Abstracts Brochure - CERN

THPCH — Poster Session 29-Jun-06 16:00 - 18:00
Impedance and Beam Stability Study at the Australian Synchrotron
We present the preliminary results of an impedance
study of the Australian Synchrotron R.T. Dowd, M.J. Boland, G. LeBlanc, M.J. Spencer, Y.E. Tan (ASP)
storage ring. Beam stability thresholds have
been determined and an overall impedance budget set. Broad-band impedance has been evaluted for various components
of the vacuum chamber, using both analytical formulae and results from MAFIA simulations. Narrow band
resonances have also been investigated, with particular attention paid to higher order modes in the RF cavities and
their effect on multi-bunch instabilities.
Instability Studies Using Evaluated Wake Fields and Comparison with Observations at
SOLEIL
Beam instability is predicted for SOLEIL using
the impedance data obtained through R. Nagaoka (SOLEIL)
component-wise numerical evaluations. The
paper also attempts to make the first comparison with measurements. A key issue for SOLEIL has been to acquire
precise knowledge of impedance up to a few tens of GHz, due to short bunches, of chambers which are essentially
3D and additionally NEG coated to a large part, which is expected to enhance the reactive part of the resistive-wall
impedance. The predictability of instabilities with the data attained thus becomes a large concern. Wake potentials
computed with a 3D code are transformed to impedances and decomposed into a series of resonators, inductive and
resistive components, to deduce the wake functions, while for NEG coated chambers, they are numerically Fourier
transformed from analytically derived impedances. Both time and frequency domain simulations are performed to
predict the longitudinal and transverse instabilities in single bunch, as well as resistive-wall instabilities in multibunch
as a function of chromaticity. A multibunch tracking is also performed to investigate the filling pattern dependence
of the latter.
Recent Studies of Geometric and Resistive-wall Impedance at SOLEIL
Coupling impedance studies are of great importance
for SOLEIL not only to avoid beam R. Nagaoka, J.-C. Denard, M.-P. Level (SOLEIL)
instability, but also to ensure protection of a
concerned chamber from EM fields excited by the beam. This paper deals with components that required particularly
such efforts, which include BPMs, ceramic chambers, and a vertical scraper. The heat deposited in the BPM buttons
is investigated as a function of the gap between a button and an electrode, button diameter and thickness. High temperatures
on a vacuum tight feed-through would be a problem, affecting the measurement stability at high currents.
Coupling of a trapped mode among successively passing bunches is also investigated. To evaluate the heat deposited
in a titanium coated ceramic chamber, its impedance is analytically solved using the field matching technique. The
solution obtained justifies the image current model that assumes a constant image current density across the coating
when the skin depth is greater than the coating thickness. The azimuthal image current distribution is pursued with
Piwinski’s formalism for flat chambers. The paper also deals with components not treated earlier such as a stripline.
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