Brady B. Gall

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.

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.

High voltage production from shaped piezoelectric transformers and piezoelectric transformer based circuits

Compact ion and electron beam sources are potentially useful for many applications. Compact beam generators require both compact charged particle diodes and compact high voltage generators. Piezoelectric transformers are currently being developed at the University of Missouri to convert medium voltage at radio frequency to high voltages. Piezoelectric transformer geometries, including tapered and wedge shaped crystals operated in bipolar output mode, are being studied. Results demonstrating resonant properties of these crystals will be presented. High voltage operation, in excess of 25 kV peak voltage, will also be presented. Additionally, various voltage multiplying circuit topologies and crystal geometries are being studied to maximize the voltage output from piezoelectric transformer systems.

Measurement of the internal stress and electric field in a resonating piezoelectric transformer for high-voltage applications using the electro-optic and photoelastic effects

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.

Electrical analysis of piezoelectric transformers and associated high-voltage output circuits

Piezoelectric transformers can be useful as compact, high-voltage supplies. At the University of Missouri, the effect of adding output circuits to bipolar piezoelectric transformers is being studied. These piezoelectric transformers produce output voltages in excess of 25 kV from medium-voltage, radio frequency inputs. However, the high output voltage and low output current of these devices can make it difficult to acquire an accurate electrical measurement of the output voltage without affecting the transformer ratio or resonance of the device. This paper will analyze capacitive voltage dividers as a means of diagnostic measurement and Cockcroft-Walton type circuits to increase the voltage multiplication beyond that of the piezoelectric transformer alone.

The efficiency of a helium-3 neutron detector evaluated using MCNPX

Summary form only given. A compact neutron source based on a piezoelectric transformer has been developed as part of an on-going research effort into radioisotope source replacements for oil-well logging and non-destructive testing applications. One of the key features that will dictate the viability of the device is the effective neutron yield produced during operation. A proportional counter that uses helium-3 as the active sensing material was used for the purpose of detecting neutrons. The detector response to neutrons depends upon the efficiency of the helium-3 proportional counter tube in conjunction with the solid angle subtended by the source and the detector, as well as the material system that surrounds the proportional counter. The coupled geometry effect and energy dependence of the detector efficiency make an analytical solution prone to error. Additionally, obtaining a calibrated neutron source that mimics the size and desired activity of the piezoelectric transformer-based source was not in the scope of the research. Consequently, a numerical model of the neutron source and detector was developed and subjected to Monte Carlo analysis using MCNPX using neutrons as the source particles. A sensitivity analysis of the detector position and orientation was used to determine the optimal location for high-sensitivity neutron counting.

Effects of electron beam loading on an operating piezoelectric transformer

Piezoelectric transformers (PTs) are currently being used to accelerate charged-particle beams for various applications [1, 2]. Beam interactions at the output of the PT can be treated as a parallel RC electrical load. The impedance of the load can affect the output voltage due to the small, finite amount of charge available in the PT. High output voltages necessary for beam acceleration cannot be achieved if the current output from the PT is too high. A thermionic electron emitter was used to provide a controllable beam current for testing the effects of electrical loading on the PT. The electron beam was operated in vacuum pressures of 10-6-10-5 Torr. Variations in the electron beam current were used to vary the resistive portion of the load while beam distance was used to vary capacitance. The effects of such variations were measured via bremsstrahlung interactions at the output electrode of the PT to determine output voltage and optical techniques for finding internal operating parameters such as electric field.

Laser-Optical and X-Ray Characterization of an Operating High-Voltage Piezoelectric Transformer in Multiple Vibrational Modes

A crystal of lithium niobate was configured for use as a longitudinal-mode piezoelectric transformer (PT). An electron gun was coupled to the transformer to allow for controlled release of charge to the high-voltage terminal of the PT. An electron tube extracted a beam from a thermionic emission source, and cylindrical optics focused the beam to a 2-mm spot at the PT output. Two diagnostic methods were used to measure the operation of the PT: bremsstrahlung X-ray spectroscopy and laser-optical interrogation. Both diagnostics showed that the PT is able to process up to 1.5 μA of current at 20-kV output in a continuous mode. The laser-optical method was also used to compare the mechanical stress, electric field, and developed voltage between the fundamental and secondary harmonic resonance modes.

Measurement of internal stress and electric field in a resonant piezoelectric transformer using the electro-optic and photoelastic effects

Full characterization of a piezoelectric transformer (PT) requires knowledge of internal operating parameters such as electric field, stress, and strain. Finite-element numerical models can determine these parameters with respect to the geometry of the PT and input conditions. However, experimental verification of these parameters for a resonating PT are challenging due to the mechanical vibration and electrical loading effects. An optical measurement has been designed to measure internal electric field by taking advantage of the electro-optic and photoelastic effects that also occur in lithium niobate (LiNbO3). The optical measurement can probe the PT without influencing its operation. A linearly polarized helium-neon laser beam was used to transversely probe a PT with respect to the input and output electric fields. As the piezoelectric effect caused variations in the internal electric field and strain, the superposition of the photoelastic and electro-optic effects resulted in a relative phase shift in the linearly polarized beam. The resultant phase shift was measured via intensity variations at a photodiode following a second linear polarizer. Internal parameters such as stress, strain, and electric field were calculated based on the coupled piezoelectric, photoelastic, and electro-optic equations. These results are presented for varying positions along the output section of a resonating LiNbO3 length-extensional PT.

High voltage piezoelectric system for generating neutrons

Compact electrical neutron generators are a desirable alternative to radioisotope neutron sources. A piezoelectric transformer system is presented which has been used to achieve neutron production. The two primary components of the system include a piezoelectric transformer plasma source (PTPS), which produced a deuterium plasma for ion extraction, and a high voltage piezoelectric transformer (HVPT), which generated an accelerating potential in excess of 100 k V. The system was operated at a pressure of 700 μTorr with an external gas supply providing controlled deuterium flow to the differentially pumped PTPS. Synchronized AC signals were used to independently drive each of the piezoelectric devices in burst mode with a 1 Hz rep-rate and 10% duty factor. A timing offset between the two signals was used to decrease electrical loading effects and increase neutron flux. The mechanism for neutron production was the D(d, n)3 He nuclear reaction, occurring when deuterium ions from the PTPS accelerated toward and impacted a deuterium-impregnated Pd or Ti target adhered to the output terminal of the HVPT.