Presence Of Space Charge Research Articles

Two surface multipactor with non-sinusoidal RF fields

Two-surface multipactor with a Gaussian-type waveform of rf electric fields is investigated by employing Monte Carlo simulations and 3D electromagnetic particle-in-cell simulations. The effects of the full width at half maximum (FWHM) of the Gaussian profile on multipactor susceptibility and the time dependent dynamics are studied. The threshold peak rf voltage, as well as the threshold time-averaged rf power per unit area for multipactor development, increases with a Gaussian-type electric field compared to that with a sinusoidal electric field. The threshold peak rf voltage and rf power for multipactor susceptibility increase as the FWHM of the Gaussian profile decreases. Compared to sinusoidal RF operation, the expansion of multipactor susceptibility bands is observed. In the presence of space charge, a high initial seed current density can shrink the multipactor susceptibility bands. The effect of space charge on multipactor susceptibility decreases as the FWHM of the Gaussian profile decreases. Decreasing the FWHM of the Gaussian electric field can reduce the electron population corresponding to the strength of the multipactor at saturation, at fixed time-averaged input power.

Investigation of wide-range trap energy spectra of typical aerospace polymer dielectric materials by using photo-stimulated discharge method

The presence of space charge in polymer insulation materials has emerged as a critical constraint in the advancement of polymer materials for high-voltage electrical devices. Trap properties play a pivotal role in governing the storage and transport of space charges. Thus, precise characterization of the distribution of charge trap energy spectra in polymer dielectric materials holds immense scientific significance for enhancing the properties and applications of such materials. In response, the author develops a platform utilizing the photo-stimulated discharge (PSD) method to measure the trap energy spectra of polymer dielectric materials. This platform allows for accurate, intuitive, and effective characterization of trap parameters across a broader range while preserving the original features of trap energy levels. Experimental results have shown that the laser light source employed in the platform does not induce photodegradation or photodamage to the tested samples. The validity of the PSD measurement system has been confirmed under diverse experimental conditions, yielding trap spectra and pertinent parameters for four representative polymer materials: polyimide, polyester, polyvinylidene fluoride, and polyethylene. This work presents a scientifically effective approach to characterize trap energy spectra in polymer dielectrics, providing valuable insights into the characterization of such spectra.

Effects of nanosecond impulse and step excitation in pulsed electro acoustic measurements on signals for space charge determination in high-voltage electrical insulation

The presence of space charges in high-voltage insulation can influence the operational stresses of the dielectric materials; the topic is present in various, even distant domains such as power grids or transportation segments that are represented by electromobility, rail, more-electric aircraft, and space systems. In this article, the effects of nanosecond impulse and step excitation in the pulsed electro-acoustic measurements (PEA) of space charges in high-voltage electrical insulation have been investigated. Various parameters of a pulser (such as impulse width, rise time, and sensor thickness) have been investigated by using a designed programable pulser. The investigations have been performed on a polyethylene sample in one- or double-layer configurations. In the latter, an interlayer sprinkled charge was analyzed. The experimental results have shown the main PEA effect during the transition phase of the applied voltage. It was observed that the pulse transition has a critical importance for stimulating space-charge vibrations and generating elastic waves. Hence, the slope’s rise and fall times are crucial in pulser design. The pulser width also has an impact on a signal’s magnitude as well as on its dispersion and attenuation. The correspondence between a pulse and step excitation was presented. Some advantages are attributed to the step-based excitation in terms of fewer wave reflections and interferences as well as the avoidance of the wavelet cancelation effect for a very narrow pulse width. Hence, step excitation can result in the easier postprocessing and interpretation of individual peaks in the acquired PEA signal. Another advantage may be recognizing the space-charge polarity instead of the net result. Important recommendations refer to the tuning of pulser timing settings with respect to signal attenuation (especially in thicker specimens) and the adequate adjustment of the sensor’s thickness. The presented analysis and experimental results extend the insight into PEA measurement – especially with a focus on diagnostic applications.

