Abstract
A coupled-bunch instability caused by an electron cloud has been observed in the KEKB LER. The time evolution of the instabilities just after the turning-off the transverse bunch feedback was recorded with several weak solenoid-field conditions, which are used to suppress the vertical blowup of the beam size due to the electron cloud. The mode spectra and their growth rates of the coupled-bunch instabilities were compared with simulations of electrons moving in drift space, a weak solenoid field, and a strong bending field. Mode spectra without a solenoid field support the model where the instability is dominated by the electron clouds in the drift space with a lower secondary yield of photoelectron ${\ensuremath{\delta}}_{2,\mathrm{max}}=1.0$ rather than 1.5. With the solenoid field, the behavior of unstable modes and the growth rate with the strength of solenoid field also support the simulation with lower secondary emission yield.
Highlights
In a multibunch positron storage ring with a narrow bunch spacing, electrons created by the synchrotron radiation and the secondary emission due to the absorption of primary electrons at the vacuum chamber surface build up in the vacuum chamber by successive production
We measured the unstable modes of the coupled-bunch instability caused by electron clouds
We observed the drastic change of the mode spectra, the retarded mode to advanced mode, for switching on to off of the solenoid magnets
Summary
In a multibunch positron storage ring with a narrow bunch spacing, electrons created by the synchrotron radiation and the secondary emission due to the absorption of primary electrons at the vacuum chamber surface build up in the vacuum chamber by successive production. Transverse coupled-bunch instabilities caused by electron clouds have been observed in many positron storage rings [1,2,3,4]. In KEKB LER, we observed strong transverse coupled-bunch instabilities, and an increase of the vertical beam size. Both of them are believed to be caused by the electron cloud effect [5]. Bunch-by-bunch feedback systems have been used to suppress any coupledbunch instabilities [6] Both systems showed excellent performance to suppress the electron cloud effect, resulting to achieve a very high luminosity of larger than 1:5 1034 cmÿ sÿ1 [7].
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