Author: Herr, W.
Paper Title Page
TUPFI031 Effect of Collision Pattern in the LHC on the Beam Stability: Requirements from Experiments and Operational Considerations 1409
 
  • W. Herr, G. Arduini, R. Giachino, E. Métral, G. Papotti, T. Pieloni
    CERN, Geneva, Switzerland
  • X. Buffat, N. Mounet
    EPFL, Lausanne, Switzerland
  • S.M. White
    BNL, Upton, Long Island, New York, USA
 
  Coherent instabilities of bunches in the LHC bunch train can be observed when the tune spread from beam-beam interactions becomes insufficient to ensure Landau damping. In particular these effects are seen on bunches with a reduced number of beam-beam interactions due to their collision pattern. Furthermore, such a reduction of the necessary stability can occur during the processes when the beams are prepared for collisions or during the optimization procedure. We discuss the observations and possible countermeasures, in particular alternatives to the existing beam manipulation processes where such a situation can occur.  
 
TUPFI032 Observation of Instabilities in the LHC due to Missing Head-on Beam-beam Interactions 1412
 
  • W. Herr, G. Arduini, R. Giachino, E. Métral, G. Papotti, T. Pieloni
    CERN, Geneva, Switzerland
  • X. Buffat, N. Mounet
    EPFL, Lausanne, Switzerland
 
  We report the observation of coherent instabilities on individual bunches out of the LHC bunch train. These instabilities occured spontaneously after several hours of stable beam while in other cases they were related to the application of a small transverse beam separation during a luminosity optimization. Only few bunches were affected, depending on there collision scheme and following various tests we interprete these instabilities as a sudden loss of Landau damping when the tune spread from the beam-beam interaction became insufficient.  
 
TUPFI033 Colliding During the Squeeze and β* Leveling in the LHC 1415
 
  • X. Buffat
    EPFL, Lausanne, Switzerland
  • W. Herr, M. Lamont, T. Pieloni, S. Redaelli, J. Wenninger
    CERN, Geneva, Switzerland
 
  While more challenging operationally, bringing the beams into collisions during the β squeeze rather than after presents some advantages. The large tune spread arising from the non-linearity of head-on beam-beam interactions can damp impedance-driven instabilities much more efficiently than external non-linearity such as octupoles presently used in operation. Moreover, colliding during the squeeze allows to level the luminosity, optimizing the pile-up in the experiments without changing the longitudinal distribution of collisions. Operational issues are discussed and experimental results from the LHC are presented.  
 
TUPFI034 Observations of Two-beam Instabilities during the 2012 LHC Physics Run 1418
 
  • T. Pieloni
    EPFL, Lausanne, Switzerland
  • G. Arduini, X. Buffat, R. Giachino, W. Herr, M. Lamont, N. Mounet, E. Métral, G. Papotti, B. Salvant, J. Wenninger
    CERN, Geneva, Switzerland
  • S.M. White
    BNL, Upton, Long Island, New York, USA
 
  During the 2012 run coherent beam instabilities have been observed in the LHC at 4 TeV, during the betatron squeeze and in collision for special filling patterns. Several studies to characterize these instabilities have been carried out during operation and in special dedicated experiments. In this paper we summarize the observations collected for different machine parameters and the present understanding of the origin of these instabilities.  
 
TUPFI035 Head-on and Long range Beam-beam Interactions in the LHC: Effective Tune Spread and Beam Stability due to Landau Damping 1421
 
  • X. Buffat
    EPFL, Lausanne, Switzerland
  • W. Herr, N. Mounet, E. Métral, T. Pieloni
    CERN, Geneva, Switzerland
 
  We discuss the Landau damping of coherent instabilities in the presence of betatron tune spread. This tune spread can originate from dedicated non-linear magnets such as octupoles, or through the beam-beam interaction. In the latter case we have to distinguish the contribution from head-on and parasitic beam-beam interactions and the collision pattern of different bunches plays an important role. The interplay of these sources of tune spread and the resulting stability is discussed for the case of the LHC.  
 
TUPME060 Tune Studies with Beam-Beam Effects in LHC 1703
 
  • S. Paret, J. Qiang
    LBNL, Berkeley, California, USA
  • R. Alemany-Fernandez, X. Buffat, R. Calaga, K. Cornelis, M. Fitterer, R. Giachino, W. Herr, A. Macpherson, G. Papotti, T. Pieloni, S. Redaelli, F. Roncarolo, M. Schaumann, R. Suykerbuyk, G. Trad
    CERN, Geneva, Switzerland
  • R. Miyamoto
    ESS, Lund, Sweden
 
  Funding: This work was partially supported by the U.S. LARP and the NERSC of the U.S. Department of Energy under contract No. DE-AC02-05CH11231.
In high brightness colliders, the tune spread due to the collisions has a significant impact on the quality of the beams. The impact of the working point on emittance growth and beam lifetime has been observed in beam experiments in LHC. Strong-strong beam-beam simulations that were accomplished to better understand such observations are shown. Compared to experiments, wide ranged parameter scans can be done easily. Tune footprints and scans of the emittance growth obtained from simulations are discussed. Three cases are considered: Very high intensity, moderate intensity and collisions with separated beams.
 
 
TUPWO048 Understanding the Tune, Coupling, and Chromaticity Dependence of the LHC on Landau Octupole Powering 1976
 
  • E.H. Maclean, M. Giovannozzi, W. Herr, Y.I. Levinsen, G. Papotti, T. Persson, P. Skowroński, R. Tomás, J. Wenninger
    CERN, Geneva, Switzerland
 
  During the 2012 LHC run there were several observations of unexpectedly large shifts to the tune, chromaticity, and coupling which were correlated with changes in the powering of Landau octupoles (MO). Understanding the chromaticity dependence is of particular importance given it's influence on instabilities. This paper summarizes the observations and our attempts to-date to understand the relationship between Q, Q', c- and the MO powering.  
 
WEPEA071 Performance Limitations in the Lhc Due to Parasitic Beam-Beam Encounters - Parameter Dependence, Scaling, and Pacman Effects 2672
 
  • T. Pieloni
    EPFL, Lausanne, Switzerland
  • X. Buffat, R. Calaga, R. Calaga, R. Giachino, W. Herr, E. Métral, G. Papotti, G. Trad
    CERN, Geneva, Switzerland
  • D. Kaltchev
    TRIUMF, Vancouver, Canada
 
  We studied possible limitations due to the long-range beam-beam effects in the LHC. With a large number of bunches and collisions in all interaction points, we have reduced the crossing angles (separation) to enhance long-range beam-beam effects to evaluate their influence on dynamic aperture and losses. Different β*, number of bunches and intensities have been used in several dedicated experiments and allow the test of the expected scaling laws.