M.A. Sinclair

Self magnetic pinch diode experiments at AWE

For many years AWE has used self magnetic (SM) pinch diodes on its lower voltage flash X-ray machines (MEVEX @ 0.8 MV and Mini-B @ 2 MV). With the recent emphasis on reduction of the X-ray spot size, one part of the diode research project has been to field SM pinch diodes at higher voltages. A series of experiments has been conducted on the Mogul D and Eros pulsed power drivers to in an attempt to meet the HRF source term requirements. The charging voltage on Mogul D was increased to its operational maximum and 62R with a 2.1 mm spot (AWE definition) was produced at 4.2 MV. Further shots on the EROS driver failed to match the Mogul D results because of the 110 kV prepulse on EROS. MCNP has been used to calculate dose scaling as a function of charge and voltage for the SM pinch diode. This has shown that long term HRF dose goals are achievable with this diode.

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.

Effect of applied field and rate of voltage rise on surface breakdown of oil-immersed polymers

In sub-systems of high-voltage, pulsed-power machines, the introduction of a solid into bulk liquid insulation located between two conductors is often necessary to provide mechanical support. Breakdown events on or around the surface of the solid can result in permanent damage to the insulation system. Described in the present paper are experimental results pertaining to surface breakdown of five different solid dielectrics held between plane-parallel electrodes immersed in mineral oil. The effect of varying level of peak applied field from 200 kV/cm (dV/dt 70 kV/μs) to 1 MV/cm (dV/dt 350 kV/μs) is investigated, and the breakdown voltages and times to breakdown are compared to those for an open oil gap. The time to breakdown is shown to be reduced by the introduction of a solid spacer into the gap. Rexolite and Torlon samples suffered significant mechanical damage, and consistently showed lower breakdown voltage than the other materials ¿ average streamer propagation velocity up to 125 km/s was implied by the short times to breakdown. Although ultra-high molecular weight polyethylene yielded the longest times to breakdown of the five types of liquid-solid gap, breakdown events could be initiated at lower levels of applied field for spacers of this material than those with permittivity closely matched to that of the surrounding mineral oil. Polypropylene and low-density polyethylene are concluded to provide the most stable performance in mineral oil. Due to the similarity of the applied voltage wave-shape (1/6.5 μs) to short-tail lightning impulses, the results may also be of interest to high-voltage system designers in the power industry.

Impulse-driven Surface Breakdown Data: A Weibull Statistical Analysis

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.

Impulse-Breakdown Characteristics of Polymers Immersed in Insulating Oil

Surface discharges along oil-immersed solids used as insulators and supports in high-voltage pulsed-power equipment can lead to catastrophic system failures. To achieve reliable compact pulsed-power systems, it is important to quantify the electrical fields at which surface flashover, or other types of breakdown event, will occur for different dielectric materials. This paper reports the observed behavior of samples of polypropylene, lowdensity polyethylene, ultrahigh-molecular-weight polyethylene, Rexolite, and Torlon, which were subjected to impulse voltages of peak amplitude of 350 kV and a rise time of 1 μs. The cylindrical samples were located between pairs of electrodes immersed in insulating oil. Breakdown events were studied under both nonuniformand uniform-field conditions, with sample lengths being chosen so that the breakdown events occurred on the rising edge of the impulse. Ultrahigh-molecular-weight polyethylene showed the highest average breakdown field, which is 645 kV/cm, in uniform fields, and the corresponding breakdown field was reduced to ~400 kV/cm in the nonuniform fields. Weibull plots of the various sets of results are presented, providing comparative data for system designers for the appropriate choice of dielectric materials to act as insulators for high-voltage pulsed-power machines.

Design and analysis of an enhanced MOSFET gate driver for pulsed power applications

This paper describes a novel MOSFET gate driver circuit design for pulsed power application. It is shown that MOSFET switching speed can be enhanced by using an energised inductor as a high current source in series with the gate terminal of a power MOSFET. This topology demonstrates switching speeds of less than 10ns for MOSFETs with high input gate capacitance. It is also shown that by increasing the current in the gate drive and with an optimised layout, a single gate driver can be used to drive multiple parallel connected MOSFETs synchronously without compromising their performance. The circuit design is analysed and experimental results presented when operating at an instantaneous power of 25 kW.

