Space Charge Density Research Articles

Ion Flow Field Calculation Considering the Influence of Water Drops and Wind Flow

In this work, an effective calculation method of the ion flow field that considers the impact of wind flow and water drops is presented. To be explicit, the nominal electric field is solved by the charge simulation method (CSM) whilst the space charge density is calculated adopting a second order upwind finite volume method. In addition, a method that determines the roughness factor of a conductor surface is used to enhance calculation accuracy. The influence of various properties of the water drops exerting on an ion flow field is analyzed. Eventually, a practical experiment is conducted to verify the calculated result, and the effectiveness and reliability of this method are then proved via comparing calculated and measured results.

COULOMB LIMIT FOR ACCELERATING CURRENT IN A LINAC ACCELERATING CHANNEL BASED ON COMBINED RF FOCUSING

Ion beam propagation stability in an accelerating channel based on combined RF-focusing (CRFF) at high spacecharge density is investigated. It is demonstrated that in case of grouped beam acceleration, the Coulomb current limitation for both the proton and the heavy ion beam is higher in case of a CRFF-based channel than for an initial part of any modern accelerator based on RFQ focusing.

Dielectric characterisation of epoxy nanocomposite with barium titanate fillers

High permittivity materials are currently in use for mitigation of electrical stress in high-voltage apparatus and energy storage systems. In this work, epoxy-based high permittivity nanocomposites with Barium titanate (BaTiO3) nanofillers are considered, for the purpose of stress mitigation. Uniform dispersion of the fillers in the polymer up to 10% by volume is achieved. Apart from the use of as-received fillers, the effect of using surface-functionalised nanoparticles (with 3-glycidoxypropyltrimethoxy-silane) before use is also investigated. The nanocomposite is characterised in terms of its complex permittivity, DC conductivity, short-term AC breakdown strength and space charge accumulation, to gauge its suitability for use in high-voltage insulation. Complex permittivity is measured using broadband dielectric spectroscopy over a broad frequency range of 1 mHz to 1 MHz. DC conductivity is studied from polarisation–depolarisation current measurements. Short-term AC breakdown strength tests are performed at power frequency (50 Hz). Space charge density along the sample thickness is obtained using pulsed electro-acoustic technique. A computational case-study is presented to show the feasibility of using the high permittivity nanocomposite for electric stress control in high-voltage equipment (viz., at mounting flanges of 69 kV bushings).

Investigation on space charge and charge trap characteristics of Al–epoxy nanocomposites

Optimised quantity of nano-aluminium (Al) filler mixed with epoxy resin using percolation threshold criteria was adapted to fabricate nanocomposite with desired space-charge and charge-trap properties. Surface potential variation due to charge deposition on to the insulating material under transient voltages was investigated, and it showed the improved decay rate with reduced trap depth for the nanocomposites with nanofillers addition in the range between 0.5 and 5 wt%. Threshold electric field calculated based on space-charge density variation by pulsed electroacoustic measurement was found to be between 7–9 kV/mm and maximum electric field calculated through polarity reversal test is about 9 kV/mm, for 5 wt% (for the optimised value of nanofiller) epoxy nano-Al composites.

Investigation on space charge and charge trap characteristics of gamma‐irradiated epoxy micro–nano composites

Epoxy nano–micro composite specimen prepared with micro silica and ion trapping nanoparticle, by shear mixing process, was exposed to gamma radiation and its performance for space charge and charge trap characteristics were analysed. The threshold for space charge accumulation of epoxy nanocomposites reduces and rate of space charge accumulation increases with an increase in dosage of gamma irradiation. The average growth of space charge density during poling and charge decay rate during depoling are relatively higher for gamma-irradiated specimens than the virgin specimen. The initial surface potential has a marginal reduction with increase in the dosage of gamma radiation, but the surface potential decay rate has increased significantly. Trap distribution characteristics indicate more number of shallow traps and increase in charge mobility after irradiation. The relative permittivity and loss tangent of the specimens have high impact due to gamma irradiation. The activation energy calculated from DC conductivity by Arrhenius law reduces with increment in radiation dose. Laser-induced breakdown spectroscopy reflected no change in elemental composition with gamma-irradiated specimen. The variation in plasma temperature and ion line to atomic line intensity ratio with dosage of gamma radiation have direct correlation to the Vickers hardness number of the specimens.

