Mark A. Kemp

Ferroelectric plasma thruster for microspacecraft propulsion

This paper presents a technology in microthruster design: the ferroelectric plasma thruster (FEPT). The FEPT utilizes an applied rf electric field to create plasma on the surface of a ferroelectric dielectric. Acceleration of ions from this plasma provides thrust. Advantages of the FEPT include emission of both electrons and ions leading to self-neutralization, creation of plasma, and acceleration of ions with a single power supply, and application of thrust in a short amount of time. We present the concept of the thruster, operational physics, as well as experimental results demonstrating plasma creation and ion acceleration. These results along with plasma spectroscopy allow us to calculate thruster parameters.

Development of a terawatt test stand at the University of Missouri for fast, multichannel switching analysis

The University of Missouri Terawatt Test Stand (MUTTS) began assembly in January 2003. Construction of MUTTS is progressing rapidly with the design and development of its high energy Marx bank. The Marx bank consists of 32, 100 kV, 0.7 /spl mu/F capacitors switched by 16 Physics International T508 spark gaps. The Marx is switched into two parallel 7 nF, intermediate storage capacitors, which are fired into a dummy load through a fast multi-channeling output switch. The Marx stores 100 kJ and can deliver a voltage of 2 MV at 500 kA into a 4 /spl Omega/ load delivering 1 TW to the load. Initial testing will be of a multichanneling 2 MV output switch, which scales nicely to a 6 MV switch design for future very high energy machines at Sandia National Laboratories. The output switch is to reliably multichannel, or close with many parallel arc channels. The goal is to adapt an existing multichanneling switch to create a multichanneling output switch with significant operational advantages, including lower inductance, compared to existing multichannel switches. The target switch inductance is 100 nH or less. The facility and tank were assembled from January to June 2003, with testing to begin in July 2003. Simulations of the test stand and specifications of the output switch will be presented. Electrode configurations and switch augmentations that will facilitate a reliable multi-channeling switch will be introduced. Details describing the development of the MUTTS facility will be included.

Thrust Measurements of the Ferroelectric Plasma Thruster

This paper details the calculation and measurement of average thrust in the ferroelectric plasma thruster. The effect of applied voltage on thrust is evaluated. In addition, the construction and calibration of a pulsed thrust stand is presented. Characteristics of the thruster include an average power of 6 W, thrust of 68-87 muN, calculated specific impulse of 1450 s, and a thruster mass of 5.7 g. Additional benefits of the thruster include impulse bits less than 1 nN ldr s, only one power supply needed for operation, and the ability to emit both electrons and ions.

Piezoelectric-Resonance Effect in a Radio-Frequency-Driven Ferroelectric Plasma Source

The effect of driving frequency on the minimum voltage required to form plasma in a radio-frequency ferroelectric plasma source is examined. When the driving frequency of the plasma source is less than the translational effect radial resonant frequency, the applied electric field and background pressure have an increased effect on plasma formation. Applied-voltage frequencies that are close to the radial mechanical resonance of a ferroelectric/piezoelectric disk cause plasma formation to occur at reduced voltages. In one case, a 75% reduction in the breakdown voltage is realized by altering only the frequency of the applied voltage.

Experimental study of the multichanneling self-break section of the rimfire switch

Many accelerators at Sandia National Laboratories utilize the Rimfire gas switch for high-voltage, high-power switching. In addition, there are many multichannel closing switches used in pulsed power applications. This paper presents a study of the multichanneling section of the Rimfire switch. The electrical effects of multichanneling and a method to force multichanneling are presented. In addition, an objective curve fitting method is utilized to deduce switch inductance from the measured data.

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.

Modeling and analysis of the rimfire gas switch

Many accelerators at Sandia National Laboratories utilize the Rimfire gas switch for high-voltage, high-power switching. Future accelerators will have increased performance requirements for switching elements. When designing improved versions of the Rimfire switch, there is a need for quick and accurate simulation of the electrical effects of geometry changes. This paper presents an advanced circuit model of the Rimfire switch that can be used for these simulations. The development of the model is shown along with comparisons to past models and experimental results.

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.

Effect of Piezoelectric Resonance in a Ferroelectric Plasma Source

Ferroelectric plasma sources have been studied extensively for the past few decades. Potential applications include cathodes for traveling wave tubes or high-energy electron beams, triggers for pulsed power gas switches, and microspacecraft propulsion. In addition to liberation of ions and electrons from the ferroelectric itself, one hypothesized source of particles is from plasma generated on the ferroelectric surface. Factors identified influencing the plasma formation include triple point enhancement due to microgaps and surface charges generated by polarization switching. In this paper, we detail the utilization of piezoelectric resonance to generate plasma with reduced applied voltages. To study this effect, an rf bipolar voltage is applied across the ferroelectric. A non-linear device impedance is observed around the radial resonant frequency of the disk. The plasma breakdown voltage is shown to correlate with the impedance of the device. This effect is similar to what occurs in Rosen-type piezoelectric transformers manufactured with the secondary in a dielectric barrier discharge geometry. In our experiments, the plasma is generated in backgrounds of both vacuum and atmospheric pressure air. In addition, both lead zirconate titanate and lithium niobate ferroelectrics are utilized.