Abstract
Photonic crystal (PhC) defect cavities that support an accelerating mode tend to trap unwanted higher-order modes (HOMs) corresponding to zero-group-velocity PhC lattice modes at frequencies near the top of bandgaps. The effect is explained quite generally by photonic band and perturbation theoretical arguments. Transverse wakefields resulting from this effect are observed (via simulation) in a 12 GHz hybrid dielectric PhC accelerating cavity based on a triangular lattice of sapphire rods. These wakefields are, on average, an order of magnitude higher than those in the 12 GHz waveguide-damped Compact Linear Collider copper cavities. The avoidance of translational symmetry (and, thus, the bandgap concept) can dramatically improve HOM damping in PhC-based structures.
Highlights
Photonic crystals (PhCs) have recently attracted interest from the accelerator community [1,2,3,4,5,6,7,8,9,10,11,12] for the following reasons: (1) PhCs enable the construction of accelerator cavities using dielectric materials. (2) PhC cavities intrinsically provide a wakefield damping mechanism
This basic concept has led to the design of many defect cavity PhC acceleration schemes, a sample of which can be seen in the list of references above
These subtleties were uncovered in a thorough comparison between wakefield damping in a 12 GHz hybrid PhC cavity based on a triangular lattice of sapphire rods (Fig. 4) and a 12 GHz cavity from the main linac of the Compact Linear Collider, (CLIC) which uses side-coupled waveguides to damp higherorder modes (HOMs) [21,22,23,24]
Summary
Photonic crystals (PhCs) have recently attracted interest from the accelerator community [1,2,3,4,5,6,7,8,9,10,11,12] for the following reasons: (1) PhCs enable the construction of accelerator cavities using dielectric materials. (2) PhC cavities intrinsically provide a wakefield damping mechanism. A simple argument from photonic band theory gives reason to believe that PhC cavities will have low wakefields It says that, given a PhC with a band gap, a defect cavity will confine only one (ideally accelerating) mode to the defect; all other higher-frequency modes will propagate through the crystal and contribute minimally to the wakefields. This work looks in more detail at wakefield suppression in PhC cavities and reveals subtleties that can undermine the benefits implied by the simple band gap argument These subtleties were uncovered in a thorough comparison between wakefield damping in a 12 GHz hybrid PhC cavity based on a triangular lattice of sapphire rods (Fig. 4) and a 12 GHz cavity from the main linac of the Compact Linear Collider, (CLIC) which uses side-coupled waveguides to damp HOMs (see Fig. 1) [21,22,23,24]. Appendix B discusses accelerating mode figures of merit (peak surface fields, accelerating efficiency, etc.) for each cavity type considered in this work
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