High-intensity effects on longitudinal bunch merging in hadron synchrotrons

Longitudinal rf manipulation schemes have been widely employed for achieving various beam experiments and applications in heavy ion or proton (hadron) synchrotrons. For high-intensity hadron beams, longitudinal space charge and cavity beam loading play a key role in beam intensity limitations since they may cause beam oscillations and longitudinal emittance growth. Efficient schemes to compress such intense bunched beams and minimize the emittance blow-up during those manipulations are of practical concern. In this article, the behavior of the particle distribution in the presence of space charge and beam loading during bunch merging is investigated via a generalized elliptical bunch model and particle-in-cell (PIC) simulations. Possible schemes to minimize these intensity effects are discussed. As an application, parameters for the longitudinal rf manipulation scenario in the upcoming second phase of the China Spallation Neutron Sources (CSNS-II) are proposed. It is shown that for (slow cycling) storage rings, with an optimized set of longitudinal parameters, the emittance growth due to intensity effects can be largely dampened and a high compression efficiency is achieved. For rapid cycling synchrotrons, fast bunch merging can be achieved via a desynchronization method. The agreement between the analytical elliptical model and the PIC simulation results indicates that the extended model can be employed for the study of the intensity effects during longitudinal rf manipulations in hadron synchrotrons.

SPECTROSCOPY AND DYNAMICS OF NONLINEAR PROCESSES IN RELATIVISTIC BACKWARD-WAVE TUBE WITH ACCOUNTING FOR EFFECTS OF SPACE CHARGE, DISSIPATION AND WAVE REFLECTIONS

An effective universal, approach to solving the problems of quantitative modelling and analysis of the fundamental characteristics of spectroscopy and dynamics of the nonlinear processes in relativistic microwave electronics devices, in particular, a relativistic backward wave tube (RBWT) has been developed and implemented. It has been performed modelling, analysis, and prediction of chaotic dynamics of RBWT with simultaneous consideration of not only relativistic effects, but also the effects of dissipation, presence of space charge, wave reflections at the ends of the decelerating system, etc in a wide range of changes at different values of control parameters, which are characteristic for the distributed relativistic electron-wave self-oscillating systems. From physical viewpoint the transition to chaos in the dynamics of the studied RBWT occurs according to the scenario everywhere in the sequence of period-doubling bifurcations, but with the growth of relativism, the dynamics become fundamentally complicated with the alternation of quasi-harmonic and chaotic regimes (including the appearance of the "beak" effect on the bifurcation diagram) and the transition everywhere intermittency to the high-D attractor.

Polluted Barrier Effect on the Electric Field Distribution in Point-Plane Air Gaps under AC Applied Voltage: Based on Experimental Model

The aim of this paper is to study the effect of the surface condition of an insulating barrier on the electric field distribution in point-plane air gaps with the presence of a space charge, under AC voltage. The pollution was modelled as a uniform conductive layer on the barrier surface. Electric field analysis was carried out by changing the conductivity, permittivity, and thickness of the pollution layer. Using the Finite Element Method (FEM), the geometric model has been implemented in COMSOL Multiphysics software. This method is used to solve the partial differential equations that describe the field with the presence of space charge. The electric field increases when the conductivity and thickness of the polluted layer increases. Uniform pollution on the side of the high voltage point greatly reduces the insulation quality of the barrier. In addition, a limit level of pollution, from which its minimal electric strength is equivalent to that of a conductive barrier, has been determined. This model has been validated by comparing with the experimental results of a point-barrier-plane configuration with a distance between electrodes equal to 5cm. The distribution of the electric field predicted by the numerical model is in accordance with the experimental results. The latter indicate that this model has a great contribution in the physics of discharges in the air under various polluted environments.

