Space Charge Electric Field Research Articles

Density irregularities, currents, and magnetic fields generated by pulsed local rf heating of a magnetoplasma: Disturbances in rf antenna vicinity

The dynamics of narrow, field-aligned magnetoplasma irregularities is studied, which develop under the action of a short rf pulse. The laboratory experiment is aimed at demonstrating the rapid, so-called “unipolar” plasma transport mode, which is accompanied by excitation of eddy currents, in the case of localized rf heating of plasma electrons. The experimental parameters are chosen in a special way. The size of the heating spot, determined by the diameter of the loop antenna, exceeds the electron gyroradius significantly but is smaller than the ion gyroradius. The rf pulse duration encompasses several electron collision times but is shorter than the gyroperiod of ions. As a result, the electrons, which are strongly magnetized, acquire energy in rf antenna vicinity and can escape the heating region only along the magnetic field B0. In turn, collisionless ions can travel across B0 under the action of space-charge electric fields. For these conditions, redistribution of the plasma occurs with “unipolar” transport coefficients and is accompanied by excitation of electric currents. Weak plasma density disturbances, which are less than 5% of the background, are measured precisely with a microwave resonator probe. Parallel electron currents are obtained from magnetic probe measurements; the ion current across B0 is restored from the density profile modifications in their dynamics. It is shown that the ions traveling across B0 with a velocity about one third of the ion-acoustic velocity can easily close the current loop, which is driven by the parallel motion of heated electrons. This regime of plasma irregularities evolution is discussed in application to previous laboratory measurements, as well as to active ionospheric experiments.

Effect of TiO2 size factor on the electrical properties of polyethylene matrix dielectrics

Abstract In this study, the TiO2 nanoparticles and TiO2 nanofibers were chosen as inorganic fillers to dope into the linear low density polyethylene (LLDPE). The TiO2 nanofibers were prepared by the electrospinning method, the diameter and length of TiO2 nanofibers were about 100 nm–200 nm and 3 μm–4 μm, respectively, and the size of the purchased TiO2 nanoparticles was about 100 nm–200 nm. The microstructure of TiO2 fillers and their composites was analyzed by the X-ray diffraction (XRD) and scanning electron microscopy (SEM). The breakdown field strength and space charge characteristics of the LLDPE/TiO2 nanoparticles and LLDPE/TiO2 nanofiber composites with different sizes of inorganic fillers were investigated. The results showed that the LLDPE/TiO2 nanofiber composites had higher breakdown strength than LLDPE/TiO2 nanoparticle composites at the same doping content. The breakdown strength of 0.5 wt% LLDPE/TiO2 nanofiber composites (237 kV/mm) was higher than that of pure LLDPE (223 kV/mm) and 0.5 wt% LLDPE/TiO2 nanoparticle composites (205 kV/mm). In addition, the space charge accumulation and electric field distortion in the composites were significantly relieved by doping TiO2 nanofibers compared with that of doping TiO2 nanoparticles.

Pattern Transition in a Surface Microdischarge in Atmospheric-Pressure Helium

The surface microdischarge in atmospheric helium is studied for the dynamic evolution of plasma patterns in the glow mode. It is found that there are two distinct patterns in the positive half cycle. This indicates that the influence of space-charge electric field caused by surface deposited charges not only exists in the negative half cycle for the evolution process, but also exists between the half cycles. It is in agreement with the phenomenon which the originating regions of discharge in positive half cycle depend on the number of the current pulse in negative half cycle. The emission pattern of the positive half cycle is dominated by the last strong current pulse in the negative half cycle.

Investigation of photoconductivity and electric field distribution in CZT detectors by time-of-flight (TOF) and charge extraction by linearly increasing voltage (CELIV)

Although cadmium zinc telluride (CZT) is the most widely used room-temperature photoconductor for low flux radiation sensing, utilization of this promising material in high flux applications, like computed tomography is challenging, due to possible dynamic polarization. Indeed, trapping of the X-ray-generated charge may cause space charge build-up and electric field distortion. Combining the conventional time-of-flight and the photo-generated charge extraction by linear increasing voltage (photo-CELIV) techniques, we track changes of the main electrodynamic parameters (such as electric potential and intrinsic electric field) of CZT under various fluxes. In addition, we investigate transport properties of electrons and holes in a wide range of temperatures and electric fields. Multiple trapping was found to govern the hole transport, while electrons conduct electrical current via various scattering mechanisms.

