Investigation of electrically tunable diode-based nonlinear transmission lines as pulse shaping networks for microwave systems at megawatt peak powers
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Abstract
The pulsed power community is actively concentrating on the miniaturization of high-power microwave (HPM) systems, driven by the growing demand for applications on oceanic vessels, satellites, and unmanned aerial vehicles. One promising approach to achieve system miniaturization involves utilizing a Nonlinear Transmission Line (NLTL) as a Pulse Shaping Network (PSN) to shift spectral energy distributions to higher frequencies. This work focuses on a subset of NLTL: diode-based nonlinear transmission lines (D-NLTL), which use the reverse bias p-n junction of a diode as a nonlinear permittivity. The presented research aims to enhance frequency generation, power output, and network efficiency, with a simultaneous focus on the design process employed in D-NLTL production. The first of three studies focused on improving our comprehension of D-NLTL dynamics with the aim of establishing guidelines for component selection and device fabrication. A proof of concept for the design process is showcased through a 40-cell D-NLTL prototype, registering a measured center frequency of 256 MHz. Additionally, the significance of factoring in estimations for parasitic losses in D-NLTL simulation is underscored through comparison between lossy and lossless simulations. The omission of parameters such as inductor coil capacitance and resistance, along with diode lead inductance and shunt capacitance, leads to simulations predicting frequency generation 2-2.5 times higher than prototype measurements. The second study employed a direct current (DC) bias in the D-NLTL network, facilitating use of bipolar excitations. The impact of the DC bias was observed in the flattening of the C(V) curve, reducing the level of impedance mismatch at the source-line interface. This resulted in a measured center frequency of 263 MHz, accompanied by an in-network amplitude of 8 V generated from a 10 V peak-to-peak square wave. Comparison with an unbiased D-NLTL, subjected to a 10 V monopolar pulse, revealed that the bipolar line exhibited a 37% increase in center frequency and a 2.67x increase in voltage measured after the source line interface. Finally, results are presented for two high-voltage D-NLTL networks based on high-current epoxy diodes of models K100F and K50F. Both networks exhibited megawatt (MW) peak powers, with a measured peak power of 2.88 MW for the K50F D-NLTL and 2.33 MW for the K100F D-NLTL. Demonstration of L-band (1-2 GHz) frequency generation was marked by Bragg cutoff frequencies of 1.44 GHz of the K50F line and 1.58 GHz for the K100F line. Additionally, the K100F D-NLTL showcased a UHF (0.3-1 GHz) band center frequency of 756 MHz. The work presented within this dissertation contributes to the pulsed power community by enhancing the design process of a D-NLTL, introducing a method for mitigating effects of the source-line impedance mismatch, and demonstrating the capability of a D-NLTL to handle both UHF to L-band content and MW peak powers. The suggestion is put forward that high frequency and power D-NLTL work should prioritize use of high current epoxy diodes (or similar) as a nonlinear element. Further, simulation techniques aimed at increased agreement with measurement are highlighted throughout each study.
Table of Contents
Introduction -- Theoretical background -- Experimental method and prototype fabrication -- Diode-based nonlinear transmission line design considerations -- Electrically tunable diode-based nonlinear transmission line -- UHF band frequency generation at MW peak powers using diode-based nonlinear transmission lines -- Conclusions and future work
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Ph.D. (Doctor of Philosophy)