Double-negative metamaterial research for accelerator applications

Material properties are central to the design of particle accelerators. One area of advanced accelerator research is to investigate novel materials and structures and their potential use in extending capabilities of accelerator components. Within the past decade a new type of artificially constructed material having the unique property of simultaneously negative permittivity and permeability has been realized, and is under intense investigation, primarily by the optical physics and microwave engineering communities [C.M. Soukoulis, Science 315 (2007) 47; D.R. Smith, J.B. Pendry, M.C.K. Wiltshire, Science 305 (2004) 788; J.B. Pendry, A.J. Holden, W.J. Stewart, I. Youngs, Phys. Rev. Lett. 76 (1996) 4773]. Although they are typically constructed of arrays of discrete cells, as long as the condition that the wavelength of applied radiation is significantly greater than the cell dimensions is met, the material mimics a continuous medium and can be described with the bulk properties of permittivity, , and permeability, m. When the permittivity and permeability are simultaneously negative in some frequency range, the metamaterial is called double negative (DNM) or left-handed (LHM) and has unusual properties, such as a negative index of refraction. An investigation of these materials in the context of accelerators is being carried out by IIT and the Argonne Wakefield Accelerator Facility [S. Antipov, W. Liu, W. Gai, J. Power, L. Spentzouris, AIP Conf. Proc. 877 (2006); S. Antipov, W. Liu, J. Power, L. Spentzouris, Design, Fabrication, and Testing of Left-Handed Metamaterial, Wakefield Notes at Argonne Wakefield Accelerator, hhttp://www.hep.anl.gov/awa/wfnotes/wf229.pdfi]. Waveguides loaded with metamaterials are of interest because the DNM can change the dispersion relation of the waveguide significantly. For example, slow backward waves can be produced in a DNM-loaded waveguide without having corrugations. This article begins with a brief introduction of known design principles for realizing a DNM [J.B. Pendry, A.J. Holden, W.J. Stewart, I. Youngs, Phys. Rev. Lett. 76 (1996) 4773; D.R. Smith, et al., Phys. Rev. Lett. 84 (2000) 4184; J.B. Pendry, A.J. Holden, D.J. Robbins, W.J. Stewart, IEEE Trans. Microwave Theory Tech. 47 (1999) 2075], along with a description of the experimental verification of the basic DNM properties of our designs. We then present our waveguide analysis, starting with the case of a waveguide loaded with a truly continuous medium that is dispersive and anisotropic. We show that the dispersion relation of a waveguide with frequency regions of negative ðoÞ and negative mðoÞ has several interesting frequency bands. While a DNM approximates a continuous medium, it is still made up of discrete elements. We discuss some implications of the discrete nature of the material for the behavior of a loaded waveguide, particularly at frequencies below the cutoff frequency of the waveguide. We conclude by describing our experimental program at present and in the near future. This includes testing the excitation of TM modes in a DNM loaded waveguide in the interesting frequency bands, both on the bench and from particle beam excitation. r 2007 Elsevier B.V. All rights reserved. PACS: 41.20.Jb; 41.60.Bq; 84.40.Az