Multidimensional effects on proton acceleration using high-power intense laser pulses

Dimensional effects in particle-in-cell (PIC) simulation of target normal sheath acceleration (TNSA) of protons are considered. As the spatial divergence of the laser-accelerated hot sheath electrons and the resulting space-charge electric field on the target backside depend on the spatial dimension, the maximum energy of the accelerated protons obtained from three-dimensional (3D) simulations is usually much less than that from two-dimensional (2D) simulations. By closely examining the TNSA of protons in 2D and 3D PIC simulations, we deduce an empirical ratio between the maximum proton energies obtained from the 2D and 3D simulations. This ratio may be useful for estimating the maximum proton energy in realistic (3D) TNSA from the results of the corresponding 2D simulation. It is also shown that the scaling law also applies to TNSA from structured targets.

Ball lightning dynamics and stability at moderate ion densities

A general mechanism is presented for the dynamics and structure of ball lightning and for the maintenance of the ball lightning structure for several seconds. Results are obtained using a spherical geometry for air at atmospheric pressure, by solving the continuity equations for electrons, positive ions and negative ions coupled with Poisson’s equation. A lightning strike can generate conditions in the lightning channel with a majority of positive nitrogen ions, and a minority of negative oxygen ions and electrons. The calculations are initiated with electrons included; however, at the moderate ion densities chosen the electrons are rapidly lost to form negative ions, and after 1 µs their influence on the ion dynamics is negligible. Further development after 1 µs is followed using a simpler set of equations involving only positive ions and negative ions, but including ion diffusion. The space-charge electric field generated by the majority positive ions drives them from the centre of the distribution and drives the minority negative ions and electrons towards the centre of the distribution. In the central region the positive and negative ion distributions eventually overlap exactly and their space-charge fields cancel resulting in zero electric field, and the plasma ball formed is quite stable for a number of seconds. The formation of such plasma balls is not critically dependent on the initial diameter of the ion distributions, or the initial density of minority negative ions. The ion densities decrease relatively slowly due to mutual neutralization of positive and negative ions. The radiation from this neutralization process involving positive nitrogen ions and negative oxygen ions is not sufficient to account for the reported luminosity of ball lightning and some other source of luminosity is shown to be required; the plasma ball model used could readily incorporate other ions in order to account for the luminosity and range of colours reported for ball lightning. Additionally, ‘phantom plasma balls’ may well be generated and go unnoticed due to very low luminosity; luminous ball lightning may be the exception. Finally, the mechanism described here may also be active in the dynamics of bead lightning.

Unified Lattice Boltzmann Method for Electric Field–Space Charge Coupled Problems in Complex Geometries and Its Applications to Annular Electroconvection

A lattice Boltzmann method (LBM) is developed to solve the electric field–space charge coupled problems. Instead of solving the macroscopic charge conservation equation and the Poisson's equation for electric potential, two discrete lattice Boltzmann equations are formulated and solved. A nonequilibrium extrapolation scheme is used to treat the boundary conditions with complex geometry. Our method is validated with several test cases for which analytical solutions and/or reference numerical results exit. An attractive feature of this methodology lies in its natural coupling with the LBM for the fluid flow. As a demonstration and also an application, the unipolar injection induced electroconvection of dielectric liquids in annular geometries is considered. The different flow patterns of the concentric and eccentric configurations are highlighted.

Study of Space Charge Dynamics in Oil and Pressboard Composite System under Different Polarity Reversal Voltages

The impacts caused by the polarity reversal of the applied voltage on space charge dynamics and electric field enhancement across an oil gap have been studied in an oil gap and single-layer pressboard composite insulation system. The experimental results validate that the electric field enhancement across the oil gap immediately after the application of the reversed electric field is directly dependent on the number of residual charges within the insulation system at the moment of the application of the reversed stress. Therefore, the electric field enhancement in the system is strongly dependent on the magnitude of the applied electric field, the duration of polarity reversal and the degradation of the mineral oil. An estimation of the enhanced electric field immediately after polarity reversal based on the charge decay characteristics has been proposed and successfully applied to calculate the electric field enhancement in an aged oil sample under 20 kV/mm, which cannot be accurately measured due to the very fast space charge dynamics in the aged oil under a strongly enhanced electric field.

