On priorities of cathode and anode contaminations in triggering the short-pulsed voltage breakdown in vacuum

Modern theoretical notations on electrical breakdown in vacuum consider cathode triggering mechanisms to be most responsible on short-pulsed (<1 /spl mu/s) breakdowns while anode mechanisms to be responsible in a part on DC and long-pulsed breakdowns. Following those notations, we tried to reveal conditions at which either mechanism steps aside to another one. The study involved several experimental techniques including the anode-probe surface scanning, pulsed electron-beam surface melting in vacuum for surface cleaning, and intentional dust particle contamination of electrode surfaces. Breakdown tests were performed using a pulser capable of producing 220 kV quasi-square pulses that were adjustable to /spl sim/30 to 80 ns pulse length. Our experiments showed that cathode emission sites are responsible for breakdowns at relatively low hold-off fields. At higher electric fields of up to 1 MV/cm, the anode share in the mechanism of triggering breakdown becomes probably more significant than the cathode mechanism.     

On the variety of triggering mechanisms of high-voltage breakdown in vacuum

The paper is devoted to study of the mechanisms responsible for vacuum breakdown as a whole and the total voltage effect in particular. Experiments at DC and pulsed voltages were carried out. It has been shown that there is no manifestation of the total voltage effect at DC voltages up to 20 kV. The strong dependence of hold-off on anode temperature was recognized at DC voltages while pulsed hold-off turned out to be almost the same with heating the electrodes. This gives a basis to consider gas desorption as an insufficient factor in the initiation of pulsed breakdown. An attempt to enhance hold-off with electrostatic removing of loosely bound particles with assistance of electron flow from a thermionic cathode was undertaken in the work. The approach turned out to be ineffective.     

Relationship between electrode surface roughness and impulse breakdown voltage in vacuum gap of Cu and Cu-Cr electrodes

The relationship between the impulse breakdown voltage of vacuum gaps and electrode surface roughness was investigated for the purpose of controlling the surface roughness on HV conductors. The roughness of mechanically polished Cu and Cu-Cr electrodes was measured with a roughness meter, and the relationship between the breakdown voltage and surface roughness was obtained for plate-to-plate gaps. The discharge-conditioning effect increased with reduction in the surface roughness. The breakdown voltage depended not only on the roughness of the cathode but also on the anode surfaces. Reducing the surface roughness was found not to be an effective way to increase the breakdown voltage for non-uniform field gaps after discharge conditioning.     

Electrical insulation characteristics of cold dielectric high temperature superconducting cable

For the optimization of electrical insulation design for high temperature superconducting (HTS) cable, evaluation of electrical insulation characteristics especially for butt gap of LN/sub 2/ impregnated cold dielectric (CD) which consists of the wrapped tape insulation impregnated with LN/sub 2/ plays an important role. This paper presents partial discharge (PD) inception and breakdown characteristics in LN/sub 2/ impregnated butt gap model which modeled a weak point of the wrapped tape insulation impregnated with LN/sub 2/ and cable model with short length with polypropylene laminated paper (PPLP/sup /spl reg//), Nomex/sup /spl reg// paper and cellulose paper. PD current pulse was found to have a steep rise time of /spl sim/ ns and amplitude of /spl sim/ tens /spl mu/A at PD inception voltage region. Little dependency of breakdown stress on the insulating material is found. PD inception stress is almost independent of insulation thickness of 1 to 3 mm. The requirement insulation thickness for 66 kV class HTS cable is estimated to be /spl sim/ 5 mm under PD-free condition from viewpoint of long-term reliability.     

