Session BM10: Mini-Conference on Electromagnetic Metamaterials for High-Power Applications I

Novel electromagnetic structures for high-power dispersion engineering

High power sources of coherent electromagnetic radiation have been an active area of research for decades, and have driven advances in directed energy, radar, and communications applications. At the highest levels of peak power, however, much of this research has been focused on oscillators driven by intense electron beams. One reason for this is the large amount of free energy associated with the space-charge of these intense beams provides ample means to drive an amplifier into self-oscillation. Recent advances in metamaterials, transformation optics, and photonic band gap structures have the potential to dramatically increase our capability to engineering the electromagnetic properties of the circuit, provide greater control of the beam-wave interaction, and offer new avenues to reach high power amplification. A notable challenge with this concept, however, is the capability of these structures to handle an intense RF and DC environment while retaining their physical integrity and electromagnetic properties. These challenges include electrical breakdown of the structures, AC and DC beam-loading, and melting due to losses in the configurations. Fundamental study of the physics of high-power sources based on metamaterials is needed to realize the potential of these advanced electromagnetic geometries.   

Progress on the Omniguide Traveling-Wave Tube Experiment

A wide-band millimeter-wave traveling-wave tube (TWT) amplifier is being developed at Los Alamos National Laboratory. We have designed, fabricated, and tested a novel W-band traveling-wave tube (TWT) amplifier based on a slow-wave cylindrically-symmetric photonic band gap (PBG) structure, or an ``omniguide.'' The omniguide represents a one-dimensional periodic system of concentric dielectric tubes. The tubes were fabricated with silica dielectric and held with a copper waveguide input and output. Cold-test results were found to be in excellent agreement with the design demonstrating very large bandwidth. Hot-test results have yielded gain which is consistent with the theory. One concern with dielectric tubes was their susceptibility to damage when struck by electrons. Our experiments show no serious damage, even after hundreds of shots. Gain experiments are continuing at Los Alamos.   

Photonic Bandgap Metallic Structures for High Power Microwave Applications

High power applications of photonic bandgap (PBG) structures include particle accelerators and vacuum electron devices. Metallic PBG structures are promising for high gradient linear accelerators to suppress high order mode wakefields. The MIT 17.1 GHz PBG based accelerator demonstrated a gradient of 35 MV/m for the input power of 2 MW. The gradient is limited by pulsed heating of rods in the PBG structure. The pulsed heating experiments at 11.4 GHz were conducted at SLAC. Gradients of 110 MV/m were measured and the breakdown rate vs. gradient was determined experimentally. An improved 11.4 GHz PBG structure with reduced pulsed heating by using elliptical rods has been built and is under test. Pulsed heating in a new 17.1 GHz PBG structure will be experimentally studied at MIT. The PBG structures may be used in vacuum electron devices. A new PBG gyrotron experiment has been designed -- a 1 kW, 250 GHz gyroamplifier with a gain of 50 dB. Research has been conducted on a non-gyrotron high frequency PBG based device, an overmoded W-Band (94 GHz) TWT.   

Implicit time-domain simulation of metamaterials on GPUs

Metamaterials present a challenge to 3D electromagnetic simulation due to their sub-wavelength structural features, demanding spatial grid cell sizes typically $\approx\lambda/50$. This is similar to the situation found modeling conventional slow-wave structures, such as TWTs. For explicit, finite-difference time-domain (FDTD) techniques, numerical stability further dictates the use of very small time steps, leading to long simulation times for wave propagation in metamaterials. We present simulations using a new alternating direction implicit (ADI) FDTD algorithm [1,2] implemented efficiently for high performance graphics processing units (GPUs). Our method uses a complex-envelope representation for the field amplitudes to factor out the rf timescale, and is absolutely stable. Consequently, we are able to use time steps comparable to the rf period for narrow-bandwidth simulations, and reduce simulation times by orders of magnitude compared to conventional FDTD on CPUs. Simulation results will be presented for a number of metamaterial structures. \newline [1] S. J. Cooke {\em et al.}, Int. J. Numer. Model., 22, 187 (2009) \newline [2] M. Botton {\em et al.}, IEEE Trans. Plasma Sci., 38 (6), 1439 (2010)   

Waveguide-based complementary metamaterial for high-power microwave generation and particle acceleration

A novel over-moded metallic structure based on sub-wavelength meta-waveguides patterned by complementary metamaterials (C-MTM) is proposed as a potential electromagnetic structure for particle acceleration and radiation generation. One of the advantages of this structure is planar fabrication which is advantageous for short-wavelength applications. A transmission line theory of the mode propagation and interaction with the electron beam is developed. It is shown that the structure supports a negative-index accelerating mode that can resonantly interact with a relativistic electron beam. In the context of radiation generation, the proposed structure can be utilized as a Backward Wave Oscillator. Using COMSOL simulations, it is shown that the beam-mode interaction results in the mode's exponential growth. Mode competition and the excitation of deleterious transverse wakes will also be discussed. Both positive and negative index TM modes strongly interacting with the beam are identified.   

Split Ring Resonators for Below Cut-off Energy Propagation

Metamaterials remain an intense subject of research for a variety of applications, ranging from tailoring the electromagnetic response of a medium in a novel fashion to allowing propagation of electromagnetic power in structures operating below the standard cut-off frequency. Generally, a metamaterial consists of an arrangement, generally periodic, of sub-wavelength, resonant structures. The physical configuration of these structures dictates the electromagnetic response of this artificial medium in such a fashion as to generate a response not typically found in nature. One open question for metamaterials consists of the applicability of these artificial media to high power, both average and peak, operation. We discuss experiments on a split-ring resonator geometry in a below cut-off waveguide, considering the ways in which defects will manifest themselves in the response of the metamaterial. Examination of these defects gives great insight into the basic physical operation of metamaterial structures at finite scales, pointing to the fact that such arrays act more as coupled oscillators than homogeneous media.   

Accelerator applications of engineered media

Material properties are vital for the field of accelerator R{\&}D. Euclid Techlabs in collaboration with the Argonne Wakefield Accelerator facility team has investigated accelerator applications of metamaterials, photonic band gap structures, nonlinear, paramagnetic and ferroelectric materials. In this paper we will present results of our work on advanced accelerating structures and accelerating applications of engineered materials. These will include wakefield test of a photonic band gap accelerating structure, design of a dielectric loaded structure with built in tunable paramagnetic absorption mechanism and beam diagnostics applications of metamaterials.