Vacuum Surface Flashover Research Articles

Discharge characteristics for various discharge patterns under negative lightning impulse voltage in vacuum

In order to develop higher voltage vacuum interrupters (VIs), electrical insulation performances in vacuum should be improved. The discharge in VI has various and complex patterns, such as breakdown in vacuum gap and surface flashover on a solid insulator. In this paper, we investigated three discharge patterns: the discharge in vacuum gap, the discharge in vacuum gap via shield, and the surface flashover on solid insulator, respectively. By measuring shield potential and insulator surface potential, we analyzed the discharge characteristics for various patterns, and clarified the relevant discharge development process. We found that the breakdown development process in vacuum gap depends on the gap length and the surface flashover development process on a solid insulator depends on the applied voltage and surface distance.

Control and suppression of impulse surface flashover in vacuum based on discharge development mechanisms

Surface discharge on solid insulator in vacuum is one of the significant issues for the development of vacuum circuit breakers (VCBs) and vacuum interrupters (VIs). For the development of higher voltage VCBs and VIs, control and suppression techniques of the surface flashover are required. The aim of present study is to suppress the surface flashover, based on physical mechanisms of flashover development. Impulse surface flashover in vacuum, starting with explosive electron emission (EEE) from cathode, develops with the formation of transient charge on the insulator as well as the development of secondary electron emission avalanche (SEEA) along the surface toward anode. To reduce the EEE current from the cathode and the SEEA development, we changed the electrode geometry such as thickness of alumina ceramics and tip radius of cathode rod, respectively, and installed a plane electrode as the back electrode of the cathode rod. As the results, EEE current was reduced by the field relaxation effect of transient charging due to EEE, and SEEA was also suppressed by the perpendicular electric field of the plane electrode. When the flashover was suppressed by using the cathode back electrode, transient charge amount was smaller on alumina dielectrics at the end of the plane electrode area. From the above results, it would be an effective technique for the control and suppression of surface flashover in vacuum to enlarge the perpendicular electric field for the insulator.

Initiation of vacuum insulator surface high-voltage flashover with electrons produced by laser illumination

In this paper, experiments are described in which cylindrical vacuum insulator samples and samples inclined at 45° relative to the cathode were stressed by microsecond timescale high-voltage pulses and illuminated by focused UV laser beam pulses. In these experiments, we were able to distinguish between flashover initiated by the laser producing only photo-electrons and when plasma is formed. It was shown that flashover is predominantly initiated near the cathode triple junction. Even dense plasma formed near the anode triple junction does not necessarily lead to vacuum surface flashover. The experimental results directly confirm our conjecture that insulator surface breakdown can be avoided by preventing its initiation [J. G. Leopold et al., Phys. Rev. ST Accel. Beams 10, 060401 (2007)] and complement our previous experimental results [J. Z. Gleizer et al., IEEE Trans. Dielectr. Electr. Insul. 21, 2394 (2014) and J. Z. Gleizer et al., J. Appl. Phys. 117, 073301 (2015)].

Investigation Into the Operating Characteristics of Fused Quartz Vacuum Surface Flashover Switch

Vacuum surface flashover switch is a promising technique scheme of high repetitive frequency fast-closing switch applied to dielectric wall accelerator. In this paper, the switch is experimentally investigated, and a nanosecond pulse with 2.02-ns effective rise time is obtained. For a cylinder-fused quartz dielectric, the breakdown field of the switch is near 55 kV/cm, and the value for the frustoconical-shaped dielectric exceeds 120 kV/cm with a lifetime up to 69000 discharges. The repetitive frequency ability of the switch is better than 100 Hz, and the breakdown voltage increases slightly with higher frequency.

A multi-functional high voltage experiment apparatus for vacuum surface flashover switch research.

