Abstracts Brochure - CERN

MOPCH131
MOPCH132
MOPCH133
26-Jun-06 16:00 - 18:00 MOPCH — Poster Session
SNS Ring Commissioning Results
M.A. Plum, A.V. Aleksandrov, S. Assadi, W. Blokland, I.E. Campisi,
P. Chu, S.M. Cousineau, V.V. Danilov, C. Deibele, G.W. Dodson, J.
Galambos, M. Giannella, S. Henderson, J.A. Holmes, D.-O. Jeon,
S.-H. Kim, T.A. Pelaia, T.J. Shea, A.P. Shishlo, Y. Zhang (ORNL)
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The Spallation Neutron Source (SNS) comprises
a 1.5-MW, 60-Hz, 1-GeV linac, an accumulator
ring, associated beam lines, and a
spallation neutron target. Construction began
in 1999 and the project is on track to be
completed in June 2006. By September 2005
the facility was commissioned up through the end of the superconducting linac, and in January 2006 commissioning
began on the High Energy Beam Transport beam line, the accumulator ring, and the Ring to Target Beam Transport
beam line up to the Extraction Beam Dump. In this paper we will discuss early results from ring commissioning
including a comparison of achieved vs. design beam machine parameters and the maximum beam intensity achieved
to date.
Coupled Maps for Electron and Ion Clouds
Contemporary electron cloud models and
U. Iriso, S. Peggs (BNL)
simulations reproduce second order phase
transitions, in which electron clouds grow
smoothly beyond a threshold from "off" to "on". In contrast, some locations in the Relativistic Heavy Ion Collider
(RHIC) exhibit first order phase transition behaviour, in which electron cloud related outgassing rates turn "on" or
"off" precipitously. This paper presents a global framework with a high level of abstraction in which additional
physics can be introduced in order to reproduce first (and second) order phase transitions. It does so by introducing
maps that model the bunch-to-bunch evolution of coupled electron and ion clouds. This results in simulations that
run several orders of magnitude faster, reproduce first order phase transitions, and show hysteresis effects. Coupled
maps also suggest that additional dynamical phases (like period doubling, or chaos) could be observed.
An Analytic Calculation of the Electron Cloud Linear Map Coefficient
The evolution of the electron density during
U. Iriso, S. Peggs (BNL)
multibunch electron cloud formation can often
be reproduced using a bunch-to-bunch
iterative map formalism. The coefficients that parameterize the map function are readily obtained by fitting to results
from compute-intensive electron cloud simulations. This paper derives an analytic expression for the linear map
coefficient that governs weak cloud behaviour from first principles. Good agreement is found when analytical results
are compared with linear coefficient values obtained from numerical simulations. This analysis is useful in predicting
thresholds beyond which electron cloud formation occurs, and thus in determining safety regions in parameter space
where an accelerator can be operated without creating electron clouds. The formalism explicitly shows that the
multipacting resonance condition is not a sine qua non for electron cloud formation.