Positive Ion Flow Research Articles

Radial Ordering of Fine Particles in Plasma under Microgravity Condition

Microgravity experiments on fine-particle plasma were carried out and the ordering and behavior of fine particles were analyzed under the microgravity condition as well as under a change in gravitational conditions. Fine particles aligned in radial directions were observed under microgravity, although they were aligned in the vertical direction under the gravity condition. It has been suggested by comparison with the result obtained under microgravity that under the gravity the mutual influence between positive ion flow and negative fine particles results in the vertical alignment of fine particles and in the vertical flow of ions toward fine particles along with the deformation of plasma.

Effects of a weak transverse magnetic field on negative ion transport in negative ion sources

The effects of a weak transverse magnetic field on negative ion (H−) extraction in a negative ion source have been studied by two-dimensional electrostatic particle simulation. A particle-in-cell model is used which simulates motion of charged particles in their self-consistent electric field. The extraction of H− ions is significantly improved by the weak transverse magnetic field in simulation as well as in experiments. Since electrons are deflected by the weak magnetic field, more H− ions arrive instead of electrons in the region close to the plasma grid in order to ensure plasma neutrality. The presence of the weak magnetic field produces important modifications in positive ion flow and, as a result, in the structure of the spatial potential which collects H− ions.

Intrinsic sheath edge conditions for sheath instability in low-pressure electronegative plasmas

The conditions at the sheath edge that could lead to instability in the plasma–wall boundary in low-pressure electronegative plasmas are considered in a comprehensive manner. The relevant range of the ratio of the negative ion density to the electron density at the sheath edge was derived on the basis of a fluid model and it was clarified how the parameter range are affected by the positive and negative ion temperatures, by the geometries of plasmas (plane-parallel, cylindrical, and spherical), by the directions of the positive ion flow (converging or diverging), and by the effects of ionization and momentum transfer collisions. It was found that in any geometry the instability would be induced in conditions that could be realized without difficulty in low-pressure plasmas.

Aldosterone as a determinant of cardiovascular and renal dysfunction.

Aldosterone is a major regulator of extracellular fluid volume and the principal determinant of potassium metabolism 1,2,3,4,5. These effects are mediated by the binding of aldosterone to the mineralocorticoid receptor in target tissues, primarily the kidney. Volume is regulated through a direct effect on the collecting duct, where aldosterone promotes sodium retention and potassium excretion. The reabsorption of sodium ions produces a fall in the transmembrane potential, thus enhancing the flow of positive ions (such as potassium) out of the cell into the lumen. The reabsorbed sodium ions are transported out of the tubular epithellium into the renal interstitial fluid and from there into the renal capillary circulation. Three primary mechanisms control aldosterone release—the renin-angiotensin system, potassium, and adrenocorticotropic hormone. The renin-angiotensin system controls extracellular fluid volume via regulation of aldosterone secretion. In effect, the renin-angiotensin system keeps the circulating blood volume constant by causing aldosterone-induced sodium retention during volume deficiency and by decreasing aldosterone-dependent sodium retention when volume is ample. In recent years there has been a radical shift in our view of aldosterone's effects on the heart, the vasculature and the kidney6,7,8,9. Aldosterone's endocrine properties have taken on a broader perspective, involving non-classic actions in non-epithelial cells found in non-classic target tissues6, 10,11,12,13,14,15. The traditional concept, that aldosterone is synthesized only in the adrenal glomerulosa cell and acts almost exclusively on the kidney to modify sodium and potassium homoeostasis, needs to be expanded. There is increasing evidence that aldosterone can have an effect on vascular remodelling and collagen formation, and a non-genomic action to modify endothelial function. Among the most intriguing effects of aldosterone are its impact on fibrosis and activity associated with a cell surface receptor in certain target tissues, including endothelial cells 6, 7, 16,17,18,19. These actions contribute substantially to the pathophysiology of congestive heart failure (CHF), as well as progressive renal dysfunction. This new information has increased interest in the development of an antagonist to block aldosterone's effect, not just because of its diuretic action but primarily because of its potential cardiovascular and renal protective effects. In this review I consider the broad spectrum of non-genomic effects of aldosterone. It is becoming increasingly evident that these effects, occurring independently of haemodynamic factors, contribute to enhanced cardiovascular risk manifested by congestive heart failure and progressive renal disease. I also discuss the clinical trials with selective aldosterone receptor antagonists that are currently underway in patients with left ventricular hypertrophy, essential hypertension and systolic hypertension, and in those who have experienced myocardial infarction. Such trials will enhance our understanding of the role of aldosterone in the pathophysiology of cardiovascular disease. Also, selective aldosterone receptor antagonism holds promise for a reduction in cardiovascular and renal disease morbidity and mortality, and for enhancement of patient wellbeing.

