Equipotential Regions Research Articles

Nanoscale Visualization of a Photoinduced Plasmonic Near-Field in a Single Nanowire by Free Electrons.

Swift electrons can undergo inelastic interactions not only with electrons but also with near-fields, which may result in an energy loss or gain. Developments in photon-induced near-field electron microscopy (PINEM) enable direct imaging of the plasmon near-field distribution with nanometer resolution. Here, we report an analysis of the surface plasmonic near-field structure based on PINEM observations of silver nanowires. Single-photon order-selected electron images revealed the wavelike and banded structure of electric equipotential regions for a confined near-field integral associated with typical absorption of photon quanta (nℏω). Multimodal plasmon oscillations and second-harmonic generation were simultaneously observed, and the polarization dependence of plasmon wavelength and symmetry properties were analyzed. Based on advanced imaging techniques, our work has implications for future studies of the localized-field structures at interfaces and visualization of novel phenomena in nanostructures, nanosensors, and plasmonic devices.

A synchroscan streak tube with high deflection sensitivity

A synchroscan streak tube with high spatiotemporal resolution and high deflection sensitivity is proposed, which contains several innovation designs. Some measures are taken to improve the imaging performances of the streak tube. Firstly, in order to obtain a high deflection sensitivity, the difference in voltage between the photocathode and anode and the length of the equipotential region of the streak tube are reduced as much as possible. Secondly, by introducing a hyperfine grid behind the cathode, reasonably designing the voltages applied to the six-electrode electrostatic focusing system, and moving the electron beam crossing point to the entrance of the deflection plates, the temporal dispersion and the temporal distortion of the streak tube are reduced, and the spatiotemporal resolution of the streak tube is improved. Besides, the streak tube is technically analyzed by tracking the temporal and spatial distribution of electrons under an operating voltage of 7000 V with the aid of computer simulation technology (CST) software. The results show that the deflection sensitivity is 125 mm/kV, the physical temporal resolution is better than 1.83 ps @MTF = 10%, and the static spatial resolution on the photocathode is better than 38 lp/mm @MTF = 10% over the effective photocathode area with a size of 10 mm × 4 mm. By applying a synchronous scanning voltage with a repetition frequency of 250 MHz to the deflection electrode, the results show that the dynamic spatial resolution of the streak tube is better than 16 lp/mm, the limit of the dynamic temporal resolution is 1.39 ps, and two rectangular electron pulses with a size of 10 mm × 20 μm and an interval of 2.3 ps emitted from the photocathode can be well resolved by the streak tube. In addition, the experimental measurements are conducted with a streak tube developed in our laboratory. The results demonstrate that the photocathode of the streak tube can work in the entire visible light region, and the response in the short wavelength region is significantly better than that in the long wavelength region. The static spatial resolution of this streak tube is 40 lp/mm in the center of the photocathode. The temporal resolution of this streak tube is 5.55 ps measured under a synchronous scanning voltage with a repetition frequency of 75 MHz.

Study of a SiC Trench MOSFET Edge-Termination Structure with a Bottom Protection Well for a High Breakdown Voltage

A novel edge-termination structure for a SiC trench metal–oxide semiconductor field-effect transistor (MOSFET) power device is proposed. The key feature of the proposed structure is a periodically formed SiC trench with a bottom protection well (BPW) implantation region. The trench can be filled with oxide or gate materials. Indeed, it has almost the same cross-sectional structure as the active region of a SiC trench MOSFET. Therefore, there is little or no additional process loads. A conventional floating field ring (FFR) structure utilizes the spreading of the electric field in the periodically depleted surface region formed between a heavily doped equipotential region. On the other hand, in the trenched ring structure, an additional quasi-equipotential region is provided by the BPW region, which enables deeper and wider field-spreading profiles, and less field crowding at the edge region. The two-dimensional Technology Computer Aided Design (2D-TCAD) simulation results show that the proposed trenched ring-edge termination structures have an improved breakdown voltage compared to the conventional floating field ring structure.