A multifunctional high voltage apparatus for experimental researches on surface flashover switch and high voltage insulation in vacuum has been developed. The apparatus is composed of five parts: pulse generating unit, axial field unit, radial field unit, and two switch units. Microsecond damped ringing pulse with peak-to-peak voltage 800 kV or unipolar pulse with maximum voltage 830 kV is generated, forming transient axial or radial electrical field. Different pulse waveforms and field distributions make up six experimental configurations in all. Based on this apparatus, preliminary experiments on vacuum surface flashover switch with different flashover dielectric materials have been conducted in the axial field unit, and nanosecond pulse is generated in the radial field unit which makes a pulse transmission line in the experiment. Basic work parameters of this kind of switch such as lifetime, breakdown voltage are obtained.

Optimization of the vacuum insulator stack of the MIG pulsed power generator

The MIG multi-purpose pulsed power machine is intended to generate voltage pulses of amplitude up to 6 MV with electron-beam loads and current pulses of amplitude up to 2.5 MA and rise time '00 ns with inductive loads like Z pinches. The MIG generator is capable of producing a peak power of 2.5 TW. Its water transmission line is separated from the vacuum line by an insulator stack. In the existing design of the insulator, some malfunctions have been detected. The most serious problems revealed are the vacuum surface flashover occurring before the current peaks and the deep discharge traces on the water-polyethylene interface of the two rings placed closer to the ground. A comprehensive numerical simulation of the electric field distribution in the insulator of the MIG generator has been performed. It has been found that the chief drawbacks are nonuniform voltage grading across the insulator rings and significant enhancement of the electric field at anode triple junctions. An improved design of the insulator stack has been developed. It is expected that the proposed modification that requires no rearrangement of either the water line or the load-containing vacuum chamber will provide higher electric strength of the insulator.

Vacuum surface flashover: experiments and simulations

It is conjectured that vacuum dielectric surface flashover can be avoided by preventing its initiation. Assuming that the flashover process is initiated by the impact of charged particles on the insulator surface, by deflecting these away from the surface, the development of a flashover avalanche can be repressed. As evidence of this, we present the results of experiments where high-voltage fast-rising microsecond timescale voltage pulses are applied on vacuum insulator samples where we methodically manipulate the magnitude and the orientation of the electric fields at the cathode and anode triple junctions and along the insulator surface.

Mode Transition of Vacuum Arc Discharge and Its Effect on Ion Current

The mode transition of a pulsed vacuum arc discharge, which is from vacuum surface flashover to non-surface vacuum breakdown or vice versa, is studied by simply adjusting a trigger resistor. This approach provides a possibility to research the transition discharge process. Since the transition process is smooth and controllable, the transition mechanism and its effect on the performance of an ion source can be investigated via various diagnosis experiments. The experimental results show that the mode transition occurs when the resistance is in the range of 0–10Ω. With the mode transition from surface flashover to non-surface vacuum breakdown, the vacuum arc discharge becomes more intense and the ion current produced by the Ti cathode ion source increases by two times. The related physical mechanism is also discussed in detail.

Measurements of an expanding surface flashover plasma

A better understanding of vacuum surface flashover and the plasma produced by it is of importance for electron and ion sources, as well as advanced accelerators and other vacuum electronic devices. This article describes time-of-flight and biased-probe measurements made on the expanding plasma generated from a vacuum surface flashover discharge. The plasma expanded at velocities of 1.2–6.5 cm/μs, and had typical densities of 1010–1012 cm−3. The expansion velocity of the plasma leading edge often exhibited a sharp increase at distances of about 50 mm from the discharge site. Comparison with biased-probe data suggests that, under most conditions, the plasma leading edge was dominated by negative ions, with the apparent increase in velocity being due to fast H− overtaking slower, heavier ions. In some cases, biased-probe data also showed abrupt discontinuities in the plasma energy distribution co-located with large changes in the intercepted plasma current, suggesting the presence of a shock in the leading edge of the expanding plasma.

