• L. Bottura, M.C.L. Buzio, N. Catalan-Lasheras, L. Deniau, M. DiCastro, S.D. Fartoukh, M. Giovannozzi, P. Hagen, J.-P. Koutchouk, M. Lamont, J. Miles, RV. Remondino, N.J. Sammut, S. Sanfilippo, F. Schmidt, D. Sernelius, R.J. Steinhagen, M. Strzelczyk, E. Todesco, R. Tomás, W. Venturini Delsolaro, L. Walckiers, J. Wenninger, R. Wolf, P. Xydi
  CERN, Geneva

The start-up of the LHC has provided the first field test for the concept, functionality and accuracy of FiDeL, the Field Description for the LHC. FiDeL is primarily a parametric model of the transfer function of the main field integrals generated by the series of magnets in the LHC powering circuits, from main optical elements to high-order harmonic correctors, both superconducting and normal-conducting magnets. In addition, the same framework is used to predict harmonic errors of both static and dynamic nature, and forecast appropriate corrections. In this paper we give a description of the level of detail achieved in the model and the rationale adopted for the LHC start-up. Beam-based measurements have been used for an assessment of the first-shot accuracy in the prediction of the current setting for the main arc magnets*.

*The work reported has been performed by the authors and the FiDeL Team

• S.M. White, H. Burkhardt, S.D. Fartoukh, T. Pieloni
  CERN, Geneva

The LHC beams will cross each other and experience perturbations as a result of the beam-beam effect at the interaction points, which can result in emittance growth and halo creation. The beam-beam force is approximately linear for small offsets and highly non-linear for larger offsets with peaks in growth close to 0.3 and 1.5 σ separation. We present a study of the process of going into collisions in the LHC and use simulations to investigate on possible emittance blow-up. We analyze how the crossing scheme can be optimized to minimize the collapsing time of the separation bumps for given hardware constraints.

• R. Tomás, M. Aiba, S.D. Fartoukh, F. Franchi, M. Giovannozzi, V. Kain, M. Lamont, G. Vanbavinckhove, J. Wenninger, F. Zimmermann
  CERN, Geneva
• R. Calaga
  BNL, Upton, Long Island, New York
• A. Morita
  KEK, Ibaraki

In 2008 beam successfully circulated in the LHC. Thanks to an excellent functioning of the BPM system and the related software, injection oscillations were recorded for the first 90 turns at all BPMs. The analysis of these data gives the unique opportunity of evaluating the periodic optics and inferring possible error sources.

• F. Zimmermann, S.D. Fartoukh, M. Giovannozzi, V. Kain, M. Lamont, Y. Sun, R. Tomás
  CERN, Geneva
• R. Calaga
  BNL, Upton, Long Island, New York

In early LHC commissioning, linear and "higher-order" polarity checks were performed for one octant per beam, by launching suitable free betatron oscillations and then inverting a magnet-circuit polarity or strength. Circuits tested included trim quadrupoles, skew quadrupoles, lattice sextupoles, sextupole spool-pieces, Landau octupoles, and skew sextupoles. A nonzero momentum offset was introduced to enhance the measurement quality. The low-intensity single-pass measurements proved sufficiently sensitive to verify the polarity and the amplitude of (almost) all circuits under investigation, as well as the alignment of individual trim quadrupoles. A systematic polarity inversion detected by this measurement helped to pin down the origin of observed dispersion errors. Later, the periodic "ring dispersion" was reconstructed from the full first-turn trajectory of an injected off-momentum beam, by removing, at each location, the large incoming dispersion mismatch, forward-propagated via the optics model. Various combinations of inverted trim quadrupoles were considered in this model until reaching a good agreement of reconstructed dispersion and prediction.

• S.D. Fartoukh
  CERN, Geneva

As for the final focus systems of linear colliders, the chromatic aberrations induced by low-beta insertions can seriously limit the performance of circular colliders. The impact is two-fold: (1) a substantial off-momentum beta-beating wave travelling all around of the ring leading to a net reduction of the mechanical aperture of the low-beta quadrupoles but also impacting on the hierarchy of the collimator and protection devices of the machine, (2) a huge non-linear chromaticity, essentially Q’’ and Q’’’, which, when combined with the geometric non-linear imperfection of the machine could substantially reduce the momentum acceptance of the ring by sending slightly off-momentum particles towards non-linear resonances. These effects will be analyzed and illustrated in the framework of the LHC insertions upgrade and a strategy for correction will be developed, requiring a deep modification of the LHC overall optics.

• K. Fuchsberger, S.D. Fartoukh, B. Goddard, O.R. Jones, V. Kain, M. Meddahi, V. Mertens, J. Wenninger
  CERN, Geneva

The 3 km long TI 8 transfer line is used to transfer 450 GeV proton and ion beams from the SPS to LHC collider. As part of a detailed optics investigation program the chromaticity of the transfer line was measured. Kick response data of the transfer line was recorded for various extraction energy offsets in the SPS. The quadrupolar and sextupolar field errors over the whole transfer line dipoles, a systematic error of the main quadrupole strengths and the initial momentum error were estimated by a fit. Using the updated model, the chromaticity of the line was then calculated.