Current Step Generation and Measurement with Rise-time in the Range of Nanoseconds

A current step generator based on a charged coaxial cable is designed and tested for characterizing impulse current shunts. This thesis has developed a traceable calibration infrastructure for fast shunts and other current sensors, defined measurement techniques for a current step and improved the test procedure and measurement capabilities. For calibration of shunts, current coil sensors are used in the measurement circuits. Since no calibration services are currently available for impulse current measuring systems, a best circuit combination is proposed for current step generation with a rise time of less than 5 ns, along with a proposed reference shunt that aims to provide the best and most stable measurement results with negligible noise, oscillations, and droop in the measured current step. Based on techniques found in the literature, current steps are generated, and different sensors were used to measure the generated steep front current steps. The generation system consists of a 110-m long, 50-Ω coaxial cable and a spark gap. Various spark gap switches, including the SF6 spark gap, are used for generating current steps. With the coaxial cable charged from one end, a current step is generated after reflecting back from the open end with a step length of twice the cable transmission delay. The cable is than discharged to the shunt (or coil) through the spark gap. The measurement system consists of shunts and coil current sensors, 5:1 and 6.6:1 attenuators based on the requirement of the sensors. The recording instrument is a 1-GHz, 8-bit, 1-GS/s digitizer. The proposed step generator can produce current steps with a stable current of up to 100 A. The rise time of the step varies from 1.6 ns to 15 ns, depending on the spark gap used for switching. The produced current is constant within 0.5% for a step length of 960 ns generated with a coaxial cable 110 m in length. To improve the test procedure and measurement capabilities, the thesis also analyzed factors affecting current step measurement, such as the type of coaxial cable, type of connection, extra shielding, clearances, interference sources, media of the spark gap, and the spark gap electrode distance (arc length). It is found that the measurement system and the rise time of current step is affected by many factors, including the coaxiality of the connection, impedance mismatch, interference, clearances, stray capacitances, and stray inductances. These results will enable future standardization of impulse current sensors

Fast start of oscillations in a short-pulse relativistic magnetron driven by a transparent cathode.

The magnetron has been a major component of radar systems since its introduction in World War II. The newer radar techniques require high peak power (GW) and short microwave pulses (few ns). To serve as a microwave source for short-pulse applications it is imperative that the magnetron needs to have both fast start and fast rate of build-up of oscillations. Both of these factors are contingent on the cathode geometry. The transparent cathode was invented at the University of New Mexico in an endeavor to improve the start time and increase the rate of build-up of oscillations in short-pulse relativistic magnetrons. The construction of the transparent cathode involves the removal of longitudinal strips of material from a hollow cathode. The resultant geometry has manifold advantages the first and the foremost of which is that it makes the cathode transparent to E_theta, thereby greatly increasing its amplitude where electrons are emitted. Hence one would expect faster rate of build-up of oscillations. Secondly, this geometry simultaneously gives rise to several different forms of priming: cathode priming, electrostatic priming and magnetic priming. The number of cathode strips is chosen so that it would excite a particular mode of interest (e.g. 6 strips would favor the formation of 6 spokes). The cathode strips may be oriented azimuthally in a manner that the electron bunches from the cathode strips would be released into the favorable phase of the mode of interest where efficient exchange of energy between the electrons and the RF fields could take place. The highlights of this dissertation are proof-of-concept computer simulations demonstrating the benefits of the transparent cathode in an A6 magnetron driven by a transparent cathode that have validated the simulations

Analysis of an Experimental Pulsed Plasma Accelerator

The purpose of this study was to gather and analyze data taken from a laboratory Pulsed Plasma Accelerator to understand its operation, and also for future comparison of the experimental data with predictions made with computer simulations based on the codes named MACH2 and GEMS. This comparison will allow for further validation of these two computer simulations in order to more accurately model the laboratory thruster. Data was collected from three different instruments: B-Dot probes; a laser interferometer; and Rogowski coils. These devices measure the time dependent current sheets, the electron number density, and the capacitor discharge current respectively. The B-Dot probes were an excellent source of data once the correct procedure for their analysis was determined. Using this data and that of the main Rogowski coil, a total resistance and a total inductance of the thruster was determined to be 0.009 Ω and 4.5Xl0-8 Henry, respectively. Then, using a simple circuit analysis, the current sheet was modeled as a damped sine wave based on the experimentally determined thruster impedance, for use in the MACH2 computer simulation. In the future, a straight comparison of the electron number density from MACH2 will be made once new developments of MACH2 are completed. Recommendations for future development of both the MACH2 and GEMS computer simulations are provided in the final chapter of this thesis

