Abstracts Brochure - CERN

THPCH — Poster Session 29-Jun-06 16:00 - 18:00
Further Studies on Betatron Sidebands due to Electron Clouds
We have observed vertical betatron sidebands
in the transverse beam spectra of
positron bunches at the KEKB LER which
are associated with the presence of electron
J.W. Flanagan, H. Fukuma, Y. Funakoshi, S. Hiramatsu, H. Ikeda,
K. Ohmi, K. Oide, M. Tobiyama (KEK)
clouds in single-beam studies*, and which are also associated with a loss of luminosity when the KEKB beams are in
collision**. The sidebands may be signals of a fast head-tail instability due to short-range wakes within the electron
cloud, providing a diagnostic for exploring the mechanism for transverse beam blow-up due to electron clouds. We
report here on further studies on the behavior of the sidebands under varying beam conditions, including varying
initial beam size below the beam blow-up threshold, chromaticity, RF voltage and fill pattern.
*J. W. Flanagan et al. PRL 94, 054801 (2005).**J. W. Flanagan et al. Proc. PAC05, p. 680 (2005).
The Effect of the Solenoid Field in Quadrupole Magnets on the Electron Cloud Instability
in the KEKB LER
The electron cloud instability which causes
vertical beam blowup in the KEKB LER is
largely suppressed by applying a weak solenoid
field to vacuum chambers in the drift
H. Fukuma, J.W. Flanagan, T. Kawamoto, T. Morimoto, K. Oide, M.
Tobiyama (KEK) F. Zimmermann (CERN)
region. However the blowup is still observed when the bunch spacing is reduced to 3.27 rf buckets or shorter. A
question is where the remaining electron clouds are. To investigate the electron clouds in the quadrupole magnets,
solenoids made of flat cables were developed and installed in 88 quadrupole magnets. The field strength of the
solenoid is 17 Gauss. The effect of the solenoid field on the blowup is now under beam study. So far no clear effect
of the solenoids on the luminosity and the sideband accompanied by the blowup is found. We report on the solenoid
system, the results of the experiments and comparison of the experimental results with simulations.
Dependence of Transverse Instabilities on Amplitude Dependent Tune Shifts
In the Photon Factory electron storage ring,
transverse instabilities have been observed in T. Miyajima, K. Harada, Y. Kobayashi, S. Nagahashi (KEK)
multi-bunch operation mode. The instabilities
can be suppressed by amplitude dependent tune shifts, which are induced by the sextupole, octupole and higher
order magnetic field. Since four octupole magnets have been installed in the PF ring, we can control the tune shifts,
which is caused by the octupole magnetic field, independently of chromaticities, which is caused by sextupole magnetic
field. In order to research the suppression mechanism of the instabilities, we measured the dependence of
the instabilities on the tune shifts, which are induced by the octupole field. The threshold of the tune shifts, which
suppress the instabilities, were observed in the measurement, and it depended on the filling pattern of the bunch
train and the beam current per bunch. In addition, we will present the results of the measurement before and after
the reconstruction for the straight-sections upgrade at the PF ring, which was carried out in 2005.
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