Browsing by Subject "Pulsed power systems"
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Item Current density and time resolved emittance measurement of an electron beam extracted from a plasma edge cathode(Texas Tech University, 1995-12) Stemprok, RomanThe Plasma Edge Cathode concept can produce high electron current densities while avoiding or delaying plasma closure of the extraction gap of an electron source. A plasma beam is generated by a surface flashover from a spark plug. An obstacle partially intercepts the emitted plasma jet at a distance of approximately 200 mm from the plasma source and causes a stationary transverse plasma boundary from which electrons are extracted. The plasma should experience negligible extemal electric forces as long as the extraction is space charge limited. Thus, it should be possible to avoid the plasma closure of the extraction gap and to obtain an extracted electron current density over 100 AJcnfi. Experimental electron beam density measurements support this theoretical prediction. For an electron beam with a diameter of about 2.5 cm the extraction current is smooth and electron current densities up to 100 A/cm were obtained repeatedly with a duration up to 2.5 jis for an extraction gap of 4 mm. For a 7.5 cm diameter beam an electron beam current density was measured from 2 to 7 A/cm^ at an increased extraction gap of 15 mm. The pepper-pot method was used to measure the emittance of the extracted electron beam. An extraction gap of 15 mm together with an extraction voltage less than 20 kV yielded a relatively smooth pattem on the scintillator. The images were recorded with a framing camera. Then they were digitized with a scanner and a computer code determined the emittance. A two-frame intensifier camera with an exposure time down to 5 ns was built to record the electron beam pattem. A 50 ns exposure time was used to determine the effective emittance. Approximately half of the 100 shots taken had a suitable exposure and separated spots for evaluation. At higher extraction voltages the pattem was irregular and indicated some instability of the plasma surface. The measured normalized emittance is on the order of EJ,^^ = 5 X 10"^m^ rad^. The normalized brightness was found typically to be B = 3 X10^ A m~^ rad"^. The trace space density measurements show a nearly parallel electron beam. A major difficulty with the emittance measurements was the limited reproducibility from shot to shot.Item Explosively-driven helical magneto-cumulative generators(Texas Tech University, 2002-08) Holt, Thomas AllenExplosively-Driven Helical Magneto-Cumulative Generators (HMCGs) have a wide variety of applications in today's pulsed power field. Though widely used, the physics of the anomalous flux loss associated with such devices is still poorly understood. Presented within are several different geometries of HMCGs designed to provide insight to, and possibly explain the anomalous flux loss. An overview of common loss mechanisms is provided as a baseline for determining whether poor performance reflects the presence of the anomalous loss or other loss mechanisms which are avoidable when proper design procedures are followed. The work presented herein was solely funded by the Explosively-Driven Power Generation MURI program funded by the Director of Defense Research & Engineering (DDR&E) and managed by the Air Force Office of Scientific Research (AFOSR).Item Insulator flashover at cryogenic temperatures(Texas Tech University, 1997-05) Stewart, Tina D.This thesis describes the development and construction of a cryogenic surface flashover system. A brief discussion of surface flashover theory and the expected characteristics of cryogenic surface flashover is included in this thesis. Although the apparatus from a previous project was used, minor modifications were made to adapt the apparatus for this research. Therefore, a brief description of the apparatus is followed by a detailed description of the two dififerent set-ups tested for this research. And fínally, results are presented for the two cooling systems. First experiments do not show discemible dififerences to flashover initiation at room temperature for Lexan samples.Item Power conditioning in inductive energy storage systems(Texas Tech University, 2000-08) Heeren, TammoThe pulsed power conditioning system (PPCS) is one of the key enabling technologies for utilizing the energy output of a magnetic flux compressor generator. This thesis shows the results of comprehensive experimental studies using an inductive energy storage system with an exploding wire fuse. The first part of the thesis deals with an analysis of the current Inductive Energy Storage System, concerning inductance and resistance, then build a basis to evaluate the measurements taken. Different fuse parameters, such as wire size, number of wires, and materials is discussed, and the influences of those parameters on the fuse performance are shown. Finally, results of an optical study of the heating and explosion phase in the 100 nanosecond regime are shown. The thesis will show that silver as wire material outperforms copper and aluminum regarding peak load current, and peak load current risetime, and that thinner wires in general perform better.Item Reflex-triode geometry of the virtual cathode oscillator(Texas Tech University, 2003-12) Lara, Matthew BAn eight-stage four-hundred kilovolt Marx generator, in connection with a 60 nanosecond pulse-forming line, is constructed and utilized as a pulsed source to power a triode version of the Virtual Cathode Oscillator (Vircator). Eight . I uF capacitors, charged to 50kV each, are switched in series by dry-air pressurized spark gaps. The energy from the bank charges a 23 ohm oil transmission line, breaking a peaking gap when the maximum voltage is reached, delivering a 60 ns-400kV pulse to the diode. The design of the "Reflex-Triode" geometry Vircator is based upon claims of high efficiency by Didenko et al.