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ABSTRACT: This paper discusses the effect of surface coatings on exploding wire behavior. Three different surface coatings of different thicknesses and materials have been studied, each with a 99.99% pure silver conducting core. Experimentally, the wires are subjected to peak current densities in excess of 107 A/cm2 on a microsecond time scale. High Speed intensified Charge-Coupled Device (iCCD) images. A theoretical one-dimensional finite difference model has been developed to predict wire behavior and determine the mechanism(s) responsible for the deviations in behavior induced by the presence of a surface coating.
Physics of Plasmas 03/2012; · 2.15 Impact Factor
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ABSTRACT: The increased popularity of high power microwave systems and the various sources to drive them is the motivation behind the work to be presented. A stand-alone, self-contained explosively driven high power microwave pulsed power system has been designed, built, and tested at Texas Tech University's Center for Pulsed Power and Power Electronics. The system integrates four different sub-units that are composed of a battery driven prime power source utilizing capacitive energy storage, a dual stage helical flux compression generator as the main energy amplification device, an integrated power conditioning system with inductive energy storage including a fast opening electro-explosive switch, and a triode reflex geometry virtual cathode oscillator as the microwave radiating source. This system has displayed a measured electrical source power level of over 5 GW and peak radiated microwaves of about 200 MW. It is contained within a 15 cm diameter housing and measures 2 m in length, giving a housing volume of slightly less than 39 l. The system and its sub-components have been extensively studied, both as integrated and individual units, to further expand on components behavior and operation physics. This report will serve as a detailed design overview of each of the four subcomponents and provide detailed analysis of the overall system performance and benchmarks.
The Review of scientific instruments 02/2012; 83(2):024705. · 1.52 Impact Factor
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ABSTRACT: Current research on long-wavelength laser interaction with magnetized plasmas is summarized. Special attention is given to laser beam guiding in plasma density minima, to plasma heating and to investigations of laser-produced plasmas in strong magnetic fields. The basic theoretical and experimental background aad research are discussed. Reactor studies based on lassr heated plasma systems are reviewed. Some areas of possible future research are mentioned.
Nuclear Fusion 01/2011; 16(6):999. · 4.09 Impact Factor
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ABSTRACT: Local cavity resonances of fast wave propagation in a hydrogen-deuterium tokamak plasma have been investigated experimentally. The results demonstrate short damping lengths of the fast waves on the high field side of the hybrid layer, and the existence of a local cavity resonance on the low field side, even for a low hydrogen minority concentration (5%) in a deuterium plasma. The strong wave damping disappears when the hybrid layer is moved towards the edge of the plasma.
Nuclear Fusion 01/2011; 27(1):151. · 4.09 Impact Factor
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ABSTRACT: A novel modeling and simulation method for flux-trapping flux-compression generators (FT-FCGs) is presented, which utilizes PSpice circuit-simulation software to solve complex differential equations derived from circuit analysis. The primary motivation for the model development is the desire for a technique to rapidly design and prototype FT-FCGs for use as drivers in high-power microwave sources. The derivation of FT-FCG equations will be given, both in the ideal (lossless) and nonideal cases. For the nonideal case, three flux conservation coefficients are added to the equations to account for intrinsic flux loss in the circuit. Time-varying inductance curves are calculated using zero-dimensional models found in literature and adapted to fit this model. A simple FT-FCG design is used as an example to show the steps taken to complete a simulation. The same design was also fabricated and tested for comparison with predicted results from the model. A comparison of the waveforms acquired through simulation and experiment was found to result in good agreement for a given set of values for the flux conservation coefficients. A discussion of the derived equations, both lossless and nonideal, is given, as well as a discussion on the investigation of the impact of the three flux constants on the circuit. Analysis is offered on the results of this investigation, and conclusions are given on the effectiveness of this model to predict FT-FCG behavior.
