Rf distribution system for a set of standing-wave accelerator structures

Sami G Tantawi

doi:10.1103/physrevstab.9.112001

Abstract

In this paper, we study the rf feeding system for a set of standing-wave accelerator structures. To avoid the initial reflections produced by the structures, sometimes these structures are fed in pairs through a four-port 3-dB hybrid. We present an extension to this system for an arbitrary number of accelerator structures and show it is always possible to cancel the reflection back to the source. The necessary and sufficient condition for this to happen depends only on the spacing between accelerator structures. In this system, the structures are not fed in a binary hierarchal system, rather in series with a set of directional couplers designed to extract an equal amount of power to each accelerator structure in the set. We study the sensitivity of such a system to errors resulting from the differences in accelerator structure spacing. We also study the sensitivity of the system to component imperfections, such as the finite directivity of the directional couplers, and the residual reflections from the loads that are attached to these couplers. We also study the system under fault conditions, such as a breakdown in an accelerator structure or a feed waveguide.

Highlights

Unlike traveling-wave accelerator structures, standingwave accelerator structures give a high-reflected signal at the beginning of the input pulse
To avoid the initial reflections produced by the structures, sometimes these structures are fed in pairs through a four-port 3-dB hybrid
The structures are not fed in a binary hierarchal system, rather in series with a set of directional couplers designed to extract an equal amount of power to each accelerator structure in the set

Summary

INTRODUCTION

Unlike traveling-wave accelerator structures, standingwave accelerator structures give a high-reflected signal at the beginning of the input pulse This is usually managed by using a microwave circulator at the input port of the structure. If the distance between one of the proper ports of the 3-dB hybrid and the first accelerator structure is equal to the distance between the second accelerator structure and the second proper 3-dB hybrid port, the reflected signal is directed to the fourth port of the hybrid where it is absorbed in a load. This isolates the structures from the rf source. This type of rf distribution system is cost effective in long linacs containing many accelerator structures, such as the ILC [3], because it eliminates circulators and simplifies the rf waveguide distribution system

SYSTEM ANALYSIS

Residual reflection due to structure failure

Residual reflection and coupling between structures due to component errors

CONCLUSION