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In this Letter, we present a study of the confinement properties of point-defect resonators in finite-size photonic-bandgap structures composed of aperiodic arrangements of dielectric rods, with special emphasis on their use for the design of cavities for particle accelerators. Specifically, for representative geometries, we study the properties of the fundamental mode (as a function of the filling fraction, structure size, and losses) via 2-D and 3-D full-wave numerical simulations, as well as microwave measurements at room temperature. Results indicate that, for reduced-size structures, aperiodic geometries exhibit superior confinement properties by comparison with periodic ones.A promising approach to the development of new types of compact, efficient microwave particle accelerators is to realize structures having higher frequencies of operation, since this can increase the accelerating field gradient and reduce the power consumption 1 . Unfortunately, in a particle accelerator, the higher the frequency, the stronger the excitation (by the wakefield effect) of higher order modes (HOMs), with a significant reduction of the beam stability. Nowadays accelerators operate at frequencies < ∼ 1 GHz, and they usually rely on waveguide-based HOM dampers in order to efficiently suppress the wakefields. However, at higher ( > ∼ 10 GHz) working frequencies, the standard configurations used for HOM damping become rather cumbersome or even technically unfeasible.In the past, open metallic photonic-crystal (PC) cavities, based on periodic arrangements of inclusions, have been proposed as candidates for a new generation of accelerating cells since they exhibit electromagnetic (EM) responses that can be highly selective in frequency. This property (and others related) arises from the formation of photonic bandgaps (PBGs), whereby multiple scattering of waves by periodic lattices of inclusions acts to prevent the propagation of EM waves within certain frequency ranges. In these structures, an open resonator can be easily created by introducing a lattice defect, e.g., by removing one inclusion. Unlike conventional closed cavities, such a point-defect PBG cavity can be designed to support only one bound mode (strongly localized within the defect region), and mostly extended HOMs. This suggests the possibility of using PBG-based cavities for effective HOM wakefield suppression, without the need for mode couplers or detuning. Indeed, a prototype of a large-gradient accelerator that relies on a metallic PBG structure has been successfully fabricated and experimentally characterized 2 . Superconducting prototypes have been also tested at 4 K, showing quality factors up to ∼ 10 5 , limited by radiation losses only 3 . All-dielectric or hybrid-dielectric PBG structures could also be used, thereby eliminating or reducing the characteristic metallic losses at the frequency of operation, even if at the expense of an increased radiative contribution 4 .Moreover, it should be observed that spatial periodicity is not an essential ingredient...

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One of the main problems in the design of high intensity rf accelerators is the presence of higher order modes (HOMs) which might degrade the beam quality. Accelerating cavities require HOMs suppression, whereas keeping a high quality factor (Q) for the fundamental mode. Both these requirements can be hardly met in closed metallic cavities, especially at high frequency where the configurations become cumbersome and technically unviable. Cavities based on photonic band gap (PBG) concepts showed that HOMs can be efficiently suppressed without affecting the fundamental mode. We present here simulations and measurements at room and cryogenic (77 K) temperature on some metallic (Copper) and metallo‐dielectric (Copper + sapphire) PBG based cavities, in order to study their rf behavior as resonant element in particle accelerators. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2486–2491, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22016

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Starting from the electric fields produced by a point charge and a dipole traveling inside a circular vacuum chamber, in this paper we derive a formalism for a complete set of equations that describe the electromagnetic fields and the longitudinal and transverse coupling impedances arising by the interaction of a beam with a perfectly conducting pipe in the case of elliptic geometry. The expressions, which are valid for any frequency and beam energy, are written in terms of expansions of Mathieu functions, allow to range from a circular geometry to the parallel plates, and show an interesting parallelism with the well-known expressions for a circular pipe. We also obtain that, under the approximation of low frequency, the formalism allows us to derive the Laslett coefficients for parallel plates, circular and elliptic beam pipe.