Abstracts Brochure - CERN

WEOFI — Beam Dynamics and Electromagnetic . . . 28-Jun-06 11:30 - 12:30
WEOFI — Beam Dynamics and Electromagnetic Fields
Beam Dynamics Measurements in the Vicinity of a Half-integer Resonance
The operating point of the betatron tune set
near a half-integer is a crucial parameter to
make high luminosity in electron/positron
ring colliders. Dynamic beam-beam effects
T. Ieiri, J.W. Flanagan, H. Fukuma, H. Ikeda, Y. Ohnishi, K. Oide,
M. Tobiyama (KEK)
would change the optics parameters of the colliders, depending on the betatron tune and the beam-beam parameter.
On the other hand, existence of the half-integer stopband makes the beam unstable. Therefore, beam behavior near
a half-integer might provide interesting issues from the viewpoint of beam dynamics. We measured a frequency
response of the beam across a half-integer for measuring the betatron tune at KEKB. A sharp spike just at a halfinteger
was observed in the tune spectrum. We believe that the spectrum would be a nonlinear resonance caused
by some off-momentum particles in a bunch, not by a coherent motion of a whole bunch. The horizontal beam size
measured using a synchrotron radiation monitor indicated a slight increase when the tune approached a half-integer.
The variations in the beam size are discussed, considering both dynamic beam-beam effects and a beta beat due to
the half-integer stopband.
RF Phase Modulation Studies at the LNLS Electron Storage Ring
In this work we present a set of measurements
of the effectiveness of RF phase mod- N.P. Abreu, R.H.A. Farias, P.F. Tavares (LNLS)
ulation on the second harmonic of the RF
frequency as a mechanism to damp longitudinal coupled-bunch instabilities. We also propose a theoretical model
of the damping mechanism, in which the increase of the spread in synchrotron frequencies inside the bunches produced
by phase modulation is responsible for damping the centroid dipolar coherent motion caused by an external
excitation, which could be a Higher Order Mode (HOM) of the RF cavities driving the coupled bunch motion. We
measured the coherent synchrotron oscillation damping of a single bunch under two circumstances, with and without
phase modulation, and determined the amount of extra damping due to the modulation. With this experiment we
could also measure the frequency of small oscillations around the stable islands formed by phase modulation and
its behavior when the RF phase modulation amplitude and frequency are changed. We performed measurements of
Beam Transfer Function (BTF) to observe the effects of phase modulation over the stable area for coherent oscillations
and compared the results with a theoretical model.
Beam Dynamics Simulation in Electron Rings in the Regime of Strong RF Focusing
The concept of strong RF focusing has been
recently proposed to obtain locally short
bunches in electron/positron colliders, by
modulating the longitudinal bunch dimen-
L. Falbo (INFN-Pisa) D. Alesini, M. Serio (INFN/LNF) M. Migliorati
(Rome University La Sapienza)
sions along the rings. To study the single bunch dynamics, a macroparticle numerical code has been written which
simulates the effects of the objects generating broad band impedance along the ring and the effects of the coherent
synchrotron radiation in dipoles and wigglers. The obtained results are shown and discussed.
Chair: P. Emma (SLAC, Menlo Park, California) 271
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