Electron Space Charge Research Articles

Optical properties of planar immersion objectives in electron impact ion sources

The ion optical properties of an immersion ion source have been calculated in the presence of electron space charge. The ion source was an electron impact ionizer with planar geometry and a crossed beam configuration, assuming a well defined atomic beam. We present the focal properties and the third-order aberration coefficients, as a function of both the electron beam current and the ion extraction geometry. The image position is relatively unchanged by the electron beam or extraction geometry; however, there are significant changes to the magnification. The aberrations indicate that it is possible to preserve a spatial resolution in a typical imaging ion source of over a 6 mm field of vision. .

Thickness dependence of the poling and current–voltage characteristics of paint films made up of lead zirconate titanate ceramic powder and epoxy resin

A specially prepared paint made up of lead zirconate titanate (PZT) ceramic powder and epoxy resin was coated on an aluminum plate and was cured at room temperature, thus forming the paint film of 25–300 μm thickness with a PZT volume fraction of 53%. The paint film was then poled at room temperature, and the poling behavior was determined by measuring the piezoelectric activity as a function of poling field. The poling behavior shows that the piezoelectric activity obtained at a given poling field increases with an increase in the film thickness from 25 to 300 μm. The current–voltage characteristic of the paint film, on the other hand, shows that the increase in the film thickness leads not only to an increase in the magnitude of the current density at a given electric field but also to an increase in the critical electric field at which the transition from the ohmic to space-charge-limited conduction takes place. This fact indicates that the amount of the space charge of electrons injected into the paint film decreases as the film thickness increases. Furthermore, comparison of the current–voltage characteristic of the paint film with that of a pure epoxy film reveals that the space charge is accumulated largely at the interface between the PZT and epoxy phases in the paint film. On the basis of this finding, a model is developed for the poling behavior of the paint film by taking into account a possible effect of the space-charge accumulation and a broad distribution of the electric field in the PZT phase. This model is shown to give an excellent fit to the experimental data of the piezoelectric activity obtained here as a function of poling field and film thickness.

Electron Transport and Frozen Concentration Gradients in a Mixed Valent Viologen Molten Salt

This paper shows that electrolytically generated crossed concentration gradients of viologen (2+) and viologen (1+) in films of its mixed valent molten salt on interdigitated array electrodes can be thermally and stably frozen in place in the interelectrode gaps. A study is presented of properties of the gradient-containing films, as compared to films that are non-mixed valent (V{sup 2+}) or that are mixed valent but lack concentration gradients. Comparisons of charge transport measurements show that the 2-5 {mu}m wide concentration gradients are persistent in the molten salt at lowered temperatures, where ionic motions are quenched relative to electron hopping as shown by differences in conductivity between mixed valent and non-mixed valent films as the temperature is lowered from +50 to -70{degree}C. Differences in the magnitude and shape of high-field current-voltage curves taken from mixed valent nongradient and V{sup 2+}/V{sup +} gradient-containing samples are interpreted with an electron-hopping model that includes a parameter for kinetic dispersity. Differences between the capacitance of the mixed valent and non-mixed valent phase of the viologen molten salt are consistent with the formation of an electronic space charge at the metal/redox conductor interface. 18 refs., 8 figs., 1 tab.

Heating and failure of niobium tip cathodes due to a high-density pulsed field electron emission currenta)

This article presents the investigative results of pulsed threshold current densities of niobium tip cathodes at temperatures ranging from 4.2 to 300 K. It has been shown that the superconducting-to-normal phase state transition of the cathode has no effect on pulsed threshold current densities. Estimations have been made for the factors limiting the current from a superconducting emitter. The experiments have shown that the cooling of the cathode from 300 to 4.2 K changes the dynamics of cathode heating considerably and leads to a steady increase in the threshold current densities with the delay time range of 10−9–10−6 s. Numerical analysis of thermal processes in the emitter has shown that the above effects are not due to the difference between the electron and the phonon temperatures (the electron–phonon relaxation time does not exceed 10−9 s), but to the more intense heat transfer processes and the reduction of Joule heating, with the cooling of the cathode. It has been shown that the space charge of emitted electrons produces a considerable effect on the temperature and current density distribution in the tip apex. Investigations have shown that the mechanism of the tip cathode failure depends on the delay time range. The simple relationship for estimation of average pulsed threshold emission current density of the tip cathode for a delay time range of 10−9–10−7 s is presented.

