Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV Journal Impact Factor & Information

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ISSN 1093-2941

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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.
    Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV 06/2015; 117(23). DOI:10.1063/1.4922862
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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.
    Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV 11/2014; DOI:10.1109/DEIV.2014.6961661
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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.
    Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV 11/2014; DOI:10.1109/DEIV.2014.6961621
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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.
    Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV 11/2014; DOI:10.1109/DEIV.2014.6961664
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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.
    Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV 11/2014; DOI:10.1109/DEIV.2014.6961644
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    ABSTRACT: The problem of an electronic devices collapse is particularly relevant for electronics, operated under conditions of significant changes in temperature, pressure, and other critical factors. Currently, the main cause of failure of electrical equipment is microscopic electrical discharges between closely spaced conductive elements of printed circuit board (PCB). The final stages of the development of such micro-discharges are arcs characterized by a large quantity of energy, which leads to the destruction of the circuit elements. This paper considers two-dimensional theoretical model of self-discharge between the current-carrying PCB elements for a wide range of gas pressures (from sub-atmospheric pressure to closed vacuum conditions), and initial concentration of electrons, which are characteristics of the operating conditions of spacecrafts and other large vacuum systems. The basic advantages of the proposed model are demonstrated (such as a wide variety of boundary conditions types and geometric of the discharge gap shapes, the scalability of the critical parameters of the environment, simple representation of surface reactions, etc).
    Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV 11/2014; DOI:10.1109/DEIV.2014.6961611
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    ABSTRACT: Low temperature glow discharge wall conditioning (GDC) using H2 gas is effective in reduction of oxygen and carbon (low-Z) contain impurities on near surface region of vessel wall. The high retention of hydrogen in vessel wall/components due to long operation of H2 GDC increases hydrogen out-gassing during tokamak operation and affects the production of high temperature plasma. The hydrogen retention can be reduced using inert gas GDC by sputter cleaning for short duration. But in that case the out-gassing rate of inert gas increases, that again impairs the plasma performance. To overcome above problems, the GDC with hydrogen-inert gas mixture can be used for better removal of C and O surface contaminants and low hydrogen retention in surface. In ADITYA tokamak, H2-GDC is carried out regularly after plasma operation, while the GDC with argon-hydrogen (Ar-H2) mixture has been experimentally tested to observe the reduction of oxygen and carbon impurities along with low hydrogen retention. In Ar-H2 GDC, the reason being the formation of ArH+ hydride ions, which has quite long life and more energy compared to H2+ ions formed in H2 GDC for breaking the bond of wall molecules. A systematic comparative study of H2 GDC and Ar-H2 Mixture GDC by changing the mixture ratio has been carried out in ADITYA tokamak. The relative levels of oxygen and carbon contain impurities have been measured using residual gas analyzer in both GDC's. We have observed a substantial reduction in oxygen and carbon impurities with a significant improvement in wall condition with Ar-H2 GDC compared to the H2 GDC. The effect of wall conditioning by Ar-H2 GDC on the performance of high temperature plasma operation will be presented in this paper.
    23rd International Symposium on Discharges and Electrical Insulation in Vacuum (ISDEIV-2014), Mumbai, India; 09/2014
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    ABSTRACT: Aditya tokamak is a limiter tokamak device with a torus vacuum vessel having square cross section. The existing Aditya tokamak is being upgraded into a machine with divertor operation and good plasma control to support the future Indian Fusion program in a big way. The up-gradation of Aditya tokamak to diverter tokamak needs additional magnetic field coils for diverter operation, close to the vessel. To accommodate the coils, the existing vessel of square cross section will be replaced with a circular cross section. This vacuum vessel will be made up of stainless steel 304L. It is bakeable up to 150° C. The ultimate vacuum is ∼ 10−9 torr. Two numbers of identical semi torus will be assembled to torus shaped vessel. The end flanges of the semi torus are connected with sealing double Viton ‘O’ rings to form a torus. The major radius of torus vessel is 750 mm &inner radius is 305 mm. Its wall thickness is ∼8 mm. The vacuum vessel will have 110 various sizes of ports/openings, including 4 tangential ports. The nonstandard ports like rectangular and triangular ports are blanked with wire seals. In this paper, the design of vacuum vessel for Aditya Upgrade tokamak is presented.
    23rd International Symposium on Discharges and Electrical Insulation in Vacuum (ISDEIV-2014), Mumbai, India; 09/2014
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    ABSTRACT: Vacuum breakdown is a very common problem during the operation of any electron gun. Arcing occurs due to high voltage break down of cathode - anode gap. This not only spoils the job, it also causes lot of damage to the electronic component of the system and damage to the emitter. In this work, a detailed study of the cause of the high voltage breakdown in the gun in the electron beam evaporation system, analysis of the electrical parameters of arc discharge inside the electron gun due to high voltage break down and suitable solution to the arc discharge problem has been presented. Power supplies need to be protected against surges arising out of high voltage breakdown. Ferrite beads have been used to suppress the high frequency content of the breakdown signal. High frequency content of the signal in the MHz range can be attenuated to lower range of KHz by use of ferrite bead thereby protecting against high frequency surges. On the other hand during breakdown there is a sudden increase in the current which needs to be limited without interruption in the process. Possible use of superconductor as fault current limiter has been analyzed in this work.
    Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV 09/2014; DOI:10.1109/DEIV.2014.6961613
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    ABSTRACT: To keep the uniform distribution of the transient recovery voltage (TRV) across each vacuum interrupter (VI) is very important for the breaking capability of double-break vacuum circuit breakers (VCBs). According to the transient equivalent circuit of double-break VCBs, a mathematical model describing the TRV distribution characteristics was established; it revealed that the bias of TRV distribution was caused by the stray capacitance and the imbalanced post-arc plasma characteristics in each VI. The validity of the TRV distribution model was proved by simulation results based on a vacuum arc model. The experimental results under different values of grading capacitors demonstrated their effectiveness in improving the even degree of the TRV distribution in double-break VCBs. However, too large grading capacitor significantly magnified the amplitude and duration of the reignition current, which was detrimental to the successful breaking of double-break VCBs. It is advisable for this reason to limit the value of grading capacitors to those ranges which can guarantee sufficiently improve the TRV distribution.
    Discharges and Electrical Insulation in Vacuum (ISDEIV), 2012 25th International Symposium on; 11/2012