HIGH EFFICIENCY AXIAL DIFFRACTION OUTPUT SCHEMES FOR THE A6 RELATIVISTIC MAGNETRON

High-power microwave research strives for compact and highly efficient vacuum diode-driven sources. MAGIC particle-in-cell (PIC) computer simulations have shown that the performance of the well-known A6 relativistic magnetron with radial power extraction through one or more of its cavities can be improved by instead using axial power extraction through a mode-converting horn antenna, resulting in improved efficiency (30% improved to 70%) and greater output power handling capability (sub-gigawatt improved to multi-gigawatt) without breakdown. In addition, axial extraction results in a more compact profile that is compatible with mounting permanent magnets, which eliminate the need for bulky pulsed electromagnets or cryo-magnets and greatly enhance system efficiency. To this end, a variety of technologies were simulated and tested in experiment, the latter which required the design, construction, testing, and calibration of new diagnostics, pulsed power systems, and hardware, such as the complex A6 magnetron with diffraction output horn antenna (MDO). The primary goal of the experiments was to verify simulated 70% efficiency and greater than 1 GW of output power from the MDO. A less expensive vii compact MDO\u27 variant, essentially an A6 magnetron with a flat-plate mode converter and \u03c0-mode strap was also simulated. Although both the MDO and the compact MDO are compatible with permanent magnets fitted around their exteriors, an effective configuration was simulated for the compact MDO, promising reduced size and increased efficiency of the total microwave system. In addition, both versions of the MDO were susceptible to bombardment of leakage electrons on their output windows; cathode endcaps were developed and tested to mitigate this issue. Finally, to further improve output power, a rodded metamaterial-like cathode that showed improved power in other relativistic magnetrons was also considered by simulation in an A6 magnetron with radial extraction.\u2

Design and operation of a harmonic gyrotron based on a cusp electron gun

Strathclyde theses - ask staff. Thesis no. : T13121This thesis presents the results of successful operation of a 2nd harmonic gyrotron based on a cusp electron gun. The numerical and experimental results agreed well with the gyrotron design parameters. Two gyrotrons based on a cusp electron gun were designed: the first gyrotron operated at the 2nd harmonic and the second gyrotron was studied to look at the scaling of this concept for operation at the 7th harmonic at a frequency of 390 GHz. The cusp electron gun was used to produce the electron beam in the gyrotron which was annular in shape. The electron beam had a voltage of 40 kV, a current of 1.5A and a velocity ratio (perpendicular component to horizontal component) of 1.5. The experimental results from the first cusp electron gun and measurements of the high quality electron beam with ~8% velocity spread and ~10% alpha spread are presented. Analytical, numerical and experimental results of a DC harmonic gyrotron are presented. The 3D PIC code MAGIC was used to simulate the interaction of the harmonic gyrotron such as the TE71 mode at the 7th cyclotron harmonic with the large orbit electron beam with the beam thickness and beam spread introduced into the simulation. The interaction cavity of both gyrotrons was in the form of a smooth cylindrical waveguide. The relationship between the cavity dimensions and cavity Q values has been studied for optimized output at the design mode with the aim of suppressing other competing modes. A linear output taper was designed with low mode conversion at the gyrotron output. A Vector Network Analyzer with high frequency millmetre wave heads was used to measure the millimeter wave properties of the gyrotron cavity. Experiments were conducted using the electron gun for the harmonic gyrotron. The gyrotron and electron gun were built as well as the interlock and safety system, pulsed power supply and magnet, the cooling and vacuum system. Millimetre wave radiation was measured for the 2.6 mm diameter cavity gyrotron operating at the 2nd harmonic at a magnetic field of 2.08 T. Experiments demonstrated that the harmonic gyrotron was sensitive to the magnetic field and electron beam parameters. Millimetre wave radiation from 108GHz to 110GHz was measured with the use of a W-band rectifying crystal detector and high pass cut off filters. The frequency of the measured millimeter wave radiation agreed very well with the design and predictions of theory.This thesis presents the results of successful operation of a 2nd harmonic gyrotron based on a cusp electron gun. The numerical and experimental results agreed well with the gyrotron design parameters. Two gyrotrons based on a cusp electron gun were designed: the first gyrotron operated at the 2nd harmonic and the second gyrotron was studied to look at the scaling of this concept for operation at the 7th harmonic at a frequency of 390 GHz. The cusp electron gun was used to produce the electron beam in the gyrotron which was annular in shape. The electron beam had a voltage of 40 kV, a current of 1.5A and a velocity ratio (perpendicular component to horizontal component) of 1.5. The experimental results from the first cusp electron gun and measurements of the high quality electron beam with ~8% velocity spread and ~10% alpha spread are presented. Analytical, numerical and experimental results of a DC harmonic gyrotron are presented. The 3D PIC code MAGIC was used to simulate the interaction of the harmonic gyrotron such as the TE71 mode at the 7th cyclotron harmonic with the large orbit electron beam with the beam thickness and beam spread introduced into the simulation. The interaction cavity of both gyrotrons was in the form of a smooth cylindrical waveguide. The relationship between the cavity dimensions and cavity Q values has been studied for optimized output at the design mode with the aim of suppressing other competing modes. A linear output taper was designed with low mode conversion at the gyrotron output. A Vector Network Analyzer with high frequency millmetre wave heads was used to measure the millimeter wave properties of the gyrotron cavity. Experiments were conducted using the electron gun for the harmonic gyrotron. The gyrotron and electron gun were built as well as the interlock and safety system, pulsed power supply and magnet, the cooling and vacuum system. Millimetre wave radiation was measured for the 2.6 mm diameter cavity gyrotron operating at the 2nd harmonic at a magnetic field of 2.08 T. Experiments demonstrated that the harmonic gyrotron was sensitive to the magnetic field and electron beam parameters. Millimetre wave radiation from 108GHz to 110GHz was measured with the use of a W-band rectifying crystal detector and high pass cut off filters. The frequency of the measured millimeter wave radiation agreed very well with the design and predictions of theory

