Contact temperature and erosion in high-current diffuse vacuum arcson axial magnetic field contacts

We have investigated the surface heating effects of drawn vacuum arcs for several industrial designs of axial magnetic field (AMF) contacts, using near infrared (IR) photography of the Cu-Cr arcing surfaces with an image-intensified charge-coupled device (CCD) camera and an IR pyrometer. This enables detailed contact temperature mapping immediately after a half-cycle of arc current. The very homogeneous temperature distribution observed at current zero stands in contrast to the visually nonhomogeneous high-current diffuse arc, which was studied in separately reported experiments using high-speed digital photography and arc voltage measurements. The peak temperature at current zero increased relatively linearly with the peak current I_P, and reached well beyond the melting range. We combine the temperature maps with a heating model to determine the thermal sheath thickness after arcing and its dependence on I_P. The results suggest that near the interruption limit of AMF contacts, the interaction of the stable high-current arc with the anode and cathode is dominated by processes induced by flowing liquid metal, which redistributes the heat input from the axially concentrated arc over most of the contact surface. Furthermore, the flow of liquid metal off the cathode and anode faces contributes to the overall contact erosion     

Technological progress of axial magnetic field vacuum interrupters

New methods have been developed for investigating vacuum arcs applied to the improvements of vacuum interrupter electrodes. Electrode surface temperature measurements reveal the anode surface melting condition at the current interruption and its influence on interruption capability, and a digital image processing technology gives us a clear understanding of the arcing phenomenon. Measurements and test results have shown the advantages of axial magnetic field electrodes in increasing the interruption capability of vacuum interrupters. It is confirmed that axial magnetic field electrodes are superior to other types of electrodes, and we believe that these new investigation methods promote the optimization of the electrode structure and the realization of higher voltage and current rating vacuum interrupters     

Axial magnetic field contacts with nonuniform distributed axial magnetic fields

It is well known that axial magnetic fields (AMFs) can keep vacuum arc in diffuse mode at high current. According to our recent research and other published papers, it has been found that vacuum arc can be maintained in high-current diffuse mode at much higher current if nonuniform AMF is applied, that the axial magnetic field is higher at contact periphery than at center. The influence of spatial distribution of AMF on vacuum arc is mainly studied in this paper. Furthermore, two types of AMF contacts with new structures to generate nonuniform AMF are presented.     

The influence of unipolar axial magnetic field on the behavior of vacuum arcs

The behavior of vacuum arcs under the influence of unipolar axial magnetic field (AMF) has been investigated. In experimental investigations, the vacuum arc mode is studied at different arc currents by using high-speed charge-coupled device (CCD) video images. In spite of the AMF, first sign of arc constriction appears at relatively small currents of about 15 kA (RMS). Three different arc modes were found. Based on generalized Ohm's law, the current density distribution in the vacuum arc with unipolar axial magnetic field is computed using three-dimensional finite-element method (FEM) software. The calculated current distribution is confirmed by the vacuum arc appearance taken from CCD video film. The distribution of AMF can be optimized by such experiments and theoretical analysis.     

Physical and theoretical aspects of a new vacuum arc controltechnology-self arc diffusion by electrode: SADE

Our new vacuum arc control technology SADE doubles the high current interruption capability of our conventional axial magnetic field technology. First, we describe the vacuum arc motion behavior recorded by a high speed charge-coupled device video camera. This arc behavior is closely related to axial magnetic field intensity. In particular, it depends on the profile of the externally generated axial magnetic field. The anode spot is likely to move to the highest magnetic field intensity. Second, we describe analytical results for concentration of vacuum arc at the anode side contact surface. This analysis implies the possibility of an ideal magnetic field profile and intensity for vacuum arc control. Finally, we describe experimental results for vacuum arc control compared with the physical and theoretical results mentioned above, and we show a practical electrode configuration for vacuum interrupters and its application in a high current interruption experiment     

Influence of bias magnetic field on the vacuum arc in double‐break vacuum circuit breakers

