Thesis (MSc)--Stellenbosch University, 2003. / ENGLISH ABSTRACT: With technological advances in solid-state switches the modernization of conventional pulsed power supplies utilizing thyratrons has become possible. A novel pulsed power supply is designed where two series-stacked IGBTs (rated at 1700 V and 300 A each) are used to replace a thryatrondriven pulsed power supply for a mini CO2 TEA laser. The mini CO2 TEA laser is firstly characterized in order to optimize the design of the pulsed power supply. It is found that stable laser discharges can be obtained with electrode voltage rise-times of 150 ns or less. Furthermore, the optical output energy and the efficiency of the laser have been significantly increased by reducing the peaking capacitor size of the pulsed power supply. The two stacked IGBTs switch a C-C transfer loop and the generated pulse is stepped up by a pulse transformer and is compressed by a two-stage magnetic pulse compression unit. Theoretically and through measurements it is shown that automatic voltage sharing between the two IGBTs can be obtained if the turn-on time of the IGBTs is much faster than the transfer time of the switched C-C transfer loop. Lifetime tests reveal that high-power IGBTs are able to reliably switch pulses with peak currents between 4 and 5 times the rated average current of the device. Under laser fault conditions, i.e. laser arcing or missing laser discharges, the reliability of the pulser can be increased by using over-voltage snubbers. In addition, it was found that the internal diode of IGBT modules can eventually fail under the prevalence of laser fault conditions. A modular construction approach is used where components of the pulsed power supply are preassembled. A new rectangular layout of the magnetic pulse compression unit is utilized in order to minimize size and simplify the final construction and assembly. / AFRIKAANSE OPSOMMING: Nuwe ontwikkeling in vastetoestand-skakelaars het die modernisering van laser-pulskragbronne moontlik gemaak. ’n Nuwe kragbron wat gebruik maak van twee seriegeskakelde IGBT’s is ontwerp om ’n Tiratron-gedrewe laserkragbron te vervang, wat vir ’n mini-C02-TEA-laser gebruik word. Die laser is vooraf eers gekarakteriseer om sodoende die laserkragbron te optimeer. Daar is bevind dat stabiele laserontladings verkry kan word met spanningstygtye van 150 ns of minder. Verder kan die uittree-energie van die laser beduidend verhoog word deur die uittreekapasitore (eng.: peaking capacitors) van die laserkragbron te verminder. Die twee serie-geskakelde IGBT’s skakel ’n C-C oordraglus. Die spanning van die gegenereerde puls word deur ’n pulstransformator verhoog en die stygtyd van die puls word met ’n twee-stadium magnetiese pulskompressor verlaag. Teoreties en deur metings kan getoon word dat eweredige spanningsverdeling tussen die twee IGBT’s outomaties verkry kan word indien die skakeltye van die IGBT’s baie vinniger as die oordragstyd van die C-C oordraglus is. Toetse het getoon dat IGBT’s pulse met piekstrome van tussen vier tot vyf keer die gespesifiseerde gemiddelde stroom betroubaar kan skakel. Tydens laserfouttoestande kan die betroubaarheid van die IGBT verhoog word deur oorspanningsgapsers te gebruik, maar onder fouttoestande wat voortduur, kan die IGBT se interne diode vemietig word. Die komponente van die laserkragbron is as modules vervaardig, wat op ’n maklike wyse gemonteer kan word. ’n Nuwe reghoekige uitleg is gebruik vir die konstruksie van die magnetiese pulskompressor, waarmee die grootte van die pulskompressor geminimeer en die konstruksie vergemaklik is.

