eighth day of the experiment, total protein contents in all treated plants were much lower compared to control. Particularly noticeable was the loss of approx. 8 μg/mL or 15 μg/mL in shoots or roots of plants treated with 100 mM Pb-EDTA. We also focused our attention on the activity of alanine transaminase (ALT), aspartate transaminase (AST) and urease. Activity of the enzymes increased with increasing length of the treatment and applied concentration of lead (II) ions. This increase corresponds well with a higher metabolic activity of treated plants. Contents of cysteine, reduced glutathione (GSH), oxidized glutathione (GSSG) and phytochelatin 2 (PC2) were determined by high performance liquid chromatography with electrochemical detection. Cysteine content declined in roots of plants with the increasing time of treatment of plants with Pb-EDTA and the concentration of toxic substance. Moreover, we observed ten times higher content of cysteine in roots in comparison with shoots. The observed reduction of cysteine content probably relates with its utilization for biosynthesis of GSH and phytochelatins, because the content of GSH and PC2 was similar in roots and shoots and increased with increased treatment time and concentration of Pb-EDTA. Moreover, we observed oxidative stress caused by Pb-EDTA in roots where the GSSG/GSH ratio was about 0.66. In shoots, the oxidative stress was less distinctive, with a GSSG/GSH ratio 0.14. We also estimated the rate of phytochelatin biosynthesis from the slope of linear equations plotted with data measured in the particular experimental group. The highest rate was detected in roots treated with 100 μM of Pb-EDTA. To determine heavy metal ions many analytical instruments can be used, however, most of them are only able to quantify total content of the metals. This problem can be overcome using laser induced breakdown spectroscopy, because it is able to provide a high spatial-distribution of metal ions in different types of materials, including plant tissues. Data obtained were used to assemble 3D maps of Pb and Mg distribution. Distribution of these elements is concentrated around main vascular bundle of leaf, which means around midrib.

Genetically Encoded Sensors to Elucidate Spatial Distribution of Cellular Zinc

Transition metals are essential enzyme cofactors that are required for a wide range of cellular processes. Paradoxically, whereas metal ions are essential for numerous cellular processes, they are also toxic. Therefore cells must tightly regulate metal accumulation, transport, distribution, and export. Improved tools to interrogate metal ion availability and spatial distribution within living cells would greatly advance our understanding of cellular metal homeostasis. In this work, we present genetically encoded sensors for Zn2+ based on the principle of fluorescence resonance energy transfer. We also develop methodology to calibrate the probes within the cellular environment. To identify both sources of and sinks for Zn2+, these sensors are genetically targeted to specific locations within the cell, including cytosol, plasma membrane, and mitochondria. Localized probes reveal that mitochondria contain an elevated pool of Zn2+ under resting conditions that can be released into the cytosol upon glutamate stimulation of hippocampal neurons. We also observed that Zn2+ is taken up into mitochondria following glutamate/Zn2+ treatment and that there is heterogeneity in both the magnitude and kinetics of the response. Our results suggest that mitochondria serve as a source of and a sink for Zn2+ signals under different cellular conditions.

Spatial Distribution of Nitrate and Nitrite Anions at the Liquid/Vapor Interface of Aqueous Solutions

Depth-resolved ion spatial distributions of nitrate and nitrite anions in aqueous solution have been quantitatively measured using X-ray photoemission spectroscopy on a 15 microm aqueous liquid jet containing 3 M NaNO(3), 3 M NaNO(2), or an equimolar mixture of the two. The surface region, which extends to photoelectron kinetic energies of 400-500 eV, is partially depleted in anions relative to the bulk 3 M concentration. The nitrate and nitrite solutions exhibit similar depth-dependent anion profiles. The results presented here are compared with recent molecular dynamics simulations of a NaNO(3) solution and are found to agree at high photoelectron kinetic energies. At shallower probe depths, the experiment measured a surface anion concentration less than that predicted by theory. Possible origins of the discrepancy are discussed in terms of the confined size of the simulation box and uncertainties that remain in regard to the inelastic mean free path of photoelectrons in aqueous media. The importance of our findings is discussed in relation to the observed increase in photochemical activity of nitrate-containing aerosols in the atmosphere.