Kinetic model of an inverted sheath in a bounded plasma system

A one-dimensional, kinetic model of inverted sheath formation in a plasma system bounded by two infinitely large planar electrodes (the source and the collector) has been developed for the first time. It is assumed that ions and electrons are injected into the system from the source with half-Maxwellian distributions, and emitted electrons are also injected from the collector with a half-Maxwellian distribution. It is assumed that the potential increases monotonically from the source to the collector. Consequently, the distribution functions of ions, electrons, and emitted electrons anywhere in the system can be written as functions of the potential. Zero and first moments of the distribution functions give particle densities and fluxes. From these, the floating condition for the collector is derived and the Poisson equation is written. The first integrals of the Poisson equation give the conditions for the electric field at the source and at the collector. The model consists of five basic equations: (1) collector floating condition, (2) neutrality condition at the inflection point of the potential, (3) source electric field condition, (4) collector electric field condition, and (5) Poisson equation. The model contains nine parameters. Five of them are plasma parameters: (1) ion mass μ, (2) ion temperature τ, (3) ion source strength α, (4) temperature of emitted electrons σ, and (5) emission coefficient ε. Then there are two potentials, (1) floating potential of the collector ΨC and potential at the inflection point ΨP and (2) electric fields, (1) electric field at the collector ηC and (2) electric field at the source ηS. If five of them are selected, the other four can be found from the system of equations (1)–(4). Numerical solutions of the Poisson equation give axial profiles of the potential, electric field, and space charge density. The model can be used for parametric analysis of the inverted sheath formation. Usually μ, τ, α, ε, and σ are selected and then ΨC, ΨP, ηC, and ηS are found from the system of equations (1)–(4). This means that the particle densities are selected independently, but the potentials and electric fields are then calculated in a self-consistent way with the selected parameters.

Mathematical Modeling of the Effect of Water Splitting on Ion Transfer in the Depleted Diffusion Layer Near an Ion-Exchange Membrane.

Water splitting (WS) and electroconvection (EC) are the main phenomena affecting ion transfer through ion-exchange membranes in intensive current regimes of electrodialysis. While EC enhances ion transport, WS, in most cases, is an undesirable effect reducing current efficiency and causing precipitation of sparingly soluble compounds. A mathematical description of the transfer of salt ions and H+ (OH−) ions generated in WS is presented. The model is based on the Nernst–Planck and Poisson equations; it takes into account deviation from local electroneutrality in the depleted diffusion boundary layer (DBL). The current transported by water ions is given as a parameter. Numerical and semi-analytical solutions are developed. The analytical solution is found by dividing the depleted DBL into three zones: the electroneutral region, the extended space charge region (SCR), and the quasi-equilibrium zone near the membrane surface. There is an excellent agreement between two solutions when calculating the concentration of all four ions, electric field, and potential drop across the depleted DBL. The treatment of experimental partial current–voltage curves shows that under the same current density, the surface space charge density at the anion-exchange membrane is lower than that at the cation-exchange membrane. This explains the negative effect of WS, which partially suppresses EC and reduces salt ion transfer. The restrictions of the analytical solution, namely, the local chemical equilibrium assumption, are discussed.

Influence of Thermal Aging on Space Charge Accumulation and Dissipation of Cross-Linked Polyethylene With Different Cross-Linking Agent Contents

This article presents the influence of thermal aging on space charge accumulation and dissipation of cross-linked polyethylene (XLPE) with different cross-linking agent contents. In this article, dicumyl peroxide (DCP) and low-density polyethylene (LDPE) respectively are used as cross-linking agent and basic experimental material. Additionally, an improved PEA method is used to measure space charge densities during voltage application and removal when thermal aging time respectively is 0, 5, 15, 30 and 80 days due to higher accuracy. And experimental results indicate thermal aging can introduce negative charge traps into pure LDPE and make space charges easier to inject and accumulate in pure LDPE and XLPE, but space charge densities in XLPE increase more rapidly as thermal aging time increases, moreover, thermal aging can simultaneously increase densities of deep and shallow charge traps in pure LDPE and XLPE, etc. And we have also explained the change of space charge accumulation and dissipation of pure LDPE and XLPE after thermal aging from the perspective of micro-structure change of pure LDPE and XLPE through the quantitative analysis of average charge densities. Besides, theoretical model of the average charge decay has also been summed up, and this model can perfectly reflect the dissipation of space charges in pure LDPE and XLPE of different thermal aging time.