The operation characteristics of axial emission ionization gauges similar to the construction of Chen et al

The operation characteristics of some axial-emission gauges similar to the construction of Chen and Suen were studied in the high vacuum range. Electron space charge effects arose in the case of high cathode potentials, yielding a strong dependence of the sensitivity on the emission current. To avoid this dependence it is necessary to operate the gauge with low cathode potentials or to increase the voltage between cathode and grid. Both the cathode potential and the anode voltage influence the sensitivity, which passes a maximum with increasing cathode voltage. The position of this maximum is shifted by the value of the anode voltage. Both effects are discussed.

Plasma turbulence enhanced current collection: Results from the plasma motor generator electrodynamic tether flight

The plasma motor generator (PMG) experiment, launched June 26, 1993, was a tethered system of two identical plasma contactors connected via a 500‐m conducting tether. The experiment was designed to demonstrate the ability of plasma contactors to provide a low‐impedance connection between a spacecraft and the ionosphere for both the electron emission and collection. The flight data indicate that plasma contactors enhance electron collection and emission by both neutralizing the electron space charge and scattering electrons across the geomagnetic field lines. Up to a 0.3 A steady current flowed along the tether in a circuit completed through the ionosphere. An analytical model for plasma contactor interaction with a background plasma which incorporates electron scattering by plasma waves is compared with the flight data. Good agreement between the model and the data is achieved for an effective scattering frequency equal to one twentieth of the local plasma contactor plasma frequency.

Unified HBT base push-out and base-collector capacitance model

The presence of electron space-charge in the collector region of a transistor has the effect of offsetting the collector doping thereby giving a reduced effective doping concentration. This effect can modify the base-collector junction capacitance and for high current densities will give rise to base push-out. This paper describes the implementation of a unified junction capacitance and push-out model for the microwave HBT biased in the forward active mode. The threshold functions for pushout and full collector depletion are given and their significance for device and circuit design is discussed. An approximate description of the effect of pushout on the HBT forward time constant is described which accounts for the substantially increased diffusion constant in the pushout region.

Potential Profiles in an Electron-Beam-Excited Plasma

Potential profiles are measured in an electron-beam-excited plasma (EBEP) system with the high-current low-energy electron beam. It is found that there is a potential step around the anode. The space potential at the center of the plasma is more negative than that at the edge plasma by the space charge of beam electrons. It is shown that the potential dip of the radial profile changes into a hill by increasing the beam energy. The radial profile becomes flat with increase of gas pressure.

Bistability effect in laser-transistor resonant-tunneling structure

The analysis of the operation of a laser-bipolar transistor incorporating a resonant-tunneling structure (RTLBT) is presented. An analytical model of RTLBT is proposed and used. It is shown that RTLBT can exhibit a bistable operation in a certain range of collector voltage ( V ̄ C < V C < V ̃ C) where the collector and the base currents, and the stimulated photon emission intensity are double-valued functions of the collector voltage. The physical nature of the bistability effect is associated with a mobile electron space charge in the collector that influences the electron transparency of the resonant-tunneling structure and the pumping conditions of the laser active region.