Inception and Propagation of Electrical Trees in the Presence of Space Charge in HVAC Extruded Cables

This paper presents the space charge impact on the inception and propagation of electrical trees in cross-linked polyethylene (XLPE) insulation via simulations and experimentation. A 3D finite element analysis (FEA)-based modeling is proposed to simulate electrical trees via a needle embedded on the XLPE insulation. The proposed FEA model demonstrates the influence of the space charge magnitude and polarity on the initiation of partial discharges (PD). Then the critical parameters at the tip of an 8 µm. tree are to be examined. Experimentation is necessary to verify the simulation results acquired and study the mechanism of electrical trees at the inception and propagation stage. Therefore, in the experiment, a needle with a curvature radius of 5 µm. is embedded in the XLPE as a method of simulating defects, such as protrusions, voids, or cracks found in cable insulation. The power source supplies several high AC voltages on the 20 kV (rated voltage) cable under test for observing the inception and propagation behavior of the electrical trees. Phase-resolved partial discharge (PRPD) patterns are extracted at the inception and propagation stages of the electrical tree to determine the severity of the PDs occurring. A relationship is obtained for the PD magnitude with respect to the applied AC voltage while considering the positive and negative polarities of the PDs. This relationship then allows for the acquisition of the maximum electric field-tree length characteristics. The goal of the simulation of electrical trees is to provide the reader with a better understanding about the effect of space charge magnitude and polarity to comprehend the behavior of the treeing inception and propagation mechanisms. The injection and extraction of space charges is a complex phenomenon that requires further visualization through simulations. Therefore, the results of both the simulation and experiment are compared with the aim of establishing a clear relationship between space charge distribution and tree initiation.

Approximate universality in the electric field variation on a field-emitter tip in the presence of space charge

The electric field at the surface of a curved emitter is necessary to calculate the field emission current. For smooth parabolic emitting tips where space charge is negligible, variation of the electric field at the surface is known to follow the generalized cosine law. Here, we investigate the validity of the cosine law in the regime, where space charge due to emitted electrons is important. Particle-in-cell (PIC) simulations with an emission algorithm based on the cosine law are employed for this study. It is shown that if EP and EL be the field at the apex of tip with and without space charge, respectively, then for ϑ=EP/EL≥0.9, the average relative deviation of the electric field from the cosine law is less than 3% over the endcap. Thus, an emission scheme based on the cosine law may be used in PIC simulations of field emission of electrons from curved emitter tips in the weak space charge regime. The relation between ϑ and normalized current ζ for curved emitters in this regime is also investigated. A linear relation, ϑ=1−δζ (where δ is a constant), similar to that obtained theoretically for flat emitting surfaces is observed, but the value of δ indicates that the extension of the theory for curved emitters may require incorporation of the field enhancement factor.

Investigation of internal fields in organic semiconductors in the presence of traps

In an organic semiconductor optoelectronic device, the built-in field within the active layer is typically determined by the difference in contact potentials of the device. However, the presence of space charges and trap states contribute to the electric field within the thin film. Depending on the maximum applied forward voltage, the trap states can be charged, inducing hysteresis in the optoelectronic response of the system. In this work, we investigate the electric fields inside organic photovoltaic device structures, in the presence of traps, using electroabsorption (EA) spectroscopy. Comparing simulations with our experimental results, we explained the origin of hysteresis in the electroabsorption signal as a function of applied DC bias. We solved Poisson’s equation to estimate the densities of trapped carriers in the active layers. The filled trap densities in poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)] (PPDT2FBT) were found to be ∼1×1017 and ∼6×1016 cm−3, respectively. From the transient EA measurements, the estimated values of energies of the trap states with respect to the HOMO level were 0.82 and 0.76 eV in P3HT and 0.70 and 0.64 eV in PPDT2FBT, which indicated the presence of midgap traps in these organic semiconductor thin films. Such trap induced changes in the internal fields within the active layers, affect the mobility and carrier transport in the organic optoelectronic devices. The midgap traps lead to exciton quenching and also act as non-radiative recombination centers, resulting in reduction in luminescence efficiency of the active layers.

