Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV

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ISSN 1093-2941

Publications in this journal

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    ABSTRACT: Vacuum arc cathode spot motion was investigated on a “roof-shaped” aluminum cathode under an external magnetic field. The cathode included a flat roof-top and four sloped sides, inclined by an angle α. A high speed camera was used to observe cathode spot motion. The spot velocity on the slopes and the distribution of cathode spots on the roof and slopes was determined. It was obtained that under a magnetic field, the spot motion on the roof was slow (<1 m/s) and mainly random, while on the slopes fast retrograde motion was observed. This velocity increased linearly with the magnetic field and decreased slightly with α.
    No preview · Article · Jun 2015 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: The paper presents measurement results for cathode spot (CS) velocity V under action of a tangential magnetic field on a clean surface (refined by preceding arcing) of the cathode made of cupper-chromium (CuCr30) composition in vacuum. Measurements were performed in the range of currents I = (30-250) A in arcs of different lengths, h = 2 mm and 4 mm. The induction of tangential magnetic field varied in the range B = (0.02-0.15) T. The dependence of the CS velocity on the magnetic field induction was found to be linear: V = KB. The constant K depends on the arc current for currents I < 2I0. (where I0 is the current per spot at B = 0). The maximum value of K (at I > 2I0) depends on the arc gap length. These results are similar to the results obtained for CS on cathodes made of pure metal, but are at variance with the result of the work by Y.H. Fu (1990) for the CuCr25 cathode. The obtained results are compared with similar results obtained previously in the experiments with different pure metals: Cu, Mo and W.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: Prior to flasshover across solid dielectrics under steady-state high voltage excitation in vacuum, there are charging phenomena occurring on insulator surface, which significantly affect the developing process of flashover. In this paper, based on the secondary electron emission avalanche model (SEEA) and Monte Carlo simulation method, the theoretical analysis of surface charge density on insulation surface is made. A two-dimensional Mont Carlo simulation on surface charge accumulation under DC voltage excitation is provided. The results reveal that surface charging process can be divided into three stages including initial accumulation, fast multiplication and final stable stage. The positive charges exist in a small surface region near the cathode. While the flashover process develops, positive charges accumulate near the anode and the amount of positive charges is an order of magnitude larger than before. With flashover process developing, the peaks of positive charges move toward cathode. In the final stage, the SSEE phenomena are achieved and surface charges are invariable and reach up to a stable distribution.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: A summary of the 2014 Dyke Award presentation is given in the form of an Extended Abstract. Ion energy data and theories are reviewed. It is shown that the potential hump theory can be reconciled with the gasdynamic acceleration theory by considering traveling potential humps, each of which is associated with an explosive emission event. Using information from pulsed laser plasma expansion, it is appears natural and consistent to have traveling humps whose heights are related to the local power dissipation of the explosive plasma-forming events. In this way, ion velocities (energies) are material-dependent following the Cohesive Energy Rule and exhibit fractal (statistical) features.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: A summary of the 2014 Dyke Award presentation is given in the form of an Extended Abstract. Ion energy data and theories are reviewed. It is shown that the potential hump theory can be reconciled with the gasdynamic acceleration theory by considering traveling potential humps, each of which is associated with an explosive emission event. Using information from pulsed laser plasma expansion, it is appears natural and consistent to have traveling humps whose heights are related to the local power dissipation of the explosive plasma-forming events. In this way, ion velocities (energies) are material-dependent following the Cohesive Energy Rule and exhibit fractal (statistical) features.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: We studied erosion traces on the surface of single-crystal silicon wafer cathodes in single vacuum discharges. The surface orientations of the wafers were {100} and {111}. The vacuum discharges were generated by short voltage pulses (20-80 ns, 200 kV) in gaps 1.5-3 mm wide. Each discharge resulted in several symmetric erosion patterns oriented along crystallographic directions. It was supposed that the formation of erosion patterns was much contributed by acoustic phenomena.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: Microwave breakdown phenomenon across dielectric window at the vacuum/dielectric interface limits the transmission of HPM and becomes a bottleneck of HPM technology development. Its avoidance or suppression is a major concern among researchers on HPM sources. Currently, the most effective approach is geometrical modification with the goal of altering the trajectories of electrons. In this paper, periodic rectangular grooves perpendicular to microwave electric field, are applied to improve the threshold of breakdown. To clarify the mechanism, the SEEA simulation model based on dynamic analysis and particle-in-cell (PIC) is built in this paper. The grooves change the electrons trajectories and eventually resonant multipactor condition may no longer be satisfied. The suppression effect is influenced by the grooves and each given width relates to an effective power range for suppression. Dielectric breakdown experiments under HPM of S-band at 2.86 GHz are conducted with the grooved dielectric window. The results of the experiment are consistent with theoretical analysis and simulation.