The Mushroom: A half‐sky energetic ion and electron detector

AbstractWe present a time‐of‐flight mass spectrometer design for the measurement of ions in the ~30 keV to 10 MeV range for protons (up to ~40 MeV and ~150 MeV for He and heavy ions, respectively) and ~30 keV to 1 MeV range for electrons, covering half of the sky with 80 apertures. The instrument, known as the “Mushroom,” owing to its shape, solves the field of view problem for magnetospheric and heliospheric missions that employ three‐axis stabilized spacecraft, yet still require extended angular coverage; the Mushroom is also compatible with a spinning spacecraft. The most important new feature of the Mushroom is the method through which uncomplicated electrostatic optics and clean position sensing combine to permit many apertures to fit into a compact, low‐mass sensor head (or wedge), several of which (ideally eight) compose a full instrument. Most of the sensor head's volume is an empty, equipotential region, resulting in the modest 250 g mass of each 10‐aperture wedge. The Mushroom is capable of separating ion species across most of its energy range and angular field of view. For example, separation of the neighboring 3He and 4He isotopes is excellent; the full width at half maximum mass resolution has been measured to be 0.24 amu to 0.32 amu, respectively. Converting this to a Gaussian width σm in mass m, this represents a σm/m mass resolution better than 0.04. This separation is highly desirable for the flight program for which the first Mushroom was built, the Solar Probe Plus mission. More generally, we estimate the mass resolution to be σm/m ≈ 0.1, but this is energy, mass, and angularly dependent. We also discuss the solid‐state detector stack capability, which extends the energy range of protons and helium, with composition, to ~100 MeV.

Temporal distortion analysis of the streak tube

Streak cameras applied to inertial confinement fusion research and flashless imaging lidar require large working areas. However, the larger the working area, the bigger the temporal distortion is. And the temporal distortion has a great influence on the detecting precision of the streak camera, resulting in an image distortion on the screen. Yet previous streak camera design work emphasized shorter time resolution and higher special resolution with paying less attention to the temporal distortion extent. Key factors that may affect the temporal distortion are thoroughly analyzed in this paper. We calculate the electric field of a small-size streak tube with the aid of the Computer Simulation Technology Particle Studio software which is a three-dimensional electromagnetic simulation software based on finite integration technology. Axial electric field distributions at different distances to the axis of the small-size streak tube are displayed. The electron trajectories launched from different points on photocathode of the streak tube are tracked through interpolating pre-calculated electromagnetic field to the particle position. It is known that curved photocathode can reduce the temporal distortion, so we calculate the temporal distortions of streak tubes whose radii of curvature of the photocathode are 30 mm, 35 mm, 40 mm, 45 mm, 50 mm, and 55 mm respectively to ascertain how the curvature influences the temporal distortion. The results show that the temporal distortion is mainly produced in the photocathode-to-deflector region, and it is negligible in the equipotential region. Also, bigger radius of curvature of the photocathode leads to a positive temporal distortion, and smaller one leads to a negative temporal distortion. And the absolute value of the temporal distortion increases with the increase of the slit length. The small-size streak tube whose radius of curvature of the photocathode is 40 mm owns the smallest temporal distortion. We also calculate the temporal distortions of electrons launched from the different positions of the photocathode with different initial energies, and the initial energy has little influence on the temporal distortion. To sum up, the dominating factor that produces the temporal distortion is the curvature of the photocathode. The slit image under a ramp sweeping voltage on screen is curved due to the temporal distortion. And the bigger the temporal distortion, the greater the curvature of the slit image is. Besides, a linear relation between the temporal distortion and deflection of the slit image is displayed. The spatial resolutions of the streak tubes with the radii of curvature of the photocathode 30 mm, 40 mm, 50 mm are calculated respectively. And the small-size streak tube whose radius of curvature of the photocathode is 30 mm has the highest spatial resolution. The radius of curvature of the streak tube photocathode should be carefully selected according to actual requirements for the streak camera. Through the analysis we provide a significant guidance for streak tube design.

Analytic study of electron flows with a virtual cathode

The formation of a virtual cathode is studied with one-dimensional analytic self-consistent dynamic models describing the pulsed injection of an electron beam into equipotential regions: half-space or a planeparallel gap.

Modified Poisson solver for the simulation of the silicon–oxide interface in semiconductor detectors

We present a modified Poisson solver for depleted semiconductor detectors that takes into account the effects of possible accumulation of mobile charge at the silicon–oxide interfaces. The solver is based on a physical model that closely approximates the correct boundary condition at the silicon–oxide interface. The model assumes that the silicon–oxide interface is divided into an equipotential region, where the electron layer is located, and a fully depleted region. The actual extension and potential of the electron layer region are approximated with the desired accuracy by an iterative procedure. This model has been implemented in 2- and 3-D Poisson solvers. The comparison with a 2-D drift–diffusion simulator has shown the accuracy of the proposed method. The modified Poisson solver has shown to be useful in giving accurate solutions to 3-D design problems at high CPU speed.

Analytic modeling of the conductance in quantum point contacts with large bias.