Effect of Semiconductive Layer on Space Charge Accumulation in XLPE

Long time DC pressure on high voltage cables will lead to the accumulation of space charge in XLPE cables, thus endangering cable insulation. In order to study the effect of the thickness of semiconducting layer on the space charge in XLPE, the space charge in 10kV and 220kV XLPE sample with different thickness of semiconducting layer was measured and compared based on PEA method. Firstly, the samples were pressurized to the specified voltage, then kept this voltage for 30 minutes, then depressurized to 0, and lastly maintained for 90 minutes. The variation of space charge distribution during the pressurized stage was analyzed with the space charge density as the characteristic parameter. The results show that the space charge near the anode and cathode is accumulated by the semi-conductive coating during the period of maintaining pressure; the thicker the semi-conductive layer is, the more obvious the accumulation of space charge is; the longer the time of maintaining pressure, the more space charge accumulates.

Effect of Nonuniform Emission on Miram Curves

Analysis of temperature-limited flow, space-charge-limited flow, and the transition between them using a simple planar diode with a thermionic cathode, in which the cathode surface has spatially nonuniform emission properties, is presented. Our theoretical results, which are derived from a model based on solutions to the Vlasov and Poisson equations, compare well with the results of particle-in-cell simulations. We find that the location and the shape of the knee in the anode current versus temperature characteristic (Miram or “rollover” curve) are significantly affected by non-uniformities in the space-charge density in the A–K gap, but are relatively unaffected by the electron motion parallel to the electrode surfaces. In particular, emission from an actively emitting region is strongly affected by the forces (or lack thereof) exerted by the space-charge of the electrons emitted by their neighbors. Perhaps, most remarkably, we find that the limiting current reaching the anode is approximately given by the classical 1-D Child–Langmuir law, even if a significant fraction of the cathode surface is non-emitting.

Numerical simulation of air flow in needle-to-cylinder electrohydrodynamic device

The paper presents the methodology and results of numerical simulation of the flow induced by corona discharge obtained with open-source environment Multiphysics Object-Oriented Simulation Environment (MOOSE) framework released by Idaho National Laboratory. Coupled system of governing partial differential equations is solved for the values of electric potential φ and space charge density ρq. The solution is used to calculate the spatial distribution of body force acting on the fluid as FEHD = ρq∇φ. The body force is subsequently introduced as a source term in Navier-Stokes equations solved in MOOSE for the value of fluid velocity. The results are presented as velocity profiles at the outlet of the flow generator and integrated to yield volumetric flow rate and the efficiency of the device. Comparison of the results with data available in literature reference for similar configuration shows satisfactory agreement.

Effect of measurement frequency on admittance characteristics in Al/p-Si structures with interfacial native oxide layer

Al/p-Si/Al diodes with interfacial native oxide layer were formed. Their frequency induced admittance-voltage measurements were made. The frequency-dependent density distribution of interface states has been determined from the corrected characteristics by considering the series resistance effect which masks the interface trap loss. The majority carrier density corresponding to the depletion and inversion parts of the C-2-V curve, was determined 1.82 x 1014 and 4.48 x 1014 cm-3 at 1000 kHz, respectively. The fact that the carrier density obtained from the inversion part of the plot is higher than that obtained from the depletion part can be related to the increase in the density of negative space charge in the depletion region.The value of was determined as 0.95 eV from the same plot. Interface state density decreased from 4.31 x 1012 eV-1cm-2 at 100 kHz to 7.30 x 1011 eV-1 cm-2 at 1000 kHz, because the interface charges do not follow the ac signal and do not contribute to capacitance values in high frequencies.