Optically Induced Space-charge and Conductivity Gratings in Wide-bandgap Semiconductors

We report optical excitation of space-charge and conductivity gratings in wide-bandgap semiconductors. The approach is based on the illumination of semiconductor material with an oscillating interference pattern formed of two light waves, one of which is phase modulated with frequency ω. The non-steady-state photocurrent flowing through the short-circuited semiconductor is the measurable quantity in this technique. The alternating current results from the periodic relative shifts of the photoconductivity and space charge electric field gratings which arise in the volume of the crystal under illumination. The experiments are carried out in β-Ga2O3 crystal and the main parameters of the photoinduced carriers are determined.

The role of bulk charge transport processes in electrical tree formation and breakdown mechanisms in epoxy resins

Electrical treeing experiments have been conducted at different temperatures and levels of absorbed moisture in Araldite CY1311 epoxy resin samples above their glass transition temperature, i.e. when the resin was in a flexible state. The fractal dimension of the electrical trees obtained and the rate of tree growth were found to depend on the environmental factors: temperature and humidity. It has also been found that at certain levels of temperature and moisture absorbed in the samples, a transition occurs from electrical treeing degradation to breakdown by thermal runaway. Complementary investigations of the dielectric properties of the same epoxy resin system have revealed that a bulk quasi-dc (QDC) charge transport mechanism takes place above the glass transition temperature, and we show that the characteristic features of the dielectric response are related to the shape of the electrical treeing degradation and the transition to thermal breakdown. This is explained qualitatively through the effect of the bulk QDC charge transport process in modifying the local space charge electric field distribution.

Space charge behaviors of PP/POE/ZnO nanocomposites for HVDC cables

Polypropylene (PP) has been paid much attention due to its high melting point, excellent electrical insulting performance and thermoplastic property, which is potential to replace the XLPE as HVDC cable insulating material. Blending PP with polyolefin elastomer (POE) is an effective way to modify its stiffness and brittleness at room temperature. However, space charge behavior of PP/POE blend, as a great concern under dc stress, is not clear and needs further investigation. To research the space charge behaviors, pure PP, PP/POE blend and its nanocomposites with different contents were prepared. Then, mechanical properties, permittivity constant, breakdown strength, volume resistivity, space charge behaviors and trap level distribution were investigated. The results indicate the addition of POE enhances the mechanical flexibility of PP greatly, and nano-sized ZnO doping has little effect on the mechanical flexibility of PP/POE blend. The nanocomposites show lower permittivity constant, higher breakdown strength and higher volume resistivity than the PP/POE blend. Compared to pure PP, the space charge accumulation and electric field distortion get severe in PP/POE blend. However, by nanoparticles doping, space charges are remarkably suppressed, which is related to the increased trap level density in nanocomposites. It indicates the PP/POE/ZnO nanocomposites have much potential for HVDC cable application, which show high mechanical flexibility as well as excellent electrical performance.

Impulse space charge and dielectric characteristics of an Al2O3 nanoparticle suspension in propylene carbonate using various electrode materials

We tested the impulse breakdown voltage of Al2O3 “nano-modified” propylene carbonate between different electrode materials. At any given concentration, the breakdown voltage was highest with stainless steel electrodes, followed by copper, and then aluminum. The space charge and electric field distributions were measured too. Results show that less space charge was injected by the electrodes, and the electric field was less distorted, than in pure propylene carbonate. However, the hoped-for reduction of the influence of the electrodes did not take place. Substantial differences in the space charge density and electric field distortion remained between the different electrode materials.

TCAD simulation of Low Gain Avalanche Detectors

In the present work, detailed simulation using Technology Computer Aided Design (TCAD) tool, Silvaco for non-irradiated and irradiated LGAD (Low Gain Avalanche Detector) devices has been carried out. The effects of different design parameters and proton irradiation on LGAD operation are discussed in detail. An already published effective two trap bulk damage model is used to simulate the radiation damage without implementing any acceptor removal term. The TCAD simulation for irradiated LGAD devices produce decreasing gain with increasing fluence, similar to the measurement results. The space charge density and electric field distribution are used to illustrate the possible reasons for the degradation of gain of the irradiated LGAD devices.

