Patrik Appelgren

A 10-GW Pulsed Power Supply for HPM Sources

A research activity involving the detailed consideration of novel high voltage transformers (HVTs) for pulsed-power applications has recently begun at Loughborough University (LU). Although the main goal is the demonstration of a compact and lightweight unit employing magnetic self insulation under vacuum conditions, the initial stage of the work is directed towards the development of a conventional air-cored HVT as a main component in a compact power supply for HPM sources. In cooperation with the Swedish Defence Research Agency (FOI), the power supply has been tested with a HPM source of the vircator type. The power source for the system uses a 70 kJ/25 kV capacitor bank and an exploding wire array to generate a 150 kV voltage pulse in the primary circuit of the HVT. A pressurised SF6 spark gap in the secondary circuit sharpens the high-voltage output, so that pulses approaching 500 kV and with a rise time below 100 ns are generated on a 20 Omega high-power resistor. The peak power produced by the power supply is in excess of 10 GW. Measurements provided by various diagnostic techniques are analysed with the aid of a detailed numerical code. Experimental results are presented from final testing of the system, where a reflex triode vircator replaces the 20 Omega resistor. Measurements made of the microwave emission using free-field sensors are presented for various electrode configurations. Comments are made with the microwave emission from the same vircator powered by a Marx generator at FOI.

Characterization and Ignition of ADN-Based Liquid Monopropellants

Monopropellant propulsion systems for space applications have relied almost exclusively on hydrazine. Hydrazine is however highly toxic, volatile and carcinogenic, and thus costly safety measures are required. In the last few years there has been considerable interest in Europe and in the USA in finding a possible substitute, since a non-toxic monopropellant would offer substantial cost savings. ADN-based liquid monopropellants seem to be a promising alternative to hydrazine, being substantially easier to handle and having a 10% higher specific impulse, and up to 60% higher density-impulse, than hydrazine. To be able to replace hydrazine, ADN-based monopropellants must be as easy to ignite. Hydrazine and ADN-based liquid propellants are very different, and thus new ignition methods must be developed. This paper presents the results from a characterization of ADN-based liquid propellants, as well as the results from electrical ignition experiments in which the propellant was resistively heated to its ignition temperature. It was found that substantially less electric energy was needed than expected. This is due to local phenomena close to, or on the surface of, the electrodes. Very fast ignition was obtained, in most cases below 2 ms.

Pulsed power transmission line transformer based on modern cable technology

A high-voltage transmission-line pulse transformer has been constructed based on modern cable technology. The transformer has been successfully tested for output powers of 0.5 GW. The high-voltage cable is equipped with a resistive layer (semicon) on the inner conductor and on the inside of the outer conductor. Semicon cables are commonly used in high-voltage transmission of electrical power. The pulse transformer was built using a coaxial semicon cable, with the inner conductor used as secondary winding and the screen as primary winding. Such a transmission-line transformer works in the same way as an ordinary transformer. The input is transformed to the desired output using a step-up or a step-down configuration. An output voltage of 85 kV with 1-/spl mu/s duration was achieved into a 15 /spl Omega/ load. Because the windings are coaxial the magnetic leakage is kept low and, therefore, the coupling coefficient is high. This type of transformer is useful in applications where weight is an important factor. Another advantage is the simple design and that it can be manufactured at a low cost.

Study of a Compact HPM System With a Reflex Triode and a Marx Generator

To study the performance of compact systems for microwave generation, a series of experiments have been performed with a microwave source powered directly by a Marx generator. The system consists of a 20-stage 400-kV/400-J Marx generator, a powerful 40-kV charger, a reflex triode, and a vircator-type microwave source. Different parametric studies were performed such as variation of the anode-cathode distances and the emitting area of the cathode. The results have been analyzed and compared to an equivalent electric circuit model of the system. The experiments, generating microwave frequencies between 3 and 5 GHz, can be fairly accurately reproduced by the model both in terms of discharge currents and microwave frequencies