When double‐break vacuum circuit breakers (VCBs) interrupt the fault current, the series arc will generate their individual magnetic fields in different breaks. The magnetic field in one break will influence the arc in another break if the magnetic field is strong enough or the two breaks are very close. In this case, an interactive magnetic field effect happens. This field is also called the bias magnetic field (BMF). BMF can cause anode erosion and affect the performance at current zero. The distribution of BMF and the optimal configuration of the double‐break VCBs were obtained by the electromagnetic field simulation using the Ansoft Maxwell software. Based on the simulated magnetic field data, in the experiments, the interaction between the series vacuum arcs in double‐break VCBs was equivalent to the interaction between a single vacuum arc and the magnetic field generated by a Helmholtz coil. A high‐speed CMOS camera was used to record the trajectory of the vacuum arc plasma under different BMFs with different types of contacts. The results show the BMF can increase the arc voltage, and the arc becomes unstable. When the BMF becomes stronger, the arc voltage increases, and the arc becomes more unstable. In addition, for different types of contacts, the development process of the arc and the influence level are different under the same BMF. For a Wan‐type transverse magnetic field (TMF) contact or strong BMF, metal sputtering is evident and anode erosion becomes serious. For a cup‐type axial magnetic field (AMF) contact, the influence of BMF on the series arc plasma in double‐break VCBs is less than that of the Wan‐type TMF contact. The results of this work may be helpful for the design of compact double‐break VCBs.     

The Investigation of Copper-Chromium Contacts in Vacuum Interrpters Subjected to an Axial Magnetic Field

The characteristics of a vacuum arc between CuCr contacts under an axial magnetic field have been investigated. Test samples were made of CuCr contacts, and the arcing voltage was measured. The arcing voltage of the CuCr contacts under an axial magnetic field is lower than that of pure copper contacts by 10-20 V. From the measurement of the post-arc current and interruption test results, it was found that the arc concentrates and a part of the electrode melts at a rather low current, but that the electrode melting does not affect the interrupting capability. Also, the insulation characteristics were measured. With respect to voltage conditioning, high-current conditioning improved the breakdown voltage by 50 percent for a 20-mm gap and by 100 percent for a 3-mm gap. These test results show that CuCr contacts, used with axial magnetic fields, are promising for use in high-voltage and high-power vacuum interrupters.     

四注行波管周期永磁聚焦系统的优化设计

根据电子注参数、布里渊磁场公式及周期聚焦系统轴向磁通密度峰值经验公式,计算出永磁聚焦系统轴向峰值磁通密度,利用Ansoft Maxwell 3D软件建立了四注行波管周期永磁聚焦系统仿真模型,对模型结构尺寸进行了参数化分析,研究了周期永磁聚焦系统结构尺寸对聚焦系统通道内磁通密度的影响,确定了磁聚焦系统结构最佳尺寸配合,优化设计出了四注行波管周期永磁聚焦系统。其电子注通道中心轴线上轴向磁通密度峰值B_z=309 mT,横向磁通密度B_t=2.86 mT,B_t/B_z=0.92%。     

Influence of interrupted current amplitude on the post-arc currentand gap recovery after current zero-experiment and simulation

For commercial vacuum circuit interrupter valves with radial field contact, the post-arc current (PAC) waveform was measured after short circuit current interruption applying a transient recovery voltage (TRV) of about 15 kV/s (RRRV). Keeping the current decline di/dt at current zero constant, the power frequency (PF) current amplitude Iˆ_PF was varied from 0.5 up to two times of the rated short circuit current. Significant influence of Iˆ_PF on the gaps memory is shown in particular effecting the post-arc current duration which varies in a range between 2 and 8 μs. Based on the existing physical models an interpretation of the results is given. It is shown that the memory effect influences the gap recovery time. The experimental results were further used to determine the parameters of the sheath growth model described by Andrews and Varey (1971). The initial ion density is fitted according to the current ramp di/dt before current zero (CZ) and according to the measured Q_PF=∫i _PF×dt during the arcing phase. The results of two different test procedures were applied in order to separate the effect of current ramp di/dt and Q_PF=∫i_PF×dt. The influence of both parameters and of the rate of rise of the recovery voltage du/dt (RRRV) on the sheath edge velocity could be demonstrated by simulation     

Cathode processes in free burning and stabilized by axial magneticfield vacuum arcs

Collective behavior of the cathode spots (CS) has been investigated in free burning and stabilized by axial magnetic field (AMF) vacuum arcs. Experiments carried out proved previously discovered phenomenon of CS group formation in free burning arc to be a general phenomenon for a short high-current vacuum arc. The dependency of CS group size in the developed are on arc current for different contact materials has been analyzed. Application of AMF with even relatively low intensity strongly affects on cathode processes. In short arcs, it hinders formation of the CS group and consequently reduces thermal stress applied to the electrodes. It has been revealed that high current vacuum arc under the action of AMF can exist only at current densities exceeding certain minimal value that depends on AMF intensity, contact gap, and does not depend on current itself. The dependency of this minimal (or normal) current density on AMF intensity has been studied for short and long vacuum arcs. A qualitative model of the cathode spot dynamics has also been proposed     

