Space-charge Field Research Articles

Ultimate transverse power of pulsed low-voltage gyrotron beam

Low operating voltage is highly attractive for medium-power millimeter-wave gyrotrons since it can reduce their size and cost, increase their safety, and, thus, improve usability for applications. However, at low voltages, the voltage depression caused by DC space-charge fields significantly limits the electron current and transverse power in the beam. Moreover, this current limitation is more pronounced for a beam with a higher pitch factor. As a result, for a given anode voltage, there is a pitch factor at which the transverse beam power in the gyrotron cavity is the maximum. This ultimate transverse power is found analytically in the non-relativistic approximation. Such a power is reached when the pitch factor calculated without taking into account voltage depression is only 0.82; voltage depression decreases the axial electron velocities, thus, increasing the actual pitch factor value in the cavity up to 1.4. As a result of this effect, high power and high efficiency cannot be obtained simultaneously in a low-voltage gyrotron. Using particle-in-cell simulations, two variants of low-voltage (5 kV) gyrotrons have been designed, namely, a device with higher power and an optimal pitch factor of 0.82 in the cavity and a device with a high pitch factor and high efficiency, but lower power.

Very long wave infrared quantum cascade detector with a twin-well absorption region

We report a very long wave (14 μm) infrared quantum cascade detector based on a twin-well coupled absorption region operating at temperatures up to 130 K. By introducing two coupled absorption quantum wells that have the same width, the absorption strength and responsivity of the detector are increased relative to the single-well design. At 77 K, we observe a responsivity of 4.06 mA/W at zero bias, which is 4.27 times that of the single-well counterpart. The responsivity is further optimized for reverse bias operation, so that the obstruction of space charge field to electron transport is compensated. The photocurrent reaches a maximum value at 77 K for an applied bias of −1.3 V, and responsivity as high as 23.76 mA/W, which is 5.85 times that under zero bias, is obtained.

Space Charge Measurement of 23-mm-Thick XLPE Cable at Ambient and High Temperatures

Space charge measurement was performed on a 23-mm-thick cross-linked polyethylene (XLPE) cable at ambient and high temperatures using a newly developed high-sensitivity measurement device and a correction method for cables under temperature gradient. The space charge and electric field distribution were obtained with a spatial resolution of 0.5 mm (2%) at ambient and high temperatures. In addition, we propose an approach to obtain the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> system’s impulse responses to maintain correction accuracy for long-term measurement. The reference waveforms were measured before and after 3 h of testing voltage application. It showed that sufficient time before the measurement is necessary for accurate reference waveforms. To obtain a suitable time for the measurement, the method using cross correlation was proposed.

Self-consistent formation and steady-state characterization of trapped high-energy electron clouds in the presence of a neutral gas background

This study considers the self-consistent formation and dynamics of electron clouds interacting with a background neutral gas through elastic and inelastic (ionization) collisions in coaxial geometries similar to gyrotron electron guns. These clouds remain axially trapped as the result of crossed magnetic field lines and electric equipotential lines creating potential wells similar to those used in Penning traps. Contrary to standard Penning traps, in this study, we consider a strong externally applied radial electric field which is of the same order as that of the space-charge field. In particular, the combination of coaxial geometry, strong radial electric fields, and electron collisions with the residual neutral gas (RNG) present in the chamber induce non-negligible radial particle transport and ionization. In this paper, the dynamics of the cloud density and currents resulting from electron–neutral collisions are studied using a 2D3V particle-in-cell code. Simulation results and parametric scans are hereby presented. Finally, a fluid model is derived to explain and predict the cloud peak density and peak radial current depending on the externally applied electric and magnetic fields, and on the RNG pressure.

Modified Charge Transport Model Under High-Frequency Unipolar Square Wave Voltage

As the main insulation material of the power electronic transformer (PET), polyimide (PI) is easy to accumulate space charge under the continuous high-frequency pulse voltage, which will threaten the operation of PET and cause electric field distortion. At present, the microscopic mechanism of charge transport under high-frequency voltage is unclear. Numerical simulation is an effective method to explore the transport characteristics of space charge. The simulation of charge transport under DC voltage is mainly based on the bipolar charge transport (BCT) model. However, the charge transport behaviors under high-frequency pulse voltage cannot be simulated by the traditional models accurately. Given this, based on the BCT model, the High-Frequency Voltage Charge Transport (HFCT) model is proposed, considering nonlinear charge mobility of the polymer and the effect of the frequency. Finally, the space charge and electric field distribution under square pulse wave voltages with different rising times are simulated based on the HVCT model, concluding that the reduction of rising time will promote the accumulation of space charge. In view of this, the HVCT model is expected to optimize the insulation design of PET and provide a reference for the dynamic simulation of space charge.