Particle-in-cell techniques for the study of space charge effects in the Advanced Cryogenic Gas Stopper

Linear gas stoppers are widely used to convert high-energy, rare-isotope beams and reaction products into low-energy beams with small transverse emittance and energy spread. Stopping of the high-energy ions is achieved through interaction with a buffer gas, typically helium, generating large quantities of He+/e− pairs. The Advanced Cryogenic Gas Stopper (ACGS) was designed for fast, efficient stopping and extraction of high-intensity, rare-isotope beams. As part of the design process, a comprehensive particle-in-cell code was developed to optimize the transport and extraction of rare isotopes from the ACGS in the presence of space charge, including He+/e− dynamics, buffer gas interactions including gas flow, radio-frequency carpets, and ion extraction through a nozzle or orifice. Details of the simulations are presented together with comparison to experiment when available.

Diagnostic aspects of partial discharge measurement at very low frequency: a review

Electrical insulation of high voltage (HV) power equipment/apparatus plays very important roles in the sound functioning of power systems. Examining the insulation condition through partial discharge (PD) measurements has considerable importance as the presence of PDs in any HV systems stands for a sign of defects and degradations in electrical insulation. Generally, the PD measurements are performed with a supply voltage of normal power frequency (PF-50/60 Hz). As an attractive alternative, the PD measurement with a very low frequency (VLF–1 Hz or lower) applied voltage has emerged as a powerful diagnostic tool. A PD event is a random physical process, it depends mostly on the availability of free electrons that triggers the PD and the presence of space charge usually originated after a discharge. As the discharge characteristics vary with the supply voltage frequency; thus, the existing interpretation knowledge at 50 Hz may or may not be directly applicable to other test frequencies, particularly in the VLF range. Therefore, to provide a general insight, this paper presents various diagnostic aspects of PD measurements, and at the end, various discharge sources (e.g., void, surface, and corona) and their behaviours under varying the test voltage frequencies are discussed in detail.

The Effect of the Axial Heat Transfer on Space Charge Accumulation Phenomena in HVDC Cables

To date, it has been widespread accepted that the presence of space charge within the dielectric of high voltage direct current (HVDC) cables is one of the most relevant issues that limits the growing diffusion of this technology and its use at higher voltages. One of the reasons that leads to the establishment of space charge within the insulation of cables is the temperature dependence of its conductivity. Many researchers have demonstrated that high temperature drop over the insulation layer can lead to the reversal of the electric field profile. In certain conditions, this can over-stress the insulation during polarity reversal (PR) and transient over voltages (TOV) events accelerating the ageing of the dielectric material. However, the reference standards for the thermal rating of cables are mainly thought for alternating current (AC) cables and do not adequately take into account the effects related to high thermal drops over the insulation. In particular, the difference in temperature between the inner and the outer surfaces of the dielectric can be amplified during load transients or near sections with axially varying external thermal conditions. For the reasons above, this research aims to demonstrate how much the existence of “hot points” in terms of temperature drop can weaken the tightness of an HVDC transmission line. In order to investigate these phenomena, a two-dimensional numerical model has been implemented in time domain. The results obtained for some case studies demonstrate that the maximum electric field within the dielectric of an HVDC cable can be significantly increased in correspondence with variations along the axis of the external heat exchange conditions and/or during load transients. This study can be further developed in order to take into account the combined effect of the described phenomena with other sources of introduction, forming, and accumulation of space charge inside the dielectric layer of HVDC cables.

Understanding beam distributions in hadron linacs in the presence of space charge

Space charge forces are a significant contribution to the beam dynamics in high intensity hadron linacs, often causing beam halo that results in beam loss. Currently, there is limited success in modeling beam distributions in high intensity hadron linacs at the RMS level, and no demonstrated capability for measuring or modeling distributions at the level of beam halo. This article discusses the current state of the art capability in this area, and explores next steps in both simulation and beam instrumentation that will enable measurement and modeling of beam halo in high intensity hadron linacs.