Recent technical developments in high-voltage and high-power vacuumcircuit breakers

Explanations are given regarding recent technical topics in the development of vacuum circuit breakers. The area-effect concept is introduced to aid in insulation designs of vacuum interrupters. By using Cu-Cr prepared through a special method, the voltage performance of vacuum interrupters has been remarkably improved. Through the development of axial magnetic field electrodes, large-current interruption has been made possible. The design concept for determining contact radii has been established. A computer simulation method for generating switching overvoltage is described, thereby indicating the possibility of realizing large-current interrupting vacuum interrupters with low switching overvoltages     

The Gamitron - A High Power Crossed-Field Switch Tube for HVDC Interruption

A high power crossed-field discharge device has been developed for use as a high voltage direct current interrupter. This device operates at low pressure (0.05 Torr), conducting current at a fixed voltage (~500 V) only in the presence of a weak magnetic field (~100 G) which is substantially perpendicular to the electric field between the electrodes. When the magnetic field is removed, ionization ceases and current interruption results. Physical phenomena occurring in this device have been investigated, including the glow-to-arc transition, gas cleanup, and high voltage breakdown. Based on the results of these investigations, switch tubes have been developed and successfully tested at the 2 kA, 100 kV level, with recovery rates in excess of 2 kV/?s. The availability of such devices will make possible many applications including HVDC circuit breakers, ac current limiters, and practical inductive energy storage.     

复合外场中磁光材料(BiTm)3(FeGa)5O12薄膜畴壁运动特性的研究

通过对BiTm)₃(FeGa)₅O₁₂膜施工加低频交变磁场,匀速率增加的直流磁场和同时对该膜施加这两种磁场(复合外场),用照相划线读数方法和通过电荷耦合器件(CCD)-计算机作数字化处理获得磁畴壁(DW)的相对百分数。结果表明:(1)复合外场下DW运动规律中存在交互作用项;(2)低频交变磁场幅值(140A/m)远低于等效阻力场(10³A/m)时,DW可以运动,但不同步,频率大于1Hz时明显滞后,(3)利用图像转换有利于提高实验结果的分辨率。     

Analysis of the deflection system for a magnetic-field-immersedmagnicon amplifier

A linear analysis of the electron-beam deflection system in a magnicon amplifier is presented. The system consists of identical cavities, one driven and the remainder passive, separated by a drift space and immersed in an axial magnetic field. The cavities contain a rotating TM₁₁₀ mode. The length of each cavity is πν _z/ω, and that of the drift space is πν_z/ω_c, where ω is the RF frequency, ω_c is the relativistic gyrofrequency in the guide field, and ν_z is the mean axial velocity of the beam electrons. The linearized electron orbits are obtained for arbitrary initial axial velocity, radial coordinate, and magnetic field. The small-signal gain and the phase shift are determined. The special case where ω_c/ω=2 has unique features and is discussed in detail. For the NRL magnicon design, a power gain of 10 dB per passive cavity is feasible. Results from numerical modeling of a magnicon with two passive cavities are presented. Operation of the output cavity at the fundamental and higher harmonics of the input drive frequency is briefly discussed     

极化电压对聚丙烯压电驻极体膜压电性能的影响

压电驻极体是具有压电效应的微孔结构空间电荷驻极体材料,其压电性能与材料的微结构和空间电荷密切相关.本文首先利用压缩气体膨化工艺对聚丙烯(PP)的微结构进行改性,然后利用接触极化方法,研究了极化电压与PP膜空间电荷密度之间的关系,及其对压电性能的影响.结果表明对于极化前厚度为100μm的PP膜,其内部建立有序空间电荷分布的阈值极化电压为2 kV;一旦有序空间电荷建立起来,PP膜即具有压电效应.随着极化电压的提高,PP膜的空间电荷密度逐步增大,压电效应显著增强.当峰值电压为8 kV时,PP膜电极上的电荷密度、准静态压电系数和品质因数FOMv(d33·g33)分别为0.56 mC/m2,379 pC/N和8.6(GPa)-1.PP压电驻极体膜的FOMv比聚偏氟乙烯(PVDF)铁电聚合物膜高2个量级以上,且声阻抗非常低(～0.025 MRayl),因此该压电膜在超声波发射-接收系统或脉冲-回波系统中具有明显的优势.     

