Explore the vast range of titles available.

In this study, the space charge characteristics in the polycyclic aromatic compounds doped cross-linked polyethylene (XLPE) composite were analysed by integration current (Q(t)) method and quantum chemical calculation. Experimentally, the space charge behaviours of XLPE composites modified by the three selected polycyclic aromatic compounds during polarisation and depolarisation process at 25 and 80°C were measured by Q(t) method, respectively. The energy levels and 3D potential distributions of the three polycyclic aromatic compounds were calculated by density functional theory. The experimental and calculation results indicate that the polycyclic aromatic compound C with deep carrier traps and stronger polarity exhibits outstanding ability to reduce space charge injection than the others at both 25 and 80°C. Generally, 4,4′-bis (dimethyl amino) benyil has great potential as the organic additive for DC cable insulation from the view of space charge suppression.

The beam transport process plays a crucial role in the design and realization of negative-ion-based neutral beam injection (N-NBI) systems for thermonuclear fusion. In particular, space charge compensation (SCC), where positive ions generated by collisions between negative ions and background gas mitigate space charge effects, serves as a critical mechanism influencing beam divergence and envelope evolution. In this work, the SCC process in N-NBI systems and its impact on beam transport are investigated using a 3D simulation model based on a Vlasov beam-tracing algorithm. Systematic simulations show that SCC evolution and downstream beam optics are jointly controlled by background gas pressure and beam energy, with higher pressure and higher energy strengthening SCC and mitigating divergence and envelope growth downstream of the extraction system. A key physical observation is that collisionless phase mixing drives the transverse velocity distribution of secondary ions toward a Maxwellian distribution, enabling an effective transverse ion temperature that quantitatively correlates with SCC degree. The residual electric field leaking downstream of the extraction system locally weakens SCC over a finite extent, producing negligible beam optics impact for the ITER-relevant 1 MeV case at pressures of 5×10−3 Pa and 1×10−2 Pa, but non-negligible transport sensitivity at 200 keV under the same pressure range. These results provide new insight into the interplay between SCC, secondary-ion dynamics, and beam transport, with direct relevance to the modeling and optimization of high-energy NBI beamlines.

Dephasing and non-radiative decay processes limit the performance of a wide variety of quantum devices at room temperature. Here we illustrate a general pathway to notably reduce the detrimental impact of these undesired effects through photonic design of the device electrodes. Our design facilitates a large Purcell enhancement that speeds up competing, desired radiative decay while also enabling convenient electrical gating and charge injection functions. We demonstrate the concept with a free-space optical modulator based on an atomically thin semiconductor. By engineering the plasmonic response of a nanopatterned silver gate pad, we successfully enhance the radiative decay rate of excitons in a tungsten disulfide monolayer by one order of magnitude to create record-high modulation efficiencies for this class of materials at room temperature. We experimentally observe a 10% reflectance change as well as 3 dB signal modulation, corresponding to a 20-fold enhancement compared with modulation using a suspended monolayer in vacuum. We also illustrate how dynamic control of light fields can be achieved with designer surface patterns. This research highlights the benefits of applying radiative decay engineering as a powerful tool in creating high-performance devices that complements substantial efforts to improve the quality of materials.By engineering the plasmonic response of a nanopatterned silver gate electrode, the radiative decay rate of excitons in a tungsten disulfide monolayer can be enhanced via the Purcell effect, creating high modulation efficiencies at room temperature.

The aging of cable insulation presents a significant threat to the safe operation of cables, with space charge serving as a crucial factor influencing cable insulation degradation. However, the characteristics related to space charge and conduction current behavior during thermal aging remain unclear. This study focused on the thermal aging of cross-linked polyethylene (XLPE) material and utilizes a combined pulse electro-acoustic (PEA) and conduction current testing system to analyze the space charge and conduction current characteristics in the sample under varying electric fields and temperatures. The average charge density, short-circuit residual electric field, electric field distortion rate, and conduction current were studied. The findings indicate that the space charge in the samples following thermal aging is predominantly governed by the injected charge. The amorphous region of XLPE decreases, while the cross-linking degree increases after aging, thereby facilitating charge carrier migration within the sample and reducing the generation of charge carriers through thermal pyrolysis. The minimum temperature required for charge injection is reduced by thermal aging. Furthermore, modifications in conduction current, residual electric field, and average charge density indicate that thermal aging has the potential to alter the microstructure and trap characteristics of XLPE. This study provides empirical evidence to elucidate the underlying mechanism of cable insulation aging.

On-site installation of accessories on extruded polymeric high voltage cables in a common practice. The procedure requires the shaping of the physical interface between the cable insulation surface and the pre-molded accessory body. On such interfaces, rough surfaces should be avoided in order to limit space charge accumulation in the insulation, which affects the cable performance by reducing insulation life-time, creating conditions for local field enhancement, and, respectively, the formation of possible breakdown path e.g. by electrical treeing. Space charge measurements on cable insulation peelings were undertaken to assess the space charge injection and accumulation on interfaces with varying degrees of surface roughness in order to improve understanding on this subject. The results of the measurements confirm the hypothesis regarding the enhancement of charge injection from rough surfaces when electric field strength exceeds a certain level. The accumulated charge density in the material is shown to strongly depend on the field strength and temperature in both polarization and subsequent depolarization measurements. These results emphasize that a bipolar charge transport model that incorporates field and temperature dependencies of charge injection, trapping, detrapping, and recombination processes needs to be adopted to accurately describe the observed electric conduction phenomena.