The Industrial Applicability of PEA Space Charge Measurements, for Performance Optimization of HVDC Power Cables

Cable manufacturing industries are constantly trying to improve the electrical performance of power cables. During the years, it was found that one of the most relevant degradation factors influencing the cable lifetime is the presence of space charge in the insulation layer. To detect the accumulated charge, the pulsed electro-acoustic (PEA) method is the most used technique. Despite the wide use of the PEA cell, several issues are still present. In particular, the PEA output signal is strongly disturbed by the acoustic waves reflections within the PEA cell. This causes the distortion of the output signal and therefore the misinterpretation of the charge profiles. This, in turn, may result in an incorrect cable characterization from the space charge phenomenon point of view. In 2017, due to the proved degradation effect of the space charge accumulation phenomenon, the IEEE Std 1732 was developed. This standard describes the steps to be followed for the space charge measurement in cables specimens during pre-qualification or type tests. Therefore, cable manufacturing industries started to take a particular interest in these measures. In the light of this, the aim of the present work is to highlight that the enacted standard is not easily applicable since various problems are still present in the PEA method for cables. In particular, in this work, the effect of multiple reflected signals due to the different interfaces involved, but also the effect of the signal attenuation due to cable dielectric thickness, as well as the effect of the PEA cell ground electrode thickness in the output charge profile, are reported. These issues have been demonstrated by means of an experimental test carried out on a full-size cable in the Prysmian Group High Voltage laboratory. To better understand the PEA cell output signal formation, a PEA cell model was developed in a previous work and it has been experimentally validated here. In particular, simulations have been useful to highlight the effect of the reflection phenomena due to the PEA cell ground electrode thickness on the basis of the specimen under test features. Moreover, by analyzing the simulation results, it was possible to separate the main signal from the reflected waves and, in turn, to calculate the suitable ground electrode thickness for the cable specimen under test.

Temperature gradient effect on the space charge behaviour in multilayers of oil and pressboard

The presence of space charge within oil and pressboard (PB) can lead to distortion of electric field within converter transformers. Electric field distortion can cause materials to be overstressed which accelerates degradation, and can result in breakdown. Therefore, it is important to analyse the factors that affect space charge formation and dissipation such as temperature, moisture, ageing, multilayers and electric fields. This paper focuses on the temperature gradient and multilayers of oil and PB on space charge. Considering the inhomogeneous acoustic wave velocity caused by a temperature gradient and different dielectric properties of multilayers of materials, an improved space charge recovery algorithm has been developed, in which the distortion of the acoustic wave caused by the attenuation, dispersion, different dielectric properties and temperature effects have been considered. It has been found that for two layers consisting of PB and oil with the bottom electrode fixed at 20 °C, the electric field in the oil increases under the temperature gradient while decreases at ambient temperature. For a three layered sample consisting of PB, oil, and PB with the bottom electrode at ambient temperature, the electric field gradient exists under the temperature gradient and the maximum electric field occurs near the cathode while occurs near the anode at ambient temperature.

Beam-loss driven injection optimization for HIAF-BRing in the presence of space charge

BRing is the booster ring of the HIAF project in which the beam intensity is designed up to 1.0 × 1011 ppp (238U34+). In order to reach the required intensities, an efficient injection scheme is crucial and the beam loss during injection must not exceed the limits defined by machine protection and dynamic vacuum requirements. An injection scheme called two-plane painting multiturn injection is consequently proposed, which exploits both the horizontal and vertical acceptance of the BRing by a tilted electrostatic septum. The influence of the space charge on the injection process is important for the two-plane multiturn injection scheme which should provide the required intensity with a minimal beam loss. The impact of the space charge on the efficiency of the injection process is investigated using a modified ORBIT code. In order to achieve lower beam loss, the particle swarm algorithm is proposed for a detailed injection optimization with space charge effect. Detailed studies on the two-plane painting multiturn injection scheme and the dependence on the lattice tune, the injected beam current, and the injected beam emittance are also presented. The results provide an important guideline for the design optimization of the injection system for the HIAF-BRing.