百纳秒脉冲下水压对水开关击穿特性的影响

以俄罗斯西伯利亚研究院通过实验总结出的液体介质气泡击穿理论的全新阐述作为理论基础,开展了300 NS短脉冲下高压强水介质开关的击穿实验,获得了该条件下水介质击穿场强及耐压时间与水中压强关系的数据。结果表明在300 NS脉冲电压实验平台下,当水介质场强变化率为2~3 MV/CM时,水介质击穿电压和耐压时间均随着水中压强的提高而呈上升趋势,且趋势越来越显著。     

Impulse breakdown voltages of triggered multistage vacuum gap

Several sealed-off triggered vacuum gaps are connected in series to improve hold-off voltage. The characteristics of impulse breakdown voltage of these series-connected gaps are investigated experimentally. The sum hold-off voltage of series-connected gaps decreases to a unit hold-off voltage when the maximum value of voltage division ratio across the gaps increases to unity. Self-breakdown probability of the series-connected gaps is always higher than that of a single gap under the same conditions. Hence, stage efficiency of the multistage gap decreases with increasing number of stages. Its value is 90 percent with 2-stage gap and 75 percent with 5-stage gap, respectively, under the same voltage division ratio and the same gap length (2.0 mm) in each stage. Triggered breakdown voltage of 2- or 3- stage gap is several hundred volts when all gaps are triggered simultaneously at the peak of the main impulse wave and a working voltage range is nearly 100 percent in this case. The working voltage range decreases with number of stages. Its value is 45 percent with 3-stage gap and 15 percent with 5-stage gap, respectively, when one triggered gap is fired for switching.     

Breakdown conditioning characteristics of long gap electrodes in a vacuum

An improvement in dielectric strength is required in vacuum circuit breakers (VCBs) intended for use in higher voltage systems. In order to develop higher voltage VCBs, it is important to improve the dielectric strength in a vacuum based on consideration of the vacuum breakdown mechanism. Particularly for gaps longer than 10 mm, little is known about the breakdown mechanisms and their quantitative analyses in a vacuum. This paper discusses the breakdown conditioning characteristics of long gap electrodes, under a non-uniform electric field in a vacuum. We treat gap lengths of up to 50 mm in this paper. The conditioning characteristics are investigated under impulse voltage applications. A negative standard lightning impulse voltage was applied to rod-plane electrodes made of Cu-Cr and SUS304 for different tip radii and gap distances until the conditioning effect was completed. We observed illumination spots on electrodes at each breakdown during the conditioning process and calculated the corresponding breakdown field strengths. Experimental results revealed that the tendency of breakdowns associated with long gaps is different from that for the short gaps. As a result, we clarified that the breakdown field strengths are nearly constant at 110-120 kV/mm at the distances longer than 10 mm, and the breakdown field strength is at its maximum when the gap distance is about 5 mm.     

Design of a cylindrical high-voltage high-temperature vacuum insulator

A design for a high temperature, HV insulator is presented. This design exploits the temperature dependent thermal and electrical properties of alumina and a unique structural insulator design to increase the high voltage hold-off characteristics of the insulator, reduces thermal flux through the insulator, and alleviate stresses caused by mismatch between insulator and electrode thermal expansion coefficients. The designed insulators insulate graphite electrodes at temperatures approaching 1000/spl deg/C and potentials up to 50 kV. In this paper, the general methodology of designing these insulators is explored, and some limited performance data is presented.     

Surface switching characteristics of variable permittivity dielectrics

Flashover voltage, lifetimes, and switch performance of insulators utilizing square thin and thick film electrodes were examined to determine the viability of using thin electrodes for reliable surface discharge switching. Gold, silver, and platinum were sputtered (0.25 /spl mu/m) and screen printed (15 /spl mu/m) onto Al/sub 2/O/sub 3/, TiO/sub 2/, and modified BaTiO/sub 3/ (MBT), then tested in air at 10/sup 5/ Pa, under vacuum (10/sup -3/ torr), and while immersed in an insulating fluid, SF-2 (manufactured by 3M). For the measured range of 0.5 to 3 mm in air, the flashover voltage for all three insulators was found to have a linear dependence on the electrode separation distance with 15 /spl mu/m thick screen printed electrodes and a square root dependence with 0.25 /spl mu/m thick sputtered electrodes. Delay times of approximately 20 ns with a corresponding jitter of 6 ns were observed across all three insulators under triggered flashover. Insulators in air with sputtered electrodes had lifetimes of approximately 5 flashovers for dc flashover and 40 for triggered flashover. Screen printed TiO/sub 2/ and MBT had dc lifetimes of approximately 10 flashovers in air, and 3 flashovers in vacuum and SF-2. Screen printed TiO/sub 2/ and MBT had triggered lifetimes of greater than 200 flashovers in air, and <3 flashovers in vacuum and SF-2. Screen printed Al/sub 2/O/sub 3/ had dc and triggered lifetimes of greater than 200 flashovers in air, vacuum and SF-2. Insulator failure during dc flashover was determined to be due to the formation of a conductive channel between the anode and cathode. Formation of the channel was attributed to insulator thermal and dielectric properties and the presence of vaporized electrode species in the gap region during flashover.     