IEEE Transactions on Plasma Science 09/2010; · 1.17 Impact Factor
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ABSTRACT: Recent efforts at the Center for Pulsed Power and Power Electronics at Texas Tech University have been focused on the development of a compact and explosively driven High Power Microwave, HPM, system. The primary energy source (other than the seed energy source) driving the microwave load in this system is a mid-sized, dual-stage helical flux compression generator, HFCG. The HFCG has a constant stator inner diameter of 7.6 cm, a length of 26 cm, with a working volume of 890 cm<sup>3</sup>. Testing at the Center has revealed energy gains in the 30's and 40's with output energy levels in the kilo-joules regime into loads of several micro-Henries. Over the last few years, close to one hundred shots have been taken with these generators into various loads consisting of dummy inductive loads, power conditioning systems, and HPM sources. Throughout these tests, the working volume of the HFCG, i.e. the volume in between the wire stator and the explosive-filled aluminum armature, was filled with SF<sub>6</sub> at atmospheric pressure. This was primarily done do avoid electrical breakdown in the generator volume during operation, resulting in flux loss. Recent design updates enable pressurizing the generator volume to pressures up to 0.5 MPa, which is needed, for instance, to replace the SF<sub>6</sub> with other gases such as air or nitrogen. The performance of the dual-stage HFCG with pressurized working volume (SF<sub>6</sub> and N<sub>2</sub>) is presented in this paper along with an analysis of the maximum electric field amplitude held off in the volume during operation. The design technique to seal the HFCG will also be briefly discussed.
Power Modulator and High Voltage Conference (IPMHVC), 2010 IEEE International; 06/2010
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ABSTRACT: Recent progress in the development of a compact, portable, explosively-driven high power microwave source is presented. The envelope to which the system must fit has a 15 cm diameter, which means each sub-system fits within this dimension, with an optimized overall length. The system includes an autonomous prime energy source, which provides the initial energy for a two-stage, flux-trapping helical flux compression generator (FCG). Typical output from the FCG is several kilojoules into a 3 μH inductor. The amplified energy from the generator, after pulse conditioning, is used to drive a virtual cathode oscillator (vircator). Recorded voltages at the vircator with this arrangement were greater than 200 kV in experiments, where radiated output powers of greater than 100 MW have been measured. Voltages of at least 300 kV, with an electrical output power of 4 GW or greater, were generated by the FCG driven pulsed power source into a water resistor load with an impedance similar to the operating impedance of the vircator. A description of each component of the compact microwave source will be given, along with waveforms from tests performed with the components independent of the rest of the system. Data from experiments with the fully integrated microwave system will be shown, and analysis will be offered to detail the performance of the system in its present state.
Power Modulator and High Voltage Conference (IPMHVC), 2010 IEEE International; 06/2010
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ABSTRACT: A pulsed ring-down source array (PRDS), also called a Resonant Antenna-Source System, is a robust high-power transient RF transmitter that provides substantial power in the far field region. In a typical PRDS, a high potential is stored in the device structure and discharged through a switch, generating a damped ringing radiated waveform. Using an array of PRDS antennas, the radiated far field power density can be increased far beyond that possible with a single source. The array's performance is dependent on the individual antenna elements, along with the main switch triggering jitter and the jitter of the trigger source. The array being tested is composed of four antennas spaced a quarter wavelength apart from one another in a straight line. The PRDS element is a half coaxial monopole antenna tuned for ~100MHz. The switch used is a custom made trigatron spark gap pressurized up to 100 PSI of N<sub>2</sub>. The results presented from a working example of a PRDS array will enable future development of the system.
Power Modulator and High Voltage Conference (IPMHVC), 2010 IEEE International; 06/2010
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ABSTRACT: A collaborative effort at Texas Tech University on high power RF transmitters has directly translated to the development of phased array pulsed ring down sources (PRDS). By operating an array of PRDS, peak radiating power on target can theoretically be multiplied by the number of sources. The primary limitation on the application of the array concept is the jitter with which the individual sources can be fired. An ideal jitter of a small fraction of the risetime is required to accurately synchronize the array to steer and preserve the risetime of the radiated pulse. This paper describes in detail the implementation of a GPS based timing system that will synchronize the individual antennas to operate at different geo-locations to function in a coordinated fashion to deliver the peak power of each element to a single position. Theoretical array performance is shown through Monte Carlo simulations, accounting for switch jitter and a range of GPS timing jitter. Each module will include a control unit, low jitter pulser, low jitter spark gap, antenna element, as well as a GPS receiver. The location of each module is transmitted to a central controller, which calculates and dictates when each element is fired. Low jitter in the timing of the GPS reference signal is essential in synchronizing each element to deliver the maxim power. Testing using a preliminary setup using GPS technology is conducted with both 1 pps and 100 pps outputs. Jitter results between modules are recorded to ~10 ns without any correction factors. With the timing and geospatial errors taken into account, the proposed concept will show usable gains of up to several hundred MHz.
