Numerical analysis for the free-boundary current reversal equilibrium in the AC plasma current operation in a tokamak

In recent decades, tokamak discharges with zero total toroidal current have been reported in tokamak experiments, and this is one of the key problems in alternating current(AC) operations.An efficient free-boundary equilibrium code is developed to investigate such advanced tokamak discharges with current reversal equilibrium configuration. The calculation results show that the reversal current equilibrium can maintain finite pressure and also has considerable effects on the position of the X-point and the magnetic separatrix shape, and hence also on the position of the strike point on the divertor plates, which is extremely useful for magnetic design, MHD stability analysis, and experimental data analysis etc. for the AC plasma current operation on tokamaks.

Current bifurcation, reversals and multiple mobility transitions of dipole in alternating electric fields

Anomalous transports of dipole in alternating electric fields are investigated by means of numerical calculation of its average angular velocity(or current). Our results show that the alternating electric fields can make the dipole exhibit many interesting transport behaviors. There exist current bifurcation and multiple current reversal phenomena about frequency of the alternating electric fields in the system in the absence of constant bias force, while many platforms appear in the curve of its average angular velocity vs. the force, i.e., multiple mobility transitions phenomenon in the presence of the constant force, dependent on frequencies of the alternating electric fields. Further investigation indicates that the multiple mobility transitions are attributed to the traveling forces on the dipole. Intrinsic physical mechanism and conditions for the characteristic dynamical behaviors to occur are also discussed in detail. These findings will possess crucial significance for optimizing heating control in the alternating electric fields.

Single-phase-to-ground fault protection based on zero-sequence current ratio coefficient for low-resistance grounding distribution network

Definite-time zero-sequence over-current protection is presently used in systems whose neutral point is grounded by a low resistance(low-resistance grounding systems).These systems frequently malfunction owing to their high settings of the action value when a high-impedance grounding fault occurs.In this study,the relationship between the zero-sequence currents of each feeder and the neutral branch was analyzed.Then,a grounding protection method was proposed on the basis of the zero-sequence current ratio coefficient.It is defined as the ratio of the zero-sequence current of the feeder to that of the neutral branch.Nonetheless,both zero-sequence voltage and zero-sequence current are affected by the transition resistance,The influence of transition resistance can be eliminated by calculating this coefficient.Therefore,a method based on the zero-sequence current ratio coefficient was proposed considering the significant difference between the faulty feeder and healthy feeder.Furthermore,unbalanced current can be prevented by setting the starting current.PSCAD simulation results reveal that the proposed method shows high reliability and sensitivity when a high-resistance grounding fault occurs.

Progressive current degradation and breakdown behavior in GaN LEDs under high reverse bias stress

The progressive current degradation and breakdown behaviors of GaN-based light emitting diodes under high reversebias stress are studied by combining the electrical, optical, and surface morphology characterizations. The current features a typical “soft breakdown” behavior, which is linearly correlated to an increase of the accumulative number of electroluminescence spots. The time-to-failure for each failure site approximately obeys a Weibull distribution with slopes of about 0.67 and 4.09 at the infant and wear-out periods, respectively. After breakdown, visible craters can be observed at the device surface as a result of transient electrostatic discharge. By performing focused ion beam cuts coupled with scan electron microscope, we observed a local current shunt path in the surface layer, caused by the rapid microstructure deterioration due to significant current heating effect, consistent well with the optical beam induced resistance change observations.

Transport mechanism of reverse surface leakage current in AlGaN/GaN high-electron mobility transistor with SiN passivation

The transport mechanism of reverse surface leakage current in the AlGaN/GaN high-electron mobility transistor（HEMT） becomes one of the most important reliability issues with the downscaling of feature size.In this paper,the research results show that the reverse surface leakage current in AlGaN/GaN HEMT with SiN passivation increases with the enhancement of temperature in the range from 298 K to 423 K.Three possible transport mechanisms are proposed and examined to explain the generation of reverse surface leakage current.By comparing the experimental data with the numerical transport models,it is found that neither Fowler-Nordheim tunneling nor Frenkel-Poole emission can describe the transport of reverse surface leakage current.However,good agreement is found between the experimental data and the two-dimensional variable range hopping（2D-VRH） model.Therefore,it is concluded that the reverse surface leakage current is dominated by the electron hopping through the surface states at the barrier layer.Moreover,the activation energy of surface leakage current is extracted,which is around 0.083 eV.Finally,the SiN passivated HEMT with a high Al composition and a thin AlGaN barrier layer is also studied.It is observed that 2D-VRH still dominates the reverse surface leakage current and the activation energy is around 0.10 eV,which demonstrates that the alteration of the AlGaN barrier layer does not affect the transport mechanism of reverse surface leakage current in this paper.