直流电絮凝法处理印染废水的研究

为提高传统化学法和生物法处理色度高、颜色变化频繁、化学需氧量(COD)值大和pH值高的印染废水的效果,试验研究了用直流电絮凝法处理实际印染废水并分析了COD、pH值和COD的变化以及极板和电能的消耗。试验结果表明,电絮凝法处理效果好,对pH值要求低,工艺简单,操作方便,成本低。     

铁电体触发赝火花开关实验研究

本文阐述了铁电体触发开关的工作原理,进行了铁电体触发开关的实验研究。得出了气压与开关导通时间和延时的关系,并对此实验结果进行了分析。在开关间距为3mm、真空度为2.5Pa、阴极腔的深度为19mm、小孔直径为4mm的条件下测得开关的抖动小于3ns。实验结果表明触发电压的高低对开关抖动的影响很大,随着触发电压的升高开关的抖动越来越小。当触发电压达到6kV左右时,开关的抖动小于1ns。     

Nanosecond, electrical triggering of water switches

Spark gaps, which use water as a switching medium, allow fast closing and high repetition rate operation for high power and high voltage switching. They are usually operated in the pulse-charging mode and have a large jitter. To reduce the jitter, a trigger electrode with positive polarity is used to initiate breakdown between a negative high voltage electrode and a ground electrode. The trigger electrode consists of a tungsten wire with a diameter of 50 μm, enclosed in glass tubing. At the triple point where the three dielectrics (glass, water and metal) meet, the electric field is greatly enhanced. From the triple point, a trigger voltage of +16 kV at a pulse duration of 100 ns produces two streamer branches traveling towards the high voltage and ground electrodes. These two streamer branches bridge the main gap and serve as preionized channels for the initiation of the breakdown between the main electrodes. Arc transition develops along the two channels and completes the switching. It allows for the gap to be fired at various voltages independent of the gap?s self-breakdown voltage. The delay between application of the trigger pulse and the electrical breakdown is on the order of hundred nanoseconds. The jitter in breakdown is on the order of ten nanoseconds. It is less by more than an order of magnitude than that obtained with pulse charging.     

Surface-discharge characteristics in a ferrite-cathode diode

The surface-discharge characteristics of a HV diode with a planar ferrite cathode are described. This experimental setup consists of a constant HV power supply, a polarity inversion line pulser with a 200 m coaxial cable, a turbomolecular pump and a discharge chamber. The chamber was of a diode type which was evacuated by a turbomolecular pump with a pressure of 13.3×10^-3 Pa and was composed of a tungsten disk anode, a planar ferrite cathode, and a polymethylmethacrylate tube body. The cathode was set perpendicular to the anode plane, and the space between the anode and cathode electrodes had a value of 0.5 mm. The coaxial cable of the pulser was charged from 10 to 40 kV by the power supply, and the rectangular pulses with output voltages of -1× the charged voltage were produced with short rise times of ~20 ns. The effective duration of the pulse was determined by the cable length and had a value of ~1 μs. The process of the flashover investigated in this report consisted of two stages. In the first stage, because the discharge current flowed inside the cathode plate, the current was primarily regulated by the cathode impedance in proportion to the resistance. Next, the current substantially increased with corresponding growth of the surface discharge on the ferrite cathode. The duration of the initial current increased with the thickness of the cathode and the diameter of the contact electrode. In the present work, the maximum current was determined by the characteristic impedance of the pulser cable since the discharge impedance in the diode substantially decreased during the surface discharge     

