Session CM10: Mini-Conference on Electromagnetic Metamaterials for High-Power Applications II

Metamaterial Mediated Energy Exchange between Wave and Beam in the High-Power regime

In this presentation we discuss metamaterial mediated energy exchange between charged particle beams and electromagnetic waves in the high-power regime, using coupled mode theory and a Pierce's approach. We present a modified form of Pierce's theory which takes into account the presence of the metamaterial and the intrinsic loss associated with metamaterials. We examine the unit-cell surface current distribution and loss in a metamaterial consisting of Complementary Split Ring Resonators (CSRR), focusing on the differences in loss between metamaterials in waveguide and free-space. We also discuss experimental and numerical parameter extraction techniques and the validated of each approach in the different cases of metamaterials in waveguide and in free space.   

Enhanced RF Losses and Field Enhancements Due to Surface Roughness

Metal-based metamaterials support a variety of electromagnetic modes with a complex interplay between the RF electric and RF magnetic field. One such interplay is exemplified by surface roughness. This paper presents an accurate evaluation of the power absorption, and of field enhancements, due to small surface roughness on a metallic structure. Absorption through both the RF electric field and RF magnetic field components of the electromagnetic mode are accounted for, self-consistently. The surface roughness is assumed to be hemispherical, with a radius much less than the free space wavelength of the electromagnetic mode. This roughness may assume an arbitrary value of permittivity, conductivity, and permeability, however. Simple scaling laws for the power absorbed, and for the RF electric field and RF magnetic field enhancements, are given in terms of the surface roughness' skin depth, which may assume an arbitrary value ranging from zero (perfect conductor) to infinity (insulator). [Zhang et al., J. Appl. Phys. 105, 114908 (2009)].   

High-power sub-mm oscillator based on surface field cavity: Bridging the THz gap

Oscillators capable of producing high-power radiation from the mid-GHz to low-THz frequency range will be presented. Such devices are attractive for a range of applications including: active sensing and biochemical spectroscopy. Oscillators capable of producing the required output power at these frequencies are not presently widely available. One of the reasons is the necessity to use high-voltage, high-current density electron beams. An output pulse power of 10kW is sufficient for many applications; compact sources are sought which dictates the use of a relatively low voltage (50kV) electron beam of moderate current. The use artificial materials which have tailored properties optimised for high power operation will be discussed. The use of metamaterials in high power applications meets a number of challenges such as coherence of the radiation scattering from the metamaterial, metamaterial overheating and field enhancement. To overcome some of the challenges an 2D cylindrical surface structure is suggested.   

What's the Meta?

The term ``Metamaterial'' was first coined in 1999 by Rodger Walser, University of Texas [B. Munk, {\it Metamaterials: Critique and Alternatives\/} (John Wiley \& Sons, New York, NY, 2009)]. His definition is as follows: {\it Metamaterials are macroscopic composites having man-made, three-dimensional, periodic cellular architecture designed to produce an optimized combination, not available in nature, of two or more responses to specific excitation.\/} He chose ``meta'' as the prefix from the Greek work meta meaning beyond. The taxonomy of meta-materials is a problem. It seems that there is no one satisfactory definition that does not restrict a class of worthy meta-materials. This presentation reviews past work on the interaction of high power electromagnetic radiation with 1-D photonic crystals. The purpose of this work was to demonstrate the use of a dielectric to focus the microwave wavebeam and to reflect it quasi-optically. In these experiments using a short pulse SINUS-6 accelerator-driven backward-wave oscillator (BWO) no deleterious effects of the high power electromagnetic fields were observed on the photonic crystal. In addition, planned experiments for an overmoded BWO whose slow wave structure is made from individual wires will be described. This latter has novel mode selection features, in addition to high power handling capability.   

High-Power Microwave Metamaterials for Phased-Array, anti-HPM, and Pulse Shaping Applications

We present a class of metamaterials capable of operating under extremely high power microwave (HPM) fields. The proposed metamaterials are in the form of periodic arrangements of unit cells with sub-wavelength dimensions and periodicities. Each unit cell is composed of multiple thin impedance sheets providing non-resonant reactive surface impedances separated from each other by ultra-thin dielectric spacers. The proposed structures can be designed to operate as spatial filters with highly selective spectral responses capable of handling power density levels in the order of 1.0 MW/cm2. Such filters could ultimately be used as counter high-power-microwave devices in HPM systems. In addition to their filtering characteristics, over a certain frequency range, the unit cells of these structures could be treated as spatial phase shifters or true-time-delay units (TTDU). We will discuss how tunable versions of these TTDUs can be used to design wideband, tunable HPM lenses for phased-array applications. Finally, we will discuss the possibility of using non-linear versions of these HPM lenses for pulse shaping applications at high-power-microwave levels.   

Overmoded Dielectric Photonic Crystal Cavities for High-Power Microwave Applications

Photonic crystal structures are attractive for use in high-power microwave applications. Experiments have demonstrated use of cavities based on metallic lattices for millimeter wave/THz generation and high-gradient particle acceleration. Due to unique dispersion properties and the large number of materials available, dielectric lattices are particularly flexible for engineering of frequency band gaps; for example, they allow the design of overmoded cavities that have no lower-order mode competition. The use of such oversized cavities offers critical advantages for experiments at high frequencies, addressing key issues such as pulsed heating and parasitic higher-order mode (HOM) excitation. We present a specific design of an accelerator cavity operating in a TM$_{02}$-like mode at 17 GHz, formed by a 2D dielectric lattice between metal plates. The use of dielectric rods reduces magnetic pulsed heating on the inner rods, which has been shown to cause breakdown in metal-rod structures. Wakefield simulations show greatly reduced HOM excitation relative to a conventional pillbox cavity. We discuss high-power testing at 17 GHz at MIT and future topics of study, including 3D photonic crystal microwave devices, exotic dielectric materials, and combined metal and dielectric lattices.