Peter Norgard

Investigation of the Piezoelectric Effect as a Means to Generate X-Rays

The piezoelectric effect is analyzed as a means to produce X-rays. A mass of crystalline piezoelectric material is used to convert a low-voltage input electrical signal into a high-voltage output signal by storing energy in a longitudinally vibrating mechanical wave. Output energy is extracted in the form of a high-voltage electron beam using a field-emission diode mounted on the surface of the crystal. The electron beam produces X-rays via bremsstrahlung interactions with a metallic surface.

A High Pressure Flowing Oil Switch for Gigawatt, Repetitive Applications

A repetitive oil switch for directed energy applications has been developed in a joint effort between teams at the University of Missouri - Columbia, Alpha Omega Power Technologies and the Boeing Company. The switch is operated at test pressures to 17.24 MPa (2500 psi), flow rates to 0.72 L-s-1 (11.4 gpm), charge voltages to -300 kV and discharge energies to 275 J per pulse at 20 pps. An examination of the electrodes after 250,000 shots with the original design led to the design of an insert device which resulted in higher performance fluid flow within the switch. The flow shaper-enhanced switch was tested for 150,000 shots, the results of which are presented in the following paper. Electrode lifetime has been evaluated for stainless steel under the original and enhanced fluid flow conditions and is reported.

Development of high power; high pressure, rep-rate, liquid dielectric switches

The University of Missouri-Columbia (UMC) is developing high power liquid dielectric switches intended to address future high power microwave (HPM) applications. Although requirements encompass a broad parameter space, the initial switch concept focuses on a 250-300kV output switch operated at 100 pps that will be scaled to 1MV. Failure to clear high electric field regions prior to the next charge cycle results in prefires, thereby limiting the maximum achievable repetition rate. Elevating the operating pressure, hence minimizing the bubble size and temporal properties, has alleviated this problem. This paper presents the design philosophy, modeling, and experimental results obtained from a single shot prototype operated in oil at pressures ranging from atmospheric pressure to greater than 13.8 MPa (2000 psi).

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.

Compact neutron generator driven with high-voltage piezoelectric transformer

Summary form only given. A lithium niobate piezoelectric transformer was used to generate neutrons using a deuterium-deuterium (D-D) nuclear reaction. Deuterium gas was flowed into a vacuum chamber at pressures between 10−4 and 10−3 Torr. The deuterium gas was ionized and electric fields generated by the transformer accelerated deuterons into a grounded, deuterium impregnated target. A suite of diagnostics, including a bremsstrahlung x-ray measurement, electrooptic probe, and helium-3 neutron detector were used to evaluate the generator voltage, electric fields, and neutron production rates.

An electrooptic probe to determine internal electric fields in a piezoelectric transformer.

A technique using the electrooptic effect to determine the output voltage of an optically clear LiNbO(3) piezoelectric transformer was developed and explored. A brief mathematical description of the solution is provided, as well as experimental data demonstrating a linear response under ac resonant operating conditions. A technique to calibrate the diagnostic was developed and is described. Finally, a sensitivity analysis of the electrooptic response to variations in angular alignment between the LiNbO(3) transformer and the laser probe are discussed.

An in-depth investigation into the effect of oil pressure on the complete statistical performance of a high pressure, flowing oil switch