Impedance computation of cryogenic vacuum chambers using boundary element method

A two-dimensional boundary element method for calculating the impedance of infinitely long cryogenic vacuum chambers of general cross section is presented. The formulation is based on combining Kirchhoff's boundary integral representation of an electromagnetic field with the surface impedance boundary condition for the anomalous skin effect, which can occur in metals at cryogenic temperature. As a result, the electromagnetic field in the chamber is expressed as a superposition of the direct and indirect space charge fields and resistive-wall wakefield. This feature allows us to compute the corresponding three impedance contributions separately. This method does not assume a specific transverse charge density such as uniform and Gaussian distributions as well as the ultrarelativistic approximation. A technique for computing the impedances and the form factors in the method is also described. The presented method is applied to circular, rectangular, elliptical, and racetrack-type vacuum chambers. The geometric effect of the cryogenic vacuum chamber cross section on the resistive-wall impedance is shown. The effect of beam velocity is demonstrated for the racetrack-type cryogenic vacuum chamber.

Hole-electron competition investigated by holographic recording in Bi12TiO20 crystal under applied electric field

Hole-electron competition was induced in an undoped Bi12TiO20 crystal using coherent pre-illumination at 532 nm. Photorefractive recording and erasure at 639.7 nm under the action of external electric field E0 were used to theoretically and experimentally investigate the competition within the sample band-gap. The good agreement between theory and experimental data allowed to obtain, in both regimes without and with pre-illumination, the saturation space-charge fields Eq and holes and electrons effective donors concentrations (ND)eff's. The phase shift between the electronic gratings and the hole grating was also computed and revealed a competition behavior between the applied electric field effect of increasing the characteristic lengths of electrons and holes moving in opposite directions in the conduction and valence bands, respectively, and the intensity of decreasing such lengths. The computed holes donor density indicated a strong contribution of the coherent pre-illumination to the hole-electron competition mechanism.

Beam emittance growth due to the strong space-charge field at low energy of a high-intensity ion linac and its mitigation using an octupole magnetic field

Abstract In the low-energy region of a high-intensity ion linac, a strong space-charge field causes a rapid beam emittance growth over a short distance of only a few meters. The beam emittance growth leads to beam loss and machine activation, raising a serious issue for regular maintenance of the accelerator component and beam power ramp-up. We studied the mechanism of beam emittance growth due to the space-charge field based on three-dimensional particle-tracking simulation and theoretical considerations. Numerical simulations of the high-intensity H− (negative hydrogen) linac at the Japan Proton Accelerator Research Complex shows that the nonlinear terms in the space-charge field directly cause beam emittance growth and beam halo formation. We also propose a method to mitigate the beam emittance growth by using an octupole magnetic field, which arises as one of the nonlinear terms in the space-charge field. By applying this method in the simulation we have succeeded in mitigating the beam emittance growth.

Investigation on effect of semiconducting screen on space charge behaviour of polypropylene‐based polymers for HVDC cables

High voltage direct current (HVDC) power transmission cable is critical for realising sustainability through renewable energy revolution. Eco-friendly thermoplastic polypropylene (PP)-based polymers/nanocomposites are regarded as promising candidates for replacing current thermoset crosslinked polyethylene (XLPE) cables. As an essential component of the extruded HVDC cable for improving conductor/insulation interface and suppressing charge injection to insulation at high DC electrical stresses, developing semiconducting (SC) screens that are compatible with PP-based insulation is of similar importance but has not been well studied yet. This work aims at designing PP-based semiconducting screens and investigating space charge behaviours of SC/PP/SC sandwich specimen to unfold the effect of semiconducting materials, bonding methods, applied DC electric field, and temperature on charge injection, accumulation, transportation, and dissipation in PP-based insulation. Although conventional thermal, mechanical, and low field electrical characterisations demonstrated that all of the developed semiconducting materials meet the performance criteria of commercial semiconducting materials, their space charge and local electric field distribution varied significantly at high DC fields. Compared with the traditional non-bonded configuration used at lab-scale, charge injection was enhanced in hot-pressed SC/PP/SC samples with tightly bonded interfaces, which better reflects the real situation in extruded cables. High temperature further intensified charge injections. Besides, our results also revealed that high temperature and electric field strongly influence charge mobilities and consequently their distribution and local electric field in PP-based insulations.

