C.J. Buchenauer

Nonlinear Transmission Lines for High Power Microwave Applications - A Survey

Nonlinear transmission lines (NLTLs) have been used successfully to produce high power microwave (HPM) oscillators over the past 20 years. The advantages of such devices include compact structures in a narrow band radiator, frequency agility, and relatively high power. The key component in such devices is the nonlinear material used in the transmission line (TL). Attempts have been made using both nonlinear dielectrics and magnetic materials in NLTLs, with magnetic materials producing the biggest successes, to-date. This paper presents an overview of the work that has been done to-date in NLTLs with an emphasis on the body of knowledge directly applicable to HPM. A brief summary of the documented efforts using both dielectric and magnetic materials will be offered. Then, the authors present their analysis of what is needed to make further progress along the path towards higher power devices. This paper presents a roadmap for research deemed necessary from an engineering point-of-view to accomplish this goal. In particular, the need for characterizing such materials and building proposed testbeds for accomplishing this goal will be discussed. Finally, we will discuss a third design option - a hybrid approach that combines the features of nonlinear dielectrics and nonlinear magnetic materials for the purpose of producing robust HPM oscillators.

A control theory approach on the design of a Marx generator network

A Marx generator is a well-known type of electrical circuit first described by Erwin Otto Marx in 1924. It has been utilized in numerous applications in pulsed power with resistive or capacitive loads. To-date the vast majority of research on Marx generators designed to drive capacitive loads relied on experimentation and circuit-level modeling to guide their designs. In this paper we describe how the problem of designing a Marx generator to drive a capacitive load is reduced to that of choosing a diagonal gain matrix F that places the eigenvalues of the closed-loop matrix A+BF at specific locations. Here A is the identity matrix and B characterizes the elements of the Marx generator and depends on the number of stages N. Due to the special structure of matrix F, this formulation is a well-known problem in the area of feedback control and is referred to as the structured static state feedback problem. While the problem is difficult to solve in general, due to the specific structures of matrices A and B, various efficient numerical algorithms exist to find solutions in specific cases. In a companion paper by Buchenauer [1] it is shown that if certain conditions hold, then setting the natural frequencies of the circuit to be harmonically related guarantees that all the energy is delivered to the load capacitor after a suitable delay. A theorem formalizing this result is presented. Earlier aspects of this research have been published in two theses [2,3].

Studies of Dielectric Breakdown Under Pulsed Power Conditions

In an effort to develop transmission lines with higher energy storage capabilities for compact pulsed power applications, ceramic dielectrics and their electrical breakdown strength (BDS) are being developed and studied. Results of research to-date show that the dense titania ceramics with nanocrystalline grain size (-200 nm) exhibit significantly higher BDS as compared to ceramics made using coarse grain materials when tested under DC conditions. Pulsed testing under similar electric field stresses have been performed and found comparable behavior [1]. Furthering the research has led to consider the electrical breakdown strength (BDS) of materials such as ceramic/epoxy composites. These ceramic/epoxy materials are of interest. This material seems to be more flexible, robust, and might have increased breakdown strength as compared to dense titania ceramics. The powders are available with nominal particle sizes of 50 nm to 400 nm. The crystalline ceramic powders have uniform spherical morphology, precise stoichiometry and high ceramic purity. The effects of rise time of high voltage pulsed power on the breakdown of the ceramic/epoxy composite material will also be the focus of interest. This paper describes our test results, reviews the statistics that are used to analyze the data, and relates our understanding to what has been accumulated in the literature to-date in the context of dielectric breakdown.

X-band relativistic BWO with frequency tuning

Recently, considerable attention has been given to the development of relativistic high-power microwave sources with the capability of broad-band frequency tuning. We have demonstrated frequency tunability via computer simulations and experiments in an X-band backward wave oscillator (BWO) with a modified cavity reflector.

Arc switch modeling with complex-impedance loads

Summary form only given. An arc switch driving a real-impedance load produces an asymmetric voltage waveform with a long dissipative tail as the plasma arc heats and expands. A two-step process is suggested by some measurements. Shunt capacitors have been added to spark switches to hasten arc heating and expansion. Improvements in waveform symmetry and closure properties are possible with more complex networks as well. The design of such networks will require a detailed understanding of switch behavior. Switching performance is analyzed with pSPICE models of the arc switches and the networks. Switch behavior is modeled after the equation (d/R)/sup m/=k /spl int//sub 0//sup t/ I/sup n/ dt, where R is the time-dependent arc switch resistance, I is the arc switch current, d is the arc length, and k is a constant for the particular switch medium. Several switch models are investigated, including those of Braginskii (m=1, n=2/3), Rompe and Weitzel (m=2, n=2), and Vlastos (m=5/3, n=2). Shunt capacitors, shunt stub transmission lines, and more complex networks are employed to improve switching performance. Waveform symmetry, late-time flatness, and rise time can be improved significantly. Modest increases in dV/dt are observed for some models. The relevance of this analysis depends critically upon the accuracy of arc switch models. To improve the accuracy of models, experiments are planned that measures the dynamic impedances of arc switches during closure.

ZT-P: an advanced air core reversed field pinch prototype

The ZT-P experiment, with a major radius of 0.45 m and a minor radius of 0.07 m, was designed to prototype the next generation of reversed field pinch (RFP) machines at Los Alamos. ZT-P utilizes an air-core poloidal field system, with precisely wound and positioned rigid copper coils, to drive the plasma current and provide plasma equilibrium with intrinsically low magnetic field errors. ZT-Ps compact configuration is adaptable to test various first wall and limiter designs at reactor-relevant current densities in the range of 5 to 20 MA/m/sup 2/. In addition, the load assembly design allows for the installation of toroidal field divertors. Design of ZT-P began in October 1983, and assembly was completed in October 1984. This report describes the magnetic, electrical, mechanical, vacuum, diagnostic, data acquisition, and control aspects of the machine design. In addition, preliminary data from initial ZT-P operation are presented. Because of ZT-Ps prototypical function, many of its design aspects and experimental results are directly applicable to the design of a next generation RFP. 17 refs., 47 figs.

Raman Spectra of F Centers

The Raman spectra of F centers are of particular interest because they provide the most direct information about the phonons responsible for the F center’s line-width. In addition, they yield information on the manner in which the vibrational modes of the host lattice are perturbed by the presence of F centers.

Design and optimization of a lowimpedance pulsed power marx generator to drive a relativistic x-band magnetron

Summary form only given. The relativistic magnetron provides a low-impedance (10-20 U) load that inherently needs matching to a low-impedance pulsed-power supply (gigawatt or greater power) to operate with maximum efficiency. One of the most common and reliable pulsed-power drivers used to accomplish this is a Marx generator. The University of New Mexico previously used a modified Pulseradreg 110 A electron beam accelerator to produce high-power microwaves from a backward wave oscillator (BWO), which was a high-impedance load (130 U). This driver will be used for planned experiments driving an X-band relativistic magnetron. The main part of the Pulserads assembly is a Marx generator composed of capacitors, spark gaps and resistors electrically arranged in an 11-stage Marx circuit. The output impedance of the Marx generator is about ~35 U (which is why a shunt resistor was required to facilitate operation with the BWO), maximum charging voltage of each single stage is <100 kV, and total stored energy is about 600 Joules. In this presentation results of the design and optimization efforts aimed at reducing the intrinsic impedance of the Marx generator are discussed. In order to provide power from the Marx generator to an X-band relativistic magnetron, a coaxial transmission line and a vacuum-insulator interface will be used and their design will be discussed