A high pressure, flowing oil dielectric switch was developed for high performance, high voltage switching, and extensively evaluated by a team at the University of Missouri—Columbia. The switch was designed to produce a continuous train of nanosecond-rise, electrical impulses, with a peak output power ranging up to several gigawatts, and at repetition frequencies ranging up to several kilohertz. High pressure, flowing oil was proposed for the switching medium as a means of enabling rapid recovery of the insulating properties of the dielectric following electrical breakdown. The switch was developed for self-breakdown operation, with an anticipated lifetime of greater than 107 switching cycles. An experimental study of the statistical performance of the high pressure, flowing oil switch was conducted over a range of oil pressures from 0.5 – 10 MPa (72 – 1450 psig), oil flow rates from 10 – 40 Lpm (2.6 – 10.6 gpm), peak modulator charge voltages from 12.5 – 25.0 kV, and gap separations from 0.40 – 1.00 mm, utilizing self-breakdown at repetition frequency of 2 Hz. In this paper, we review the effects of operating the switch over the full range of oil pressures at constant modulator charge voltage, constant gap separation, and constant oil flow rate in a study of the complete statistical performance of the high pressure oil switch. The breakdown characteristics of the high pressure oil switch demonstrate remarkable properties, including a linear dependence relating the oil pressure to both the mean and maximum breakdown electric field strength, independence of the minimum break-down electric field strength on oil pressure, and a decidedly non-linear relationship between oil pressure and breakdown electric field strength standard deviation.

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A high-pressure oil switch for high-energy systems has been developed and tested to repetition rates up to 100 pps in burst mode and test voltages to -300 kV. The switch utilizes high-pressure flowing synthetic oil and advanced mechanical designs to achieve high-performance operation at pressures to 17.24 MPa (2500 psi). Experimental data is presented for both single-shot performance as well as continuous repetition rate performance to 20 pps. Electrode lifetime is evaluated for stainless steel and Elkonite to demonstrate electrode lifetime. Evidence is presented in support of the dependence of switching performance on the geometry of the switch

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The high output voltages from piezoelectric transformers are currently being used to accelerate charged particle beams for x-ray and neutron production. Traditional methods of characterizing piezoelectric transformers (PTs) using electrical probes can decrease the voltage transformation ratio of the device due to the introduction of load impedances on the order of hundreds of kiloohms to hundreds of megaohms. Consequently, an optical diagnostic was developed that used the photoelastic and electro-optic effects present in piezoelectric materials that are transparent to a given optical wavelength to determine the internal stress and electric field. The combined effects of the piezoelectric, photoelastic, and electro-optic effects result in a time-dependent change the refractive indices of the material and produce an artificially induced, time-dependent birefringence in the piezoelectric material. This induced time-dependent birefringence results in a change in the relative phase difference between the ordinary and extraordinary wave components of a helium-neon laser beam. The change in phase difference between the wave components was measured using a set of linear polarizers. The measured change in phase difference was used to calculate the stress and electric field based on the nonlinear optical properties, the piezoelectric constitutive equations, and the boundary conditions of the PT. Maximum stresses of approximately 10 MPa and electric fields of as high as 6 kV/cm were measured with the optical diagnostic. Measured results were compared to results from both a simple one-dimensional (1D) model of the piezoelectric transformer and a three-dimensional (3D) finite element model. Measured stresses and electric fields along the length of an operating length-extensional PT for two different electrical loads were within at least 50 % of 3D finite element simulated results. Additionally, the 3D finite element results were more accurate than the results from the 1D model for a wider range of electrical load impedances under test.

Olefin Synthetic Oil: A New Paradigm in Repetitive High-Pressure Oil Switches

A high power, rep-rate, liquid dielectric switch has been designed to meet future requirements for a variety of directed energy applications. A flowing, high-pressure liquid dielectric was chosen for the design of a megavolt class switch intended to operate continuously at pulse repetition rates in excess of 100 pps. This paper reports on the design efforts and initial testing of a full size prototype, 250 kV switch. The design capitalizes on lessons learned from experiments with a single shot concept validation high pressure liquid dielectric switch. Design efforts include extensive electric field modeling, circuit model simulations, and fluid flow simulations. The design facilitates fast rise times, quick switch recovery, and long electrode life. Rep-rate testing of the high pressure dielectric switch includes testing at rep-rates up to 100 pps and voltages up to 250 kV. The design of the pulser used to test the rep-rate switch is described in a separate conference paper. Switch diagnostics include D-dot probes and a Rogowski current probe. Results from the experimental switch tests will be compared to circuit models and the data presented showing the operating characteristics of the switch for various pressure ranges