Mode confinement in photonic quasicrystal point-defect cavities for particle accelerators

Abstract

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...

Isotropic properties of the photonic band gap in quasicrystals with low-index contrast

“…Like photonic crystals, PQC's can demonstrate directive emission, 5 mode confinement, 6 and superlensing. 7 A one-dimensional photonic band gap has been observed in dielectric multilayers stacked according to a Fibonacci series. 8 Dielectrics arranged according to quasiperiodic geometries such as octagonal (8-fold), decagonal (10-fold), and dodecagonal (12-fold) are shown to have a two-dimensional PBG.…”

Calculation of wakefields in a 17 GHz beam-driven photonic band-gap accelerator structure

“…This radiative damping decreases the Q factor of the dipole modes, making the wake damp faster. This kind of PBG structure has been extensively researched theoretically and experimentally [23][24][25]. A 17 GHz six-cell traveling-wave PBG accelerator structure has previously been designed, simulated, and tested at the MIT Plasma Science and Fusion Center (PSFC) [8,9].…”

X-band photonic band-gap accelerator structure breakdown experiment

“…Recent advances in the study of photonic structures for accelerator applications include research on hybrid structures consisting of metallic and dielectric components [10,11]; research on quasicrystals [12]; research on disordered photonic structures [13]; research on truncated, optimized photonic structures [14]; and theoretical research on wakefields [15].…”