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: A 100 MeV electron linear accelerator is proposed to be built by APPD, BARC, Mumbai, India for neutron generation purposes. This paper presents the results of electron beam tracking for a 100 MeV standing wave electron linac to determine preferential operation parameters from the results of beam dynamics studies.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: The ability to withstand high voltages in vacuum is one of the key reasons for its choice as an insulation medium for distribution-class voltage fault interrupters. However, the dielectric performance of vacuum is still not fully understood, sometimes leading to contradictory results. In this paper, high voltage withstand test results are presented. The subsequent location and quantity of the discharge locations on the electrode surfaces are compared with computer electric field models to establish a relationship between electric field, surface area, and likelihood of failure.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: The diffuse, or spotless, mode of current transfer has been observed on cathodes of vacuum arcs under conditions where the average cathode temperature was high enough, about 2000 K. It has been known for many years that none of the known mechanisms of current transfer to cathodes of vacuum arcs and, in particular, of the electron emission is capable of producing the current densities of the order of 105-106 Am-2 deduced from the experiment. A fresh attempt to clarify this question is made in this work. Cathodes made of chromium are considered, on which the most of the experiments have been performed. It is shown that an account of the difference between values of thermionic and photoelectric work functions given in the reference literature allows one to significantly reduce the deviation between the theory and the experiment. Unfortunately, data on thermionic work function available in the literature refer to the cathode surface temperatures below 1400 K, which is significantly smaller than measured temperatures of the chromium cathodes of vacuum arcs operating in the spotless mode. Therefore, further experimental data are needed in order to clarify this effect.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: With the fast development of multi-terminal high-voltage direct-current (HVDC) transmission system, a fast HVDC interruption technique is urgently demanded to break the short circuit current in the multi-terminal HVDC transmission system with a rapid rising rate. In this paper, an interruption scheme for HVDC based on vacuum switch and artificial current zero technique is proposed. The modularized main circuit breaker is composed of multiple modules in series, each module consists of a vacuum circuit breaker with fast operation mechanism, a RC snubber branch, and a metal oxide arrester. A triggered sphere gap is adopted for the commutation switch to achieve the bi-directional interruption. A backup circuit breaker is used to interrupt the residual current flowing in the circuit caused by the weak arc extinguishing capacity of the sphere gap. The interruption scheme is modeled and simulated. The current commutation process is analyzed.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: This paper highlights the development of Rod Pinch (RP) diode for flash X-ray generation as intense radiographic source at BARC, Vizag. The typical RP diode employed used a small diameter (1-2 mm) anode rod extended through a cathode circular aperture (5-6 mm inner diameter). The diode chamber is maintained at 10-5 Torr vacuum by a rotary backed diffusion pump. Experiments performed on a modified Kali 1000 Pulsed Power System (300 kV, 30 kA, 100 ns) were aimed at optimizing the source by maximizing the figure of merit (dose @ 1m in rad/ spot diameter2 in mm2) with minimizing of the diode impedance. The typical electron beam parameters used in the experiments are 240-270 kV, 20-25 kA, 100 ns, with a few hundreds of kA/cm2 current density. The optimization resulted in a configuration with tungsten anode rod having dimensions of a 1.6 mm diameter, tapering extension length 5-25 mm beyond the graphite cathode aperture (Cathode disk ID = 5 mm, thickness = 3mm) to produce a radiation dose of 150-200 milli rad at 1 m distance having an estimated spot-size of 1-2 mm. The radiation emitted from a rod-pinch diode is measured using Thermoluminscence dosimeters (TLDs) at an angular interval of 15° on either side of the rod in horizontal and vertical plane.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: The fast electrical explosion of a single metal wire has important applications in m plasma and the voltage collapse, is very important to subsequent evolvement of the core-corona structure. In this paper, the energy deposition in the initial stage of the electrical explosion of Aluminum (Al) wire, which is widely used in relevant research and applications, is investigated experimentally. Experiments are conducted in vacuum (∼10-2 Pa) chamber. The pulse current with magnitude of ∼1kA and rising rate of 80-170A/ns is generated by a capacitor bank with charging voltage up to 40kV. The deposited energy is calculated by integrating the products of resistive voltage and current before voltage collapse. The influence of current rising rate on energy deposition is investigated. The experimental result of energy deposition is also compared with the simulation result of a one-dimensional MHD model, which is developed to investigate the explosion process of metal wire.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: A 50 ampere beam current having a rise time lesser than 500 pico-seconds with its energy 50 keV is generated using field emission mechanism. Beam duration was 2 ns. CsI coating on carbon fibre cathode was done using plasma spray coating. The results obtained from this cathode were compared to the carbon fibre cathode and the reason behind higher current density in the case of CsI coating has been explained.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: Vacuum interrupters (VI's) are the primary switching technology for medium voltage (1-52kV) electrical power systems. The extensive field service of VI's is now raising interest in determining the end of life for these devices, with the primary concern being the loss of vacuum. The manufacturing method of the VI's can affect the calculated long-term life. In particular, there are three main design groups to evaluate. The key differences between these three groups are the processing temperature and permeability of the housing to hydrogen and helium. Many potential failure modes occur independently of the field application or lack thereof, and can be evaluated based data from VI's in long-term storage or after operations, such as seen during development and certification testing. The field returns history from VI manufacturers also provided critical information on the VI lifetime. These various factors can be combined to identify risk factors that could alter the estimated life.