圆极化TE11输出模式旋向可在线切换的相对论磁控管模拟研究

提出了一种圆极化TE₁₁输出模式旋向可在线切换的相对论磁控管。该器件互作用区结构采用同腔型磁控管结构,输出结构采用全腔提取结构,励磁系统采用Helmholtz线圈磁场系统。本文利用全腔提取结构的模式激励理论对该器件的输出模式成分进行了理论分析,利用粒子模拟软件对该器件的工作性能进行了模拟研究。粒子模拟结果表明:在外加电压770 kV和外加轴向磁场0.2 T（方向与微波轴向输出方向同向）的条件下,该器件的工作模式为5π/6模,工作频率为2.35 GHz,输出功率为3.86 GW,功率效率达到55.5%,输出模式为右旋圆极化TE₁₁模式且模式纯度达到99%以上;当外加轴向磁场与微波轴向输出方向反向时,该器件的输出模式即可在线切换为左旋圆极化TE₁₁模式,而其他输出性能基本保持不变。     

Thermal Quantum Discord in Pure Dzyaloshinskii-Moriya Model with Magnetic Field

We investigate the effects of the directions of Dzyaloshinskii-Moriya (DM) interaction vector and magnetic field on the quantum discord in the pure DM model. For different directions of DM vector, we find that there are different optimal parameter components of magnetic field. Moreover, we find that the optimal parameter components rules are the same for the Hamiltonian H₁ and H₂. According to the rules, for a certain axial DM vector, we can get the maximal quantum discord by adjusting the direction of the external magnetic field, which is feasible under the current experimental technology.     

150kV脉冲X射线测试系统

基于Marx发生器原理设计了150kV脉冲X射线测试系统,该系统采用正负极充电的双边Marx发生器线路,Marx发生器设计为15级同轴结构,采用紧凑低电感设计来获得窄脉冲的输出。实验结果表明,X光机设计合理,获得了窄脉宽、高幅度的高压脉冲输出:在充电电压为20kV的情况下,X射线管电压150kV;X射线脉宽约60ns;25cm处剂量约7.8×10-6 C/kg;焦斑直径2.5mm。搭配计算机X射线摄影系统,成像面积可达30cm×40cm,分辨力大于1lp/mm,可以满足一般低能闪光照相的需要。     

Evolution of the gas-puff Z-pinch column

The evolution of a neon implosion of low energy discharge (4 kJ, 40 kV, 150 kA, 1.1 μs) was studied using X-ray, schlieren, and high speed electrooptical visible gated quadro camera diagnostics. The geometry, an helical structure of pinched column, two steps of pinching, and X-ray emission have been studied. The hypothesis of an axial component of magnetic field generation, the helical shape of the magnetic and electric field lines, and the release of magnetic energy during the second pinching of the column are discussed     

Study of fast switching processes due to electric and magnetic fields-an NMR approach

Solid state NMR techniques have been developed to investigate dynamic molecular effects (e.g., molecular reorientations) due to simultaneously applied external electric fields on electrically sensitive materials such as liquid crystals (LC), liquid crystalline polymers (LCP) and polymeric electrets. Such effects can be observed only on relatively thin systems (10-200 μm). That means that many scans are necessary to achieve a sufficiently high signal-to-noise-ratio in the spectra (500-1000 scans). If the material is also magnetically sensitive, the electric field can be used to orient molecules in a starting orientational state and by switching-off the voltage to access fast reorientation processes in the magnetic field B₀. Until now, the behaviour of orientable molecular systems under the influence of electric fields has been investigated by means of a more or less quasistatic approach (LCP: 100 V, electrets: 1 kV) in equilibrium states. The achievable time resolution depends on the desired signal-to-noise-ratio. For the case of proton NMR this means a time resolution of about 10 min. However, very often switching processes occur on a much shorter time scale. Using conventional techniques it is impossible to observe fast (ca. 100 μs) electrically or magnetically induced reorientation processes. In this work, we present a concept to overcome the problems outlined above and to extend the area of our current in situ NMR investigations on thin electrically-switched or poled polymeric layers. The basic idea is to include synchronized electric pulses during the NMR experiment using the preparation and/or mixing periods of a 1D or 2D pulse sequence for the application of an orienting field (electric or magnetic) and to use the reversibility of the molecular switching phenomenon to achieve a sufficient signal-to-noise-ratio. The techniques extend the range of possible investigations from about 100 μs to approximately T₁ for correlated spectra (and to longer times of applied fields for uncorrelated spectra). Results are shown for a nematic LC and a nematic polymer having a similar side chain.