Erratum [Space charge and electric field in thermally aged multilayer joints model

This paper is concerned with space charge accumulation and electric field distribution at XLPE/EPDM interfaces after thermal aging. XLPE, EPDM and multilayer XLPE/EPDM/XLPE flat samples have been manufactured and aged thermally during 240 and 480 h at 105 °C. Material degradation by oxidation during aging has been assessed by FTIR spectroscopy. The conductivities and permittivities of the samples have been measured before and during ageing. Using the values of conductivity and permittivity values obtained experimentally, the charge density values at the XLPE/EPDM interfaces and the electric field distribution across a multilayer XLPE/EPDM HVDC cable joint model has been calculated for different thermal aging times. The results show that the surface charge density values vary with the ageing time (decrease at aging beginning and then increase) and determine important enhancements of the electric field, in the vicinity of the inner but also of the outer semiconductor. The simultaneous presence of the volume and interfacial charges determines a less increase of the electric field for homocharges and more important for heterocharges, both for unaged and aged samples.

Positive streamer initiation from raindrops in thundercloud fields

AbstractThe threshold field for the electric gas discharge in air is ≈26 kVcm−1atm−1, yet the maximum field measured (from balloons) is ≈3 kVcm−1atm−1. The question of how lightning is stimulated is therefore one of the outstanding problems in atmospheric electricity. According to the popular idea first suggested by Loeb and developed further by Phelps, lightning can be initiated from streamers developed in the enhanced electric field around hydrometeors. In our paper, we prove by numerical simulations that positive streamers are initiated, specifically, around charged water drops. The simulation model includes the kinetics of free electrons, and positive and negative ions, the electron impact ionization and photon ionization of the neutral atmospheric constituents, and the formation of space charge electric fields. Simulations were conducted at air pressure 0.4 atm, typical at thundercloud altitudes, and at different background electric fields, drop sizes, and charges. We show that the avalanche‐to‐streamer transition is possible near drops carrying 63–485 pC in thundercloud fields with intensity of 10 kVcm−1atm−1 and 15 kVcm−1atm−1 for drops sizes of 1 mm and 0.5 mm, respectively. Thus, the electric field required for the streamer formation is larger than the measured thunderstorm fields. Therefore, the results of simulations suggest that second mechanisms must operate to amplify the local field. Such mechanisms could be electric field space variations via collective effects of many hydrometeors or runaway breakdown.

Sheet Electron Beam Transport in a Metamaterial-Loaded Waveguide Under the Uniform Magnetic Focusing

We have theoretically investigated the transport of a sheet electron beam focused by a uniform axial magnetic field in a metamaterial-loaded waveguide. A theoretical model has been proposed to analyze the sheet electron beam transport. In this model, we have used CST simulations to obtain the mean potential on the metamaterial as the boundary condition. According to the boundary condition, the space-charge electric field and the beam curling have been theoretically analyzed. It is found that the beam curling can be greatly reduced by decreasing the spacing between the metamaterial and the sheet electron beam. This model is verified using the CST simulations. This paper can pave the way for developing brand new metamaterial-based vacuum electron devices with a sheet electron beam.

Space charge and electric field in thermally aged multilayer joints model

This paper is concerned with space charge accumulation and electric field distribution at XLPE/EPDM interfaces after thermal aging. XLPE, EPDM and multilayer XLPE/EPDM/XLPE flat samples have been manufactured and aged thermally during 240 and 480 h at 105 °C. Material degradation by oxidation during aging has been assessed by FTIR spectroscopy. The conductivities and permittivities of the samples have been measured before and during ageing. Using the values of conductivity and permittivity values obtained experimentally, the charge density values at the XLPE/EPDM interfaces and the electric field distribution across a multilayer XLPE/EPDM HVDC cable joint model has been calculated for different thermal aging times. The results show that the surface charge density values vary with the ageing time (decrease at aging beginning and then increase) and determine important enhancements of the electric field, in the vicinity of the inner but also of the outer semiconductor. The simultaneous presence of the volume and interfacial charges determines a less increase of the electric field for homocharges and more important for heterocharges, both for unaged and aged samples.