Effects of polarization-reversed electromagnetic ion cyclotron waves on the ring current dynamics

Electromagnetic ion cyclotron(EMIC)waves are widely believed to play an important role in influencing the radiation belt and ring current dynamics.Most studies have investigated the effects or characteristics of EMIC waves by assuming their left-handed polarization.However,recent studies have found that the reversal of polarization,which occurs at higher latitudes along the wave propagation path,can change the wave-induced pitch angle diffusion coefficients.Whether such a polarization reversal can influence the global ring current dynamics remains unknown.In this study,we investigate the ring current dynamics and proton precipitation loss in association with polarization-reversed EMIC waves by using the ring current-atmosphere interactions model(RAM).The results indicate that the polarization reversal of H-band EMIC waves can truly decrease the scattering rates of protons of 10 to 50 keV or>100 keV in comparison with the scenario in which the EMIC waves are considered purely left-handed polarized.Additionally,the global ring current intensity and proton precipitation may be slightly affected by the polarization reversal,especially during prestorm time and the recovery phase,but the effects are not large during the main phase.This is probably because the H-band EMIC waves contribute to the proton scattering loss primarily at E<10 keV,an energy range that is not strongly affected by the polarization reversal.

Zero-sequence Current Suppressing Strategy for Dual Three-phase Permanent Magnet Synchronous Machines Connected with Single Neutral Point

Dual three-phase permanent-magnet synchronous machines(DTP-PMSM)connected with a single neutral point provide a loop for zero-sequence current(ZSC).This paper proposes a novel space vector pulse width modulation(SVPWM)strategy to suppress the ZSC.Five vectors are selected as basic voltage vectors in one switching period.The fundamental and harmonic planes and the zero-sequence plane are taken into consideration to synthesis the reference voltage vector.To suppress the ZSC,a non-zero zero-sequence voltage(ZSV)is generated to compensate the third harmonic back-EMF.Rather than triangular carrier modulation,the sawtooth carrier modulation strategy is used to generate asymmetric PWM signals.The modulation range is investigated to explore the variation of modulation range caused by considering the zero-sequence plane.With the proposed method,the ZSC can be considerably reduced.The simulated and experimental results are presented to validate the effectiveness of the proposed modulation strategy.

Morphology and tribological properties of Ni/n-SiO_2 composite coatings by pulse-reverse current brush-plating

Ni/n-SiO2 composite coating was electrodeposited by brush-plating with pulse-reverse current（RC）. The morphology, hardness, and tribological properties of the coating were investigated and compared with those of Ni and composite coatings electrodeposited with direct current（DC）. The results indicate that Ni/n-SiO2 composite coating electrodeposited by RC, because of RC and the nano powders, has denser coating, finer crystal grains, higher hardness（HV650.0, nearly 1.5 times higher than that of Ni coating electrodeposited by DC） and lower friction coefficient（nearly 0.62）, as a result, in the wearing experiment, the Ni/n-SiO2 composite coating electrodeposited by RC has the least worn loss. So this kind of coating has better wear resistance. And RC electro brush-plating can be used as a new technology of brush-plating in the area of wear resistance.

CRYSTALLIZATION OF SODIUM TUNGSTEN BRONZE USING ELECTROLYSIS WITHREVERSE CURRENT~＋

Utilizing the technique of electrolysis with reverse current,sodium tungsten bronze (Na_xWO_3) crystals were prepared from a molten mixture of sodium tungstate (Na_2WO_4 ) and tungstic trioxide (WO_3) at 800℃. Crystal structure, lattice parameter, and s

MHD Equilibrium with Reversed Current Density and Magnetic Islands Revisited:the Vacuum Vector Potential Calculus

The solution of Grad-Shafranov equation determines the stationary behavior of fusion plasma inside a tokamak. To solve the equation it is necessary to know the toroidal current density profile. Recent works show that it is possible to determine a magnetohydrodynamic （MHD） equilibrium with reversed current density （RCD） profiles that presents magnetic islands. In this work we show analytical MHD equilibrium with a RCD profile and analyze the structure of the vacuum vector potential associated with these equilibria using the virtual casing principle.