An improved model for prediction of the cone-jet formation in electrospray with the effect of space charge

The presence of space charge can cause a non-negligible effect in some electrospray applications, which, however, was ignored in previous numerical studies. In the present study, the formation of the cone-jet in the electrospray process is investigated with considering the space charge effects. The governing equations of fluid flow and electrostatics are coupled in the numerical method. The charge transport equation is implemented with no simplification and the volume of fraction (VOF) method is employed to track the liquid-gas interface. An improved model of considering the effect of charged droplet emitted from the tip of cone-jet on the external electric field is proposed. By considering the space charge effects, the improved model can achieve better accuracy in predicting the shape of the cone-jet by reducing the mean relative error from 12.5% to 4.2%. The flow field in the cone-jet is obtained and the vortices are found to play an important role in the cone-jet formation. In terms of the current produced by the cone-jet, the prediction error is reduced from 19 %-39% to 10 %-16% by considering the space charge. Further validation is conducted by comparing the simulated results with the prediction of the scaling laws. It was found that the prediction of the improved model fits better with the scaling laws based on both the analytical and experimental results. The new model can provide more reliable and detailed information for the simulation of other processes in the electrospray, i.e., the jet breakup and droplets motion.

Space-charge-induced Seebeck effect in solid dielectrics

We show theoretically that, in the presence of space charge, the Seebeck coefficient of a homogeneous plane-parallel capacitor could go beyond corresponding limits of the linear-response regime. In this study, we consider the coexistence of p-type and n-type carriers and interrelate the carrier concentrations via the mass-action law. If only Ohmic conductivity is considered or if the distribution of space charge is taken to be uniform, the Seebeck coefficient, which is the same as that given by Onsager’s reciprocal relations, takes on a magnitude between 0 and kB/q, where kB is the Boltzmann constant and q is the electronic charge. But in the nonlinear-response regime which corresponds to any space-charge-limited scenario with a nonuniform distribution of space charge, Onsager’s reciprocal relations break down and the magnitude of the Seebeck coefficient takes on a value between kB/q and 2kB/q, exceeding the limit of the linear-response regime.

Resonance identification studies at the CERN PS

In view of the LHC Injectors Upgrade (LIU) and the challenging high brightness target beam parameters, a broad range of possible working points for the Proton Synchrotron (PS) is being investigated. High order resonances have been identified, both structural resonances driven by space charge due to the lattice harmonics of the PS, and resonances excited by multipolar components in the machine. This paper provides a summary of the performed tune scan studies, covering both experimental and simulation results. Furthermore, non-linear analysis techniques have been used to characterize the resonances and their effect on the beam in presence of space charge.

A Modified Algorithm for the Simulation of Charge Behavior in Water Tree Aged Cross-Linked Polyethylene Cable

In this paper, a modified algorithm is introduced to explain the process of charge formation in water tree aged cable insulation with respect to the dielectric properties of water tree areas and the cylindrical geometries of cable insulation. The algorithm combined with the classic bipolar charge transport model represents a novel attempt to simulate charge behavior in insulation with water tree. The finite-difference method is employed to solve for the electric field in the presence of space charge, and the finite-volume method is employed to solve the charge continuity equation. The dielectric parameters in this paper are not instantaneous near the interface but are functions of the radius. To overcome the Courant–Friedrichs–Lewy condition, the Crank–Nicolson method is used to separate the governing equations in the time domain. To validate the accuracy of the model, this paper also presents the space charge behavior in water tree aged cross-linked polyethylene cable samples measured by the full-scale pulsed electro-acoustic method. The homocharge is injected under a high electric field. During the depolarization process, the shallow trap charge gradually disappears, whereas the homocharge continues to form a positive peak signal. Simulations show that the homocharge accumulates in the tip of the water tree because the dielectric characteristics of the material change during the water-aging process. The model describes the behavior of space charge in the water tree aged insulation of cylindrical geometries. Improvements will be needed in the following research.