Abstract
In-vacuum undulators have been widely operated in many synchrotron radiation facilities across the world. They usually are required to be operated at a smaller magnet gap than those of other undulators. Thus, operating challenges including impedance effects on the stored electron beam are introduced by these devices. In this paper, we report the efforts in solving the problem of coupled-bunch instabilities caused by an in-vacuum undulator in the SPEAR3 storage ring. Using beam based measurements, cold rf measurements, and numerical simulations, the source of the beam instabilities is characterized as trapped modes in the vacuum chamber. Using numerical models, we explored several approaches to reduce the strength of the trapped modes and found that ferrite dampers were the most effective and simplest way for mode damping in our SPEAR3 in-vacuum undulator. The results of the first rf cold measurement on an in-vacuum undulator equipped with these ferrite dampers agree well with numerical simulations.
Highlights
Since the first in-vacuum undulator (IVU) was designed and operated in KEK [1] in the early 1990s, IVUs have been installed in various synchrotron radiation (SR) light sources around the world in order to expand the radiation spectrum to higher energies without the need to increase the beam energy [2,3,4,5,6,7,8]
In an IVU, the permanent magnet rows are inside a vacuum chamber, so the magnet gap is free from physical limitations and can be designed to be as small as possible
At SPEAR3, we observed coupled-bunch instabilities caused by one of our IVUs, we were motivated to conduct thorough studies on their sources. This led us to conduct some pioneering R&D studies in understanding and controlling the beam coupling impedances caused by the trapped rf modes in the IVU chamber
Summary
Since the first in-vacuum undulator (IVU) was designed and operated in KEK [1] in the early 1990s, IVUs have been installed in various synchrotron radiation (SR) light sources around the world in order to expand the radiation spectrum to higher energies without the need to increase the beam energy [2,3,4,5,6,7,8]. We discovered that the beam size increase was caused by transverse coupledbunch instabilities driven by the trapped resonant modes inside the IVU chamber. Passive methods have been used to damp problematic rf modes caused by the ridge waveguide structure for the storage ring These rf modes, found earlier in some beam chambers in the Advanced Photon Source (APS) due to the small gap between the beam chamber and the antechamber [26], corrupted the output from the beam position monitors. Our study will be beneficial to the SR community in understanding the trapped rf modes inside an IVU chamber and preventing possible beam instabilities caused by them. V, we briefly discuss our study of possible alternate solutions to damp the trapped modes in an IVU chamber
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