Tomas Hurtig

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.

The penetration of plasma clouds across magnetic boundaries : The role of high frequency oscillations

Experiments are reported where a collision-free plasma cloud penetrates a magnetic barrier by self-polarization. Two closely related effects, both fundamental for the penetration mechanism, are studied quantitatively: anomalous fast magnetic field penetration (two orders of magnitude faster than classical), and anomalous fast electron transport (three orders of magnitude faster than classical and two orders of magnitude faster than Bohm diffusion). It is concluded that they are both mediated by highly nonlinear oscillations in the lower hybrid range, driven by a strong diamagnetic current loop which is set up in the plasma in the penetration process.

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.

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 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 ...

Numerical experiments on plasmoids entering a transverse magnetic field

Plasma from the Earths magnetosheath has previously been observed inside the magnetosphere. Inhomogeneities in the magnetosheath plasma, here called plasmoids, can impact the magnetopause and doing so set up a polarizing field that allows it to penetrate the magnetopause and enter the magnetosphere. A set of simulations of plasmoids with different dimensions is presented in this paper. For plasmoids that are longer than those previously published, waves propagating upstream from the barrier are found. It is also found that the penetration process causes the part of the plasmoid that is upstream of the barrier to rotate. The role of plasmoid width and cross sectional shape in penetration is studied, and for plasmoids that are less than half an ion gyroradius wide, the plasmoid is compressed to obtain a vertically oriented elliptical cross section, regardless of the initial shape. When the initial plasmoid width exceeds the ion gyroradius, the plasmoid still penetrates through a mechanism involving a potential that propagates upstream from the magnetic barrier.

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.