Power Modulator and High Voltage Conference (IPMHVC), 2010 IEEE International; 06/2010
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ABSTRACT: Exploding wire arrays as fast switches are of interest for explosive pulsed power applications utilizing magnetic flux compression. This type of opening switch has proven effective in producing pulses of several hundred kilovolts into vacuum diode loads. The research presented here discusses an exploding wire array capable of producing single digit kilo-joules, 100 nanosecond pulses when driven by a 45 kilo-Amp current with a waveform closely resembling typical magnetic flux compression output. For this specific parameter range, the optimal fuse design was developed based on the experimental behavior of the fuse under variation of parameters such as wire spacing, shielding, and quenching medium. Each fuse is composed of several silver wires arranged in a straight wire cylindrical array and is typically pressurized in a chamber filled with about 0.6 MPa of SF<sub>6</sub>. The tradeoff between wire spacing and voltage output was addressed by designing four different fuse termination pairs each with a diameter that increased wire spacing from 5 to 20 mm in 5 mm increments. A wire shield test was also conducted as an extension to the wire spacing experiment to uncover any mutual radiative effects between wires on fuse opening behavior. The optimum fuse design, including the optimum fuse wire diameter, will be discussed with a 20 Ohm resistive load as well as a vacuum diode load with similar impedance.
Power Modulator and High Voltage Conference (IPMHVC), 2010 IEEE International; 06/2010
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ABSTRACT: The design and testing of an integrated front-end power and control system for helical flux compression generators (HFCG) is presented. A current up to 12 kiloamps needs to be pushed into the 5.8 microhenry field coil of the HFCG to establish the necessary seed flux for generator operation. This current is created with the discharge of a 5 kilovolt, 50 microfarad metalized polypropylene film capacitor using a single-use semiconductor closing switch. Once peak current/flux is obtained in the seed coil an exploding bridge wire (EBW) detonator is initiated with a discharge from a 1 kilovolt, 500 millijoule capacitor array contained in the compact fire set. Both capacitances, seed and fire set, are charged using a rapid capacitor charger system. The rapid capacitor charger is a solid state step up converter supplied by lithium-ion polymer (LiPo) batteries. It provides the 5 kilovolts and 1 kilovolt dual output voltages required for the compact seed source and compact fire set, respectively. The rapid capacitor charger operates at an average output power of 3 kilowatts and charges both capacitances simultaneously in under 250 milliseconds. The rapid capacitor charger is reusable if protected from the explosive detonation.
Power Modulator and High Voltage Conference (IPMHVC), 2010 IEEE International; 06/2010
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ABSTRACT: Pressure simulations have been performed for several experimental explosively driven ferroelectric generators, with 2.54-cm-diameter PZT EC-64 discs as the ferroelectric material, using the hydrodynamic code system CTH, developed by Sandia National Laboratories. An empirical relationship was found between the results of the pressure simulations and the output voltages of the experimental generators, and an algorithm was generated to convert the simulated pressure into open-circuit voltage. This empirical algorithm has been applied to simulations of different experimental ferroelectric generators, and the results show a good correlation when compared to the corresponding experimental open-circuit output voltages. The experimentally achieved output voltages normalized for a thickness range from 14 to 34 kV/cm.