Vacuum Surface Flashover of High Gradient Insulators Under Nanosecond Pulse

Vacuum surface flashover of the high gradient insulator (HGI), which consists of many alternative layers of conductors and insulators, was found to generally start from one or several dielectric layers adjacent to the anode under nanosecond pulse by using the fast photography. A simple model is proposed to explain the experimental observations. The secondary electron emission avalanche (SEEA) along the dielectric surface adjacent to the anode will be greatly enhanced due to the upstream SEEA starting from the traditional and the intermediate cathode triple junctions. The 15-mm-thick HGI samples with different numbers of dielectric layers were tested and the samples with six and eight dielectric layers showed the best performance. The experimental result proves that as the number of dielectric layers reaches a certain level, the performance of the HGI will decrease as the number of dielectric layers increases.

Transient Charging Characteristics in Surface Flashover Development Process due to Explosive Electron Emission in Vacuum

For the development of higher voltage vacuum circuit breakers (VCB) and vacuum interrupters (VI), surface discharge phenomenon in vacuum is one of the most significant issues. We measured the charge activity on alumina dielectrics in vacuum through ultra-high speed optical and electrical measurements. We found out that the surface flashover in vacuum starts with an explosive electron emission (EEE) with the higher current density, which is the electron emission from the cathode, and results in the surface flashover development. Transient charge on the alumina dielectrics formed by electrons emitted by EEE influences the distortion of electric field distribution around the cathode rod and then the subsequent flashover development. Especially, in the case of the shorter gap between the cathode rod and the alumina dielectrics, the electron emission could be suppressed by the relaxation effect of the electric field due to the transient charge accumulation.

Expansion of a Surface Flashover Plasma

A strong electric field applied parallel to the surface of a dielectric in vacuum can lead to the formation of a vacuum surface flashover discharge, which will produce light, neutral gas, and plasma. Plasma formed in this way can be used for a number of applications, including as a source of electrons, but the plasma's expansion away from the discharge site will impact its usefulness for those applications. Here, we report measurements on the expansion of a surface flashover plasma generated from a pulsed discharge in vacuum, using electric fields on the order of 10 MV/m applied across a Plexiglas sample. Expansion velocity and energy distribution measurements were made using a moveable, biased detector probe. The temporal structure of the plasma arriving at the probe was found to have a characteristic shape, indicating the presence of three different components of the plasma having different velocities in the range of 1–9 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm cm}/\mu{\rm s}$</tex></formula> . The pulser charging voltage was found to have a noticeable effect on the plasma energy distributions, but only a small effect on the plasma expansion velocity.

Variation in time lags of vacuum surface flashover utilizing a periodically grooved dielectric

In a wide variety of high-power pulsed devices, vacuum surface flashover plays an important role in the system performance. The capability of grooved dielectric to hold off high-voltage pulse is investigated experimentally by using a long pulse electron beam accelerator, which is capable of generating a quasi-square wave with pulse duration of 180 ns and output voltage of 100-400 kV. It is found that both concavely and convexly grooved dielectrics increase promisingly the flashover time lags. With groove width of 2 mm, depth of 2 mm, period of 4 mm, electrode gap spacing of 100 mm, and diode voltage of 265 kV, the flashover time lag for polycarbonate is found to increase from 93.0 ns to 182.6 ns. While for polytetrafluoroethylene, its flashover time lag is increased from 45.0 ns to 81.2 ns, and for high-density polyethylene, its flashover time lag is increased from 72.4 ns to 100.5 ns. Electrical and optical diagnoses on temporal evolution of flashover channel confirm that surface flashover in vacuum is characterized by three-phase development, namely the secondary electron emission avalanche (SEEA), impact ionization in desorbed gas, and the final plasma driven breakdown. Based on this analyzing, it is found that the increase of flashover time lags is mainly due to the suppressing of SEEA process by using grooved structures.

