Abstract
We discuss the nature of the electromagnetic fields excited by the beams in the beam pipe of an interaction region. In trying to find an optimum geometry for this region with a minimum of electromagnetic wave excitation, we have discovered one mode, which remains even in a very smooth geometry. This mode has a longitudinal electrical component and can be easily excited by the beam. By analyzing the structure of this mode we have found a way to absorb this mode. The work was done in connection with a proposal for a future electron-positron collider.
Highlights
One of the ways to reach a higher luminosity in colliders is to increase the beam currents
During the design of the interaction region (IR) we can try to diminish these effects by making the geometry of the IR very smooth and reduce the number of trapped higher order modes (HOMs). In analyzing this problem we found that one mode will still stay in the IR even when the geometry of the metal chamber is very smooth
In this paper we describe a study focused on reducing the beam impedance of the interaction region beam pipe in the proposed Future Circular electron-positron Collider (FCC-ee) [15]
Summary
One of the ways to reach a higher luminosity in colliders is to increase the beam currents. It can happen that the mode resonant frequency may be equal to the frequency of some of the revolution harmonics In this case, the amplitude of the electromagnetic field of this mode will grow until it reaches the value determined by the loaded quality factor together with the loss factor of this mode. HOMs have a large quality factor, of order more than 1000, and the frequency of the mode is in resonance with some harmonic of the revolution frequency, local heating in the IR can reach tens of kW of power. We carefully describe this mode and its field structure This follows by studying three different beam pipe models proposed for the FCC-ee IR.
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