Causes of nonstability and nonreproducibility in widely used Bayard–Alpert ionization gauges

This is the second in a set of related reports on (1) nonstable behavior of widely used ionization gauges, (2) causes of nonstability and nonreproducibility in widely used Bayard–Alpert (BA) gauges, and (3) a stable and reproducible BA gauge design with approximately a tenfold improvement in both stability of gauge calibration and reproducibility gauge-to-gauge compared to older ionization gauge designs. Computer simulations of electron and ion trajectories are utilized to illustrate a variety of causes of changes in BA gauge sensitivity with time of operation and in reproducibility gauge-to-gauge. Both computer simulation and experiment confirm that the ion production and collection rates in a BA gauge are far more sensitive to small changes in electrode geometry and potentials than has been commonly realized. Any change in geometry or potential of any electrode or insulator in the vicinity of electron or ion trajectories will affect gauge sensitivity as will any change in electron or ion space charge. Changes in the distribution of emission along the cathode will affect sensitivity if large axial field components are present in the anode volume. If these physical changes occur from gauge to gauge, the sensitivity of the gauges will differ, and this will cause them to give different pressure indications when exposed to the same actual gas density. If the changes occur with time of operation in a given gauge, the gauge sensitivity will change with use and the pressure indication will be in error even if the gauge has been carefully calibrated. This new understanding has been used to develop a significantly more stable and reproducible BA gauge.

Three-dimensional particle-in-cell simulations of applied-B ion diodes

The three-dimensional particle-in-cell code quicksilver [Seidel et al., Computational Physics, edited by A. Tenner (World Scientific, Singapore, 1991), p. 475] has been used to study applied-B ion diodes. The impedance behavior of the diode in these simulations is in good agreement with both analytic theory and experiments at peak power. The simulations also demonstrate the existence of electromagnetic instabilities which induce divergence in the ion beam. Early in time, there is an instability at high frequency relative to the ion transit time τi, and the resulting beam divergence is low. However, later in time, the system makes a transition to an instability with a frequency close to 1/τi, and the ion beam divergence rises to an unacceptably high value. The transition is associated with the build-up of electron space charge in the diode, and the resulting increase in the beam current density enhancement (J/JCL). Using different schemes to inhibit the electron evolution, the transition has both been postponed and permanently eliminated, resulting in Li+1 ion beams with a sustained divergence of ∼10 mrad at an energy of ∼10 MeV.

Controlled, Periodic Domain Formation in Strontium Barium Niobate Induced by Volume Holograms

We demonstrate an all-optical technique of controlled, periodic ferroelectric domain reversal induced by electronic space charge fields associated with photorefractive volume holograms. This technique may be used to selectively fix and update individual holograms in photorefractive crystals.

Electron emission from metals under fast multicharged ion bombardment

Two models of secondary electron emission (the thermospike model and the cascade model) are considered in the paper. The models are shown to need correction for electron emission blocking by the electron space charge near the surface. Auger processes in cascades must also be taken into account.

Total pressure measurement down to 10−12 Pa without electron stimulated desorption ion errors

In order to measure a total pressure as low as 10−12 Pa, the avoidance of electron stimulated desorption (ESD) ion errors is more important than the x-ray limit, because even if we can contrive to reduce ESD ions in a gauge, we do not know what fraction the ESD ion error makes in the measured pressure value. This is the largest defect in the reliability of the measurements. Therefore, the development of the ion spectroscopy gauge which avoids ESD ion errors, was recalled with a historical review of the limiting processes in a hot-cathode gauge. The most advanced ion spectroscopy gauge was constructed with a fine platinum alloy spherical grid ion source, immersed in a copper block with stainless steel flanges. Gas phase ions were effectively separated from ESD ions by the actions of electron space charge in the grid and a hemispherical 180° ion energy analyzer. The x-ray limit of the advanced gauge was approximately 2.5×10−13 Pa, and the outgassing rate of 2×10−9 Pa ℓ/s (about 1/100 that of an ordinary nude Bayard–Alpert gauge) was achieved.

Electron space charge effects in ion sources for residual gas analysis

An electron impact ionization source suitable for residual gas analysis (RGA) with a quadrupole mass spectrometer has been studied both experimentally and by computer simulation. The electronic space charge is shown to play a role in limiting the extracted current from RGA ion sources and the simulation treats this aspect of the problem self-consistently. Under certain source conditions, the ion extraction efficiency is observed to decrease for electron currents above approximately 1 mA-well below the current at which space charge limiting of the electron emission is expected. The observed effects are well reproduced by the simulation. It is shown that whereas the electron trajectories are only weakly perturbed by space charge effects, the loss of ionization efficiency can be attributed to drastic changes in the ion trajectories.