Emittance growth caused by phase modulation in bunched electron beam cooling

The demand for electron coolers operating at much higher energies than previously achieved necessitates the use of radio-frequency (RF) fields for electron acceleration resulting in the bunched electron beams. Differences in the time structure of ion and electron bunches and variations in their relative timing significantly affect the cooling process. Experiments on ion beam cooling with a short electron bunch have been carried out in the CSRm storage ring at IMP (Lanzhou). Longitudinal painting by the electron bunch phase modulation is found leading to a significant ion beam loss. We show that the space charge field variations of the bunched electron beam play a major role in the ion beam emittance growth while cooling. In this paper, we study the dependency of the ion beam emittance growth on the electron beam parameters and present results of numerical simulation which support experimental observations.

Validation of the smooth step model by particle-in-cell/Monte Carlo collisions simulations

Bounded plasmas are characterized by a rapid but smooth transition from quasi-neutrality in the volume to electron depletion close to the electrodes and chamber walls. The thin non-neutral region, the boundary sheath, comprises only a small fraction of the discharge domain but controls much of its macroscopic behavior. Insights into the properties of the sheath and its relation to the plasma are of high practical and theoretical interest. The recently proposed smooth step model (SSM) provides a closed analytical expression for the electric field in a planar, radio-frequency modulated sheath. It represents (i) the space charge field in the depletion zone, (ii) the generalized Ohmic and ambipolar field in the quasi-neutral zone, and (iii) a smooth interpolation for the transition in between. This investigation compares the SSM with the predictions of a more fundamental particle-in-cell/Monte Carlo collisions simulation and finds good quantitative agreement when the assumed length and time scale requirements are met. A second simulation case illustrates that the model remains applicable even when the assumptions are only marginally fulfilled.

Theory and Practice of Creating a Two Energy Outputs Magnetron

This paper presents the methodology for 3-D simulation and design of the magnetron with two energy outputs. The principal functionality of the modified magnetron, both theoretically and experimentally, has been determined. The central distinguishing feature of this magnetron (for example, unlike the conventional magnetron with one output) is the availability of second energy output in the anode block. The mathematical models of both the resonant anode block and the electron-wave interaction are described. The dispersion characteristics of the anode block for cavities various geometries are given. It is supposed that forming the total RF field in the interaction space results from the interference of RF fields excited severally in the resonant anode block consisting of the small and large cavities (resonators). For PIC-simulation of electron-wave interaction in the dual-output magnetron, the non-linear system of equations is stated as a self-consistent system containing the equation of motion (for electron stream), the equation of excitation (for RF field), and Poisson’s equation for calculating the space-charge field. The fundamental feature of the self-consistent system of equations is a new algorithm for determining the Coulomb interaction forces. The implementation of the mathematical model made it possible not only to gain new knowledge about the physical processes in the magnetron but also to determine its output characteristics. On operating frequency of ∼ 13.34 GHz, at an anode voltage of 495 V, a magnetic field of ∼ 0.25 T, and with air cooling of the magnetron, there were obtained the following limiting values: the RF output power of ∼ 14.6 W and the power conversion efficiency of ∼ 40.8%. The use of the second energy output allowed extending the magnetron’s functionality and implementing the modes of the frequency tuning (adjunction) and its stabilization. The simulation results are in good agreement with the experiment. To the 100th anniversary of magnetron

Modeling the saturation of the multipactor effect in a dielectric-loaded parallel-plate waveguide