As the ultimate fate of nearly all stars, including our Sun, white dwarfs (WDs) hold rich and informative histories in their observable light. To determine a fundamental parameter of WDs, mass, we perform the first measurement of the average gravitational redshift of an ensemble of WDs. We find a larger mean mass than that determined from the primary and expansive technique known as the spectroscopic method. The potential inaccuracy of this method has broad astrophysical implications, including for our understanding of Type 1a supernova progenitors and for constraining the age of the Universe. This motivates us to investigate the WD atmosphere models used with the spectroscopic method, particularly the input theoretical line profiles, by developing a new experimental platform to create plasmas at WD photospheric conditions (T_e ~ 1 eV, n_e ~ 10^17 cm^-3). Instead of observing WD spectra to infer the plasma conditions at the surface of the star, we set the conditions and measure the emergent spectra in the laboratory. X-rays from a z-pinch dynamic hohlraum generated at the Z Pulsed Power Facility at Sandia National Laboratories irradiate a gas cell to initiate formation of a large (120x20x10 mm or 24 cm^3) plasma. We observe multiple Balmer lines from our plasma in emission and in absorption simultaneously along relatively long (~120 mm) lines of sight perpendicular to the heating radiation. Using a large, radiation-driven plasma aides us to achieve homogeneity along our observed lines of sight. With time-resolved spectroscopy we measure lines at a range of electron densities that spans an order of magnitude, and we do this within one pulsed power shot experiment. Observing our plasma in absorption not only provides the signal-to-noise to measure relative line shapes, it allows us to measure relative line strengths because the lines share the same lower level population. This constrains the theoretical reduction factors used to describe ionization potential depression or the occupation probabilities associated with these Balmer lines. We compare our measured line shapes with the theoretical ones used in WD atmosphere models as part of the first fruits of this rich experimental platform. / text

Experiments in high energy density (HED) plasma physics have become more accessible with the increasing availability of high-intensity pulsed lasers. Extending the experiment parameters to include magnetized HED plasmas requires a field source that can generate fields of order 100 tesla. This dissertation discusses the design and implementation of a pulsed field driver with a designed maximum of 2.2 MA from a 160 kJ capacitor bank. Faraday rotation measurement of 63 T for a 1.0 MA discharge supported Biot-Savart estimates for a single-turn coil with a 1 cm bore. After modification, the field driver generated up to 15 T to magnetize supernova-like spherical blast waves driven by the Texas Petawatt Laser. The presence of the high field suppressed blast wave expansion, and had the additional effect of revealing a cylindrical plasma along the laser axis. / text

"The pulsed plasma thruster (PPT) is an onboard electromagnetic propulsion device currently being considered for use in various small satellite missions. The work presented in this thesis is directed toward improving PPT performance using a control engineering approach along with externally applied magnetic fields. An improved one dimensional electromechanical model for PPT operation is developed. This slug model represents the PPT as an LRC circuit with a dynamics equation for the ablated plasma. The improved model includes detailed derivation for the induced magnetic field and a model for the plasma resistance. A modified electromechanical model for the case of externally applied magnetic fields is also derived for the parallel plate geometry. A software package with a graphical user interface (GUI) is developed for the simulation of various PPT types, geometric configurations, and parameters The simulations show excellent agreement with data from the Lincoln Experimental Satellite (LES)-6, the LES-8/9 PPT and the Univ. of Tokyo PPT. The control objective employed in this thesis involves the maximization of the specific impulse and thrust efficiency of the PPT, which are each directly related with the exhaust velocity of the thruster. This objective is achieved through the use of an externally applied magnetic field as a system actuator. To simulate an open-loop constant-input controller the modified electromechanical PPT model is applied to the various PPT configurations. In this controller the external magnetic field was applied as constant throughout or portions of the PPT channel. For the Univ. of Tokyo PPT a magnetic field applied over the entire 6-cm long channel increases the specific impulse and thrust efficiency by 10% over the case that the filed is applied in the first 1.75 cm of the PPT channel. The magnitude of these increases compare well with the results of the UOT applied B-field experiments. For the LES-6 and LES-9 PPTs, the simulations predicts significant performance enhancements with approximately linear increases for the specific impulse, thrust efficiency and impulse bit. "