Modeling of a Small Helical Magnetic Flux-Compression Generator

In order to gain experience in explosive pulsed power and to provide experimental data as the basis for computer modeling, a small high-explosive-driven helical magnetic flux-compression generator (FCG) was designed at the Swedish Defence Research Agency. The generator, of which three have been built, has an overall length of 300 mm and a diameter of 70 mm. It could serve as the energy source in a pulse-forming network to generate high-power pulses for various loads. This paper presents a simulation model of this helical FCG. The model, which was implemented in Matlab-Simulink, uses analytical expressions for the generator inductance. The model of resistive losses takes into account the heating of the conductors and the diffusion of the magnetic field into the conductors. The simulation results are compared with experimental data from two experiments with identical generators but with different seed currents, influencing the resistive losses. The model is used to analyze the performance of the generator.

Experimental Study of Electromagnetic Effects on Solid Copper Jets

In this paper we present a study of the interaction between all electric current pulse and a solid copper jet. Experiments were performed using a dedicated pulsed power supply delivering a current ...

Comparison Between Experimental and Numerical Studies of a Reflex Triode

This paper presents a comparison of experimental and simulated results of a reflex triode driven by a compact Marx system. The experimental setup consists of a Marx system and a reflex triode together with a short output waveguide. A parametric study has been performed. The diagnostics used include current and voltage measurements and measurement of the magnetic field component of the microwave pulses using a B-dot probe. The 3-dimensional particle-in-cell simulation code MAGIC is used to numerically study the system described above. A ID model of the Marx system has been designed and this is connected to a 3D model of the reflex triode. Also included in this model is the output waveguide and part of the anechoic chamber used in the experiments. The simulated current, voltage and microwave radiation are compared to the experimental results and close qualitative agreements are usually found. Parameters like generated microwave power and microwave energy are also studied and presented in this paper.

Initial Results from Experiments with a Reflex Triode Powered by a Marx Generator

The reflex triode is a device used for the generation of high-power microwave (HPM) radiation. This study presents an experimental investigation of a reflex triode, powered by a 400 kV Marx generator, and also gives a comparison between theoretically and experimentally obtained scaling laws. The experimentally determined dominating frequency from the reflex triode with different anode-cathode gaps agree qualitatively well with the theoretical expression, but in general the experimental frequency is lower.

Interaction Between Solid Copper Jets and Powerful Electrical Current Pulses

The interaction between a solid copper jet and an electric current pulse is studied. Copper jets that were created by a shaped-charge device were passed through an electrode configuration consisting of two aluminum plates with a separation distance of 150 mm. The electrodes were connected to a pulsed-power supply delivering a current pulse with amplitudes up to 250 kA. The current and voltages were measured, providing data on energy deposition in the jet and electrode contact region, and flash X-ray diagnostics were used to depict the jet during and after electrification. The shape of, and the velocity distributions along, the jet has been used to estimate the correlation between the jet mass flow through the electrodes and the electrical energy deposition. On average, 2.8 kJ/g was deposited in the jet and electrode region, which is sufficient to bring the jet up to the boiling point. A model based on the assumption of a homogenous current flow through the jet between the electrodes underestimates the energy deposition and the jet resistance by a factor 5 compared with the experiments, indicating a more complex current flow through the jet. The experimental results indicate the following mechanism for the enhancement of jet breakup. When electrified, the natural-formed necks in the jet are subjected to a higher current density compared with other parts of the jet. The higher current density results in a stronger heating and a stronger magnetic pinch force. Eventually, the jet material in the neck is evaporated and explodes electrically, resulting in a radial ejection of vaporized jet material.

Modelling of a Small Helical Magnetic Flux Compression Generator

Summary form only given. This paper presents a 0-dimensional simulation model of the helical flux-compression generator, adapted to the FOI generator design. The model, which was implemented in Matlab-Simulink, uses analytical expressions for the generator inductance. The model of resistive losses takes into account the heating of the conductors and the diffusion of the magnetic field into the conductors. The simulation results are compared to experimental data from two experiments with identical generators but with different seed currents, which has an impact on the resistive losses. Comparison shows that the agreement between measured and calculated inductance change with time is good and that the simulated current amplification during operation agrees well in both shape and amplitude.