Sabahattin C. Yurt

Design of a metamaterial slow wave structure for an O-type high power microwave generator

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 μ. 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 e. 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 synchronou...

Similarity of Properties of Metamaterial Slow-Wave Structures and Metallic Periodic Structures

A study of the evolution of wave dispersion in systems of all-metallic periodic structures with increasing corrugation depth shows a similarity of the properties of waves in metamaterial slow-wave structures (MSWSs) and traditional metallic SWSs used in high-power microwave sources. We show that the main properties of MSWSs, such as the existence of a lowest order negative dispersion wave below cutoff, also appear in ordinary metallic periodic systems with deep corrugations. Furthermore, we find that the appearance of negative dispersion in all-metallic periodic structures with increasing corrugation depth is accompanied by a hybrid mode being identified as the lowest order negative dispersion mode.

Dispersion diagram modeling for a metamaterial-like slow-wave structure

We report results on modeling dispersion diagrams for a novel metamaterial-like design to be used as a slow-wave structure for Cherenkov microwave sources. The structure, having its spatial period much less than the wavelength, exhibits extremely low group velocity and, thus, a promise of large values of the Pierce parameter.

Power Combiner for High Power Cerenkov Devices

Two Cherenkov devices are effectively combined using a circular waveguide T-junction in a simulation study. The microwave sources are driven by the same high voltage pulser to prevent phase difference at the output of the power combiner. This passive power combiner not only combines the microwave power, but also outputs a Gaussian beam when a TM01 mode is used as the input. An efficient serpentine mode converter has been used as the basis of the power combiner design where the fields sum up at the common section of the two mode converters. A combined summation/conversion efficiency was ~90% when the output power was more than 1.8 times the input power with a Gaussian output pattern. The combined microwave power was more than 600 MW at 10.2 GHz.

O-type oscillator with metamaterial-like slow-wave structure

We report results on “hot” modeling of an O-type oscillator with a metamaterial-like periodic electrodynamic structure. The structure, having its spatial period much less than the wavelength, exhibits extremely low group velocity and, at the location of the propagating electron beam, supports the longitudinal component of electric field of a fair amplitude, thus, providing a sufficient value of the Pierce parameter.

A compact high-power microwave metamaterial slow-wave structure: From computational design to hot test validation

There is considerable interest in exploiting metamaterials to engineer dispersion relations for new beam/wave interactions that are not possible using conventional all-metal periodic structures. In this paper, we describe the computational design, and both calculated and experimental cold and hot tests of a new high-power microwave slow wave structure at the University of New Mexico to generate a record 100 MW of power in L-band in a very compact size.

Designing of an O-type BWO with a metamaterial slow-wave structure

The aim of this work is to design a novel metamaterial slow wave structure (SWS) for high power microwave (HPM) generation using a short pulse relativistic electron beam. The structure provides around 15% efficiency with 250 MW output power at 1.4 GHz frequency. It generates backward wave generation and beam-wave interaction with a TE21-like mode.

Relativistic vircator with an electromagnetic bandgap medium

Summary form only given. The virtual cathode oscillator (vircator) can be one of the most promising high-power microwave sources due to its conceptual simplicity, high-output power capability, and potential tunability. Microwaves can be generated when the current of an intense relativistic electron beam exceeds the space-charge-limiting current of the structure. In this case, a virtual cathode comprising an electrostatic potential barrier is formed. Some of the electrons in the virtual cathode lose their kinetic energy and are then reflected back toward the anode. The energy conversion efficiency of the vircator is known to be very low (single digit). In this work, an electromagnetic band gap (EBG) material is investigated to increase the efficiency, to improve the tunability, and to extract the desired field structure at the output. The EBG structure follows a slot antenna concept, or more specifically, a leaky wave slot antenna. The slot array is cut at the waveguide wall to perturb the surface currents in a desired way. The fully electromagnetic, relativistic particle-in-cell (PIC) codes (MAGIC and CST-PS) and the fully electromagnetic software tool ANSYS-HFSS were used in this study.

High power microwave source loaded by a two-spiral metamaterial structure for Cherenkov radiation

Summary form only given. Metamaterials have unusual properties such as negative refractive index that provides backward wave propagation, also called Cherenkov radiation in vacuum electronics. They also have other unique properties such as period much less than operational wavelength, low group velocity, below cutoff propagation, among others. We are interested in studying metamaterial slow wave structures (SWSs) as a new application for high power microwave sources similar to [3]. For this reason, we developed a structure comprising two parallel and transversely periodic spiral metamaterials having resonant frequency of about 4.4 GHz. Using HFSS eigenmode simulation, we obtained negative dispersion around this frequency which is a TE-like mode (Ez = 0) where the group velocity is negative and extremely low. Dimensions of one period of the two-spiral structure are 22x22x2 mm. Since it has 2 mm thickness and is all metallic, it will be less susceptible to electrical breakdown. These two parallel plates are placed inside a standard WR187 rectangular waveguide with cross section 22x48 mm and the corresponding cut-off frequency for the second mode is 6.25 GHz. This suggests below cut-off propagation for this structure. We used a constitutive parameter extraction method using Sparameters for the corresponding mode [5] in order to verify simultaneously negative ε and negative μ and we obtain double negative behavior in the frequency region 4.2-4.9 GHz, which includes our resonant frequency. Even if the structure is capable of having only negative μ, it can be considered as double negative structure when it is placed inside a cut-off waveguide according to [2].

Evolution of wave dispersion in periodic structures with increasing amplitude of corrugation

Computer simulations using the electromagnetic code HFSS show that increasing the amplitude of the periodic corrugation of slow wave structures (SWSs) leads to expanding stopbands and decreasing the maximal frequency in the first passband of a wave. Results of further increasing the amplitude decrease the cut-off frequency and result in the appearance of a backward wave in the phase interval of hd = (0 - π) (here d is the period of corrugation and h is the wavenumber). It is pertinent to note that in this case the usual interpretation of stopbands and passbands as bands of Bragg reflection or Bragg mode conversion becomes incorrect. It is also incorrect to interpret passbands and cut-off frequencies as the minimal frequencies for wave propagation. We found that backward waves appear not only in periodic systems with small period with respect to the wavelength as in metamaterial SWSs, but also in systems with large period that we demonstrate for sinusoidal and rectangular profiles of periodic corrugations. Above all, we found that cut-off frequencies for TM modes (frequencies that correspond to phases (2n+1)π, where n is the number of positive and negative spatial harmonics including n=0 on the dispersion diagram) decrease with increasing amplitude of corrugation, which is also a typical characteristic of metamaterial SWSs.