James A. VanGordon

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.

Characterization of Power Insulated-Gate Bipolar Transistors to Create a SPICE Model for Pulsed Power Applications

Power insulated gate bipolar transistors (IGBTs) are frequently used in pulsed power applications. While published data is readily available for most IGBTs under steady-state conditions, little information is known for a given IGBT regarding pulsed power conditions. This paper presents a characterization process of power IGBTs for creating IGBT SPICE models. Each IGBT will be subjected to single 5 mus pulses at 4.5 kV and 1 kA. During the pulse, characteristics such as rise time, fall time, maximum collector current, and maximum collector-emitter voltage will be measured. Additionally, the effects of the gate drive with respect to the collector-emitter characteristics will be observed. This process will be demonstrated with the IXYS IXEL40N400 and the Powerex QIS4506001 IGBTs to allow for circuit simulations with these models as circuit elements.

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.

Characterization of power IGBTS under pulsed power conditions

The power insulated gate bipolar transistor (IGBT) is used in many types of applications. Although the use of the power IGBT has been well characterized for many continuous operation power electronics applications, little published information is available regarding the performance of a given IGBT under pulsed power conditions. Additionally, component libraries in circuit simulation software packages have a finite number of IGBTs. This paper presents a process for characterizing the performance of a given power IGBT under pulsed power conditions. Specifically, signals up to 4.5 kV and 1 kA with approximately a 5 µs pulse width will be applied to a given IGBT. This process utilizes curve fitting techniques with collected data to determine values for a set of modeling parameters. These parameters are used in the Oziemkiewicz implementation of the Hefner model for the IGBT that is utilized in some circuit simulation software packages [1, 2]. After the nominal parameter values are determined, they can be inserted into the Oziemkiewicz implementation to simulate a given IGBT.

Method for Approximating Electron Beam Currents Accelerated by a Piezoelectric Transformer

Piezoelectric transformers (PTs) are currently used to accelerate charged-particle beams for various applications. 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 because of the small, finite amount of charge available in the PT; therefore, high-impedance diagnostics are required to characterize the PT. 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 at pressures of 10-6-10-5 torr. The input mechanical quality factor and the effective output electrical quality factor were used to approximate the electron beam current on the basis on the PTs equivalent electromechanical circuit. The output voltage needed for the output electrical quality factor was measured via bremsstrahlung interactions at the output electrode of the PT. Optical techniques for finding internal operating parameters such as electric field and stress were used to determine the load with curve fitting as a comparison with the quality factor diagnostic. Approximating the electron beam current with such methods will help determining the output power that such PTs can generate for future applications.

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.

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.