Experimental Verification of A6 Magnetron with Permanent Magnet

A compact A6 relativistic magnetron with diffraction output using a transparent cathode, simple mode-converter, and a permanent magnet were simulated and tested at the University of New Mexico (UNM) for the Office of Naval Research. The standard compact MDO with a simple mode converter and transparent cathode radiates a TE11 mode axially through a cylindrical horn antenna. The magnetic field, essential for magnetron operation, is provided by a Neodymium Iron Boron (NeFeB) GradeN40M rare earth magnet. The permanent magnet eliminates the need for a pulsed magnet and accompanying circuit, significantly reducing the size of the system. A permanent magnetic field is also ideal for repetitive rate firing, which was demonstrated at the Naval Surface Warfare Center Dahlgren Division (NSWCDD) High Voltage Advanced Research (HIVAR) Laboratory in Dahlgren, Virginia. An extensive sweep of input parameters was simulated using MAGIC, a three dimensional particle-in-cell virtual prototyping tool to find the ideal operational parameters to be experimentally verified. A sweep of input voltages was completed experimentally using UNM’s PULSERAD accelerator in order to compare and verify simulation results. The compact MDO with permanent magnet and transparent cathode were shipped to and assembled at the NSWCDD HIVAR Laboratory on their modulator and recommended input parameters from UNM’s simulations and experimental verification were demonstrated. Results from the experimental set up reproduced the qualitative behavior of the simulations and the simulated operating frequency of 2.5GHz was measured experimentally. The anticipated Gaussian radiation pattern from the 5 radiated TE11 mode was also experimentally verified by two ways, first by neon bulb grid array, and secondly by measuring the peak of the radiated microwave pulse relative to varying locations

Design and validation of current probes for studying RF current flux through mechanical aircraft joints

The study of electrical currents flowing through the airframe is very important from an EMC (Electromagnetic Compatibility) standpoint due to the potential to produce severe aircraft faults. As an example, direct lightning strikes are one of the main failure causes, effect on which we will orient our work. Furthermore, one of the key points where the study of currents is more complex are the discontinuities formed by the mechanical joints that attach fuselage plates, in addition to the difficulty of placing sensors to measure current flowing through these junctions. In this work, a fixture intended to emulate such discontinuities of mechanically attached panels will be developed and evaluated. Subsequently, a prototype of probes capable to measure current flow will be built and studied theoretically and experimentally. Having the prototypes validated, flat probes embedded in a PCB will be constructed to be able to be inserted into the mechanical junctions, such as screws or rivets. Finally, current probes will be verified by means of a surge generator, capable of producing a wave similar to the one originated by a direct lightning strike

Development of analytical and experimental tools for magnetic pulse welding

A key process parameter in magnetic pulse welding (MPW) is the workpiece velocity, and while some Finite Element Analysis (FEA) packages exist that are capable of modeling these processes, there is a lack of simplified analytical modeling efforts, which are attractive for their simplicity and cost. In this work, an electromagnetic actuator, named a Uniform Pressure Actuator (UPA), is analyzed, designed, constructed, and tested experimentally. The analytical model is shown to predict workpiece velocities accurately and produce an efficient forming and a robust design. Additionally, an alternative method to measure workpiece velocity is presented, implementing a fiber optic, reflectance dependent sensor. The sensor is shown to be an attractive low cost solution to measurement of high velocities in high voltage, magnetic environments, through experimental measurement in parallel with a Photon Doppler Velocimetry (PDV) system

Investigation of flat capacitor discharge electromagnetic launchers

In this thesis, studies of flat or pancake type electromagnetic launcher systems are described. The studies involved the development of several numerical models, and are supported throughout by experimental investigation. The models were based on a coaxial filamentary division technique, and the results they provided were compared with those from a commercial electromagnetic finite element modelling package. They were used to investigate some of the many possible launcher structures and power supply arrangements, as part of a wide-ranging parametric study. The aim of this thesis was to gain an insight into the factors that affect the performance of the launchers. Several different techniques were implemented to reduce the computation time. Practical experimentation provided a clear demonstration of the launcher technology, and supplied valuable model validation data. To aid the experimental work new projectile speed and yaw measurement systems were developed, and these were supported by results from a high-speed camera. A novel dual projectile launcher was tested, and was shown to improve the launch efficiency and to operate at higher energies, due to the reduction in drive coil recoil. Projectile deformation was investigated in both solid discs and flat annular projectiles

Investigation of pulsed quasi-steady MPD arc jets

Evaluation of magnetohydrodynamic arc thrusters operating in quasi-steady mode with electrode vapor as propellan