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 non-uniform-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 similar to 600 kV/cm in uniform fields, and similar to 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
Using a modified technique of transmission electron microscopy, the properties of oxide films formed on aluminum foil by heat treatment in various oxidizing atmospheres at temperatures from 400 to 600°C have been studied. Above 400°C crystalline oxide forms by nucleation and growth, and has a basically fcc lattice of parameter 7.9±0.15 A.
Abstract-Surface breakdown of oil-immersed solids chosen to insulate high-voltage, pulsed-power systems is a problem that can lead to catastrophic failure. Statistical analysis of the breakdown voltages, or times, associated with such liquid-solid interfaces can reveal useful information to aid system designers in the selection of solid materials. Described in this paper are the results of a Weibull statistical analysis, applied to both breakdown-voltage data and time-to-breakdown data generated in gaps consisting of five different solid polymers immersed in mineral oil. Values of the location parameter γ provide an estimate of the applied voltage below which breakdown will not occur, and under uniform-field conditions, γ varied from 192 kV for polypropylene (PP) to zero for ultrahigh-molecular-weight polyethylene (UHMWPE). Longer times to breakdown were measured for UHMWPE when compared with the other materials. However, high values of the shape parameter β reported in the present paper suggest greater sensitivity to an increase in applied voltage-that is, the probability of breakdown increases more sharply with increasing applied voltage for UHMWPE compared to the other materials. Analyzing peak-applied-voltage data, only PP consistently reflected a low value of β across the different sets of test conditions. In general, longer mean times to breakdown were found for solid materials with permittivity more closely matched to that of the surrounding mineral oil.Index Terms-Breakdown voltage, dielectric breakdown, flashover, pulse power systems, Weibull distribution.
The Atomic Weapons Establishment, Aldermaston, U.K., has a number of pulsed-power-driven flash X-ray machines for diagnosing the hydrodynamics of explosively-driven high-atomic-number materials. The most powerful of these machines is Mogul-E, which operates at about 10 MV and 30 kA, delivering about 400 R at 1 m in a 5-mm spot. Longer-term plans envisage the upgrading of existing facilities through the construction of a hydrodynamic research facility (HRF) with multiaxis radiography. It is proposed that the HRF will be furnished initially with three inductive voltage adder (IVA) machines operating at 14 MV, each giving 600 R at 1 m in a 5-mm spot. It is envisaged that, following further research, the outputs will be increased towards 1000 R, with a reduction in X-ray spot size. More speculative proposals involve increasing the number of machines from three to five, and/or the splitting of the output end of one or more machines to drive more than one X-ray source per machine. An overview of the research programme necessary to achieve these aims is presented. Topics covered include: experiments to investigate the performance at 5 MV of the paraxial diode and the magnetically immersed diode; split magnetically insulated transmission lines where one machine drove two X-ray sources; the design and testing of a prototype IVA module operating at 1.5 MV; the design of an upgrade from 5 MV to 10 MV of one of our existing single pulse-forming-line machines; and initial experiments to investigate the usefulness of ultrashort-pulse lasers for generating useful fluences of 2-4 MeV X-rays.Index Terms-Blumlein, chirped-pulse amplification, CPA, diodes, electron accelerator, immersed B-field diode, inductive voltage adder, IVA, marx, paraxial diode, pulse power, pulsed power, radiography, rod pinch, source size reduction.
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