P. Senior

Design, construction and testing of explosive-driven helical generators

This paper describes the development of a very efficient computer model for the design and performance prediction of explosive-driven helical generators. The model is based on simple theoretical considerations. Validation of the model is achieved by comparing the theoretical and measured performances of existing both high- and low-energy generators. It is shown that, although the basic model predicts accurately the load current history of high-energy generators, a somewhat more elaborate model is needed for low-energy devices. The model has been used in the design of a simple 1 MJ generator with an eight-section stator coil, intended for use as a current source in an investigation of high-current conditioning systems. A description is given of the construction and testing of this device. Experimental results are in accordance with predictions from the design code and establish that, when primed with 40 kJ at 50 kA from a capacitor bank and using 15 kg of high explosive, the generator is capable of delivering an output of 1 MJ at 7 MA to a coaxial load.

Studies of a very high efficiency electromagnetic launcher

This paper describes the culmination of a research activity intended to demonstrate unequivocally that in a launcher, electrostatic to kinetic energy conversion can be achieved with an efficiency exceeding 50%. Two numerical models are presented, with the first of these being a fast and simple method of establishing the optimum launcher design. The second model is based on detailed filamentary considerations, and for the first time highlights the various factors that limit the global efficiency. Results from the detailed modelling are shown to be in very good agreement with experimentally obtained data.

A novel flux compression/dynamic transformer technique for high-voltage pulse generation

This paper presents the basic concepts that underlie a fundamental research activity initiated recently at Loughborough University, U.K. A novel technique is described that enables the so-termed shock-wave-driven flux compression process to be performed inside a laboratory, without the need for any high-explosive charge, and results from preliminary proof-of-principle experiments are analyzed. Details of the necessary ancillary equipment, such as fast (TA/s) generators, electric guns, high-voltage helical transformers, and special transducers are presented, together with a study of the dielectric/metallic phase transition in aluminum powder. The paper concludes by showing how the different concepts can be combined, leading to a high-voltage pulse generator with a fast-rising output.

Transportable high-energy high-power generator

High-power applications sometimes require a transportable, simple, and robust gigawatt pulsed power generator, and an analysis of various possible approaches shows that one based on a twin exploding wire array is extremely advantageous. A generator based on this technology and used with a high-energy capacitor bank has recently been developed at Loughborough University. An H-configuration circuit is used, with one pair of diagonally opposite arms each comprising a high-voltage ballast inductor and the other pair exploding wire arrays capable of generating voltages up to 300 kV. The two center points of the H configuration provide the output to the load, which is coupled through a high-voltage self-breakdown spark gap, with the entire autonomous source being housed in a metallic container. Experimentally, a load resistance of a few tens of Ohms is provided with an impulse of more than 300 kV, having a rise time of about 140 ns and a peak power of over 1.7 GW. Details of the experimental arrangement and typical results are presented and diagnostic measurements of the current and voltage output are shown to compare well with theoretical predictions based on detailed numerical modeling. Finally, the next stage toward developing a more powerful and energetic transportable source is outlined.

Pulsed-power Technology And Experimentation At Loughborough University Of Technology

The paper introduces the pulsed-power programme of research at Loughborough University of Technology (LUT), which began some 15 years ago with the production and measurement of blast waves from exploding wires. More recently LUT have assembled facilities and techniques, both in the laboratory and on firing ranges, for investigation into methods of conditioning and sharpening the output of a flux compressor (FC) and for participating in a collaborative investigation of propellant and explosively driven magneto-hydrodynamic (MHD) devices. The various facilities and the FC conditioning techniques are described, their contributions to the MHD investigation are outlined and typical results are presented.

An ultra-fast electro-optic probe for 500 kV pulsed voltage measurements

This paper describes the use of an assembly of electro-optic components and a high-performance capacitive voltage divider in a novel voltage-measurement probe for use in short-duration 500 kV pulsed-power situations, where conventional probes are unsuitable. Complete electrical isolation is provided between the high-voltage circuit and the recording oscilloscope. Experimental results confirm a predicted rise-time of less than about 3 ns, which is significantly better than that of high-quality 90 MHz bandwidth commercially available probes for the same level of voltage.

SHARPENING A FLUX COMPRESSOR OUTPUT USING EXPLODING FOIL & PLASMA SWITCHING TECHNIQUES

The paper describes an on-going research programme into compact explosively-driven energy sources, together with the techniques and conditioning circuits used to produce sharp high-voltage output pulses. Both flux compressor (FC) and magnetohydrodynamic (MHD) sources are considered. An outline is given of FC designs and firing arrangements, and the potential and use of explosively-driven MHD sources with meatgrinder techniques for initial priming are mentioned. A description is given of output conditioning experiments, both in the laboratory and on the firing range, in which exploding and explosively-formed foil and plasma switching techniques are all used.

Empirical modelling of copper foil fuses [as pulsed power switches]

This paper describes experiments that use a fast capacitor bank to provide data for the production of simple but accurate empirical models for exploding copper foil fuses. It is demonstrated that foils of different thicknesses behave quite differently, a phenomenon previously noted only for exploding aluminium foils. Empirical models for both 17 /spl mu/m and 25.4 /spl mu/m foils were developed from the experimental data, and used successfully in computer programs which predict in detail the behaviour of the foils in a variety of switching circuits. The computer models were used to optimise the design of an exploding foil used as an opening pulsed power switch in a capacitor driven circuit, that transfers a sharp current pulse to a plasma erosion opening switch (PEOS). It was predicted that voltages exceeding six times the initial bank voltages could be produced, and experiments are described that provide evidence for this while avoiding the possibility of damage to the bank.

Analysis of helical generator driven exploding foil opening switch experiments

The paper presents results from experiments in which a 1 MJ helical flux compression pulsed power generator was used to power an exploding-foil opening switch. Although the high reproducibility of the generator performance enabled the experiments to be performed successfully, the time variation of the fuse voltage confirmed earlier work showing the different fuse behaviour when fed at the reduced energy input rate provided by a flux compressor. Consideration of the experimental data, using an empirical model for the copper foils obtained from fast capacitor bank experiments, showed that the predictions are only accurate until the fuse begins to vaporise. The knowledge gained will enable better fuse designs for flux compression conditioning to be produced, but a complete understanding of the post-burst fuse behaviour must await a detailed theoretical approach which is under development elsewhere. The paper concludes by describing a novel circuit arrangement, which avoids the post-burst fuse behaviour. Automatically operating opening and closing exploding-foil switches are used to provide the conditioning required to drive a very fast high-impedance load.

A 10 GW Tesla-Driven Blumlein Pulsed Power Generator

A repetitive 0.6 MV, 10 GW Tesla-driven Blumlein pulsed power generator, with an overall energy efficiency in excess of 90%, has been designed, manufactured and demonstrated by the Pulsed Power Group at Loughborough University. This paper describes the application of various numerical techniques used to design a successful generator, such as filamentary modeling and electrostatic and transient circuit analysis. All the major parameters of both the Tesla transformer and the Blumlein pulse-forming line were determined, enabling accurate modeling of the overall unit to be performed. The wide-bandwidth embedded sensors used to monitor the dynamic characteristics of the overall system are also presented. Experimental results obtained during this major experimental program are compared with theoretical predictions and the way ahead toward generating faster output voltage impulses is considered.