Results of dynamic aperture studies for increased beta* with beam-beam interactions

We have evaluated the dynamic aperture in the presence of beam-beam effects for alternative running scenarios, in particular for the commissioning and early operation of the LHC. For top energy of 7 TeV we have studied the effect of increased beta* up to beta* = 2 m and found the expected increase of dynamic aperture for a moderate loss of luminosity. We have further studied the possibility of head-on collisions with a small number of bunches at injection energy

Analytical calculation of the smear for long-range beam-beam interactions

The Lie-algebraic method is used to develop generalized Courant-Snyder invariant in the presence of an arbitrary number of beam-beam collisions, head-on or long-range, in a storage ring collider. The invariant is obtained by concatenating nonlinear beam-beam maps in the horizontal plane and to first order in the beam-beam parameter. Tracking evidence is presented to illustrate that with LHC parameters the invariant is indeed preserved and can be used to predict the smear of horizontal emittance observed in tracking simulations. We discuss the limits of applicability of this model for realistic LHC collision schemes

Robustness of Resonance Free Lattices Against Gradient Errors

Resonance-free lattices make it possible to cancel the effect of non-linear resonances due to systematic multipoles in an alternating gradient circular machine. These lattices are made of identical cells with specified phase advances. It is therefore mandatory to examine to what extent the property remains valid in presence of gradient errors. In the case of LHC, three times the nominal gradient errors are acceptable from the point of view of both a3 and b4 components

Efficiency for the imperfect LHC collimation system

The LHC collimation system requires a high cleaning efficiency in order to prevent magnet quenches due to regular beam diffusion. The cleaning efficiency is significantly reduced due to imperfections of the collimator jaws and the machine optics. Tracking tools have been set up to predict the cleaning efficiency in presence of multiple imperfections. The deterioration of cleaning efficiency is quantified for different errors, including collimator surface non-flatness, collimator alignment errors, beta beating, orbit errors, non-linear field errors, and chromatic effects

Correction of the Systematic b3 Error with the Resonance-Free Lattice in the LHC

The effect of the sextupole component b3 in the LHC dipoles on the resonance-free lattice has been investigated. It is shown that its dynamic aperture, without b3 spool piece correction, is close to that of the nominal LHC lattice version 6.0 with spool pice correction. A prerequisite is the addition of a few chromaticity sextupoles in the dispersion suppressors. Under this condition an increase of the b3 component by a factor of two can probably be accepted. Furthermore, a systematic relative gradient error up to one per mil can be tolerated without changing this result

Large Scale Beam-beam Simulations for the CERN LHC using Distributed Computing

We report on a large scale simulation of beam-beam effects for the CERN Large Hadron Collider (LHC). The stability of particles which experience head-on and long-range beam-beam effects was investigated for different optical configurations and machine imperfections. To cover the interesting parameter space required computing resources not available at CERN. The necessary resources were available in the LHC@home project, based on the BOINC platform. At present, this project makes more than 60000 hosts available for distributed computing. We shall discuss our experience using this system during a simulation campaign of more than six months and describe the tools and procedures necessary to ensure consistent results. The results from this extended study are presented and future plans are discussed

Numerical Optimization of Collimator Jaw Orientations and Locations in the LHC

The collimation system of LHC will consist of flat collimator jaws distributed along the IR7 lattice with the aim of limiting the maximum combined amplitudes of secondary halo particles (born along the edges of the primary collimators). The code DJ (Distribution of Jaws) computes this amplitude using a quasi-analytic algorithm (no tracking), by which the maximum initial angles are found, corresponding to trajectories escaping all secondary jaws. We report the latest version of DJ, which contains the following enhancements: (1) the orientation of each pair of jaws is a free variable (instead of using only vertical, horizontal, or 45 degrees skew jaws); (2) the minimizing method used is "simulated annealing", which, for our case of a discontinuous function of up to 32 variables, always finds a global minimum. Different initial jaw distributions lead to different final ones, but they all give essentially the same maximum halo amplitude; this seems to depend only on the number of jaws and the lattice parameters, particularly the tune-split. We discuss lattice characteristics found favorable for collimation

Initial error analysis for the LHC collimation insertion

The two cleaning insertions in the LHC, for betatron and momentum collimation, are optimized for an ideal lattice and collimator jaw setup. We have studied a collimation beam line with randomly generated jaw misalignments and quadrupole field and alignment errors, the resultant distortion of the reference orbit being corrected with the help of monitors placed near critical collimators. Different closed orbit errors and beam shapes are considered at the entrance. We report the level of errors for which no corrections are needed and the level for which corrections are not possible