Numerical Modelling of a Flyer Plate Electromagnetic Accelerator

A joint program involving the study and practical performance of a foil-flyer electromagnetic accelerator has recently been initiated by Atomic Weapons Establishment, Aldermaston, and Loughborough University. As an initial phase of the work, both 0-D and 2-D numerical models for the foil-flyer accelerator have been developed. The 0-D model, although very crude, is capable of providing an insight into the accelerator phenomena and is currently used for parametric design studies. The 2-D model is based on the well-proven Loughborough filamentary modeling technique and is capable of accurately calculating the 2-D distribution of current, velocity, acceleration, and temperature of the flyer, together with the complete distribution of the magnetic and electric fields generated during a shot. The paper presents the two models and compares typical theoretical predictions with the corresponding experimental results.

Breakdown of mineral oil: Effect of electrode geometry and rate of voltage rise

Experimental data on the propagation of streamers in mineral oil is important for the design of high-voltage systems in the power and pulsed-power industries. In the present study, breakdown voltages and pre-breakdown delay times were measured for plane-parallel electrodes, and for two non-uniform electrode arrangements. For each geometry, the breakdown characteristics were determined for impulses of rise-time 100 ns, and also rise-time 1 μs. The maximum rate of voltage rise (dV/dt) was 4 MV/μs. For the non-uniform geometries with inter-electrode gap length of 8.5 mm, the time to breakdown was 2.5-3 times longer for impulses of rise-time 1 μs than for 100 ns risetime. The time-to-breakdown data suggest that streamer propagation velocity increases with higher values of dV/dt. For example, the estimated propagation velocity for pinplane geometry with a 1 μs rise-time is 10-12 km/s. At 100 ns rise-time for the same electrode geometry, the average propagation velocity exceeds 40 km/s. The results are compared with data previously generated in parallel liquid-solid gaps, and it is concluded that the time to breakdown is longer, and that higher applied fields are required to initiate breakdown, in open oil gaps compared to the case when a solid spacer is present. The results presented are intended to provide reference data for designers of oil-immersed, high-voltage systems such as power transformers and pulsed-power supplies.

Foil-flyer electro-magnetic accelerator initial results from a new AWE pulsed power generator

An experimental programme, leading to the development of a foil-flyer electromagnetic accelerator (FFEMA), is being conducted at AWE. A high current pulsed power generator driving a foil-flyer is used to provide tailored impulse profiles into targets for testing material properties at high strain rates. This work has required the design and construction of both an experimental platform to conduct the experiments, named AMPERE, as well as the development of a number of computer models to predict the electrical and mechanical performance of the foil-flyer and its interaction with a target.

Effect of electrode geometry and rate of voltage rise on streamer propagation in mineral oil

Experimental data on the propagation of streamers in mineral oil is important for the design of high-voltage systems in the power and pulsed-power industries. In the present study, pre-breakdown delay times were measured for plane-parallel electrodes, and for two types of non-uniform electrode arrangement. For each geometry, the breakdown characteristics were determined for impulses of rise-time 100 ns, and also rise-time 1 µs. The maximum applied voltage magnitude was 400 kV, giving a maximum dV/dt of 4 kV/ns. For the non-uniform geometries with inter-electrode gap length of 8.5 mm, the time to breakdown was 2.5–3 times longer for impulses of rise-time 1 µs than for 100 ns rise-time. The time-to-breakdown data suggest that streamer propagation velocity increases with higher values of dV/dt. For example, the estimated propagation velocity for pin-plane geometry with a 1 µs rise-time is 10–12 km/s. At 100 ns rise-time for the same electrode geometry, the average propagation velocity exceeds 40 km/s. The results presented are intended to provide reference data for designers of oil-immersed high-voltage systems in both the power and pulsed-power industries.