R. L. Druce

Design and operation of a dual vircator HPM source

To use two separate vircators simultaneously for two-frequency multipulse testing, we designed and constructed a versatile modulator which drives two vircators in parallel. We present details of the design, including vircator design with particle-in-cell simulations using CST Particle Studio® and MAGIC. The system operates as planned. Microwave output of the vircators under various operating conditions fits frequency varying inversely with the anode-cathode gap, although there is considerable scatter at larger gaps, likely due to cathode plasma non-uniformity.

Dielectric Constant of Sand Using TDR and FDR Measurements and Prediction Models

The effects of soil dry density and water content are being examined through experimental time-domain-reflectometry (TDR) and frequency-domain reflectometry (FDR) methods in correlation with electromagnetic simulations. The infiltration rate (hydraulic conductivity) of water in sand is exceptionally high, resulting in heterogeneous moisture distribution through the soil. The effective dielectric constant of the soil/water/air mixture is dependent on the soils dry density and moisture content. Both TDR and FDR methods are performed on a coaxial transmission line filled with a soil/water/air mixture. The flow of the water through the soil creates a dynamic situation in which the soil/water/air electrical impedance changes over time. The resulting soil has heterogeneous water content, creating varying electrical impedance values along the length of the coaxial line. The soil compaction, i.e., dense or loose, has significant impact on the heterogeneity of the moisture content through the soil and the dry density of the soil. In each case, the effective dielectric constant is determined from the data collected from TDR and FDR experiments, and the values are compared with the predictions using established empirical models by Topp, Hilhorst, and Hendrickx. With the exception of the data represented as a function of the degree of saturation, the Hendrickx model appears to best represent the measured dielectric constants since it falls within two standard deviations of the measured data. A computer simulation technology (CST) Microwave Studio is used to supplement experimental observations of various soil moisture contents in a coaxial cell. Simulations confirm that the change in the dielectric constant through the soil is a result of the heterogeneous moisture distribution. It was found that the soil moisture content has a major impact on the resulting dielectric constant from measurements or modeling. In the coaxial-cell device, soil moisture migration during the testing period results in a heterogeneous moisture regime and a temporal dielectric constant. This is particularly exaggerated for high-hydraulic-conductivity soils such as sand.

Design and testing of wide bandgap current limiting devices

The University of Missouri in collaboration with Helava Systems Inc. have developed a concept and have shown in experiments the feasibility of a solid state switch based on the photoconductive properties of a semiconductor for radar limiters in a linear mode. Three possible device geometries were subsequently designed using CST Microwave Studio€ which would allow for matched microwave off-state transmission but provide substantial limiting of the signal in the on-state (illuminated) condition. Each design was simulated and the results compared allowing for the best possible geometry to be chosen. The chosen design allowed for greater than 99% off-state transmission and an on-state limiting of less than 1% of the incident signal. Initial experimental tests to determine the semiconductors effectiveness to act as a photoconductive switch were investigated using highly conductive silver paint. These devices were then subjected to testing and the results compared with simulated calculations in CST and MATLAB®. Through these tests, the University of Missouri has demonstrated the ability of aluminum gallium nitride (AlGaN) to act as a photoconductive switch when illuminated with 355-nm light. Experiments show a greater than two orders of magnitude drop in semiconductor channel resistance upon illumination. While further investigation into the ability of the device to obtain sub-ohm resistance levels is needed, initial tests and calculations confirm the ability of AlGaN materials to act as a current limiting device with the geometry designed by the University of Missouri.

Design and implementation of dual independent Vircators driven by a single pulsed power source

Dual, independent Vircators are currently being designed and fabricated that are to be driven by a single pulsed power source at the Center for Physical and Power Electronics at the University of Missouri-Columbia.

