Photonic band gap higher order mode coupler for the international linear collider

Abstract

A photonic band gap (PBG) higher-order-mode (HOM) coupler is proposed as an Alternative Configuration Design (ACD) for the HOM coupler for the International Linear Collider (ILC). The PBG HOM coupler uses a two-dimensional triangular PBG structure with good axial symmetry. Simulation studies of a PBG HOM coupler show that it maintains the operating mode at 1.3 GHz with 10 10 > Q. While a PBG HOM coupler provides superior damping for all the higher order modes, in principle, the effectiveness of HOM damping remains an open subject of future investigations. INTORDUCTIONAs reported in the ILC Base Configuration Design (BCD) [1], the accelerated ILC beam, if similar to the proposed in the TDR beam of the TESLA 500 GeV collider, will generate spectrum up to 0.4 THz. The beamdeposited power in a cryo-module housing 12 TTF-like 9-cell structures will be ~24 W if no synchronous excitation of parasitic modes takes place. A large fraction of the beam-deposited power (17.4 W) is in propagating modes above 5 GHz. The beam-deposited power must be removed from cryo-modules to avoid an additional heat load in 2K environment and to maintain the high quality of the accelerated beam (preserving the low emittance). This will be achieved by means of two kinds of devices: HOM couplers and beam line absorbers. Beam dynamics simulations showed that preservation of the low emittance demands suppression of high impedance dipoles to ext Q of the order of 5 10 . This suppression will also ensure stable operation if resonant excitation of some high impedance modes takes place.The ILC BCD employs the suppression scheme as proposed in the TESLA TDR [2]. The careful studies of HOM suppression at TESLA test facility (TTF) linac showed that almost all dipole modes, but two, are well damped and satisfy the specification. Besides its ineffectiveness of damping the two dipole modes, the BCD HOM coupler has other two shortcomings: a) mode asymmetry, and b) high cost. Further improvement in the HOM coupler is still required.We propose a PBG HOM coupler as an Alternative Configuration Design (ACD) for the ILC HOM coupler. The PBG HOM coupler uses a two-dimensional triangular PBG structure which is composed of metalic rods. Such a structure has superior axial symmetry. It allows all unwanted high-order modes to leak out of the structure while preserving high Q of the operating mode. Potentially, the cost of building PBG HOM couplers can be reduced due to the simple geometry of the structure.The goals of our HFSS simulation studies of the PBG HOM coupler are to determine the feasibility of confining the 1.3 operating mode in the PBG HOM coupler and to explore the effectiveness of HOM damping. The present paper reports the progress in achieving the first goal. PBG HOM COUPLER DESIGNThe schematic layout of the PBG HOM coupler is shown in Fig. 1(a). The PBG HOM couplers are mounted to both ends of the 9-cell cavity. Use of a triangular superconducting lattice design with four rows of rods is chosen for the PBG HOM coupler as shown in Fig.1 (b). The radius ...

Towards photonic crystal and metamaterial high-power microwave applications

“…There is now a growing number of research examining the use of PBG based structures for these high power applications.PBG structures have many advantages and can exceed the performance of conventional microwave structures. For example: the authors of [7] demonstrate that PC resonant cavities with a Q in excess of 10 9 can be realized, and used to accelerate particles with gradient > 30M V /m [8]; Sirigiri in [9] demonstrated a gyrotron oscillator using a PBG structure; Xu and Seviour have considered a 2 cavity PBG klystron [10]; and Smirnov and Yu have considered theoretically an output cavity of a multibeam Klystron [11]; and Zhou have designed a PC coupler for the ILC [12]. Particle accelerators generally operate at frequencies below 3 GHz.…”