The aim of this paper is to establish a numerical simulation model for the multipactor effect in a partially dielectric-loaded parallel-plate waveguide, with a focus on the investigation of multipactor saturation mechanisms for different dielectric materials with different secondary emission yield (SEY) properties. An electrostatic method involving the radio-frequency fields, space charge fields, and the dynamics of charge accumulation on the dielectric surface and solutions for electrostatic fields are proposed. The evolution of the electron number, accumulated charge, and secondary electron multiplication rate for different input voltages and SEY properties of the dielectric materials are studied using numerical calculations. The results show that two physical multipactor phenomena occur in a dielectric-loaded parallel-plate waveguide: a self-sustaining phenomenon, which means that the electron population reaches a saturation level, and a self-extinguishing phenomenon. The latter can be divided into two cases: in one, the number of electrons undergoes a process of multiplication, saturation, and reduction, and in the other, the number of electrons disappears after their population reaches a maximum. Furthermore, a multipactor susceptibility diagram for SEY curves of different dielectric materials is constructed. The results show that the multipactor effect is suppressed when the maximum of the SEY curve is less than 1.3.

Influence of Polarity Reversal Period and Temperature Gradient on Space Charge Evolution and Electric Field Distribution in HVDC Extruded Cable

During the operation of HVDC extruded cables, voltage polarity reversal (VPR) is considered one of the most severe conditions for cable insulation. In this paper, a bipolar charge transport model developed for cylindrical geometry is improved by introducing ionic carriers from impurity dissociation for the simulation of space charge and electric field in an HVDC extruded cable with thick polymeric insulation under VPR, and the construction of the geometric model is based on a practical 160 kV DC polymeric cable. The influence of polarity reversal period (PRP) and temperature gradient (TG), formed by the load current flowing through the conductor, on the space charge evolution and the electric field distribution are investigated. The mechanisms of the charge dynamics and the field distortion affected by the PRP and the TG are also discussed. The results show that under TG, the maximum transient field appears near the interface between the conductor shield and insulation in the early stage of complete VPR. In addition, the longer the PRP, the more serious the maximum transient field distortion. Moreover, an increase in TG intensifies the maximum field distortion under steady and transient states due to the enhancement of heterocharge accumulation.

Effect of the DC Electric Field Polarity and Amplitude on Partial Discharge of Oil–paper Insulated under Wedge-shaped Electrodes

As an important piece of equipment of ultrahigh-voltage direct current, the valve side of a converter transformer contains a large dc component prone to partial discharge (PD) under the action of a slightly uneven electric field, which can be simulated using a wedge electrode. To study the PD characteristics of oil&#x2013;paper insulation under a wedge-shaped electrode, a 2-D axisymmetric simulation model was established based on the bipolar-charge transport theory and the fluid dynamics drift-diffusion theory. The microscopic discharge images at nanosecond scale under different dc voltage polarities were calculated, and the change process of the space charge and electric field intensity distribution was obtained. The effects of the voltage amplitude, polarity, and dielectric constant on PD characteristics are discussed. The simulation results show that the discharge pattern and velocity exhibit a notable polarity effect. This polarity effect is related to the adsorption of the cellulose hydroxyl group to negative charge, which leads to the amplitude of the homopolar charge accumulated in negative polarity being greater than that accumulated in positive polarity under the same applied voltage intensity so that the homopolar charge has a greater barrier to the discharge. This type of adsorption is positively correlated with the difference of dielectric constant between the paperboard and insulating oil. The results of this study are of great theoretical significance for elucidating the dc discharge mechanism under wedge electrodes.

ADVANCED COMPUTING TOPOLOGICAL AND DYNAMICAL INVARIANTS OF RELATIVISTIC BACKWARD-WAVE TUBE TIME SERIES IN CHAOTIC AND HYPERCHAOTIC REGIMES

The advanced results of computing the dynamical and topological invariants (correlation dimensions values, embedding, Kaplan-York dimensions, Lyapunov’s exponents, Kolmogorov entropy etc) of the dynamics time series of the relativistic backward-wave tube with accounting for dissipation and space charge field and other effects are presented for chaotic and hyperchaotic regimes. It is solved a system of equations for unidimensional relativistic electron phase and field unidimensional complex amplitude. The data obtained make more exact earlier presented preliminary data for dynamical and topological invariants of the relativistic backward-wave tube dynamics in chaotic regimes and allow to describe a scenario of transition to chaos in temporal dynamics.

