Origin and reduction of wakefields in photonic crystal accelerator cavities

TL;DR

This paper investigates the origin of wakefields in photonic crystal accelerator cavities, explains their cause through bandgap theory, and demonstrates how breaking symmetry can significantly reduce these wakefields.

Contribution

It provides a theoretical explanation for HOM trapping in PhC cavities and proposes a method to improve HOM damping by avoiding translational symmetry.

Findings

Wakefields in PhC cavities are linked to zero-group-velocity modes at the bandgap top.

Observed wakefields are an order of magnitude higher than in traditional copper cavities.

Breaking translational symmetry enhances HOM damping in PhC structures.

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 the top of the bandgap. The effect is explained quite generally from photonic band and perturbation theoretical arguments. Transverse wakefields resulting from this effect are observed in a 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 waveguide-damped Compact Linear Collider (CLIC) copper cavities. The avoidance of translational symmetry (and, thus, the bandgap concept) can dramatically improve HOM damping in PhC-based structures.