Current–voltage characteristics in a flame plasma: analysis for positive and negative ions, with applications

A flat, cylindrical, laminar, H 2–O 2–N 2 flame in plug flow with velocity v f = 19.8 m s −1 and cross-sectional area A = 1.05 × 10 −4 m 2, at 1 atm and 2400 K, was doped with appropriate additives to give a weak, continuum, quasi-neutral plasma involving Cs +/ e − or H 3O +/ e − or H 3O +/Cl −/ e −. The flame impinged on a planar, normal, water-cooled, conducting electrode designated N (for nozzle) located a variable distance z downstream from the flame’s luminous reaction zone which is separated by a dark space or gap δ ≃ 1 mm from a water-cooled metallic burner B, the second electrode. Neither electrode has the properties of a Langmuir probe. Two types of data were measured: i– V characteristics with the current i(N) collected by the nozzle versus the applied voltage Δϕ between N and B at a fixed value of z (e.g. 20 mm); and profiles of i(N) versus z at a fixed value of Δϕ. For Δϕ < 0 (e.g. −50 V; N is negative), the electrons are stopped in the flame and the constant saturation current is controlled by the convective flow of positive ions to the nozzle: i +(N) = eAn +(N) v f . This provides a wonderfully simple and accurate measurement of the absolute density n +(N). Alternatively, if a weak solution of a Cs salt (e.g. 10 −4 M CsCl) is sprayed using a pneumatic atomizer such that Cs is completely ionized, the delivery factor of the atomizer can be calibrated. Furthermore, if the delivery factor giving n +(N) is known, v f can be determined. With Δϕ fixed (e.g. −50 V), a profile of n +(N) versus z can be obtained throughout the flame gas downstream; the density distribution is not affected by the application of the applied voltage. For Δϕ > 0 (e.g. +50 V, N is positive), the current through the flame i +(B) = − i e (N) − i −(N) (the latter term is included if negative ions are present), is controlled by the flow of positive ions of mobility μ + to the burner across the potential gradient in the burner gap g: i +(B) = eAn +(B)μ +∇ϕ g. When Δϕ is sufficiently positive to achieve a constant saturation current, n +(B) can be determined; it represents the total ion production in the flame reaction zone. When negative ions are present replacing even a large fraction of the electrons, the effect on the i– V characteristic is relatively minor; it does not appear possible to provide a separate analysis for n e and n −. However, profiles through the flame plasma clearly show the effects of negative ion processes such as ion–ion recombination, for example. For a wide range of the applied voltage Δϕ, both positive and negative, it is possible to calculate the potential distribution in the burner gap, the bulk flame plasma and in the positive ion sheath at the nozzle. This provides a quantitative understanding of the ion and electron behaviour throughout the flame.

Ion source electrode biasing technique for microsecond beam pulse rise times

Heavy ion fusion (HIF) induction accelerators require ion sources that can deliver intense heavy ion beams with low emittance. The typical pulse length is 20 μs with a rise time less than 1 μs and a repetition rate of 10 Hz. So far, the surface ionization sources have been used in most HIF induction linac designs. However, there are other ions of interest to HIF (e.g., Hg, Xe, Rb, Ar, and Ne) which cannot be produced by the surface ionization sources, but rather by volume ion sources. In this paper, we describe an experiment that uses a multicusp source with a magnetic filter to produce beam pulses that have a rise time in the order of 1 μs. By applying a positive biasing pulse on the plasma electrode with respect to the source body, the positive plasma ions can be temporarily repelled from the neighborhood of the extraction aperture, leading to a suppression of the ion beam. As the bias is removed, positive ions flow to the extraction region, enabling a fast-rising beam pulse. The beam current pulses show that there are two distinct groups of ions. An initial fast current rise time (&amp;lt;2 μs) corresponding to ions originating from within the magnetic filter region followed by a second group of ions with a longer rise time (10–20 μs) originating from the plasma bulk region. Proper positioning of the filament cathode and the magnetic filter field relative to the extraction aperture was found to be critical.

