• T. Pieloni
  PSI, Villigen
• W. Herr
  CERN, Geneva
• J. Qiang
  LBNL, Berkeley, California

Under nominal operational conditions, the LHC bunches experience small unavoidable offset at the collision points caused by long range beam-beam interactions. Although the geometrical loss of luminosity is small, one may have to consider an increase of the beam transverse emittance, leading to a deterioration of the experimental conditions. In this work we evaluate and understand the dynamics of beam-beam interactions with static offsets at the collision point. A study of the emittance growth as a function of the offset amplitude in collisions is presented. Moreover, we address the effects coming from the beam parameters such as the initial transverse beam size, bunch intensity and tune.

• M. Meddahi, I.V. Agapov, K. Fuchsberger, B. Goddard, W. Herr, V. Kain, V. Mertens, D.P. Missiaen, T. Risselada, J.A. Uythoven, J. Wenninger
  CERN, Geneva
• E. Gianfelice-Wendt
  Fermilab, Batavia

Funding: Work partly supported by Fermilab, operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy

Through May to September 2008, further beam commissioning of the SPS to LHC transfer lines was performed. For the first time, optics and dispersion measurements were also taken in the last part of the lines, and into the LHC. Extensive trajectory and optics studies were conducted, in parallel with hardware checks. In particular dispersion measurements and their comparison with the beam line model were analysed in detail and led to propose the addition of a “dispersion-free” steering algorithm in the existing trajectory correction program.

• D. Kaltchev
  TRIUMF, Vancouver
• W. Herr
  CERN, Geneva

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.

• F.W. Jones
  TRIUMF, Vancouver
• W. Herr, T. Pieloni
  CERN, Geneva

The simulation code COMBI has been developed to enable the study of coherent beam-beam effects in the full collision scenario of the LHC, with multiple bunches interacting at multiple crossing points over many turns. The parallel version of COMBI was first implemented using a soft-Gaussian collision model which entails minimal communication between worker processes. Recently we have extended the code to a fully self-consistent collision model using a Grid-Multipole method, which allows worker processes to exchange charge and field information in a compact form which minimizes communication overhead. In this paper we describe the Grid-Multipole technique used and its adaptation to the parallel environment through pre- and post-processing of charge and grid data. Performance measurements in multi-core and Myrinet-cluster environments will be given. We will also present our estimates of the potential for very large-scale simulations on massively-parallel hardware, in which the number of simulated bunches ultimately approaches the actual LHC bunch population.