Mattias Elfsberg

Experimental Studies of Anode and Cathode Materials in a Repetitive Driven Axial Vircator

Repetitive use of a high-power-microwave radiation source implies strong erosion on cathode and anode materials. Electrode-material endurance has been studied in a series of experiments with an axial virtual cathode oscillator powered by a compact Marx generator. The Marx generator is operated in a 10-Hz repetitive mode with a burst of ten pulses. Velvet and graphite was used as electron-emitting materials, and they showed markedly different pulse characteristics. The following three different anode materials were used: stainless-steel mesh, stainless-steel wires, and molybdenum wires, which all had different influence on the pulse characteristics.

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.

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

Experimental Studies of the Influence of a Resonance Cavity in an Axial Vircator

Experiments on an axial virtual-cathode oscillator (vircator) with a resonance cavity enclosing the virtual cathode are reported. The vircator is driven by a repetitive Marx generator operating in a single-shot mode. To be able to separate different radiation mechanisms, the design of the vircator allows adjustment of the cavity depth as well as the way microwave radiation is extracted. The microwave radiation is measured with a pair of free-field B-dot sensors. The maximum field strengths were registered when the bandwidth was very narrow.

Experiments on Gliding Discharge Configuration for Plasma-Assisted Combustion

Gliding discharges are of interest to be used to assist combustion processes, one reason being their ability to transit from the thermal arc mode to a nonthermal discharge mode. We report on experiments performed with a gliding discharge in a forced flow of ambient air to study the discharge mode and discharge behavior when the impedance of the discharge channel is matched to the inner impedance of the power source. Our discharge remained in the arc mode and a higher flow rate of the externally forced airflow gave better performance in terms of energy transfer to the discharge channel.

Experimental studies of the influence of a resonance cavity in an axial vircator

Experiments on an axial virtual-cathode oscillator (vircator) with a resonance cavity enclosing the virtual cathode are reported. The vircator is driven by a repetitive Marx generator operating in a single-shot mode. To be able to separate different radiation mechanisms, the design of the vircator allows adjustment of the cavity depth as well as the way microwave radiation is extracted. The microwave radiation is measured with a pair of free-field B-dot sensors. The maximum field strengths were registered when the bandwidth was very narrow.

MAGNETIC FIELD MEASUREMENT SYSTEM FOR HPM RESEARCH

One method to characterize the radiated microwave field from a high‐power microwave (HPM) source is to measure the radiated high‐level electromagnetic field in several locations at a high sampling rate registering the frequency time dependence, thus being able to determine the radiated pattern and mode. A complete free‐field measurement system for measuring the magnetic field component in high‐level electromagnetic fields has been developed at FOI.The system consists of a B‐dot sensor and a balun, both designed and constructed at FOI. The B‐dot sensor is designed as two cylindrical loop sensors with differential output. The balun is a microstrip design etched on a dual sided PTFE circuit board. Complete systems have been calibrated at SP Technical Research Institute of Sweden. A method to analyze the data from the free‐field systems has been developed.

High-Voltage Pulsed-Power Cable Generator

A cable-based 25-GW pulsed-power generator with output impedance of 2 Omega is presented. It is designed to deliver a 200-ns-long 500-kV pulse into a 10 Omega load. The primary energy storage of the generator consists of a 50-kV 20-kJ capacitor bank. The 50-kV capacitor bank is discharged into a 1 : 12 transformer. The transformer is designed to charge a pulse-forming line (PFL) to 600 kV. When charged, the PFL is discharged into a load via a spark gap. The spark gap is located in a coaxial system containing deionized water together with the cable endings of the PFL and transformer. The electric field at the cable endings is refractively graded by the high permittivity of the surrounding water. The primary and secondary windings consist of high-voltage cables that are interleaved and wound together. The PFL consists of eight 40-m-long 110-kV coaxial cables with both ends connected to the load. Each cable screen is grounded in the middle and connected in parallel. The cables have a characteristic impedance of 30 Omega. The parallel cable setup gives the PFL an impedance of 2 Omega. The total length, height, and width of the pulse generator are 4, 2, and 1.2 m, respectively.

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.