Quantitative EDS analysis in the environmental ESEM using a bremstrahlung intensity-based correction for primary electron beam variation and scatter

Quantitative EDS analysis of bulk samples in the scanning electron microscope (SEM) or electron microprobe requires, as a fundamental parameter, a stable and reproducible primary electron beam current Beam current is usually measured with a Faraday cage positioned in the electron column below the objective aperture or in the specimen holder. Reproducibility and stability within 1%/hour is a minimum condition.Primary beam current measurement in the ESEM or any high pressure SEM is difficult to measure. Electron-gas interaction in the biased chamber generates a positive ion flow highly amplified relative to the primary beam (Danilatos, 1990) and generates an x-ray signal from the gas. The latter signal amplitude is dependent on primary beam current, chamber pressure and backscatter electron signal from the specimen (Griffin et al. 1993). These interactions prevent quantification of EDS data standardised to Faraday cage primary beam current measurements or x-ray counts from a reference standard.

A model of nonthermal radiation from accreting X-ray pulsars

The formation of accelerator gaps in the magnetosphere of accreting X-ray pulsars, especially in the regions above the inner part of the accreting disk has been proposed. In this paper, we concentrate on the case where the gaps are filled with soft X-ray photons which arise from the accretion-powered thermal X-rays at the polar caps, but are Compton-degraded when they traverse the accretion matter flow. The high-energy electron current, whose existence is assumed to counter balance the positive ion (proton) flow, boosts these soft X-rays up to γ-rays by inverse Compton scattering. Only very few of these γ-rays, however, can be directly observed on Earth because most of them collide with the thermal X-ray photons from the polar caps outside the gaps

Role of drifts and global merged interaction regions in the long‐term modulation of cosmic rays

It is shown that the long‐term modulation of 0.2‐ to 3‐GV galactic and anomalous cosmic rays over the 22‐year heliomagnetic cycle is principally a combination of two solar related processes, the cumulative effect of long‐lived global merged interaction regions (GMIRs) and large‐scale particle gradient and curvature drifts in the interplanetary magnetic field. This paradigm for cosmic ray modulation is based on the observed changes in cosmic ray intensity from solar minimum to solar maximum over successive solar cycles (21 and 22) using data from 1 AU and from the outer heliosphere. For cycle 21 (when positive ions flow in over the solar pole and out along the heliospheric neutral current sheet; qA&gt;0), the 1977–1980 modulation is dominated by GMIRs. While drifts may play a role in particle transport in the heliosphere at this time, the changing inclination of the heliospheric neutral sheet has a negligible effect on the intensity of cosmic ray nuclei. In cycle 22 when this flow pattern is reversed, it is shown that drifts are an important but not dominant factor for galactic cosmic ray modulation with the current sheet related drift effects decreasing with increasing rigidity R and heliocentric distance r. Anomalous cosmic rays are much more sensitive to changes in the current sheet inclination. The observed changes in the galactic cosmic ray intensity over the 1987–1988 period due to the increasing inclination of the current sheet, α, are a factor of 3–5 smaller than predicted by the time‐dependent model of Potgieter and Le Roux (1992). The latitude variation at Voyager 1 (heliolatitude 32°) of the dependence of galactic cosmic ray intensity on α is consistent with theoretical expectations. The strong role of GMIRs and their episodic nature requires a long lifetime (1.5–1.8 years) and a magnetic structure that effectively extends over the solar poles. Such a long GMIR lifetime implies a modulation boundary of the order of ∼175 AU.