X-Ray studies on single crystal films of BaTiO3

Abstract Epitaxial crystals of BaTiO3 have been grown by activated reactive evaporation. The ultra-thin crystal less than 100 A thickness shows an elongation along the c-direction. The elongation of the crystal is discussed in terms of an electronic space charge at the surface layer in the ferroelectric semiconductor.

Study of the performance of a group of quadrupole mass spectrometers

Results are presented from a series of experiments that were designed to characterise quadrupole analyzers. Three different analyzers have been studied. The first, typical of the small quadrupoles often used for residual gas analysis, has rods 50 mm long and 6.25 mm in diameter. The second uses rods of similar diameter, but 125 mm long. The third uses rods of diameter 12 mm and length 230 mm. Simple ion sources were used. All had cylindrical ionization regions with external filaments, in a configuration similar to an extractor ion gauge. All were operated under similar conditions. The sensitivity and linearity of the instrument is found to depend in a complex way on the operating conditions of the ion source. The observed behavior is explained qualitatively by the presence of both electronic and ionic space charge in the ion source.

The bistability effect in a bipolar transistor with resonant-tunnelling collector structure

A bipolar transistor incorporating a resonant-tunnelling structure in its collector is investigated using both analytical and ensemble Monte Carlo particle models. A resonant-tunnelling transistor of this type has two current states for a certain range of collector-based voltage. The double-valued characteristics and bistability effect are connected with electron space charge in the collector region.

Ion spectroscopy gauge: Total pressure measurements down to 10−12 Pa with discrimination against electron-stimulated-desorption ions

The hot-cathode magnetron gauge by Lafferty (which presently holds the possibility of making the lowest-pressure measurements, 10−15 Pa) and the improved Helmer gauge by Benvenuti and Hauer (which carried out the lowest-pressure measurements to date, 2.5×10−12 Pa) have been examined regarding the measurement limit due to electron-stimulated-desorption (ESD) ions. On the basis of this examination, a new hot-cathode total pressure gauge, called an ion spectroscopy gauge, which substantially avoids errors caused by ESD ions down to 10−12 Pa, has been developed. Discrimination against ESD ions is accomplished by combining three technical elements: (i) a spherical grid ion source; (ii) a spherical 180° ion energy analyzer; and (iii) burying the ion source in an aluminum alloy flange. The ion source plays an important role in the generation of a large kinetic energy difference between gas-phase ions and ESD ions, by using electron space charge. The 180° analyzer, which has a high energy resolution, promotes ion focusing and tends to reduce the x-ray limit. The burying technique reduces the outgassing rate of the hot-cathode gauge. Performance measurements of the new gauge have been carried out in a new aluminum extreme high vacuum system. The effective separation of gas-phase ions from ESD ions for two variants of the gauge, one with a molybdenum grid and one with a platinum grid, was demonstrated using oxygen dosing. The performance of the platinum-grid gauge was confirmed down to 2×10−11 Pa, essentially free from errors caused by ESD ions. The x-ray limits and sensitivities of the new gauges were: below 2×10−12 Pa with a sensitivity of 4.5×10−4 A/Pa for the molybdenum grid and 5.6×10−12 Pa with a sensitivity of 1.8×10−4 A/Pa for the platinum grid.

Bifurcation of resonant tunnelling current due to the accumulated electrons in a well

The effect of electron space charges in a potential well on the transmission coefficient of resonant tunnelling of electrons through a one-dimensional double barrier is investigated using the standard transfer-matrix method. It is shown that the bistable states in the transmission coefficient appear due to the positive feedback loop to incident electron space charges in the well. The growth rate of the small perturbation in the unstable region between the bistable states is also investigated numerically.