In this study, characteristics of charge injection under extra high electric field (above 100 kV/mm) in cross‐linked polyethylene (XLPE) were investigated by experiments of conduction current and space charge. The results show that current density from low electric field to sample breakdown corresponds to space charge limited current (SCLC) theory. More specifically, Schottky current is similar to experiment current before 100 kV/mm, while the J–E curve conforms to a modified SCLC theory after 100 kV/mm. Besides, the non‐linear coefficient of J–E curve from 100 kV/mm to extra high electric field is smaller than theoretical value, and the injection depth of space charge is restricted as the field becomes higher than 100 kV/mm, which may be caused by the negative differential mobility of charge. Driven by extra high electric field, charge collides with lattice of dielectric and scatters. As a result, mean free time of charge decreases and charge mobility is reduced with the increased field. Consequently, considering the decrease in charge mobility, a mobility‐limited charge injection equation is proposed, and the validity of the proposed equation under extra high electric field is demonstrated by space charge simulation.

The oil‐pressboard insulation on the valve side of the converter transformer withstands pulsating square wave voltage. The voltage contains lots of DC components and high‐order harmonics, which is likely to cause insulation failures. The partial discharge and space charge experiments of oil‐impregnated pressboard under DC superimposed harmonic voltage are conducted to study the effects of harmonic frequency and DC components on the discharge and charge accumulation and to explore the correlation mechanism of charge dynamic behaviour and discharge. Firstly, it is found that high‐order harmonic voltage promotes the partial discharge generation, but DC component suppresses the discharge. Moreover, the charge inside the pressboard is mainly injected in the same polarity and accumulates over time. The space charge density near the electrode increases with the increase of harmonic frequency, and the large amount of charge injection leads to electric field distortion. Meanwhile, the electric field direction changes faster, and the frequent charge injection and extraction behaviours make the charge recombination intensify, which promotes the discharge. Through the above analysis, the influence law of harmonic voltage on the failure of oil‐pressboard insulation is obtained, providing a theoretical basis for the structural optimisation and condition assessment of the converter transformer insulation.

Space charge accumulates near electrodes in insulating materials under high voltage direct current. Electric stress created by charge accumulation distorts electric field, reduces the remaining useful life of insulating materials, and decreases partial discharge inception voltage that are critical for ensuring the dielectric integrity of power applications. In this article, a novel approach that mitigates space charge accumulation using electrets is presented. Electrets are used to reduce electric fields that cause charge injection and space charge accumulation in dielectric materials. The validity of using electrets for preventing space charge accumulation is numerically and experimentally demonstrated. Bipolar charge transport model is developed to simulate space charge injection and accumulation in silicone rubber without and with the incorporation of electret. Space charge experiments are performed to confirm the numerical results. The results show that electrets can effectively mitigate space charge accumulation in dielectric materials. The results also show that the incorrect use of electret such as reversing the polarity can lead to more substantial space charge accumulation. An entirely new way of preventing space charge accumulation is proposed. Incorporating an electret film into a metal‐dielectric interface prevents space charge from being injected into the main dielectric. Electrets generate electric fields that can be utilized to counter high electric field originating from high‐voltage metal surfaces, increase charge injection barrier height, and mitigate space charge injection and accumulation.

The accumulation of space charge is considered to be an important key factor to accelerate the aging of the insulation of HVDC cables. How to reduce the accumulation of space charge in the insulation layer is an urgent problem to improve the voltage level of cables. In this paper, an approach is presented to prevent charge from the inner semiconductive layer to the insulating layer, using a modified semiconductive compound by doping magnetic carbon nanofibers. Through microwave-assisted heating, Ni was deposited on the surface of carbon nanofibers which have been pre-treated. Modified semiconductive compound electrodes was used to test the injection of charges. The results showed that charge injected into LDPE insulting layerwith modified CNFs-Ni/ LDPE electrode is less than that with LDPE semiconductive layer without CNFs-Ni electrode.

Semi-conductive composites composed of carbon black-polymer play an important role in uniform electric field in high voltage direct current (HVDC) cable. They also affect space charge behaviors in the insulation material. However, the charge injection characteristics of semi-conductive composites are not detailed. In this work, the electrode structure of ‘Semi-conductive composites- Insulation material- Metal bottom’ (S-I-M) is proposed, and the currents formed by injected charges from semi-conductive composites are characterized by the thermally stimulated depolarization current (TSDC) method. Further, the experimental results based on the structure of S-I-M are compared with the traditional electrode structure of M-I-M (Metal upper electrode- Insulation material- Metal bottom electrode) and the simplified cable electrode structure of MS-I-M (Metal upper electrode-Semi-conductive electrode- Insulation material- Metal bottom electrode), respectively. The experimental results show that the semi-conductive composite plays an important role in the charge injection process and it presents a different tendency under different compound modes of temperature and electric field. For the low electric field (E ≤ 5 kV/mm) and the low temperature (T ≤ 50 °C), the current caused by the accumulated charges follows the rule, IS > IMS > IM. For the low electric field and high temperature (T > 50 °C), the current caused by the injected charges follows the rule, IMS > IM > IS. This phenomenon is closely related to the interface characterization and contact barrier.