High‐pressure scanning electron microscopy of insulating materials: A new approach

A new SEM technique for imaging uncoated non-conducting specimens at high beam voltages is described which employs a high-pressure environment and an electric field to achieve charge neutralization. During imaging, the specimen surface is kept at a stable low voltage, near earth potential, by directing a flow of positive gas ions at the specimen surface under the action of an electric bias field at a pressure of about 200 Pa. In this way charge neutrality is continuously maintained to obtain micrographs free of charging artefacts. Images are formed by specimen current detection containing both secondary electron and backscattered electron signal information. Micrographs of geological, ceramic, and semiconductor materials obtained with this method are presented. The technique is also useful for the SEM examination of histological sections of biological specimens without any further preparation. A simple theory for the charge neutralization process is described. It is based on the interaction of the primary and emissive signal components with the surrounding gas medium and the resulting neutralizing currents. Further micrographs are presented to illustrate the pressure dependence of the charge neutralization process in two glass specimens which show clearly identifiable charging artefacts in conventional microscopy.

Kelvin-Helmholtz instability in a plasma with negative ions

A dispersion relation for low-frequency electrostatic modes in a plasma with negative ions is derived for the case in which a velocity shear Kelvin-Helmholtz instability exists in the positive ion flow along the magnetic field. It is found that the negative ions have, generally, a destabilizing effect, as seen previously for ion-acoustic and electrostatic ion-cyclotron waves. The influence of the negative-ion-to-positive-ion mass mass ratio on the stability is also examined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Practical method for the purification of carrier gases for gas chromatography

For sensitive analysis of traces of inorganic gases, detectors are employed based on Penning's effect, which requires pure helium or neon as the carrier gas. These gases may be purified by an electric discharge. This method is based on cataphoresis, i.e. the change in composition of a gaseous mixture in an electric field. This change is the result of a flow of contaminant ions and is important when the ionization energy of the contaminant gas is lower than the energy of a metastable level of the carrier gas. The positive ions flow towards the cathode, which is compensated for, in the closed vessel, by a diffusive flow of neutral particles. If the electrodes are hollow and used as gas exit from the chamber, the compensating flow of neutral particles will decrease and the concentration of the contaminant in gas flowing out of the chamber through the cathode will increase. Accordingly, the concentration of the contaminant in gas flowing away through the anode becomes lower than the initial concentration. Construction of the discharge chamber and the value of the electric current depend on the initial concentration of the contaminant and the required level of purity. The experimental results show that the method may be used for the complete purification of helium and neon with low initial concentrations of contaminant. In this case, total ionization and purification can simply be obtained by a corona discharge.

Large electrical currents traverse growing pollen tubes.

Using a newly developed vibrating electrode, we have explored the electric fields around lily pollen germinating in vitro. From these field measurements, we infer that each weeted pollen drives a steady current of a few hundred picoamperes through itself. Considered as a flow of positive ions, this current enters an ungerminated grain's prospective growth site and leaves it opposite end. After a grain germinates and forms a tube, this current enters most of the growing tube and leaves the whole grain. The current densities over both of these extended surface regions are relatively uniform, and the boundary zone, near the tube's base, is relatively narrow. This current continues as long as the tube grows, and even continues when elongation, as well as cytoplasmic streaming, are blocked by 1 mug/ml of cytochalasin B. After a otherwise indistinguishable minority of tubes have grown to lengths of a millimeter or more, their current comes to include an endless train of discrete and characteristic current pulses as well as a steady component. These pulses are about 30s long, never overlap, recur every 60-100s, and seem to enter a region more restricted to be growing tip than the steady current's sink. In most ways, the current through growing lily pollen resembles that known to flow through focoid eggs.

Method of Generating Very Intense Positive-Ion Beams

The combination of multiply reflected electrons and positive ion flow in a reflex triode arrangement is analyzed. Under certain conditions it is possible to generate very intense beams of positive ions with this device. The analysis demonstrates that the energy loss and scattering of the electrons as they pass through the anode have a major effect on the ion and electron currents. Solid fractional-range anodes are shown to produce more intense ion beams than semitransparent mesh anodes.

ELECTRO-OSMOSIS IN NITELLA

Simultaneous measurements of minute electrical current and water flow through single living cells of Nitella translucens have shown that there is an electro-osmotic transport of the order of 100 moles of water per Faraday in the direction of the positive ion flow. This implies the existence of water-filled charged pores through the plasmalemma and tonoplast of the Nitella cell. The pores available for ion movement are far too few to account for the high water permeability of the cell membranes, and the possible extra turgor pressures which might be produced by an electro-osmotic flow through these pores could not be more than 10−4 atmosphere.

