Recently, considerable attention has been given to the development of relativistic high-power microwave sources with the capability of broad-band frequency tuning [1]. We have demonstrated frequency tunability via computer simulations and experiments in an X-band Backward Wave Oscillator (BWO) with a modified cavity reflector.Unlike a BWO [2] where the cavity is the same for any distance between it and the SWS inlet, we use a cavity with increasing volume as the distance between the cavity and SWS inlet increases in order to keep the resonant frequency close to the tuned frequency. The opening of the cavity into the channel for the electron beam increases as the distance between the cavity and SWS increases in order to decrease the risk of microwave breakdown even in the mid-range of frequencies where the microwave power is maximum. A frequency tunability of about 9% is achieved by changing the distance between the cavity and the SWS, varying the guide magnetic field and the gap between the electron beam and SWS (the smaller the gap, the wider the range of tunability). All these means provide radiation power of at least 100 MW with pulse shape corresponding to the voltage pulse shape. Experiments using the 450 keV SINUS-6 electron beam accelerator are in good agreement with the PIC simulations.Another convenient way of controlling the gap is to use magnetic decompression of the electron beam [1] that also gives a possibility to avoid deposition of electrons on the SWS wall when a weaker magnetic field is used. In addition, thickness of the electron beam in BWO decreases H/H c times, where H and H c are guide magnetic field in the interaction space and on the tubular thin-walled cathode, respectively.
In an effort to develop transmission lines with higher applications, the University of Missouri-Rolla (UMR) and the University of New Mexico (UNM) have undertaken a collaborative approach to developing and studying ceramic dielectrics. At UMR, the electrical breakdown strength (BDS) of Ti0,-based materials is investigated for high energy density applications. The results of research to-date show that dense titania ceramics with nanocrystalline grain size (-200 nm) exhibit significantly higher BDS as compared to ceramics made using coarse grain materials.Processing-microstructure-property relationships in Ti02 systems are found to play a role with respect to increasing the BDS. At UNM, a pulsed power system is being assembled to perform BDS studies of the ceramic materials produced at UMR. Electromagnetic simulations in support of this work will also presented. The long-term aim of this research is to enable the fabrication of large sizes of high energy density ceramics for use in pulsed power systems. , energy storage capabilities for compact pulsed power 0-7803-791 5-UO3617.W 020031EEE.
With the aim of developing novel planar transmission lines that improve prospects for compact pulsed power, the ECE Department at the University of New Mexico (UNM) has played an important role in a collaborative research project with Sienna Technologies, Inc. supported by DoE. In future envisioned pulsed power-driven systems on mobile platforms it is well know that ceramic dielectrics with large ε have great potential for decreasing the volume required for the system. Such ceramic dielectrics are also important for higher energy density capacitors. Unfortunately, ceramics with higher dielectric constant normally have lower breakdown strength (< 1.0 MV/cm) compared with organic materials or thermoplastic materials. In view of this, to achieve high energy densities (> 0.5 J/cm 3 ) in dielectrics the present trend consists of casting in a mold a composite material composed of granulated ceramic embedded in an organic dielectric matrix. In an effort to accomplish this, Sienna Technologies has been responsible for fabrication of the ceramic composite samples while UNM has been performing pulsed breakdown characterization. In addition, another objective of this work is to investigate the behavior of the dielectric constant of the composite material as ceramic materials generally present a nonlinear dependence, i.e., ε varies as a function of the applied voltage. There has been a paucity of research on this nonlinear behavior for ceramic composites and it is necessary to understand the ε variation for two basic applications in pulsed power: linear & nonlinear transmission lines (NLTLs).
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