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: Partial discharges (PDs) phenomena in medium-voltage switchgears usually exist and are worth being paid more attention. In this paper, a kind of special switchgear cabinet and two kinds of typical PD sources is designed for the PD measurements. Based on the two typical PD models, measurement sensitivity and influence factors of above four kinds of detection methods are researched and analyzed. The experimental results show that Pulsed current (PC) method is the most sensitive to PD signals but it is also more sensitive to the electromagnetic disturbance. Holes on the switchgear cabinet are important for Acoustic Emission (AE) method. The cabinet materials and sensor positions have no significant influence on Transient earth voltage (TEV) signal. Impermeability magnetic material has more significant influence on preventing Ultra-high-frequency (UHF) signals transmission than the permeability magnetic material, which is unfavorable for detection of UHF signal. Larger holes on switchgear cabinet are beneficial to the measurement of UHF method. For the needle-plane model, the phase-resolved partial discharge (PRPD) graph changes from 'ball' shape to the 'M' shape as the voltage increases. Different from needle-plane model, PRPD graph of suspended-metal model exists significant stratification phenomena.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: High energy electron beams (30-150 keV) are widely used as intense heat source in welding, melting and evaporation of refractory metals. These operations are mostly carried out in high vacuum (10-4-10-5 mbar) to aid unimpeded flow of electrons from generation to the point of application. A 30 kV, 60 kW Electron beam optical column (EOC) is designed and developed under the high power beam technology programme. The optical column consists of an electron gun and twin electromagnetic focusing lenses. The heating power of emitter, gun perveance, grid voltage control, beam focusing with the twin electromagnetic lenses and beam spot size measurements are reported in this paper. The results are compared with the design values. Operational issues during the high power melting and the strategy for automation of the electron gun are discussed.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: This paper presents an experimental investigation into the behaviour of metal-vapour arcs in vacuum with and without superimposed axial magnetic fields on copper (99.95 % pure electrolytic) electrodes. Properties of vacuum arcs have been investigated for a discharge current up to about 5 kA. Low-current arcing voltage and average current per spot have been measured. Voltage-current characteristics for high-current arc in axial magnetic field up to 0.1 T have been obtained. High-current arc voltage has been measured as a function of the arc current and the diameter of the expanding ring of cathode spots. From the measurements, an empirical relationship for the arc voltage has been derived for its linear behaviour. High-speed framing photography has been used in the investigation to record the cathode spots, plasma columns and gross features of discharge as a whole over a wide range of conditions. It has been found that there is an axial magnetic field that gives the critical plasma confinement. Cathode-spot motion has been studied and retrograde velocity vs. self-magnetic field data are reported for copper cathode.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: During the arc interruption process of vacuum circuit breakers (VCBs), the behavior of inter-electrode plasma, which has a great influence on the anode progress and further impacts the electric insulation recovery speed of VCBs, is a the key factor determining the interruption performance of VCBs. In this paper, according to the actual flowing status of vacuum arc, based on the previous two-temperature MHD model, we firstly presented an improved Magneto Hydro Dynamic (MHD) model including the standard k-ε turbulence equations and it is suitable to simulate both of the supersonic arc and subsonic arc. Then the multi-physics coupling software-COMSOL was adopted to simulate the high-current supersonic arc and subsonic arc existing in some typical commercial VCBs. After that, the spatial distributions of several important plasma parameters of supersonic and subsonic arc, such as density, ion and electron temperature, pressure and Mach number, were calculated and analyzed in detail. Finally, we obtained the distribution of energy flux density on anode surface (EFD), an important parameter which characterizes the specific influence of inter-electrode plasma on the anode status. The numerical simulation results are a great supplement with regards to the experimental results of vacuum arc and the calculated EFD could be used as a heat flux boundary condition for the analysis of anode thermal process.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV
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    ABSTRACT: In recent years, pulse power technology is developing towards the goal of high working frequency. One of the key technologies involved is the development of high power repetitive short-circuit transition switches. The triggered vacuum switch (TVS) is promising in this field because of the excellent dielectric strength recovery property of vacuum. However, the high frequency current interruption ability and vacuum dielectric strength recovery characteristic of TVS with common electrode structure have not been known. In this paper, the TVS is tested under five different high oscillating frequencies. The results show that restrike time contains strong randomness. The restrike probability is increasing with the increase of rate of current change when it passes zero. And with the increase of oscillating frequency, the interruption ability of TVS becomes weak. All these will help the development of high power repetitive short-circuit transition switches.
    No preview · Article · Nov 2014 · Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV