The cavity magnetron is the most compact, efficient source of high-power microwave (HPM) radiation. The imprint that the magnetron has had on the world is comparable to the invention of the nuclear bomb. High- and low-power magnetrons are used in many applications, such as radar systems, plasma generation for semiconductor processing, and—the most common—microwave ovens for personal and industrial use. Since the invention of the magnetron in 1921 by Hull, scientists and engineers have improved and optimized magnetron technology by altering the geometry, materials, and operating conditions, as well as by identifying applications. A major step in advancing magnetrons was the relativistic magnetron introduced by Bekefi and Orzechowski at MIT (USA, 1976), followed by the invention of the relativistic magnetron with diffraction output (MDO) by Kovalev and Fuks at the Institute of Applied Physics (Soviet Union, 1977). The performance of relativistic magnetrons did not advance significantly thereafter until researchers at the University of Michigan and University of New Mexico (UNM) independently introduced new priming techniques and new cathode topologies in the 2000s, and researchers in Japan identified a flaw in the original Soviet MDO design. Recently, the efficiency of the MDO has reached 92% with the introduction of a virtual cathode and magnetic mirror, proposed by Fuks and Schamiloglu at UNM (2018). This article presents a historical review of the progression of the magnetron from a device intended to operate as a high-voltage switch controlled by the magnetic field that Hull published in 1921, to the most compact and efficient HPM source in the twenty-first century.

The relativistic magnetron is the most compact and efficient high-power microwave source. In order to further increase the total efficiency of microwave generation in a relativistic magnetron with diffraction output with a low-energy state of electrons that is formed between two virtual cathodes (VCs) in the interaction space, the second VC at the downstream end of the interaction space is replaced with a magnetic mirror that reflects all leakage electrons back into the interaction space. Previously these leakage electrons were deposited on the anode surface without interacting with the microwaves. Particle-in-cell simulations show that this configuration of relativistic magnetron increases the electronic efficiency from 86% to 92%, and increases the total efficiency up to the maximal electronic efficiency. This is a record high efficiency of microwave generation in a relativistic magnetron or any other gigawatt-class, gigahertz-frequency HPM source.

Metamaterials (MTMs) are structured materials with subwavelength features that can be engineered to have some unique properties not found in nature, such as negative refractive index, reversed Doppler effect, and reversed Cherenkov radiation. Based on these novel MTMs, several important research groups have made great attempts to develop novel MTM-inspired vacuum electron devices (VEDs) and accelerators. Just as solid-state power devices are innovated by incessant emerging of new semiconductor materials, VEDs can also be inspired by MTMs to have very remarkable advantages, such as smaller size, higher power, higher efficiency, and/or larger gain relative to conventional VEDs, such as traveling-wave tubes, backward-wave oscillators, and klystrons. Similarly, relative to conventional accelerators, MTM-inspired accelerators have obvious advantages, such as smaller size and higher accelerating gradient. Furthermore, MTM-inspired devices have promising applications in areas such as radar, communication, electronic warfare, microwave heating, and imaging.

We describe a new O-type high power microwave oscillator that uses a metamaterial slow wave structure (MSWS) supporting waves with negative dispersion. The MSWS comprises periodically alternating, oppositely oriented split ring resonators (SRRs) connected to a metal tube where the distance between the rings is much less than a wavelength of the radiation generated. The SRRs provide negative permeability $\mathit{\mu }$ . The diameter of the metal tube is such that the generated oscillations are below cutoff for a regular waveguide with the same dimension, thus providing negative permittivity $\mathit{\epsilon }$ . A tubular electron beam propagates coaxially through this structure. The interaction space is coupled with the outer coaxial channel through gaps between the SRRs. Radiation is extracted in an endfire manner at the end of the outer channel via a conical horn section. Using particle-in-cell (PIC) simulations, it was found that the electron beam in the interaction space forms a sequence of trapped electron bunches by the synchronous operating wave. The output parameters of this oscillator for an applied voltage U = 400 kV, electron beam current I = 4.5 kA, and guide axial magnetic field B = 2 T are radiation power P = 260 MW, radiation frequency f = 1.4 GHz, and electronic efficiency η = 15% when the total SWS length L consisting of 12 split rings is 34.5 cm. The output radiation pattern corresponds to a TE₂₁-like hybrid mode. This article presents details on the simulations of this novel structure and computational and experimental cold tests of a prototype structure in preparation for experimental hot tests.

A nonlinear lumped element transmission line (NLETL) that consists of an LC -ladder network can be used to convert a rectangular input pump pulse into a series of RF oscillations at the output. The discreteness of the LC sections in the network contributes to the line dispersion while the nonlinearity of the LC elements produces the nonlinear characteristics of the line. Both of these properties combine to produce wave trains of high frequency. This paper describes an NLETL circuit model that is used to simulate RF generation for a given input pump pulse and the experiments used to validate the simulated results. The circuit model is used to study a nonlinear capacitive line that comprises nonlinear C but linear L and a nonlinear inductive line that comprises nonlinear L but linear C . Extensive and comprehensive parametric studies were carried out for the various NLETLs to understand the behavior and characteristics of these lines. Interesting observations were made, and explanations were given for their occurrence. A hybrid line that comprises both nonlinear elements L and C was also investigated using the circuit model with the goal of better matching to a resistive load. Simulations of the hybrid line indicate promising results.

A high-power-microwave (HPM) source consisting of a backward wave oscillator (BWO) based on a metamaterial slow wave structure (MTM SWS) is investigated and tested. The MTM SWS is composed of a circular waveguide with periodic loading of two complementary electric split-ring resonator (CeSRR) disks and was designed at the University of California, Irvine, CA, USA (UCI). This SWS has been studied and simulated to quantify its cold structure properties. Moreover, a BWO using the proposed SWS has been simulated using a particle-in-cell solver to evaluate its performance. In the simulations, this BWO has generated 88-MW peak power in a pulse duration on the order of 15 ns with frequency about 2.9 GHz. This SWS was fabricated using 3-D printing and copper plating technology and was tested using the SINUS-6 pulsed electron beam accelerator at the University of New Mexico, Albuquerque, NM, USA (UNM). An output of 22 MW at 3.0 GHz was observed in experiments.