IEEE Transactions on Plasma Science 05/2010; · 1.17 Impact Factor
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ABSTRACT: An evaluation of the energy density and efficiency of high-voltage capacitors, from various manufacturers, at voltages above their rated level is presented. Characteristics such as decreasing capacitance, decreasing efficiency, and increasing energy density are described. Data are taken from eight capacitors; six of which are composed of varying nonlinear ceramic materials as a dielectric, and the remaining two are wound capacitors, which were chosen to exhibit their linear characteristics. Rapid (1-100 ms) charging and discharging, similar to that of a generator operation at a repetition rate of more than 10 Hz with an <sub>~</sub>100-ns pulse, were the conditions simulated in this particular test setup.
IEEE Transactions on Plasma Science 04/2010; · 1.17 Impact Factor
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ABSTRACT: This paper presents a nonexplosive pulsed-power system that replicates the output current waveform of a flux compression generator (FCG). The primary purpose of this system is to efficiently test the power conditioning components of an explosively driven HPM system, while drastically reducing the time between tests which are inherent with explosive experiments. The power conditioning system (PCS) of the HPM system includes an energy-storage inductor, an electroexplosive opening switch (fuse), and a peaking gap and serves to match the FCG output characteristics with the HPM diode load requirements. A secondary purpose of the nonexplosive test bed is to provide data points which could be directly compared with those from explosively driven experiments. For this reason, a reflex-triode virtual cathode oscillator (vircator) was connected to the output of the nonexplosive system, and the results of which were compared with similar testing done with an FCG and a compact Marx generator. Since the behavior of the fuse is known to play a critical role in the performance of the PCS, a study was performed on the effect of different fuse designs on the overall performance of the PCS. Specifically, the quality of the electrical connection between the fuse wire array and the rest of the system was tested. Fuse design experiments were conducted with the nonexplosive test bed firing into a water resistor dummy load, which showed a 13% increase in peak load voltage and more than an 11% increase in energy transfer for fuses with improved wire-electrode connection strength. Some basic rules about fuse design, as well as conclusions on the performance of the PCS when driving an HPM load, are given.
IEEE Transactions on Plasma Science 01/2010; · 1.17 Impact Factor
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ABSTRACT: Increased customer needs and intensified global competition require intelligent and flexible automation. The interaction technology mobile robotics addresses this, so it holds great potential within the industry. This paper presents the concepts, ideas and working principles of the mobile robot "Little Helper" - an ongoing research project at Aalborg University, Denmark, concerning the development of an autonomous and flexible manufacturing assistant. To demonstrate the "Little Helper" concept a full-scale prototype has been built and experiments carried out. Experiences and knowledge gained from this show promising results regarding industrial integration, exploitation and maturation of mobile robotics.
Emerging Technologies & Factory Automation, 2009. ETFA 2009. IEEE Conference on; 10/2009
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ABSTRACT: The mechanisms and predictors of insulator degradation and erosion by pulsed high-current surface discharges are presented and discussed. Erosion and degradation depend on the insulator material, the electrode material, the ambient gas, and the presence of UV stabilizers in, or on the surface of, the insulator. Insulator degradation is the result of material decomposition into conductive metal or carbon species and is measured by a decrease in the surface breakdown voltage. Insulator erosion is measured by the material's mass loss. The performance of a large group of ceramic, polymeric, and elastomeric materials tested with graphite and molybdenum electrodes is presented in this investigation. The insulators are exposed to repetitive 300-kA 20-mus-long surface discharges. Tests are performed in atmospheric air and pure nitrogen. Various methods to rank insulators in terms of holdoff voltage degradation, mass erosion, and holdoff voltage conditioning (HVC) using the material's thermochemical properties are presented and discussed. HVC is characterized by an initial increase in surface holdoff voltage. The ranking method developed by the authors characterizes the insulator according to the holdoff degradation resistance (HDR), mass vaporization coefficient (MVC), and HVC figures of merit calculated by the material's thermochemical properties. The investigation also shows the relationship between the HDR, MVC, and HVC figures of merit.
