M.C. Enache

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.

Simple 2D model for helical flux-compression generators

This paper presents a simple but complete 2D model for helical flux-compression generators that overcomes many of the limitations present in existing zero-dimensional models. The generator circuit is effectively decomposed into separate z and; current carrying circuits, with each of the; circuits (rings) corresponding to a different current. Use is also made of a technique by which these rings are sequentially switched out of circuit. The approach proposed opens the way to a full understanding of the behavior of cascade systems of generators inductively coupled by dynamic transformers using the so-called flux-trapping technique. In addition, the model can also yield an important insight into the phenomena that differentiates the performance of small generators when primed by a capacitor, a battery, or an externally produced magnetic field. Finally, the numerical code developed in the paper can readily be adapted to model high-energy and high-current generators in which the helical coil and the armature are of variable geometry. Valuable design information is provided on the magnetic and the electric field distributions within the generator and on the likely radial and axial movements of the stator turns.

2D Modeling of Inductively Coupled Helical Flux-compression Generators—FLUXAR Systems

In a number of single-shot applications and experiments at remote locations where flux-compression generators provide the most practicable power source, it may be necessary to use cascaded generators with inductive coupling between them to provide the required power amplification. The complexity of the resulting system appears to have so far inhibited the development of any simple but complete numerical code, and has prevented a full understanding of the complex action of the overall system. The present paper meets these requirements, by extending an existing simple 2D helical generator code, and shows how this can be used to solve problems arising from the design stage of a prototype system.

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.

A fast electro-optic high-voltage sensor

This communication describes the construction and testing of a fast electro-optic high-voltage sensor, intended for use in a variety of pulsed-power applications. A 150 kV coaxial high-voltage cable is adapted to form an adjustable voltage divider, with the voltage across the lower section of the divider being measured with the aid of a Pockels cell. As developed, the sensor has been employed to measure 15 kV pulses with a rise time of less than 3 ns, and work is in hand to extend the pulse voltage measurement into the megavolt region.

A two-stage exploding-foil/plasma erosion opening switch conditioning system

When a load requiring a very fast rising current is fed from a flux-compression generator, it is necessary to introduce a conditioning circuit in order to reduce the rise time of the generator output current substantially. The final stage of this circuit will include a plasma erosion opening switch, which is the fastest known opening switch for use at the current levels involved. This paper presents results obtained during the development of a novel two-stage conditioning circuit and compares the experimental performance of the circuit with results provided by a computer simulation. A time compression of the rise time of the currents from microseconds to less than 100 ns was obtained, representing a 40-fold increase in the corresponding signals.

Studies of a very high efficiency cryogenic launcher

Cryogenic experiments, in which ring-shaped projectiles are accelerated axially by a single-turn drive coil powered by a capacitor bank, have demonstrated that more than 50% of the initially stored electrostatic energy can be converted into kinetic energy in the projectile. Results provided by a detailed numerical modeling are shown to be in close agreement with experimental data.

Three-dimensional modelling of helical flux-compression generators

Two-dimensional modelling techniques using a filamentary-mesh approach are very well established for use in helical flux-compression generator design, and provide results that have been verified experimentally. However, none of the published numerical codes appear to describe adequately certain details involved in calculating the resistance of the helical stator coil. In particular, the skin and proximity effects that arise from diffusion of the magnetic field into the conductor are clearly 3-dimensional phenomena, and their influence is only very approximately represented by either existing formulae or a 1-dimensional description. In most published codes, the formulae that are used provide merely a steady-state approximation to the proximity effect. The present paper outlines a 3-dimensional approach that has been developed for use with an existing 2-dimensional model, and which forms the first element of an overall 3-dimensional modelling code. It is shown that the scale of the 3-dimensional proximity effects in a helical coil carrying a pulsed current can be many times greater than those predicted by the conventional formulae.

Pulsed-power research at Loughborough University

This is a companion paper to the other papers at the Conference that emanate from Loughborough University, UK. It introduces a few of the other interesting activities that are underway as part of the overall pulsed power research activity.