Design and construction of a 250 kV, 100 Hz repetitive Vircator test stand

The Center for Physical and Power Electronics (CPPE) at the University of Missouri-Columbia is investigating the applicability of utilizing two separate Vircators simultaneously for multipulse testing. To facilitate this research, we have designed and constructed a versatile modulator to drive the HPM sources that allow us to drive two Vircators in parallel. The modulator consists of a thyratron-switched capacitor bank that pulse charges a water transmission line through a pulse transformer. The water transmission line is then switched into a voltage adder by a pressurized oil switch with an output risetime of 20 ns. The capacitor bank is charged to 30 kV by a bank of power supplies delivering up to 40 kJ/s and switched by a thyratron into the pulse transformer. The 1:10 pulse transformer charges the water line to 300 kV in approximately 2.5 μs. When driven by the pulse line, the 250 kV output voltage is utilized to simultaneously drive the two, 32 Ω Vircators in parallel or a 16 Ω load with a 70 ns long pulse. We will present details of the design, including particle-in-cell simulations using CST Particle Studio and SPICE circuit simulations of the Vircator design and modulator design, as well as construction details. The test results will also be presented.

Study of nonlinearity effects in simple circuits under pulsed conditions

The nonlinearities produced via RF direct injection in simple circuits comprised of diodes, transistors, and ICs, when driven with multiple frequency inputs, is under investigation. The effects of different pulse durations and frequency of the RF pulse while investigating each devices stability and nonlinear response will be reported. Previous tests have concluded that the response of these devices exhibit multiple nonlinear effects when given continuous multiple input frequency stimuli. Given the previous results, multiple injected pulse conditions are under test. In these experiments, pulse durations varying between 200ns to 1ms, and frequencies between 10MHz and 25MHz are being tested to determine the effect on each devices response. The device considered will primarily consist of a common RF amplifier. The implementation of the test fixtures, laboratory equipment, and data analysis will be discussed along with the test results and conclusions.

Measurement of sand effective dielectric constant

The effects of soil water content on dielectric constant are being examined through experimental time-domain-reflectometry (TDR) and frequency-domain reflectometry (FDR) methods in parallel with electromagnetic simulations. The effective relative permittivity (dielectric constant) of the soil, water, and air mixture is highly dependent on the soils moisture content. Both TDR and FDR methods are performed on a coaxial transmission line filled with a soil/water/air mixture. Water flowing through the mixture creates an active situation in which the soil/water/air electrical impedance changes over time. The resulting mixture has a heterogeneous water content, creating varying electrical impedance along the length of the coaxial line. The effective dielectric constant is found from the data collected from TDR and FDR measurements, and the values are compared with the predictions by the Hendrickx effective dielectric constant model. The Hendrickx fits within three standard deviations of the mean of the measured dielectric constants.

A high voltage pulsed power system for repetitive vircator testing

The University of Missouri - Columbia (UMC) is beginning research into high power microwaves and their effects upon various systems. A high power microwave (HPM) laboratory is, therefore, being established at UMC to provide the capacity to perform the tests. The initial HPM source will be based on a virtual cathode oscillator, or vircator. The vircator requires a stable pulse-top voltage and will be required to be repetitively driven. The UMC has modified an existing repetitive pulse generator to achieve the required pulse characteristics. The UMC vircators will be driven by a water pulse forming line output whose output pulse is transformed to higher voltage by an induction voltage adder cell. The coaxial water pulse forming line (PFL) has an impedance of 4.8 Ω and produces a 150 kV pulse that is 70 ns in length into a matched-impedance load. A modulator has been developed to provide the requisite 365 J energy pulse at repetition frequencies up to 40 pps in continuous mode operation. The output voltage from the PFL is delivered to a matched-impedance transmission line and transformed to 300 kV by an induction voltage adder to provide sufficient voltage for the vircator to operate efficiently. Simulations of the driver connected to a vircator model were used to verify performance. Simulation results are discussed showing driver performance under rep-rate conditions.

Development of the microwave test facility at the University of Missouri Center for Physical and Power Electronics

In 2009 the Center for Physical and Power Electronics at the University of Missouri-Columbia began to construct a facility to enable scaled tests using both low power and high power, pulsed and continuous radio frequency and microwave energy. This facility has been online since early 2010 and has been used to perform advanced tests on antennas and electronics. We are continuing to expand its capabilities both at low power and high power using HPM sources.