Positive Column Formation in Ion Sources Employing Surface Ionization

In order to gain some insight in the problem of space charge neutralization of ion beams we make use of a previous calculation to discuss the unidimensional flow of positive ions in a tube where both ions and electrons issue from an emitter surface but only ion current is collected by means of an accelerating electrode. The resulting ion flow can be neutralized to varying degrees by the electrons present. We find that even in the absence of collisional interactions the potential distribution in the tube may closely resemble the electrode sheath and positive column structure familiar to glow discharges. This is the situation to be expected whenever the ion current has values far in excess of that predicted from a zero degree of neutralization space charge limited theory.

Two-Carrier Space-Charge-Limited Current in a Trap-Free Insulator

The space-charge-limited electrical behavior of a trap-free insulator containing both mobile electrons and holes is investigated under the approximation of neglecting diffusion current. When both contacts are ohmic, the dependence of two-carrier space-charge-limited current on voltage and interelectrode spacing remains the same as in the one-carrier case; i.e., the current varies as the square of the voltage and as the inverse cube of the spacing. For given voltage and spacing, however, the two-carrier current is usually orders of magnitude larger than the one-carrier current. This is in striking contrast to the case of space-charge-limited flow of electrons and positive ions in a vacuum. The two-carrier current in an insulator can be reduced to the size of the one-carrier current either by making one contact highly blocking or by reducing the mobility of one of the carriers to zero. While the first approach yields in detail the well-known one-carrier case, the second approach differs from the one-carrier case in that no net excess space charge can be injected into the insulator. The intermediate case of a slightly blocking contact shows that, while the current may be carried mainly by one sign of carrier, the net space charge of the insulator can have the opposite sign. When both contacts are blocking, the current saturates for sufficiently high voltages. The current is smaller than the saturation value by an amount proportional to the cube of the interelectrode spacing and the inverse square of the voltage. Near such saturation, the space charge is negligible and the electric field is uniform across the insulator. It is plausible that many of the above results will also be true for an insulator with traps.

The Positive Column in a Longitudinal Magnetic Field

A magnetic field parallel to the axis of the positive column of a discharge is known to impart a rotary component on the radial motion of electrons. This reduces the flow of both positive ions and electrons to the wall, causing a corresponding decrease in the radial and longitudinal components of the electric field and in the electron temperature. It is shown here that under certain conditions Langmuir probes can be applied to explore a plasma in a magnetic field. A new method of measuring the concentration of charges is used: first relative values of ion concentration are obtained from the ion saturation currents in various magnetic fields. This result together with the electron saturation currents in zero magnetic field gives absolute values of concentration of charges in a magnetic field. Such measurements have been carried out in helium at low pressures. The effect of the magnetic field on the radial distribution of charges, the rate of ionization, the electric field, the electron temperature, the average energy loss in a collision, and the radial potential distribution is discussed. It is concluded that in zero magnetic field the Langmuir theory of free ion fall describes best the properties of the discharge plasma whereas in a magnetic field of sufficient strength Schottky's theory of ambipolar diffusion applies. When the gas becomes highly ionized, the effect of the partial pressure of the electron gas may become important so that finally both the longitudinal component of electric field and the electron temperature should become independent of the magnetic field. Previous observations of a low pressure positive column in caesium at high current densities seem to support this view.

Electron emission from langmuir probes and from the cathode of the glow discharge through gases

The current to the cathode of a direct current discharge through gases is made up of a flow of positive ions to the cathode surface together with a flow of electrons away from the electrode. It is of importance in the theory of the discharge to obtain an estimate of the relative proportions of the current carried by electrons and positive ions. Owing to the very small ionising efficiency of positive ions with the energies present in the discharge, it is necessary that the electron current should arise largely from the action of the positive ions on the surface of the cathode, and it is generally assumed that the electron emission should increase with the energy with which the ions strike the electrode. This paper presents the results of measurements made to determine the positive ion and electron currents at the surface of a simplified form of cathode, a negatively charged Langmuir probe in the positive column of an arc discharge through various gases. The theory of the cold probe developed originally by Langmuir and Mott-Smith assumes that the electron emission is zero, but it will be shown that evidence of electron emission is given both by observation of the current-voltage characteristic of such a probe and by measurement of the heating effect of the positive ions. The results will then be discussed from the point of view of the glow discharge.