Andrew Benwell

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.

Runtime and Jitter on a Laser-Triggered Spark-Gap Switch

The University of Missouri has completed a new facility, named Tiger, for pulsed-power experimentation. Tiger consists of a 2.8-MV 450-kJ Marx bank that charges up to four 7-nF intermediate storage capacitors (I-stores) in parallel. When charged, the storage capacitors are switched into a resistive load through an SF6-filled laser-triggered gas switch. This switch has been designed to study the factors affecting runtime and jitter of spark-gap switches. All experiments presented in this paper were performed with a single I-store. The test switch was operated from about 500 kV up to 1.25 MV, at switch pressures from 10 to 50 psig. A 30-mJ 266-nm Nd:YAG laser was focused between the switch electrodes to initiate breakdown in the switch. The University of Missouri has examined laser energy, percentage of self-break, and focal length to determine their relation to runtime and jitter. A short discussion of the Tiger facility is presented with experimental results of jitter and runtime tests. The end goal of this paper is to understand the factors contributing to increased jitter and runtime and, thereby, provide paths to improved switch performance.

A resonantly driven piezoelectric transformer for high voltage generation

A resonant piezoelectric transformer circuit is being developed for high voltage generation. A lithium niobate ferroelectric crystal with a rotated y-cut polarization orientation is used as a step up transformer. The rectangular shaped crystal is driven through the thickness into the extensional vibration mode. Applied voltages at low radio frequencies, 40 to 90 kHz, are used, which includes the half and full wavelength resonance frequency. The circuit design takes advantage of the piezoelectric transformation into mechanical energy to bias the primary side of the transformer. An impedance matching transformer is used to match the crystal to the driving circuit over a wide range of frequencies. The piezoelectric circuit is designed to drive high impedances at 50 to 100 kV for beam or plasma applications. Voltage and frequency characteristics with several load impedances have been measured. The results of these tests to characterize the piezoelectric transformer circuit are presented.

Dielectric flashover with triple point shielding in a coaxial geometry.

Increasing performance of vacuum insulator barriers is a common goal in pulsed power. Insulating performance is continually being improved while new methods are developed. Triple point shielding techniques have been shown to increase flashover voltage, but the role of cathode versus anode shielding is still not fully understood. Open circuit flashover characteristics were obtained for a coaxial geometry to view the effects of triple point shielding for this geometry. The tests included applying various combinations of triple point shields on zero and +45 degrees insulators. Shielding was tested at the cathode triple point outside of the dielectric and at the anode triple point inside the dielectric. The role of anode versus cathode triple point shielding was examined. Flashover voltage was observed to increase when either a cathode or anode triple point shield was applied; however, adding a shield to both regions lowered the flashover threshold. Both triple point regions were found to be important and dependent on each other for some coaxial geometries.

Particle Swarm Optimization of Pulsed Power Circuit Models

Circuit modeling is ubiquitous throughout the pulsed power discipline. Both plasma processes and systems can be modeled with circuits of varying complexities. Sometimes, circuit models need to be generated to match experimental waveforms. Particle swarm optimization (PSO) is a technique which can be utilized to automatically generate a circuit model to match experimental data. This paper details the PSO algorithm as well as two case studies of the implementation.

Flashover prevention of high voltage piezoelectric transformers

Piezoelectric transformers are used as step-up voltage elements in many devices [1, 2]. The University of Missouri is developing a piezoelectric transformer as an accelerator for an ion beam [3]. In cases where high voltage pulses are desired, discharges can result from a large electric field near triple point junctions [4, 5]. Due to the small scale of the device, conductive triple point shields are difficult to employ to prevent flashover. This paper presents an investigation of piezoelectric flashover prevention by thin film encapsulation. Dielectric material was deposited on the piezoelectric transformer both over the entire device, and at specific regions of interest. The dielectric was deposited by evaporation to eliminate gaps at the triple point. The flashover strength is evaluated depending on the dielectric type, thickness, and length. The mechanical loss incurred by the deposition is evaluated to determine if it hinders the motion of the transformer.

Initial results from the University of Missouri terawatt test stand

The University of Missouri Terawatt Test Stand (MUTTS) is fitted with a 2.7 MV multichanneling laser triggered gas switch scaled from a 4 MV switch developed at Sandia National Laboratories. The long term goals of the research at MUTTS are to improve the multichanneling reliability and jitter of the switch when the electrode rings in the cascade section are either increased in number or in diameter. Multichanneling is important for two main reasons: 1) a reduction of electrode wear and 2) reduced inductance. The test facility provides a high voltage/high current test bed for a number of experiments that are easily scalable to large accelerators, such that, we expect the results to be directly applicable to the requirements of the Z and ZR accelerators. The first series of shots at MUTTS included dummy load and open load configurations to determine parasitic circuit elements. Diagnostics include monitoring the load current and Marx total current, voltage at the Marx output, load, and Marx trigger unit. Equivalent series resistance, series inductance, Marx capacitance, and shunt resistance were determined from these diagnostics. This is used to develop a first order circuit model of the energy storage section by comparison to experimental data. Characteristics of the scaled switch and initial pulsed power tests at the facility are presented

Electrical parameters of projectile stun guns

Projectile stun guns have been developed as less-lethal devices that law enforcement officers can use to control potentially violent subjects, as an alternative to using firearms. These devices apply high voltage, low amperage, pulsatile electric shocks to the subject, which causes involuntary skeletal muscle contraction and renders the subject unable to further resist. In field use of these devices, the electric shock is often applied to the thorax, which raises the issue of cardiac safety of these devices. An important determinant of the cardiac safety of these devices is their electrical output. Here the outputs of three commercially available projectile stun guns were evaluated with a resistive load and in a human-sized animal model (a 72 kg pig).

A 2.8 MV, 600 kA pulsed power driver constructed at the University of Missouri-Columbia

Summary form only given. The University of Missouri has completed construction of a new facility for pulsed power experimentation. The pulsed driver is driven by a Marx bank consisting of 32, 100 kV, 3.1 muF capacitors. The Marx bank can store 450 kJ and is switched into any combination of one or up to four, parallel, 7 nF intermediate storage capacitors. The intermediate storage capacitors are switched with a high voltage spark gap switch into a resistive load. The driver is capable of delivering 2.8 MV at 600 kA to a 4 ohm load. Simulations indicate that the driver is capable of producing 1 MA through a load with lower impedance. Initial uses for the facility include experimentation on switch jitter and arc spectroscopy on a spark gap switch. The test results are also used in the characterization of the machine. Simulations of the driver capabilities are presented along with details of the initial results.

Investigation of Dielectric Flashover in an SF6 Filled Laser Triggered Gas Switch

A study of dielectric flashover in SF6 in a laser triggered gas switch (LTGS) is reported. Flashover occurring on the inside of the insulating barrier of the switch leading to the eventual failure of the switch was studied. The University of Missouri-terawatt test stand is testing the trigger section of a LTGS to understand and improve the flashover characteristics of these switches. A Rimfire LTGS was modified to operate with a trigger gap voltage of 938 kV. The modified switch was tested repeatedly over a range of pressures to examine the sensitivity of switch flashover to several parameters. The aspects of the switch that were tested include triple points both due to asymmetrical torque on the endplates, and due to gaps between the insulator and endplate, and the effect of slower voltage risetime. An analysis of the early findings of this study is presented