Sten E. Nyholm

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.

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

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.

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 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.

Radiated Electric Field Strength From High-Power Microwave Systems

Intentional electromagnetic interference (IEMI) is an emerging threat and a new electromagnetic environment that has to be considered in the mitigation of electromagnetic compatibility problems. This paper presents an attempt to estimate the upper limitations of IEMI systems based on high-power microwave devices. The upper limit of transmitted electromagnetic wave power is basically set by the microwave breakdown event. The estimate includes the properties of the electromagnetic wave propagation from the generation of the wave to the possible target.

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.