[l] A previously constructed TTU Vircator includes a unique E-beam source, the "brush" cathode; in which a circular array of pins is used as an explosive field emission source to produce relatively high beam currents. The anode consists of a round wire mesh through which the E-beam passes, generating a dense cloud of negative charge known as a "Virtual Cathode." This initial phase of testing is composed of basic operation of the entire system and baseline output power and efficiency measurements.Item Sub-nanosecond breakdown in argon and air(Texas Tech University, 2004-12) Crull, Eric WSub-nanosecond gaseous breakdown is of key importance in several areas including high-speed switching for pulsed power applications, fast breaking plasma limiters for RADAR protection, and the development of Ultra-Wideband (UWB) RADAR sources. This paper summarizes research done to expand the traditionally accepted range for E/p plots of similarity laws describing breakdown. Experiments were carried out using a variety of rounded electrode gap geometries and spacing combinations with over-voltages up to a factor of 35. This paper also describes modifications to the existing test set-up, which were performed in order to minimize rise time and reduce impedance mismatches. All experiments performed in this research used a pulse delivery system with short coaxial transmission lines feeding an axial or radial spark gap inside a vacuum chamber. The gap geometry, whether it was axial or radial, incorporated a conical inner and outer conductor designed to maintain a constant impedance as the transition was made from transmission line to gap. The electric field distribution at the gap in these experiments is more homogenous than that created using the tip-plane geometry of previous research. The data in this paper encompasses a relatively wide range of voltages and pressures. At each relevant combination of voltage and pressure where breakdown occurred, the conduction current was calculated. Minimum voltage - current delay times on the order of 200 ps were calculated for high E/p and compared to previously published results.Item Switching output stages for direct PCM-PWM amplification with enhanced PSRR and THD performance(Texas Tech University, 2003-12) Pate, Michael ScotThis thesis describes the process involved in designing the loop filter that is required when changing a class-D power output stage from a full-bridge configuration to a half-bridge. The loop filter designed is part of an error correction feedback network. This thesis covers the basic theoretical derivations for the device, complete circuit design and simulation and basic device layout considerations. Ideas for future versions are given for reference.Item The effect of area on pulsed breakdown in water(Texas Tech University, 2004-08) Truman, Keith PhillipThe use of water dielectrics in pulsed power systems is of considerable interest due to the favorable physical and electrical properties of water. Numerous researcher activities have explored various mechanisms of water breakdown including breakdown initiation. The consensus is that initiation of breakdown is caused by a localized field enhancement at the surface of an electrode near a microprotrusion. The research conducted here examines the effect increased area has on the breakdown strength of water. Experimental results are presented on the degree to which electrode surface area impacts the dielectric strength of water. A water gap of 4 mm was tested under pulsed conditions with a maximum electric field in excess of 1 MV/cm and pulse widths greater than 1µs. Stainless steel electrodes with a Bruce profile were used to generate a uniform electric field across the water gap. The profile of the electrodes enabled areas ranging from 0.5 cm2 to 75 cm2 while minimizing the change in capacitance of the water gap. The results of the testing are compared to existing area breakdown models. Conclusions are made as to the effect electrode surface area had on the breakdown strength of the water gap.Item Window and cavity breakdown caused by high power microwaves(Texas Tech University, 1998-05) Hemmert, David J.The transmission of high power microwaves through dielectric windows is of essential importance in their use. When an interface window fails due to surface flashover and breakdown, the power can no longer be transmitted and may reflect back into the source, possibly damaging it. In the work reported here, the physical mechanisms of surface flashover and breakdown are investigated for power levels of 10 MW/cm. A 3 MW magnetron and an S-band traveling wave resonator are coupled to produce 100 MW at 2.85 GHz in a high vacuum environment. A window geometry is established to provide a purely tangential electric field along the window surface. High speed diagnostics include forward, reverse, and local field power levels, x-ray emission, and discharge luminosity and imaging. Investigations into other window geometries as well as surface coatings and vacuum-gas interfaces are possible.