Influence of Electrode Geometry on Pulsed Surface Flashover of the Alumina Insulator in Vacuum

Research on the surface flashover of the vacuum insulation under pulsed voltage is significant for the design and manufacture of pulsed power devices. In this paper, several new electrodes are designed and manufactured, and the surface flashover voltages of the cylindrical alumina insulators are measured in vacuum in different electrodes using a single pulse. The influence of the depth of the metal of the recessed electrode on the withstand voltage of the gap is reported in detail. We find that a screening electrode could increase the relative hold-off voltage almost 50% more than a plane electrode could. The developing process of surface flashover in vacuum by microsecond pulses is also discussed. Consequently, studies on the pulsed voltage hold-off property of the electrode geometry in vacuum could be quite helpful in designing the insulating geometry for high-voltage vacuum devices.

Experimental study on gas desorption in surface flashover of solids in vacuum

The relation of desorbed gases and surface discharge in vacuum is a complex problem without reliable theory. The vacuum experimental platform of direct current and nanosecond pulse source was built, and the quadrupole mass spectrometer connected to vacuum cavity was used to analysis the out-gassing characteristics of insulation materials. Then the vacuum surface discharge experiments were done and the desorbed gases were measured in the process. By comparing the mass spectrum before discharges with ones after discharges, we obtained the out-gassing composition of several insulation materials. Meanwhile we analyzed the probable sources of different gases. Main gas components generated by vacuum surface flashover included H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O, N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , hydrocarbon fragment, CO, CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and sometimes high polymer fragment. C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> appeared in gas components when the filamentary discharge channel was formed between two electrodes. The results show that the high polymer surface may decompose and the dissolved gases may be extracted besides gas desorption in discharges. The discharge pattern has direct effect on their out-gassing characteristics. We'll thoroughly research their relation in subsequent work.

刻槽绝缘子真空表面闪络光学特性

刻槽绝缘子真空表面闪络光学特性

基于磁场闪络抑制技术的真空沿面闪络实验研究

基于磁场闪络抑制技术的真空沿面闪络实验研究

亚微秒脉冲下高梯度绝缘子的沿面闪络特性

亚微秒脉冲下高梯度绝缘子的沿面闪络特性

Improvement of surface flashover characteristics about 45° insulator configuration in vacuum by a new organic insulation structure

In the present paper, a new three-layered 45° insulation structure is proposed and investigated to further improve the surface flashover performance of the 45° vacuum- insulator configuration. This structure was prepared by inserting organic composite between the electrode and insulator. With a numerical simulation method, the electric field distributions in this structure were analyzed. The result shows that by adjusting the permittivity of the inserted layer between the electrode and insulator, the electric field at the cathode and anode triple junction can be suppressed significantly in order to prevent the appearance of the surface flashover in vacuum. On the basis of these theoretical analyses, we conducted a series of vacuum flashover experiments about the new three-layered insulation structure of polyethylene composite-polyethylene-polyethylene composite. In our experimental result, it is indicated that when the permittivity of the inserted layer change in certain range, the surface flashover performance in vacuum of the 45° vacuum-insulator configuration can be improved, e.g. the highest first impulse flashover voltage reaches to 81 kV and increases by 32% compared with the polyethylene standard samples.

New development of microstack insulator technology

Microstack insulators were developed to satisfy application demands for pulsed power system high performance and miniaturization. Using a high temperature lamination method, we manufactured two types of microstack insulators from interleaved layers of brass and polyimide (PI) and from stainless steel and fluorinated ethylene propylene (FEP). The surface state and vacuum out-gassing characteristics of these microstack insulator samples were measured. Then using a nanosecond (10 ns/30 ns) pulse source with a Marx generator and a single coaxial pulse-forming line, the vacuum surface flashover characteristics of samples were tested. The results show that flashover takes place during the rising period of the voltage, and the maximum vacuum flashover field intensity for microstack insulator samples is close to 180 kV/cm. Based on the preceding work, the high frequency dielectric constants of microstack insulators were also measured using an impedance analyzer. The results show that microstack insulators have better high frequency dielectric performance than conventional insulation materials.