"As new, smaller satellites are built, the need for improved on-board propulsion systems has grown. The pulsed plasma thruster has received attention due to its low power requirements, its simple propellant management, and the success of initial flight tests. Successful integration of PPTs on spacecraft requires the comprehensive evaluation of possible plume-spacecraft interactions. The PPT plume consists of neutrals and ions from the decomposition of the Teflon propellant, material from electrode erosion, as well as electromagnetic fields and optical emissions. To investigate the PPT plume, an on-going program is underway at WPI that combines experimental and computational investigations. Experimental investigation of the PPT plume is challenging due to the unsteady, pulsed as well as the partially ionized character of the plume. In this thesis, a triple Langmuir probe apparatus was designed and used to obtain electron temperature and density measurements in the plume of a PPT. This experimental investigation provides further characterization of the plume, much needed validation data for computational models, and is useful in thruster optimization studies. The pulsed plasma thruster used in this study is a rectangular geometry laboratory model built at NASA Lewis Research Center for component lifetime tests and plume studies. It is almost identical in size and performance to the LES 8/9 thruster, ablating 26.6 ug of Teflon, producing an impulse bit of 256 uN-s and a specific impulse of 986 s at 20 J. All experiments were carried out at NASA LeRC Electric Propulsion Laboratory. The experimental setup included triple Langmuir probes mounted on a moveable probe stand, to collect data over a wide range of locations and operating conditions. Triple probes have the ability to instantaneously measure electron temperature and density, and have the benefit of being relatively simple to use, compared to other methods used to measure these same properties. The implementation of this measuring technique is discussed in detail, to aid future work that utilizes these devices. Electron temperature and density was measured from up to 45 degrees from the centerline on planes parallel and perpendicular to the thruster electrodes, for thruster energy levels of 5, 20 and 40 J. Radial distances extend from 6 to 20 cm downstream from the Teflon surface. These locations cover the core of the PPT plume, over a range of energy levels that corresponds to proposed mission operating conditions. Data analysis shows the spatial and temporal variation of the plume. Maximum electron density near the exit of the thruster is 1.6 x 1020, 1.6 x 1021, and 1.8 x 1021 m-3 for the 5, 20 and 40 J discharges, respectively. At 20 cm downstream from the Teflon surface, densities are 1 x 1019, 1.5 x 1020 and 4.2 x 1020 for the 5, 20 and 40 J discharges, respectively. The average electron temperature at maximum density was found to vary between 3.75 and 4.0 eV for the above density measurements at the thruster exit, and 20 cm from the Teflon surface the temperatures are 0.5, 2.5, and 3 eV for the 5, 20 and 40 J discharges. Plume properties show a great degree of angular variation in the perpendicular plane and very little in the parallel plane, most likely due to the rectangular geometry of the PPT electrodes. Simultaneous electron temperature and density traces for a single thruster discharge show that the hottest electrons populate the leading edge of the plume. Analysis between pulses shows a 50% variation in density and a 25% variation in electron temperature. Error analysis estimates that maximum uncertainty in the temperature measurements to be approximately +/- 0.75 eV due to noise smoothing, and the maximum uncertainty in electron density to be +/- 60%, due to assumptions related to the triple probe theory. In addition, analysis of previously observed slow and fast ion components in the PPT plume was performed. The analysis shows that there is approximately a 3 us difference in creation time between the fast and slow ions, and that this correlates almost exactly with the half period of the oscillations in the thruster discharge current."

<p>Pulsed power generators are a key component in compact systems for generation of high-power microwaves (HPM). HPM generation by virtual cathode devices such as Vircators put high demands on the source. The rise time and the pulse length of the source voltage are two key issues in the generation of HPM radiation. This thesis describes the construction and tests of several inductive high power pulse generators. The pulse generators were designed with the intent to deliver a pulse with fast rise time and rectangular voltage. Therefore, the pulse generators are all based on discharges of transmission lines. The transmission lines used in the pulse generators are modern high voltage cables normally used in power transmission. All tested pulse generators have step up transformers as intermediate magnetic energy storage. Further, the pulse generators have contributed to the knowledge of compact designs. Closing switches is another important device in pulse generator architectures. In general, high power switching involving large currents and high voltages requires the use of robust switches. The switches used in the pulse generators of this thesis are of gas and liquid type.</p>

Thesis (Ph.D.)--University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (February 27, 2007) Vita. Includes bibliographical references.

Pulsed power generators are a key component in compact systems for generation of high-power microwaves (HPM). HPM generation by virtual cathode devices such as Vircators put high demands on the source. The rise time and the pulse length of the source voltage are two key issues in the generation of HPM radiation. This thesis describes the construction and tests of several inductive high power pulse generators. The pulse generators were designed with the intent to deliver a pulse with fast rise time and rectangular voltage. Therefore, the pulse generators are all based on discharges of transmission lines. The transmission lines used in the pulse generators are modern high voltage cables normally used in power transmission. All tested pulse generators have step up transformers as intermediate magnetic energy storage. Further, the pulse generators have contributed to the knowledge of compact designs. Closing switches is another important device in pulse generator architectures. In general, high power switching involving large currents and high voltages requires the use of robust switches. The switches used in the pulse generators of this thesis are of gas and liquid type.