Corona onset criterion and surface electric field intensity characterized by space charge density

The accurate evaluation of corona onset criterion in air gap under high voltage has important theoretical and practical significance, but it is calculated and measured inaccurately at present because of the distortion caused by space charge. In this study, space charge density is measured by contactless acoustic device and it can directly determine the corona onset voltage and surface electrical field. The direct detection of space charge can eliminate the distortion of charge when other parameters are used to measure corona onset criterion. The experimental onset voltage in this work are compared with that calculated by Peek's formula, corona current and photon number. Results demonstrate the applicability and accuracy of this method. In the other hand, maximum space charge ratio is adopted to analyze the change rule of surface electrical field. It is found that the surface field strength is not constant after corona occurs and should be lower than the onset electrical field. This study provides a feasible approach to further study the effect of space charge on corona onset criterion in air gap.

The Cooling Effect of Beam Self-Fields on the Photocathode Surface in High Gradient RF Injectors

The intrinsic slice emittance of the emitted electrons on the photocathode surface at each moment during the transient photoemission process depends on the transverse size of the slice and the mean kinetic energy of the electrons within the slice. The latter relies on the surface barrier potentials of the cathode material at a fixed wavelength of the incident light, and is thus significantly influenced by the presence of strong rf and beam self-fields at /close to the cathode surface. This is, in particular, the case in high brightness injectors for modern free electron lasers. In this article, the beam self-fields are determined in a self-consistent approach, based on which improved transverse and temporal emission distributions are obtained. The nonlinear correlations of the intrinsic surface slice emittance within the bunch are shown for multiple bunch charges. A peak to peak variation of the intrinsic surface emittance is estimated as 30% for the highest charge-density case considered in this paper. An overall reduction of the average intrinsic emittance is computed as 10% accordingly. The cooling effect on the cathode surface is enhanced as the local space-charge density rises. The impacts of the cooling effect on downstream beam qualities are demonstrated through particle tracking simulations based on the injector setup at the Photo Injector Test Facility at DESY in Zeuthen (PITZ).

Study on electric field distribution in cylindrical metal silo considering particle segregation

The cylindrical metal silo plays an important role in the industry for storing the product, while the undesired of electrostatic charge on the dielectric powder leads to dangerous electrostatic discharge and threatening human safety and causing a great economic loss. The aim of the study was to investigate the effect of the particle segregation on the electric field in the silo base on the electric field simulation. Three parameters were studied: large particle layer thickness fraction, small particle space charge density (ρS) and large particle space charge density (ρL). The effect of the particle segregation on the electric field in the silo was evaluated through comparing the electric field in the segregation case to that in the uniform case. The results showed that the particle segregation significantly affected the electric field in the silo. The space charge density of the small particle mainly influenced the electric field in the small particle layer, while that of the large particle had a significant effect on the electric field in the large particle layer. The particle segregation not only could increase the electric field strength but also could decrease the electric field, which was dependent on the relative magnitude of the large (|ρL|) and small (|ρS|) particle space charge density. Additionally, the relative growth rate (absolute value) of the maximum electric field strength increased with the increase of |ρL-ρS|.

Calculation of Ion Flow Field of Monopolar Transmission Line in Corona Cage Including the Effect of Wind

In this work, the ion flow field of a monopolar transmission line inside the corona cage of a square cross-section is iteratively calculated concerning the effects of wind. The electric field distribution is solved analytically using the charge simulation method (CSM). Meanwhile, the upwind finite volume method (UFVM) with 2nd order accuracy is presented for the distribution of space charge density. Additionally, a dual mesh grid is established in the calculation domain, the interlaced geometric construction of the mesh assures a quick and effective convergence rate. In the final part, a reduced-scaled experiment is designed to examine the feasibility and accuracy of this approach, electric field and ion current density on the bottom side are measured by field mills and Wilson plates. The data numerically computed fits well with that acquired by measurement.

