Emily A. Baxter

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.

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.

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.

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.

Charged-Particle Emission and Self-Biasing of a Piezoelectric Transformer Plasma Source

The piezoelectric transformer (PT) plasma source (PTPS) is a compact radio-frequency-driven plasma source developed for near-space and microspacecraft propulsion. The PTPS utilizes the PT effect to aid in plasma production and acceleration of charged particles to create thrust. Charged-particle emission measured with a Faraday cup and a retarding potential analyzer is presented. Low emitted ion energies are explained by self-bias of the PT output due to charged-particle transfer from the plasma to the PT surface. Finally, self-neutralization of the PTPS was investigated for a PTPS isolated from the vacuum chamber ground.

Effects of capacitive versus resistive loading on high transformation ratio piezoelectric transformers for modular design considerations

Piezoelectric transformers (PTs) can be useful as compact, high-voltage supplies. However, PTs cannot be fully utilized if the associated circuitry does not allow for the desired transformation ratio to be maintained. The effects of adding capacitive and/or resistive loads to a PT are being studied. Experiments and modeling suggest that the highest transformation ratios are achieved when both the resistive and reactive load elements are optimized, rather than only the magnitude of the impedance. Specifically, these effects are being studied to modularize the PT as a compact, high-voltage power supply for a variety of applications. This paper will analyze these loading effects and suggest a coupling method for maintaining high transformation ratios from Rosen-type PTs comprised of rotated y-cut lithium niobate.

Fabrication and characterization of high current field emitters with silicon ball-tip pins

This paper introduces a novel design of high current field emitters based on unique pin structures with a ball shaped tip. Our ball-tip pins provide relatively large surface area at the tip and increase the field enhancement factor regardless of dimension of the pin base. The ball-tip pins turn out to produce higher field emission current than that from the generic sharp-tip pins due to the significantly increased field emission sites at the ball-tip surface.

Electrostatic probe measurement of the piezoelectric transformer plasma source

Summary form only given. A compact ion source for neutron production is being developed for industrial and security applications. The piezoelectric transformer plasma source (PTPS) produces ion currents on the order of 10 μA using a cylindrical LiNbO3 (lithium niobate) crystal measuring 10 mm in diameter by 2 mm thick. The PTPS has previously been characterized to determine the impact of driving voltage, gas flow, extraction voltage, and hollow anode geometry on the ion current.The plasma generated by the PTPS was further characterized using two different electrostatic probe diagnostics. A cylindrical Langmuir probe was used to approximate the electron temperature and density in the RF plasma and a triple probe was used to determine the floating voltage of the plasma. Estimations of the electron temperature and density of the plasma determined using electrostatic probe diagnostics are presented.

Characterization of high-voltage lithium niobate piezoelectric transformers

Piezoelectric transformers (PTs) are capable of generating the high accelerating potentials required for many charged-particle beam applications. However, the PTs need to be characterized during operation as an accelerator for such beam loads. PT characteristics such as internal stress, electric field, and electric potential can provide insight into the optimal operating conditions for a given application. Measurement of internal stress can be used to ensure that the single-crystal lithium niobate (LiNbO3) PT does not exceed the fracture limit. Internal electric field and potential can be used to determine the accelerating potential created by the PT. Internal parameters such as these have been measured using an optical diagnostic that relies on the inherent photoelastic and electro-optic properties of lithium niobate. Beam currents have been determined using least-squares curve fitting with experimental and modeled results. Further characterization of the beam interactions with the PT was done by examining the direct bremsstrahlung x-ray spectra that were produced and the x-ray fluorescence peaks excited by the source. This paper presents a variety of PT characterization results under varied PT operating conditions.