G. Rumolo

Transverse impendance of LHC collimators

The transverse impedance in the LHC is expected to be dominated by the numerous collimators, most of which are made of Fibre-Reinforced-Carbon to withstand the impacts of high intensity proton beams in case of failures, and which will be moved very close to the beam, with full gaps of few millimetres, in order to protect surrounding super-conducting equipments. We present an estimate of the transverse resistive-wall impedance of the LHC collimators, the total impedance in the LHC at injection and top energy, the induced coupled-bunch growth rates and tune shifts, and finally the result of the comparison of the theoretical predictions with measurements performed in 2004 and 2006 on a prototype collimator installed in the SPS.

Transverse Mode‐Coupling Instability in the CERN Super Proton Synchrotron

A vertical single‐bunch instability has been observed in 2003 right after injection at 26 GeV/c in the CERN Super Proton Synchrotron (SPS). High‐intensity proton bunches (∼1.2 1011 p/b) with low longitudinal emittance (∼0.2 eVs) are affected by heavy losses after less than one synchrotron period. Such phenomenon has already been observed with leptons in many machines, e.g. in the SPS, or with protons at transition, e.g. in the CERN Proton Synchrotron (PS). However, to the authors’ knowledge, it is the first time with protons far from transition. The absence of transverse mode‐coupling instability in hadron machines is generally explained by three mechanisms: (i) the intensity threshold for the longitudinal microwave instability is generally lower than for the transverse mode‐coupling instability, (ii) the intensity threshold due to mode‐coupling between the two lowest azimuthal modes increases with space charge, and (iii) the intensity threshold increases with bunch length (in the long‐bunch regime). In t...

Electron cloud studies for KEKB

Electron-cloud build up, incoherent tune spread and electron-induced beam instability are likely to be responsible for the vertical beam-size increase observed at the KEKB Low Energy Ring (LER). We report on recent simulations and analytical estimates, addressing the electron-cloud evolution for various magnet configurations, comparing the beam size blow up predicted by three different simulation models (micro-bunches, multi-particle tracking with soft Gaussian approximation, particle-in-cell), and finally discussing analytical expressions for the instability threshold.

Beam Instabilities in Hadron Synchrotrons

Beam instabilities cover a wide range of effects in particle accelerators and they have been the subjects of intense research for several decades. As the machines performance was pushed new mechanisms were revealed and nowadays the challenge consists in studying the interplays between all these intricate phenomena, as it is very often not possible to treat the different effects separately. The aim of this paper is to review the main mechanisms, discussing in particular the recent developments of beam instability theories and simulations.

Scrubbing: Expectations and Strategy, Long Range Perspective

Electron cloud buildup simulations and machine experience during Run 1 showed that electron cloud effects could significantly limit the performance of the LHC when operating with 25 ns bunch spacing. Beam induced scrubbing will have to be used to lower the Secondary Electron Yield (SEY) of the beam chambers and therefore reduce electron cloud induced pressure rises, heat load and beam degradation. This contribution reviews the experience accumulated on electron cloud effects during Run 1 and define a possible scrubbing strategy to allow operation with 25 ns beams in 2015. Several measures taken during LS1 should allow for an improved scrubbing efficiency compared with Run 1. Moreover, the potential of using a dedicated scrubbing scheme based on the doublet beam, following the promising SPS tests in 2012, is described and analyzed. To conclude, possible alternatives of operation scenarios are defined, which will depend on the degree of success of the scrubbing runs.

Performance potential of the injectors after LS1

The main upgrades of the injector chain in the framework of the LIU Project will only be implemented in the second long shutdown (LS2), in particular the increase of the PSB-PS transfer energy to 2GeV or the implementation of cures/solutions against instabilities/e-cloud effects etc. in the SPS. On the other hand, Linac4 will become available by the end of 2014. Until the end of 2015 it may replace Linac2 at short notice, taking 50MeV protons into the PSB via the existing injection system but with reduced performance. Afterwards, the H− injection equipment will be ready and Linac4 could be connected for 160MeV H− injection into the PSB during a prolonged winter shutdown before LS2. The anticipated beam performance of the LHC injectors after LS1 in these different cases is presented. Space charge on the PS flat-bottom will remain a limitation because the PSB-PS transfer energy will stay at 1.4GeV. As a mitigation measure new RF manipulations are presented which can improve brightness for 25 ns bunch spacing, allowing for more than nominal luminosity in the LHC.

A SIMULATION STUDY OF THE ELECTRON CLOUD IN THE EXPERIMENTAL REGIONS OF THE LHC

The LHC experimental regions (ATLAS, ALICE, CMS and LHCb) are characterised by having a variable geometry, non-uniform magnetic field, and the presence of two beams that will collide at the Interaction Point (IP). A detailed study of electron multipacting in the experimental chambers is needed to establish the pressure increase due to electron stimulated desorption, especially critical in the experimental regions. Furthermore, knowledge of the predicted electron cloud density all along the experimental regions will allow for an estimation of its possible effects on the beam stability.

Transverse behaviour of the LHC proton beam in the SPS: an update

During the 1999 SPS run, strong transverse instabilities were observed with the LHC beam. Both the instability characteristics and the identical threshold current as for beam-induced electron multipacting led to consider the interaction of the beam with the electron cloud as a likely source. In 2000, we have measured the dependence of beam motion, beam loss, and emittance growth on bunch intensity, number of bunches, octupole strength, chromaticity, and gaps in the bunch train. We report on these recent studies and compare the beam observations with simulations of electron cloud build up and electron-induced single-bunch instabilities.

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 8b

HOW TO MAXIMIZE THE HL-LHC PERFORMANCE *

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