We have developed a model for calculating differential conductance that accounts for the effects observed at high source-drain voltage. The model uses a linear potential drop over a saddle-point potential. If the region of the potential drop is wide enough, only that region is contributing to the transmission and a simple expression is obtained. The temperature is assumed to increase linearly with the source-drain voltage and a model for an equipotential region in the saddle-point potential corresponding to the channel width is made. The result from the calculations of the conductance with this model is in good agreement with experimental results.

Far‐field potentials in cylindrical and rectangular volume conductors

The occurrence of a transient dipole is one method of producing a far-field potential. This investigation qualitatively defines the characteristics of the near-field and far-field electrical potentials produced by a transient dipole in both cylindrical and rectangular volume conductors. Most body segments of electrophysiologic interest such as arms, legs, thorax, and neck are roughly cylindrical in shape. A centrally located dipole generator produces a nonzero equipotential region which is found to occur along the cylindrical wall at a distance from the dipole of approximately 1.4 times the cylinder's radius and 1.9 times the cylinder's radius for the center of the cylinder. This distance to the equi-potential zone along the surface wall expands but remains less than 3.0 times the cylindrical radius when the dipole is eccentrically placed. The magnitude of the equipotential region resulting from an asymmetrically placed dipole remains identical to that when the dipole is centrally located. This behavior is found to be very similar in rectangular shallow conducting volumes that model a longitudinal slice of the cylinder, thus allowing a simple experimental model of the cylinder to be utilized. Amplitudes of the equipotential region are inversely proportional to the cylindrical or rectangular volume's cross-sectional area at the location of dipolar imbalance. This study predicts that referential electrode montages, when placed at 3.0 times the radius or greater from a dipolar axially aligned far-field generator in cylindrical homogeneous volume conductors, will record only equipotential far-field effects.

The behaviour of mise-à-la-masse anomalies near a vertical contact

The present paper deals with the effect on mise-à-la-masse anomalies of a vertical contact near conductors. It is assumed that the conductivity contrast of the conductors with the surrounding medium (the ratio of conductivities) is not less than 100, and the conductors thus form equipotential regions. The problem is formulated with the aid of a Fredholm's integral equation of the first kind developed for the potential, which is solved numerically by means of a method of subareas. Green function for the simple source potential in the vertical contact structure is the kernel of the integral equation. The vertical contact effects on mise-à-la-masse anomalies are illustrated with the aid of several numerical examples. The contact affects the shape of the mise-à-la-masse anomalies, and their intensity in particular. It is also common for the anomaly peaks to be shifted from over the body that has current earthing.

The principle of the invariance of brightness in electron optics

The principle that (excluding irreversible effects) the brightness (or the current per unit area per unit solid angle) of an electron beam is invariant over any equipotential region of space is established by simple considerations based on classical thermodynamics, and its value confirmed as

The measurement of Donnan potentials with cellulose and aqueous solutions

When a membrane, such as a sheet or a strip of cellulose, separates one solution of an electrolyte from another of different concentration, an electrical potential difference is established across the membrane, and this potential difference can be accurately measured by suitable experimental means. When potassium chloride is used as the electrolyte, the diffusion potential within the membrane is very small, and the latter may be regarded not as an inert sieve but as a macromolecular structure, the electrochemical properties of which are due to the presence of non-diffusible ionogenic groups, such as carboxylic acid groups. The dissociation, in an aqueous medium, of a portion of these groups causes the membrane to acquire a negative potential with respect to each of the contiguous solutions. A method is described whereby the single potential difference between the membrane and a solution of potassium chloride in which it is immersed may be determined. The experimental results are in agreement with the view that the membrane may be regarded as an aqueous region of uniform potential, and that this potential is dependent on a Donnan distribution of the mobile ions. The degree of dissociation of the non-diffusible ionogenic groups depends on the concentration of neutral electrolyte solution in contact with the membrane, since, on account of the Donnan effect, the pH of the solution imbibed by the membrane differs from that of its external environment. Values for the dissociation constant of the carboxylic acid groups in Cellophane have been obtained from the present results. When hydrochloric acid is the electrolyte, and in salt solutions of low pH, the dissociation of the carboxylic acid groups is almost entirely repressed, and the Donnan potential vanishes. The results obtained with materials of artificially enhanced carboxyl content are also in agreement with expectation, and indicate that the Donnan potential is due solely to the presence of carboxylic acid groups. The variation of the potential with the water imbibition of the cellulose lends further support to the concept of the swollen membrane as an equipotential region.