A numerical study of ponderomotive ion acceleration in a dense plasma driven by a circularly polarized high-intensity laser beam normally incident on thin foils

AbstractWe use an Eulerian Vlasov code to study the efficient ion acceleration in dense targets by the ponderomotive force of a high-intensity circularly polarized laser beam, normally incident on a dense plasma. The code solves the one-dimensional relativistic Vlasov–Maxwell equations for both electrons and ions. We follow in details the mechanism of formation and evolution of a double-layer structure, where electrons are pushed steadily in the forward direction by the ponderomotive force of the laser beam, trapping an ion population, while an induced space charge electric field pulls ions behind them, forming a double-layer structure supported by the strong ponderomotive pressure of the intense laser beam. We consider the case of a high-density deuterium plasma with n/ncr = 100, where ncr is the critical density. Three cases are studied, by varying the width of the dense target and the intensity of the laser beam (with the normalized vector potential or quiver momentum a0 = 50 and a0 = 100), to follow the physical processes involved in the ion acceleration and the final formation of a neutral plasma jet ejected from the back of the target. We follow the transition from a situation where the laser pulse radiation pressure is acting on the double layer in the target, to a situation where below a given thickness a fraction of the laser energy is transmitted through the target. The absence of noise in the Eulerian Vlasov code allows us to follow accurately the evolution of the phase-space structures of the distribution functions.

High power microwave damage mechanism on high electron mobility transistor

In this paper, the damage process and mechanism of the typical high electron mobility transistor by injecting high power microwave signals are studied by simulation and experiment methods. By using the device simulator software Sentaurus-TCAD, a typical two-dimensional electro-thermal model of high electron mobility transistor is established with considering the high-field saturation mobility, Shockley-Read-Hall generation-recombination and avalanche breakdown. The simulation is carried out by injecting the 14.9 GHz, 20 V equivalent voltage signals into the gate electrode. Then, the distributions of the space charge density, electric field, current density and temperature with time are analyzed. During the positive half cycle, a conduction channel appears beneath the gate electrode near the source side within device. It is found that the electric field is extremely strong and the current density is very large. Therefore, the temperature increases mainly occurs beneath the gate electrode near the source side. During the negative half cycle, because of the concentration of the large number of carriers induced by avalanche breakdown, the electric field is stronger than that in the positive half cycle. But the current density is lower than that in positive half cycle. Therefore, the increase of temperature is dominated by the electric field. With the effects of both strong electric field and high current density, the temperature of the transistor rises in the whole signal cycle. In addition, temperature in the positive half-cycle rises faster than that in the negative half-cycle.Furthermore, the peak temperature appears at the location beneath gate electrode near the source side because the electric field and current density are strongest in this area. When the temperature within the device is higher than 750 K, intrinsic breakdown occurs in GaAs material, so the heating process becomes quicker. With the temperature increases, the GaAs reaches its melting point, and the device fails permanently. Furthermore, taking the original phase of 0 and for example, we discuss the influences of different original phases on damage process. It is shown that when original phase is zero, the temperature increase rate is faster, and the burn-out time is shorter.Failure analysis of high electron mobility transistor devices damaged by microwaves is carried out with scanning electron microscope, and the simulation results are well consistent with the experimental results. The conclusion may provide guidance for studying high power microwave defense of low noise amplifier and rugged design of high electron mobility transistor in fabrication technology.

Numerical study of a plane poiseuille channel flow of a dielectric liquid subjected to unipolar injection

In this paper, the interaction between a plane Poiseuille channel flow and an electroconvective movement induced by the electric field is numerically investigated. A flow is generated by an inlet parabolic profile in a rectangular duct. Space charges are injected in the flow through a metallic electrode placed on one of the channel walls and brought to a given potential. Transient numerical simulations have been carried out to investigate the structure of the flow. The entire set of the coupled Navier-Stokes and EHD equations is solved using an efficient finite volume technique. The behavior of the flow subjected to an applied voltage between the two electrodes is analyzed and time evolution of the charge density distributions is presented. The interaction between the convective movement induced by space charge injection and electric field and the mainstream flow, emphasizes the appearance of periodic transverse traveling waves convected in the channel. The dynamic of the flow through the variation of the ionic mobility parameter M is investigated. For a given Reynolds number and ionoc mobility parameter M it exists a threshold value Tc of the instability parameters T above which the transverse traveling rolls appear or not. When T is increased, different flow regimes have been highlighted starting from a complete steady state up to a fully unsteady electro-plumes flow configuration.