Analysis of salts transport affected by root absorption capacity in surface — irrigated fields in the upper Yellow River basin

In order to analyze the salt transport affected by roots and its effects on soil salinity in an experimental irrigated field newly established in an alluvial valley of the Yellow River in China, spatial distribution of ions contained in waters, soils and crops relevant to these phenomena were evaluated there. During the intensive surveys conducted in year 2007–2008, the Yellow River water, irrigation canal water, groundwater, field soils and crops, etc. were sampled and their chemical characteristics such as electrical conductivity, concentrations of ions Na+, Ca2+, Mg2+, K+, Cl−, SO 4 2− and NO 3 − were measured. Irrigation seemed to cause increases in the concentrations of ions Na+, Cl− and SO 4 2− in the groundwater. Although those were also major ions contained in the field soil, the soil was classed as saline but not sodic according to the standard classification. On the other hand, K+, which is one of the major essential nutrients for plant growth, was highly concentrated in the crops, while Na+ was not concentrated because of crop’s poor ability to absorb it. The ion concentration within the plant body seemed to be reflected by the active and selective ion uptake by roots and the transpiration stream. Furthermore, salt accumulation in the surface-irrigated field largely depended on the upward transport of water and ions in the soil profile affected by root absorption capacity. The information obtained in this study will contribute to the development of scientific methods for sustainable and effective plant production in irrigated fields.

X-Ray photo- and resonant Auger-electron spectroscopy studies of liquid water and aqueous solutions

It has only recently become possible to use photoelectron spectroscopy (PES) to study the electronic structure of highly volatile aqueous surfaces. Here, we review current X-ray PES and related resonant Auger-electron decay and intermolecular Coulomb decay investigations in solution, which aim at understanding the solute–water, water–water, and solute–solute interactions at the microscopic level. Systems that will be discussed include neat liquid water, and aqueous solutions of hydroxide, hydronium, salts and amino acids. From observed chemical shifts in the main photoelectron lines or from the occurrence of spectator Auger-electron peaks, information on electron dynamics and/or energy transfer processes, and at times on the structure of the hydration complex, may be accessed. Furthermore, we discuss PES in conjunction with variable energy incident X-ray photons; with the resulting change of the (photo)electron kinetic energy, ion spatial distributions at the liquid–vapor interface can be determined.

Numerical study of ionic current in dielectric liquid layer subjected to ac voltage

Ionic current flowing in a thin layer of dielectric liquid under the action of ac voltage was studied numerically. Strongly blocking electrodes and equal mobilities of anions and cations were assumed. Field dependent dissociation and recombination were taken into account. The spatial distributions of ion concentrations and of electrical potential along coordinate normal to the layer and the ionic current density during a single voltage cycle were calculated as functions of time. At sufficiently low frequency and sufficiently high voltage, the ionic current was non-sinusoidal function of time. Significant contribution of higher harmonics were present. In certain frequency region this contribution gave rise to a bump of the current. The moment at which the bump occurred was not related with the value of ions mobility. These effects resulted from changes of spatial distribution of ions. In most cases the changes occurred simultaneously in the whole layer. In sufficiently thin layer, two gradually diffusing groups of ions wandering across the layer, one formed by cations and the other formed by anions, could be distinguished. Their appearance coincided with the maximum of the bump which was particularly high in such case. In thicker layers, similar groups of ions did not arise, although the current bumps were present.

Characterization of the spatiotemporal evolution of laser-generated plasmas

We characterize the time evolution of ion spatial distributions in a laser-produced plasma. Krypton ions are produced in strong, linearly and circularly polarized optical laser fields (1014–1015 W/cm2). The Kr+ ions are preferentially detected by resonant x-ray absorption. Using microfocused, tunable x rays from Argonne’s Advanced Photon Source, we measure ion densities as a function of time with 10 μm spatial resolution for times ≤50 ns. For plasma densities of the order of 1014 cm−3, we observe a systematic expansion of the ions outward from the laser focus. We find the expansion timescale to be independent of the plasma density though strongly dependent on the plasma shape and electron temperature. The former is defined by the laser focus, while the latter is controlled by the laser polarization state. We have developed a fluid description assuming a collisionless quasineutral plasma, which is modeled using a particle-in-cell approach. This simulation provides a quantitative description of the observed behavior and demonstrates the role of the very different electron temperatures produced by circularly and linearly polarized light. These results demonstrate the utility of this method as an in situ probe of the time and spatial evolution of laser-produced plasmas.

High-Energy Ion Emission from Deuterium Clusters by Using an Intense Femtosecond Laser ¡¤¡¤¡¤¡¤¡¤

The interaction of a femtosecond laser pulse with deuterium clusters was experimentally studied. The prepulse and the ampli ed spontaneous emission (ASE) e ects on laser absorption and ion emission were measured with the ASE time width controlled. We also measured the spatial distribution of ions along the plasma channel produced from laser interaction with clusters. We performed an experiment to generate neutrons by using laser-induced nuclear fusion, d(d, n)3He. A 10-TW femtosecond laser beam (280 mJ, 30 fs) was focused onto deuterium gas clusters and produced ions accelerated by Coulomb explosion, which resulted in a neutron yield of 3 103/shot.