391 research outputs found
Electron cloud in the CERN accelerators (PS, SPS, LHC)
Several indicators have pointed to the presence of an Electron Cloud (EC) in
some of the CERN accelerators, when operating with closely spaced bunched
beams. In particular, spurious signals on the pick ups used for beam detection,
pressure rise and beam instabilities were observed at the Proton Synchrotron
(PS) during the last stage of preparation of the beams for the Large Hadron
Collider (LHC), as well as at the Super Proton Synchrotron (SPS). Since the LHC
has started operation in 2009, typical electron cloud phenomena have appeared
also in this machine, when running with trains of closely packed bunches (i.e.
with spacings below 150ns). Beside the above mentioned indicators, other
typical signatures were seen in this machine (due to its operation mode and/or
more refined detection possibilities), like heat load in the cold dipoles,
bunch dependent emittance growth and degraded lifetime in store and
bunch-by-bunch stable phase shift to compensate for the energy loss due to the
electron cloud. An overview of the electron cloud status in the different CERN
machines (PS, SPS, LHC) will be presented in this paper, with a special
emphasis on the dangers for future operation with more intense beams and the
necessary countermeasures to mitigate or suppress the effect.Comment: 8 pages, contribution to the Joint INFN-CERN-EuCARD-AccNet Workshop
on Electron-Cloud Effects: ECLOUD'12; 5-9 Jun 2012, La Biodola, Isola d'Elba,
Ital
PyECLOUD and build-up simulations at CERN
PyECLOUD is a newly developed code for the simulation of the electron cloud
(EC) build-up in particle accelerators. Almost entirely written in Python, it
is mostly based on the physical models already used in the ECLOUD code but,
thanks to the implementation of new optimized algorithms, it exhibits a
significantly improved performance in accuracy, speed, reliability and
flexibility. Such new features of PyECLOUD have been already broadly exploited
to study EC observations in the Large Hadron Collider (LHC) and its injector
chain as well as for the extrapolation to high luminosity upgrade scenarios.Comment: 6 pages, contribution to the Joint INFN-CERN-EuCARD-AccNet Workshop
on Electron-Cloud Effects: ECLOUD'12; 5-9 Jun 2012, La Biodola, Isola d'Elba,
Ital
Summary of session on Beam Losses, Halo Generation and collimation
The session on beam losses, halo generation and collimation is the first of two sessions of the BEAM07 Workshop, which were devoted to specific CERN-GSI subjects and were meant to be the follow up of last year’s CERNGSI Bilateral Meeting on Collective Effects, which took place on March 30-31, 2006 at the GSI-Darmstadt
Simulation study of electron cloud build up in the SPS MKD kickers
During the 2008 run, an unusual behavior characterizing pressure and temperature increase in some of the dump kickers of the SPS was noticed. In particular, it was observed that 1) the MKDV2 kicker would exhibit maximum heating with 75 ns spaced LHC beams and 2) the pressure rise was specially critical in MKDV1 in presence of 50 ns spaced LHC beams [1]. While the anomalous heating of MKDV2 with 75 ns beams could be tentatively explained by the denser beam current spectrum that would more likely hit one of the kicker impedance peaks, the fast pressure rise in MKDV1 with 50 ns spaced beams was ascribed to a surface effect, namely beam induced multipacting leading to electron cloud formation. This report summarizes a simulation study that was done in order to check whether the electron cloud behavior in the dump kickers could explain the experimental observations
Operational beams for the LHC
The variety of beams, needed to set-up in the injectors as requested in the
LHC, are reviewed, in terms of priority but also performance expectations and
reach during 2015. This includes the single bunch beams for machine
commissioning and measurements (probe, Indiv) but also the standard physics
beams with 50 ns and 25 ns bunch spacing and their high brightness variants
using the Bunch Compression Merging and Splitting (BCMS) scheme. The required
parameters and target performance of special beams like the doublet for
electron cloud enhancement and the more exotic 8b4e beam, compatible
with some post-scrubbing scenarios are also described. The progress and plans
for the LHC ion production beams during 2014-2015 are detailed. Highlights on
the current progress of the setting up of the various beams are finally
presented with special emphasis on potential performance issues across the
proton and ion injector chain.Comment: Submitted for publication in a CERN Yellow Report (YR
Benchmarking headtail with electron cloud instabilities observed in the LHC
After a successful scrubbing run in the beginning of 2011, the LHC can be
presently operated with high intensity proton beams with 50 ns bunch spacing.
However, strong electron cloud effects were observed during machine studies
with the nominal beam with 25 ns bunch spacing. In particular, fast transverse
instabilities were observed when attempting to inject trains of 48 bunches into
the LHC for the first time. An analysis of the turn-by-turn bunch-bybunch data
from the transverse damper pick-ups during these injection studies is
presented, showing a clear signature of the electron cloud effect. These
experimental observations are reproduced using numerical simulations: the
electron distribution before each bunch passage is generated with PyECLOUD and
used as input for a set of HEADTAIL simulations. This paper describes the
simulation method as well as the sensitivity of the results to the initial
conditions for the electron build-up. The potential of this type of simulations
and their clear limitations on the other hand are discussed.Comment: 7 pages, contribution to the Joint INFN-CERN-EuCARD-AccNet Workshop
on Electron-Cloud Effects: ECLOUD'12; 5-9 Jun 2012, La Biodola, Isola d'Elba,
Ital
Update on fast ion instability simulations for the CLIC main linac
The specification for vacuum pressure in the CLIC electron Main Linac critically depends on the fast ion instability. In fact, the maximum tolerable pressure value in the pipe of the Main Linac is dictated by the threshold above which the fast ion instability sets in over a CLIC bunch train. Previous calculation based on ion generation from scattering ionization of the residual gas alone showed that, due to the loss of the trapping along the linac caused by the beam size shrinking from acceleration, a pressure as high as 10 nTorr could be accepted, higher than the tolerable value in the long transfer line. However, since the accelerated beam becomes transversely very small, its electric field can reach values above the field ionization threshold. When this happens, the whole space region with a sufficiently high electric field gets instantly fully ionized by the first bunch and the effect on the bunch train could be severe. We have modeled field ionization in our simulation code FASTION and re-evaluated the onset of fast ion instability in the Main Linac
High intensity effects of fixed target beams in the CERN injector complex
The current fixed target (FT) experiments at CERN are a complementary approach to the Large Hadron Collider (LHC) and play a crucial role in the investigation of fundamental questions in particle physics. Within the scope of the LHC Injectors Upgrade (LIU), aiming to improve the LHC
beam production, the injector complex will be significantly upgraded during the second Long Shutdown (LS2). All nonLHC beams are expected to benefit from these upgrades. In this paper, we focus on the studies of the transverse instability in the Proton Synchrotron (PS), currently limiting the intensity of Time-Of-Flight (ToF) type beams, as well as the prediction of the impact of envisaged hardware modifications. A first discussion on the effect of space charge on the observed instability is also being presented
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