Abstracts Brochure - CERN

THPCH — Poster Session 29-Jun-06 16:00 - 18:00
Successful Bunched-Beam Stochastic Cooling in RHIC
Stochastic Cooling of high energy and high
frequency bunched beam has been demon- J.M. Brennan, M. Blaskiewicz (BNL)
strated in RHIC at 100 GeV. Narrowing of
the Schottky spectrum and shorting of the bunch length resulted from cooling the beam for 90 minutes. The purpose
of the stochastic cooling is to counteract the fundamental limit of the luminosity lifetime of heavy ions in RHIC
which is Intra-Beam Scattering. IBS drives transverse emittance growth and longitudinal de-bunching. The major
components of the system have been tested with heavy ion and proton beams in previous runs in RHIC, demonstrating
that the difficult challenges of high frequency bunched beam stochastic cooling can be overcome. The vexing
problem of pollution of the Schottky spectrum by coherent components is solved with optimized filtering and high
dynamic range low noise electronics. A set of 16 high-Q cavities is used to achieve adequate kicker voltage in the 5
to 8 GHz band. This technique exploits the bunched beam time structure to level the microwave power requirement
and enables the use of solid state amplifiers to drive the kickers. Because RHIC did not operate with heavy ions in
the FY06 run, the system was tested with specially prepared low intensity protons bunches of 2·10 9 particles.
Trapped Modes in Tapered Vacuum Chambers
Investigation of the electrodynamic properties
of tapered vacuum chambers uncovered A. Blednykh, J.-M. Wang (BNL)
the existence of trapped modes. Inside tapered
elliptic, rectangular and circular chambers, these modes were identified and classified. To verify the theoretical
and numerical calculations, a copper prototype of the rectangular chamber was manufactured for microwave measurements.
S-parameter and bead pull measurements were performed. The experimental and theoretical results are
compared.
Transverse Impedance of Small-gap Undulators for NSLS-II
We discuss the transverse impedance resulting
from the use of small-gap undulators in A. Blednykh, S. Krinsky, B. Podobedov, J.-M. Wang (BNL)
the proposed NSLS-II storage ring. For superconducting
undulators, the impedance arises due to the tapered elliptical vacuum chamber. For in-vacuum
permanent magnet devices, the impedance results from a more complex geometry. We consider both cases and report
results obtained using the electromagnetic simulation program GdfidL.
Transverse Impedance of Tapers with Axially Symmetric and Elliptical Cross Section
We investigate the transverse impedance of
circular and elliptical tapers. Analytical es- B. Podobedov, S. Krinsky (BNL)
timates for the transverse impedance at zero
frequency are obtained by extending a perturbation approach introduced by Stupakov. The perturbation theory is
compared to EM code simulators ABCI and GDFIDL. Using the analytic and numerical results, we develop useful
parameterizations for the low frequency impedance and discuss their relevance for single bunch transverse instabilities.
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