Optical Induction and Erasure of Ferroelectric Domains in Tetragonal PMN‐38PT Crystals

AbstractPT‐relaxor ferroelectrics exhibit excellent piezoelectric and quadratic nonlinear optical properties, making them prominent candidates for realization of phononic and nonlinear photonic crystals which rely on spatially patterned ferroelectric domains. However, formation of domain patterns, especially in three dimensions, has been challenging. This paper presents the first experimental demonstration of localized ferroelectric domains and their 2D and 3D patterns inside 0.62Pb(Mg1/3Nb2/3)O3‐0.38PbTiO3 (PMN‐38PT) single‐domain crystals engineered with focused near‐infrared femtosecond laser pulses. Two types of domains are optically induced. Primary domains are formed in the focal volume of the beam, and secondary domains appearing at higher laser power, in the shape of hollow cylindrical structures, are formed around the beam. A physical mechanism of optical domain inversion involving thermoelectric and space charge fields is proposed. This study contributes to a deeper understanding of domain formation and structuring in PMN‐PT relaxor‐based ferroelectrics, paving the way to integrate electromechanical, acoustic, and nonlinear optical effects in a single crystal.

Numerical calculation of transient electric field considering space charge in oil-paper insulation of converter transformers

PurposeThe purpose of this paper is to develop a numerical simulation method based on the transient upstream finite element method (FEM) and Schottky emission theory to reveal the distribution characteristics of space charge in oil-paper insulation.Design/methodology/approachThe main insulation medium of the converter transformer in high voltage direct current transmission is oil-paper insulation. However, the influence of space charge is difficult to be fully considered in the insulation design and simulation of converter transformers. To reveal the influence characteristics of the space charge, this paper proposes a numerical simulation method based on Schottky emission theory and the transient upstream FEM. This method considers the influence of factors, such as carrier mobility, carrier recombination coefficient, trap capture coefficient and diffusion coefficient on the basis of multi-physics field coupling calculation of the electric field and fluid field.FindingsA numerical simulation method considering multiple charge states is proposed for the space charge problem in oil-paper insulation. Meanwhile, a space charge measurement platform based on the electrostatic capacitance probe method for oil-paper insulation structure is built, and the effectiveness and accuracy of the numerical simulation method is verified.Originality/valueA variety of models are calculated and analyzed by the numerical simulation method in this paper, and the distribution characteristics of the space charge and total electric field in oil-paper insulation medium with single-layer, polarity reversal of plate voltage and double-layer are obtained. The research results of this paper have the guiding significance for the engineering application of oil-paper insulation and the optimal design of converter transformer insulation.

Simulation of Cold Atmospheric Plasma Generated by Floating-Electrode Dielectric Barrier Pulsed Discharge Used for the Cancer Cell Necrosis

A numerical simulation of a pulsed floating electrode dielectric barrier discharge (FE-DBD) at atmospheric pressure, used for melanoma cancer cell therapy, is performed using a plasma model in COMSOL Multiphysics software. Distributions of electron density, space charge, and electric field are presented at different instants of the pulsed argon discharge. Significant results related to the characteristics of the plasma device used, the inter-electrodes distance, and the power supply are obtained to improve the efficiency of FE-DBD apparatus for melanoma cancer cell treatment. The FE-DBD presents a higher sensitivity to short pulse durations, related to the accumulated charge over the dielectric barrier around the powered electrode. At higher applied voltage, more energy is injected into the discharge channel and an increase in electron density and electric consumed power is noted. Anticancer activity provided by the FE-DBD plasma is improved using a small interelectrode distance with a high electron emission coefficient and a high dielectric constant with a small dielectric thickness, allowing higher electron density, generating reactive species responsible for the apoptosis of tumor cells.

Scanning ultrafast electron microscopy reveals photovoltage dynamics at a deeply buried p−Si/SiO2 interface

The understanding and control of charge carrier interactions with defects at buried insulator/semiconductor interfaces is essential for achieving optimum performance in modern electronics. Here, we report on the use of scanning ultrafast electron microscopy (SUEM) to remotely probe the dynamics of excited carriers at a Si surface buried below a thick thermal oxide. Our measurements illustrate a previously unidentified SUEM contrast mechanism, whereby optical modulation of the space-charge field in the semiconductor modulates the electric field in the thick oxide, thus affecting its secondary electron yield. By analyzing the SUEM contrast as a function of time and laser fluence we demonstrate the diffusion mediated capture of excited carriers by interfacial traps.