Space Charge Accumulation and Decay in Dielectric Materials with Dual Discrete Traps

Charge trapping and de-trapping properties can affect space charge accumulation and electric field distortion behavior in polymers. Dielectric materials may contain different types of traps with different energy distributions, and it is of interest to investigate the charge trapping/de-trapping dynamic processes in dielectric materials containing multiple discrete trap centers. In the present work, we analyze the charge trapping/de-trapping dynamics in materials with two discrete traps in two cases where charges are injected continuously or only for a very short period. The time dependent trapped charge densities are obtained by the integration of parts in the case of continuous charge injection. In the case of instantaneous charge injection, we simplify the charge trapping/de-trapping equations and obtain the analytical solutions of trapped charge densities, quasi-free charge density, and effective carrier mobility. The analytical solutions are in good agreement with the numerical results. Then, the space charge dynamics in dielectric materials with two discrete trapping centers are studied by the bipolar charge transport (BCT) model, consisting of charge injection, charge migration, charge trapping, de-trapping, and recombination processes. The BCT outputs show the time evolution of spatial distributions of space charge densities. Moreover, we also achieve the charge densities at the same position in the sample as a function of time by the BCT model. It is found that the DC poling duration can affect the energy distribution of accumulated space charges. In addition, it is found that the coupling dynamic processes will establish a dynamic equilibrium rather than a thermodynamic equilibrium in the dielectric materials.

Interface engineering on ZnO/Au based Schottky junction for enhanced photoresponse of UV detector with TiO2 inserting layer

Metal-oxide semiconductor ZnO shows enormous potential in the field of photoelectric detection. However, the adsorption of H2O/O2 on its surface unavoidably results in degradation in the photoresponse. Herein, Au–TiO2–ZnO (ATZ) ultraviolet (UV) photodetectors were designed to benefit the photosensing performance by weaken the surface state effect through tuning the Schottky junction interface with TiO2 insertion layer. As expect, 97% enhancement of IPh response together with three orders of magnitude decrease of dark current was triggered. This enhancement is owing to the efficiently separation and extraction of photoexcites under the effect of the stronger and expanding built-in field, which is a result of reduction of space charge density in barrier region induced by inhibiting H2O/O2 adsorption at the ZnO/Au interface. These findings here provide a promising method to boost performances of optoelectronic devices by interface engineering and could be extended to other semiconductor devices.

Variation of surface electric field intensity determined by space charge density at different temperatures

Surface electric field (SEF) strength plays an important role in studying corona discharge and is widely utilized in the research of corona loss and ion flow field in transmission line projects. The electrical field invariant theory after corona occurs is widely adopted to describe the SEF, but it is not consistent with the actual corona discharge situation. Thus, how to accurately evaluate the change of SEF is crucial for the design of overhead lines. Space charge caused by corona around the wire surface is closely related to the SEF; therefore, it is feasible to use the space charge to characterize SEF. In this work, a system for space charge density measurement is established in the laboratory to analyze the distribution of space charge under different voltages and temperatures. The validity of the experimental results was verified by finite element method (FEM), and the maximum charge density ratio is proposed to correct the variation formula of SEF. The experimental results show that SEF is not equal to the onset field intensity, and displays a decreasing trend after corona initiates, which can be used to rectify the electrical field invariant theory.

The influence mechanism of semiconductive material on space charge accumulation in HVDC cable accessory

To study the effect of semiconductive (SC) material on space charge accumulation in cable accessories, an accelerated aging of cable samples with cable accessories was carried out in the laboratory. The experimental results show accumulation of space charge in the cable insulation covered by a stress cone, whereas the space charge was negligible in the region wrapped by the accessory-reinforced insulation as well as regions not in contact with the accessory. With increased aging time, the space charge density gradually increased, and the axial distribution gradually extended. A comparative experiment on cable insulation slices was carried out using silicone rubber and EVA SC material as the upper electrodes to determine the cause of the space charge accumulation. The results showed that a large quantity of heteropolar charges accumulated near the silicone rubber SC upper electrode, whereas the charges near the EVA SC electrode were negligible. It was concluded that due to the weak charge conducting ability of the silicone rubber SC material used as the electrode, space charges cannot escape from the stress cone and thus accumulate on the outside of the cable insulation at the stress cone position. This paper shows that the SC material used in cable accessories plays an important role in space charge accumulation.