Possibility of comb-type rf shield structure for high-current accelerators

Y Suetsugu,K Shibata,M Shirai

doi:10.1103/physrevstab.6.103201

Y Suetsugu, K Shibata + Show 1 more

Open Access

https://doi.org/10.1103/physrevstab.6.103201

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Abstract

A novel rf shield structure for bellows chambers or gate valves in high-current accelerators is proposed. The rf shield has no thin fingers, but nested comb teeth instead at the inner surface of the beam duct. The comb-type rf shield has a higher thermal strength structurally than the conventional finger-type one. A simulation also indicates its lower impedance. Especially, the leak of a TE modelike higher order mode through the axial slits, which causes mainly heating of these components, is almost suppressed due to thick teeth. An abrasion test in a vacuum using a test piece of the comb-type rf shield shows less dust generation compared to the finger type. Another preliminary test transmitting 508 MHz microwaves through a prototype rf shield in the atmosphere found no abnormal heating or arcing. A trial model of bellows chambers with the comb-type rf shield will be installed and tested using a real beam in the KEK $B$-factory positron ring this year.

Highlights

In an accelerator vacuum system, several vacuum components, like bellows chambers and gate valves, which would have a large axial gap otherwise, are usually equipped with rf shield structures inside
The conventional rf shield is a finger type, which consists of many narrow and thin fingers surrounding the inner surface of the beam duct [1,2,3,4,5,6,7]
The temperature rise of the beam duct, should be controlled. This structural restriction will not raise any problem in an application to the gate valve, where the stroke is originally small and no bending or tilting is necessary, in principle

Summary

INTRODUCTION

In an accelerator vacuum system, several vacuum components, like bellows chambers and gate valves, which would have a large axial gap otherwise, are usually equipped with rf (radio frequency) shield structures inside. The high bunch current and the short bunch length lead to intense HOM excitation at various vacuum components These result in heating of the fingers due to Joule loss. For future higher luminosity accelerators with storage currents of over 10 A [11,12], the heating or damage of the finger-type rf shield can be a serious hardware problem. The temperature rise of the beam duct, should be controlled This structural restriction will not raise any problem in an application to the gate valve, where the stroke is originally small and no bending or tilting is necessary, in principle

Loss factors

Electric field between teeth

HOM outside of the rf shield

Abrasion test in a vacuum

Findings

Heating test