The use of high-voltage pulsed electric fields in biotechnology and medicine has lead to new methods of cancer treatment, gene therapy, drug delivery, and non-thermal inactivation of microorganisms. Regardless of the application, the objective is to open pores in the cell membrane and hence either facilitate the delivery of foreign materials inside the cell or to kill the cell completely. Pulsed power supplies are needed for electroporation, which is the process of applying pulsed electric fields to biological cells to induce a temporary permeability in the cell membrane. The applications of pulsed electric fields are dependent on the output pulse shape and pulse parameters, both of which can be affected by the circuit parameters of the pulsed power supply and the conductivity of the media being treated. <br /><br /> In this research, two Metal Oxide Field Effect Transistor (MOSFET)-based pulsed power supplies that are used for electroporation experiments were designed and built. The first used up to three MOSFETs in parallel to deliver high voltage pulses to highly conductive loads. To produce pulses with higher voltages, a second pulsed power supply using two MOSFETs connected in series was designed and built. The parallel and series MOSFET-based pulsed power supplies are capable of producing controllable square pulses with widths of a few hundred nanoseconds to dc and amplitudes up to 1500 V and 3000 V, respectively. The load in this study is a 1-mm electroporation cuvette filled with a buffer solution that is varied in conductivity from 0. 7 mS/m to 1000 mS/m. The results indicate that by controlling the circuit parameters such as the number of parallel MOSFETs, gate resistance, energy storage capacitance, and the parameters of the MOSFET driver gating pulses, the output pulse parameters can be made almost independent of the load conductivity. <br /><br /> Using the pulsed power supplies designed in this work, an investigation into electroporation-mediated delivery of a plasmid DNA molecule into the pathogenic bacterium <em>E. coli</em> O157:H7, was conducted. It was concluded that increasing the electric field strength and pulse amplitude resulted in an increase in the number of transformants. However, increasing the number of pulses had the effect of reducing the number of transformants. In all of the experiments the number of cells that were inactivated by the exposure to the pulsed electric field was measured.

The use of high-voltage pulsed electric fields in biotechnology and medicine has lead to new methods of cancer treatment, gene therapy, drug delivery, and non-thermal inactivation of microorganisms. Regardless of the application, the objective is to open pores in the cell membrane and hence either facilitate the delivery of foreign materials inside the cell or to kill the cell completely. Pulsed power supplies are needed for electroporation, which is the process of applying pulsed electric fields to biological cells to induce a temporary permeability in the cell membrane. The applications of pulsed electric fields are dependent on the output pulse shape and pulse parameters, both of which can be affected by the circuit parameters of the pulsed power supply and the conductivity of the media being treated. <br /><br /> In this research, two Metal Oxide Field Effect Transistor (MOSFET)-based pulsed power supplies that are used for electroporation experiments were designed and built. The first used up to three MOSFETs in parallel to deliver high voltage pulses to highly conductive loads. To produce pulses with higher voltages, a second pulsed power supply using two MOSFETs connected in series was designed and built. The parallel and series MOSFET-based pulsed power supplies are capable of producing controllable square pulses with widths of a few hundred nanoseconds to dc and amplitudes up to 1500 V and 3000 V, respectively. The load in this study is a 1-mm electroporation cuvette filled with a buffer solution that is varied in conductivity from 0. 7 mS/m to 1000 mS/m. The results indicate that by controlling the circuit parameters such as the number of parallel MOSFETs, gate resistance, energy storage capacitance, and the parameters of the MOSFET driver gating pulses, the output pulse parameters can be made almost independent of the load conductivity. <br /><br /> Using the pulsed power supplies designed in this work, an investigation into electroporation-mediated delivery of a plasmid DNA molecule into the pathogenic bacterium <em>E. coli</em> O157:H7, was conducted. It was concluded that increasing the electric field strength and pulse amplitude resulted in an increase in the number of transformants. However, increasing the number of pulses had the effect of reducing the number of transformants. In all of the experiments the number of cells that were inactivated by the exposure to the pulsed electric field was measured.