The influence of pressure and conductivity on the pulsed breakdownof water

The influence of hydrostatic pressure and liquid conductivity on the dielectric breakdown of water solutions subjected to high amplitude electric fields of sub-microsecond duration has been investigated. Well-defined pulses (80 kV, 3 ns risetime, 100 ns duration) have been applied to a gap (0.4 to 2.1 mm), between Rogowski profile electrodes (thus ensuring a uniform electric field), containing de-ionized water (non-distilled, and distilled and ultrasonically treated), sodium chloride solutions (0.001 to 1.0 molar), or magnesium sulfate solutions (0.01 to 0.1 molar). Breakdown in these liquids has been studied at pressures from atmospheric up to 40 MPa. The inter-electrode potential and the current response were measured indicating the time lag to breakdown, breakdown voltage, and temporal characteristics of the breakdown process. The breakdown time lag increases with increasing pressure, and is insensitive to the liquid conductivity. These findings have relevance to the ongoing discussion concerning `thermal' vs. `electronic' mechanisms for dielectric breakdown in liquids. In particular, the results suggest that breakdown evolves via `bubble' formation by field emitted currents near asperities on the cathode, and that the time for the change in liquid conductivity as a result of breakdown is limited by processes other than ionization growth (due to electron impact ionization of molecules in the `bubble') of prebreakdown electron currents     

Surface treatment of HV electrodes for superconducting cyclotronbeam extraction

At the Istituto Nazionale di Fisica Nucleare Laboratorio Nazionale del Sud of Catania, a K=800 superconducting cyclotron is in operation since 1994. The accelerator beam extraction is accomplished using electrostatic deflectors constituted by HV electrodes generating electric fields to 140 kV/cm in 6 to 8 mm gaps. The electrostatic performance is strongly dependent on the presence of high magnetic fields (up to 5 T) and of high ion energy and intensity of heavy ion beams. The behavior of these systems is mainly determined by the field electron emission effects on the cathode surface, by electron focalization processes due to the magnetic field and by heating effects due to the ion beams. A detailed study of these effects is under development starting with point, the characterization of the electrode surface vs. the material used (Ti, Cu, stainless steel, etc.) and of the best mechanical and chemical treatment available, Moreover, we are studying the electrostatic performance by using titanium surfaces and nitriding over a wide temperature range (300 to 800°C). Preliminary results concerning the use of thermal processed polished surfaces are presented and discussed     

Surface flashover of oil-immersed dielectric materials in uniform and non-uniform fields

The applied electrical fields required to initiate surface flashover of different types of dielectric material immersed in insulating oil have been investigated, by applying impulses of increasing peak voltage until surface flashover occurred. The behavior of the materials in repeatedly over-volted gaps was also analyzed in terms of breakdown mode (some bulk sample breakdown behaviour was witnessed in this regime), time to breakdown, and breakdown voltage. Cylindrical samples of polypropylene, low-density polyethylene, ultra-high molecular weight polyethylene, and Rexolite, were held between two electrodes immersed in insulating oil, and subjected to average applied electrical fields up to 870 kV/cm. Tests were performed in both uniform- and nonuniform- fields, and with different sample topologies. In applied field measurements, polypropylene required the highest levels of average applied field to initiate flashover in all electrode configurations tested, settling at ~600 kV/cm in uniform fields, and ~325 kV/cm in non-uniform fields. In over-volted point-plane gaps, ultra-high molecular weight polyethylene exhibited the longest pre-breakdown delay times. The results will provide comparative data for system designers for the appropriate choice of dielectric materials to act as insulators for high-voltage, pulsed-power machines.     