The Transport Mechanisms of Reverse Leakage Current in Ultraviolet Light-Emitting Diodes

The transport mechanisms of the reverse leakage current in the UV light-emitting diodes （380nm） are investi- gated by the temperature-dependent current-voltage measurement first. Three possible transport mechanisms, the space-limited-charge conduction, the variable-range hopping and the Poole-Frenkel emission, are proposed to explain the transport process of the reverse leakage current above 295 K, respectively. With the in-depth investigation, the former two transport mechanisms are excluded. It is found that the experimental data agree well with the Poole Frenkel emission model. Furthermore, the activation energies of the traps that cause the reverse leakage current are extracted, which are 0.05eV, 0.09eV, and 0.11 eV, respectively. This indicates that at least three types of trap states are located below the bottom of the conduction band in the depletion region of the UV LEDs.

Effects of Oxygen Concentration in Monocrystalline Silicon on Reverse Leakage Current of PIN Rectifier Diodes

The effects of initial oxygen concentration on the reverse leakage current of PIN rectifier diodes were studied.We fabricated the PIN rectifier diodes with different initial oxygen concentrations,and analyzed the electrical properties,anisotropic preferred etching by means of optical microscopy,Fourier transform infrared spectroscopy and transmission electron microscopy.It is pointed out that the reverse leakage current increases exponentially with the increasing initial oxygen concentration.Furtherly,we researched and analyzed the mechanism of the effects of initial oxygen concentration on the reverse leakage current of PIN rectifier diode.It is shown that the oxygen precipitations present in an "S" curve with increasing initial oxygen concentration after high temperature diffusion.The main reason is that the nucleation and growth of oxygen precipitation at high temperature induce bulk oxidation-induced defects (B-OSF),which are mainly dislocations,and a small amount of rod stacking faults.The density of B-OSF increases with the increasing initial oxygen concentration.The existence of B-OSF has great effects on the reverse leakage current of PIN rectifier diode.

Zero-Sequence Current Suppression Strategy for Open-End Winding Permanent Magnet Synchronous Motor Based on Model Predictive Control

Compared with the traditional three-phase star connection winding,the open-end winding permanent magnet synchronous motor(OW-PMSM)system with a common direct current(DC)bus has a zero-sequence circuit,which makes the common-mode voltage and the back electromotive force(EMF)harmonic generated by the inverters produce the zero-sequence current in the zero-sequence circuit,and the zero-sequence current has great influence on the operation efficiency and stability of the motor control system.A zero-sequence current suppression strategy is presented based on model predictive current control for OW-PMSM.Through the mathematical model of OW-PMSM to establish the predictive model and the zero-sequence circuit model,the common-mode voltage under different voltage vector combinations is fully considered during vector selection and action time calculation.Then zero-sequence loop constraints are established,so as to suppress the zero-sequence current.In the end,the control strategy proposed in this paper is verified by simulation experiments.

Steering Multiple Reverse Current into Unidirectional Current in Deterministic Ratchets

Recent investigations have shown that with varying the amplitude of the external force,the deterministicratchets exhibit multiple current reversals,which are undesirable in certain circumstances.To control the multiple reversecurrent to unidirectional current,an adaptive control law is presented inspired from the relation between multiple reversalscurrent and the chaos-periodic/quasiperiodic transition of the transport velocity.The designed controller can stabilizethe transport velocity of ratchets to steady state and suppress any chaos-periodic/quasiperiodic transition,namely,thestable transport in ratchets is achieved,which makes the current sign unchanged.

currentMKMJ25-22型自愈式脉冲电容器耐久性试验条件的验证及其重要意义

随着电力装备制造技术的不断提升,自愈式脉冲电容器作为一种关键的电子元器件,在多个领域的应用逐渐得到广泛推广。为了确保这些电容器在实际应用中的稳定性和可靠性,必须对其进行严格的耐久性试验。然而,目前主流的试验方法尚不能准确快速地反映MKMJ25–22型自愈式脉冲电容器的实际性能。针对MKMJ25–22型自愈式脉冲电容器的试验问题,文章对标行业试验标准,对试验设备的能力进行了理论分析,并在元件选取、充放电回路等方面对试验回路进行了优化设计。通过对1台MKMJ25–22型自愈式脉冲电容器进行2000次的耐久性试验,证明回路、设备、配件均能够满足MKMJ25–22型自愈式脉冲电容器的试验要求。

Effect of Rare Earth on Microstructure and Corrosion Resistance of Pulse Reversal Current Electrodeposition Ni-Co Alloy Coatings

The effect of adding RE to plating bath on microstructure and corrosion resistance of Ni-Co alloy coatings prepared by pulse reversal current electrodeposition was studied by means of SEM/EDS, electrochemical analysis and corrosion mass loss etc. The results show that adding proper RE to plating solution can promote the microstructure of coatings compacter, the surface smoother and the crystal finer, and improve the corrosion resistance. The coatings exhibite the highest corrosion resistance when the concentration of RE reaches 0.25 g·L -1. The reason of increasing corrosion resistance by adding RE was also investigated.

