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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...

HOW TO MAXIMIZE THE HL-LHC PERFORMANCE *

This contribution presents an overview of the parameter space for the HL-LHC [1] upgrade options that would maximize the LHC performance after LS3. The analysis is assuming the baseline HL-LHC upgrade options including among others, 25ns spacing, LIU [2] parameters, large aperture triplet and matching-section magnets, as well as crab cavities. The analysis then focuses on illustrations of the transmission efficiency of the LIU beam parameters from the injection process to stable conditions for physics, the minimization of the luminous region volume while preserving at the same time the separation of multiple vertices, the luminosity control mechanisms to extend the duration of the most efficient data taking conditions together with the associated concerns (machine efficiency, beam instabilities, halo population, cryogenic load, and beam dump frequency) and risks (failure scenarios, and radiation damage). In conclusion the expected integrated luminosity per fill and year is presented.

MULTITURN EXTRACTION BASED ON TRAPPING IN STABLE ISLANDS

Recently a novel approach to perform multi-turn extraction was proposed based on beam splitting in the transverse phase space by means of trapping inside stable islands. During the year 2002, preliminary measurements at the CERN Proton Synchrotron with a low-intensity, singlebunch, proton beam, confirmed the possibility of generating various beamlets starting from a single Gaussian beam. The experimental campaign continued also during the year 2003 to assess a number of key issues, such as the feasibility of trapping with high-intensity beam and capture efficiency. The experimental results are presented and discussed in detail in this paper.

Adiabatic beam trapping in stable islands of transverse phase space: measurement results at CERN Proton Synchrotron

Recently a novel approach to the problem of multi-turn extraction was proposed. It consists of splitting the beam by adiabatic capture inside stable islands created in the transverse phase space by sextupoles and octupoles. Numerical simulations indicate that such a technique should be feasible and potentially superior to the method presently used at the CERN Proton Synchrotron. During 2002, intense efforts were devoted to the experimental verification of this newly proposed extraction mode. Finally, beam capture into the islands was observed. In this paper, the extraction principle is briefly reviewed and the experimental results are presented and discussed in detail.

Nominal LHC beam instability observations in the CERN Proton Synchrotron

The nominal LHC beam has been produced successfully in the CERN proton synchrotron since 2003. However, after having restarted the CERN PS in spring 2006, the LHC beam was set-up and observed to be unstable on the 26 GeV/c extraction flat top. An intensive measurement campaign was made to understand the instability and to trace its source. This paper presents the observations, possible explanations and the necessary measures to be taken in order to avoid this instability in the future.

PICs: what do we gain in beam performance

The beam parameters in the LHC resulting from the Performance Improvement Consolidation (PIC) activities presented in (1)(2) will be briefly recalled and motivated assuming that LINAC4 will be operational as PS-Booster Injector. The corresponding limitations in the LHC are outlined. Based on the above performance an estimate of the LHC yearly integrated luminosity will be provided. The evaluation of the need and extent of the performance and reliability improvement for some of the PIC items might imply additional information: the necessary machine studies and the specific operational experience required during Run 2 will be summarized.

A concept for the lhc luminosity upgrade based on strong beta* reduction combined with a minimized geometrical luminosity loss factor

A significant reduction of beta* requires a new beam crossing scheme to overcome the consequence of the luminosity loss factor. We define in this paper the parameters of a possible solution taking advantage of an early separation scheme optionally supplemented by a weak crab crossing. Large aperture (150 mm) Nb3Sn triplet quadrupoles are chosen for their compactness, temperature margin and ability to relax the collimator gap and impedance. This concept offers high performance for a moderate increase of complexity. An open issue is the possibility of integrating dipoles inside the detectors.

Measurements of the LHC Collimator Impedance with Beam in the SPS

The transverse impedance of the LHC collimators will likely dominate the overall transverse impedance in the LHC at high energies and potentially limit the maximum intensity. A prototype collimator was recently tested in the SPS. Small, but significant tune shifts depending on the collimator position have been observed using different independent high resolution tune measurement methods. In addition trapped modes predicted from numerical simulation at the ends of the collimator jaws have been identified by bench measurement techniques as well as with the beam. We present a description of the measurements and an analysis of the results.

Simulation Aspects of the Code Benchmarking Based on the CERN‐PS “Montague‐resonance”Experiment

Measurements of emittance exchange due to the Montague resonance in the CERN Proton Synchrotron in 2003 have provided detailed data, which are suitable for benchmarking of different simulation codes for high‐intensity accelerators. We present here some characteristic features of the Montague resonance by using first simulations obtained with MICROMAP and IMPACT under simplifying conditions. The challenges for the planned code benchmarking are discussed.