M. Roybal

Experimental verification of the advantages of the transparent cathode in a short-pulse magnetron

Recently, considerable attention has been given to the development of different means of providing rapid start-of-oscillations in pulsed relativistic magnetrons. The innovative University of Michigans (UMs) methods of priming: cathode priming, magnetic priming and RF priming have shown improvement in the start of oscillations. At the University of New Mexico (UNM) we have been able to show via particle-in-cell (PIC) computer simulations fast start, fast rate of build-up of oscillations and significant improvement in the output characteristics of a magnetron using the transparent cathode. The three-dimensional PIC code MAGIC was used for the simulations. Experiments were performed using UNMs SINUS-6 accelerator with an applied voltage of 260 kV in 17 ns.

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.

Initial experimental results from a relativistic magnetron driven by transparent cathodes

The relativistic magnetron has been studied as a high power microwave source since the 1970s. Its most salient properties are its simplicity and its frequency agility. Recently, researchers at the University of New Mexico proposed that a transparent cathode can significantly improve the output characteristics of magnetrons. Since then, a comprehensive study using particle-in-cell simulations has elaborated the many benefits of using this cathode. This talk will enumerate the various benefits of operating a relativistic magnetron using a transparent cathode. In addition, the initial results of experiments at the University of New Mexico on an A6 magnetron driven by a short-pulse Sinus-6 accelerator will be presented. Planned experiments on an A6 magnetron using a longer pulse (50 ns) accelerator at UNM will also be described. Details will also be provided regarding the redesign of the longer pulse accelerator.

Short-Pulse Accelerators at UNM for Magnetron Experiments

We have demonstrated through computer simulations using the fully relativistic, three-dimensional particle-in-cell code MAGIC that the relativistic magnetron is less susceptible to mode competition when the rise time of the drive voltage is less than the characteristic cavity fill-time of the magnetron.

Design and Optimization of a Low-e mpedance Pulsed-Power Marx Generator to Drive High-Power Releativistic X-Band Magnetron

In this paper, the design and optimization efforts aimed at reducing the intrinsic impedance of the Marx generator are discussed. By reducing the number of stages of the Marxs electrical circuit from the presently configured 11-stage Marx to a 7-stage Marx the inductance can be significantly decreased. 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 the results of the design will be discussed.

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

Design and Optimization of a Low-Impedance Pulsed-Power Marx Generator to Drive High-Power Relativistic X-Band Magnetron

One class of high-current relativistic HPM devices, the relativistic magnetron [1], provides a low-impedance (10-50 Omega) load that inherently needs matching to a low-impedance pulsed-power (Gigawatt or greater) supply 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 110A electron beam accelerator [2] to produce high-power microwaves [3] from a backward wave oscillator (BWO), which was a high-impedance load (130 Omega). This driver will be used for planned experiments with 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 Omega (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 the present paper some results of the design and optimization efforts aimed at reducing the intrinsic impedance of the Marx generator are discussed. It turns out, for example, that by re-arranging the Marxs electrical circuit from the presently configured 11-stage Marx into a series-parallel connection of 12 capacitors, it is possible to decrease the impedance of the Marx generator down to ~15 Omega (implementing series-parallel connection of 2 parallel lines with 6 series capacitors in each line - 2times6 circuit), or even down to ~10 Omega (implementing series-parallel connection of 3 parallel lines of 4 series capacitors -3times4 circuit). Another feature that will be integrated in the redesign of the Pulseradreg 110 A electron beam accelerator is the use of a brazed ceramic insulator stack to facilitate high-vacuum operation. Successful completion of this upgrade will allow for experimental studies of a low-impedance relativistic X-band magnetron to begin to operate au naturel.

Studies of ceramic dielectrics for use in high power, pulsed power transmission lines

Summary form only given, as follows. Summary form only given. The development of compact pulsed power systems has been limited by the materials used in traditional pulsed power accelerators. One approach to decreasing the size of transmission lines would be to use ceramic dielectrics in lieu of traditional plastics or their derivatives. This presentation discusses a collaborative program with a ceramics group that is developing novel materials that have high breakdown strengths. The University of New Mexico program is using electromagnetic modeling techniques to design materials with distributed electrical properties for future manufacture. The program is also using pulsed testing techniques to assess the breakdown strengths of newly manufactured samples.