Analytical model for reverse characteristics of 4H-SiC merged PN Schottky (MPS) diodes

A new analytical model for reverse characteristics of 4H-SiC merged PN Sehottky diodes （MPS or 3BS） is developed. To accurately calculate the reverse characteristics of the 4H SiC MPS diode, the relationship between the electric field at the Schottky contact and the reverse bias is analytically established by solving the cylindrical Poisson equation after the channel has pinched off. The reverse current density calculated from the Wentzel-Kramers-Brillouin （WKB） theory is verified by comparing it with the experimental result, showing that they are in good agreement with each other. Moreover, the effects of P-region spacing （S） and P-junction depth （Xj） on the characteristics of 4H-SiC MPS are analysed, and are particularly useful for optimizing the design of the high voltage MPS diodes.

Current-induced synchronized magnetization reversal of two-body Stoner particles with dipolar interaction

We investigate magnetization reversal of two-body uniaxial Stoner particles, by injecting spin-polarized current through a spin-valve structure. The two-body Stoner particles perform synchronized dynamics and can act as an information bit in computer technology. In the presence of magnetic dipole–dipole interaction（DDI） between the two particles,the critical switching current Ic for reversing the two dipoles is analytically obtained and numerically verified in two typical geometric configurations. The Ic bifurcates at a critical DDI strength, where Ic can decrease to about 70% of the usual value without DDI. Moreover, we also numerically investigate the magnetic hysteresis loop, magnetization self-precession,reversal time and synchronization stability phase diagram for the two-body system in the synchronized dynamics regime.

Recent Results on Field Reversed Configurations from the Translation, Confinement and Sustainment Experiment

The field-reversed configuration (FRC) offers an attractive alternative approach to magnetically confined fusion because of its extremely high β, simple linear geometry, and natural divertor for helium ash removal. Multi-hundred eV and high density FRCs have been produced using the standard Field Reversed Theta Pinch (RFTP) method, with a confinement scaling that leads to fusion conditions. These FRCs are, however, limited to only tens of mWb fluxes and sub-msec lifetime. Recent progress has been made in building up the flux and sustaining the FRC current using Rotating Magnetic Fields (RMF) in the Translation, Sustainment, and Confinement (TCS) facility at the University of Washington. TCS has demonstrated formation and steady-state sustainment of standard, flux-confined, prolate FRCs. The RMF also provides stability for the n = 2 rotational mode, which is the dominant global instability observed experimentally. Simple calculations show that a strong radially inward force imposed by the RMF increases proportionally to any local outward deformation of the plasma cross section. Evidence of this has been experimentally demonstrated, and the effects of various RMF antenna geometries studied. High temperature FRCs could also be produced in TCS by translating high energy plasmoids formed in the normal theta pinch manner into the confinement chamber containing the RMF antennas. Extremely interesting results were obtained for this translation and capture process. The plasmoids can survive the violent dynamics of supersonic reflections off magnetic mirror structures, producing a stable high-β, near-FRC state with substantial flux conversion from toroidal to poloidal. This is a tribute not only to the robustness of FRCs, but also to the tendency of an FRC to assume a preferred state for a magnetized plasma. The magnetic helicity, as inferred by a simple interpretive model, is approximately preserved, possibly conforming to a high-β relaxation principle.

In-plane current-induced magnetization reversal of Pd/CoZr/MgO magnetic multilayers

High critical current density(>10^(6)A/cm^(2))is one of major obstacles to realize practical applications of the currentdriven magnetization reversal devices.In this work,we successfully prepared Pd/CoZr(3.5 nm)/MgO thin films with large perpendicular magnetic anisotropy and demonstrated a way of reducing the critical current density with a low out-of-plane magnetic field in the Pd/CoZr/MgO stack.Under the assistance of an out-of-plane magnetic field,the magnetization can be fully reversed with a current density of about 10^(4)A/cm^(2).The magnetization reversal is attributed to the combined effect of the out-of-plane magnetic field and the current-induced spin-orbital torque.It is found that the current-driven magnetization reversal is highly relevant to the temperature owing to the varied spin-orbital torque,and the current-driven magnetization reversal will be more efficient in low-temperature range,while the magnetic field is helpful for the magnetization reversal in high-temperature range.