IEEE Transactions on Plasma Science 10/2009; · 1.17 Impact Factor
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ABSTRACT: Recent research efforts at Texas Tech University on impulse antenna phased array has needed to develop a reliable high voltage, high repetition rate switch that will operate with ultra low jitter. An ideal jitter of a small fraction of the risetime is required to accurately synchronize the array to steer and preserve the risetime of the radiated pulse. In, we showed the initial test system with sub-ns results for operations in different gases and gas mixtures. This paper discusses in detail 50 kV, 100 Hz switch operations with different gases. The effects of gases and gas mixtures have on switch performance which includes recovery rate and in particular jitter will be investigated. Gases tested include, dry air, H<sub>2</sub>, N<sub>2</sub>, and SF<sub>6</sub>, as well as H<sub>2</sub>-N<sub>2</sub>, and N<sub>2</sub>-SF<sub>6</sub> gas mixtures. Switch jitter as a result of triggering conditions is discussed, also including a comprehensive evaluation of jitter as a function of formative delay in the various gases. The temperature of gas and its effects on switch jitter is also documented in this paper. A 50 ¿, 1 nF pulse forming line is charged to 50 kV and provides the low inductance voltage source to test the different gases. Triggering is provided by a solid state opening switch voltage source that supplies ~150 kV, 10 ns risetime pulses at a rep rate up to 100 Hz in burst mode. A hermetically sealed spark gap with a Kel-F-PCTFE (polychlorotrifluoroethylene) lining is used to house the switch and high pressure gas.
IEEE Transactions on Dielectrics and Electrical Insulation 09/2009; · 1.09 Impact Factor
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ABSTRACT: The generation of high power microwaves using explosively driven pulsed power is of particular interest to the defense community. The high energy density of explosives provides the opportunity to design pulsed power systems which occupy significantly less volume, yet provide the same output power, as traditional methods of high power microwave (HPM) production. Utilizing a flux compression generator (FCG) as explosive driver necessitates introducing an intermediate power conditioning system (PCS) that addresses the typical impedance mismatch between FCG and HPM source. The presented PCS is composed of an energy storage inductor, an opening fuse switch and a self-break peaking gap all of which needed to fit within an envelope of 15 cm diameter. Currents in the tens of kilo-amperes and voltages in the hundreds of kilo-volts have to be handled by the PCS. The design of the system, which takes up less than 11 liters of volume, as well its performance into a 20 ¿ resistive load (used to approximate the operating impedance of certain HPM sources) is presented. Approximately 6 GW of electrical peak power was delivered to the load.
Pulsed Power Conference, 2009. PPC '09. IEEE; 08/2009
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ABSTRACT: Vircator high power microwave sources are simple, robust, and require no external magnetic field, making them desirable for use in practical compact high power microwave systems. A vircator can be driven directly from the output of a low-impedance Marx generator, eliminating the need for bulky intermediate energy storage components. A compact high power microwave system has been constructed and tested at Texas Tech University utilizing a triode geometry vircator and a compact Marx Generator. The size and performance of this system is compared to a similar system previously developed at Texas Tech. The current triode vircator is housed within a six inch diameter tube which is eleven inches in length. The Marx is contained in an oil tank that is 36 inches long à 12 inches wide à 18 inches tall. Diode voltage and current, and radiated microwave waveforms are presented.
Pulsed Power Conference, 2009. PPC '09. IEEE; 08/2009
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ABSTRACT: Many pulsed power applications have demanding system requirements. Power systems for these applications are expected to provide high energy, high pulsed power and long standby times without recharge, all in a very compact package. The different properties of batteries and capacitors make them most suitable for different uses. When selecting a prime power source for compact pulsed power systems a hybrid system often provides the optimal solution, utilizing a battery for prime energy storage during standby and a capacitor for intermediate energy storage before and between operations. This system takes advantage of the best characteristics of both sources to fulfill the system requirements. The design and testing of such a compact system is discussed. The system utilizes a solid-state converter to charge a 50 ¿F polypropylene capacitor to 5 kV in under 500 ms from lithium-ion polymer (LiPo) batteries. Battery selection and testing is also covered. The battery and charger assembly occupies 1.25 L while the capacitor occupies an additional 1.4 L.
Pulsed Power Conference, 2009. PPC '09. IEEE; 08/2009