Improvement of microwave power from a virtual cathode oscillator and its repetitive operation using a metal mesh anode

A virtual cathode oscillator with a stainless‐steel mesh anode of various transparencies and wire diameters was studied experimentally for the enhancement of microwave power and its repetitive operations. The maximum microwave power observed was about 20 MW at 12 GHz for a diode voltage of 250 kV and an electron beam current of 39 kA using an anode mesh with wire diameter of 0.22 mm and a transparency of 67%. The microwave emission was enhanced by decreasing the mean angle of beam scattering when a mesh of smaller wire diameters was used in the anode. The increased transparency of the fine mesh also contributed to the enhancement of the microwave emission. Use of the mesh anode afforded the operation in several repetitive shots. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(2): 1–10, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10259     

高压电场中金属丝段的电爆现象

金属丝电爆是制备超细粉的一种新方法,为了解决通常所采用金属丝与电极接触后通入大电流的电爆方法容易发生电极烧蚀这一问题,开发了高压电场中金属丝段与电极非接触电爆的设备。通过改变电极间距、金属丝长度和电场电压,进行了系列高压电场中金属丝段电爆试验。结果表明,在高压电场中电极与金属丝的端部发生气体放电,将大电流导入金属丝段而发生电爆,可减轻对电极的烧蚀;由于金属丝段的端部与电极之间的等离子体旁路作用,金属丝段的端部残留0.5~2 mm长的金属丝不发生电爆;适于制备粉末的电场中金属丝段电爆的工艺条件是,电场电压为6、7、8 kV时,与之匹配的电极间距与丝长之差的范围分别为:1.8~4 mm、2.2~5.8 mm和3.2~8 mm。     

Parallel streamer discharges between wire and plane electrodes in water

Streamer discharges in tap water and distilled water have been generated by applying a voltage pulse from 120 to 175 kV and 500 ns duration to a wire-to-electrode configuration. Electrical and optical diagnostics were used to explore the temporal development of the streamers in tap and distilled water, at various applied voltages and both polarities. With the wire serving as anode, multiple, parallel streamer discharges were generated. The number density of these streamers along the wire decreases with decreasing electric field on the surface of the wire. The dependence of the streamer density on electric field indicates the role of field enhancement at inhomogeneous microstructures along the wire as streamer initiation mechanism. The appearance of the discharge was different for tap and distilled water. However, the measured average streamer propagation velocity from the positive wire to the grounded plane electrode, of 32 mm//spl mu/s, was independent of the water conductivity and the applied voltage. This suggests the existence of a self-sustained electric field at the streamer head. With the wire serving as cathode, only a weak light emission from the area close to the wire was observed, and streamers did not appear for the same voltage amplitude as with the positive polarity. This suggests that an ionic current flowing in the water is not dominant in the streamer propagation process.     

Influence of sand/dust contamination on the breakdown ofasymmetrical air gaps under switching impulses

This paper discusses the influence of airborne and surface-adhering sand/dust contamination on the breakdown characteristics of asymmetrical air gaps when subjected to standard switching impulses of both polarities. Extensive measurements of breakdown voltages and breakdown times are carried out under clean and polluted conditions using rod-plane and sphere-plane gaps. When positive switching impulses are applied to gaps ⩽1.8 m, the presence of sand/dust particles ⩽150 μm diameter and ⩽2 g/m³ concentrations have insignificant influence on the breakdown characteristics of the air gaps. However, the presence of such particles can greatly affect the breakdown characteristics for negative switching impulses. Under such conditions, the particles can decrease V₅₀ by ⩽40% for gaps with field factor f ⩽3.5. For gaps with 3.550 by ⩽80%. For more nonuniform field gaps, V₅₀ values for clean and contaminated conditions are similar. The particles can also significantly influence the breakdown time lag characteristics under negative switching impulses. The observed effects are primarily caused by sand/dust particles which settle on the cathode, while the particles present in the interelectrode gap or on the anode have very little influence