A study of the coherent beam–beam effect in the framework of the Vlasov perturbation theory

Most hadron synchrotrons rely on lattice nonlinearities for Landau damping of impedance driven coherent modes of oscillation. However, in a collider, the presence of beam-beam interactions strongly modifies the transverse amplitude detuning and therefore the resulting stability diagram. A numerical tool to evaluate the effect of beam-beam on the stability diagram has been developed and is used to discuss observations during different phases of the operational cycle of the Large Hadron Collider during the 2012 proton run. In particular, we show the evolution of the stability diagram when the strength of long range beam-beam interactions is increased, during the betatron squeeze. Also, we investigate the stability of beams colliding with a small transverse offset and compare to observations of instabilities when bringing the beams into collision and while leveling the luminosity.

Section: Methodsmentioning

confidence: 99%

Stability diagrams of colliding beams in the Large Hadron Collider

Buffat

Herr

Mounet

et al. 2014

“…A more detailed strong-strong 6D beam-beam simulation (available tools include COMBI [5] or Beam-Beam 3D [6]) is needed to verify if the resonance can be suppressed by the Landau damping due to chromaticity [7] or nonlinear magnets. Such simulations are usually very time consuming and beyond the scope of this paper.…”

Section: Discussionmentioning

confidence: 99%

Beam-beam effects of gear changing in ring-ring colliders

Hao

Litvinenko

Ptitsyn

2014

In ring-ring colliders, the collision frequency determines the bunch structures, e.g., the time between the bunches in both rings should be identical. Because of relatively low relativistic speed of the hadron beam in sub-TeV hadron-hadron-and electron-ions colliders, scanning the hadron beam's energy would require either a change in the circumference of one of the rings, or a switching of the bunch (harmonic) number in a ring. The later would cause so-called gear changing, i.e., the change of the colliding bunches turn by turn. In this article, we study the difficulties in beam dynamics in this gear-changing scheme.

“…This matrix is easily extended to offset collision by locally linearizing the beam-beam force and accordingly modifying the parameter ξ in Eq. (6). The long-range interactions are lumped in two locations with phase advances of AE π 2 with respect to the IP.…”

Section: A the Circulant Matrix Modelmentioning

confidence: 99%

“…However, the frequency of these modes may be well separated from the incoherent tune spread and consequently they do not profit from the large intrinsic Landau damping properties of the beam-beam interactions [6]. Under external excitation, such as machine impedance, these modes could therefore become unstable.…”

Section: Introductionmentioning

confidence: 99%

Transverse mode coupling instability of colliding beams

White

Buffat

Mounet

et al. 2014

In high brightness circular colliders, coherent and incoherent beam dynamics are dominated by beambeam interactions. It is generally assumed that the incoherent tune spread introduced by the beam-beam interactions is sufficiently large to cure any instabilities originating from impedance. However, as the two counterrotating beams interact they can give rise to coherent dipole modes and therefore modify the coherent beam dynamics and stability conditions. In this case, coherent beam-beam effects and impedance cannot be treated independently and their interplay should be taken into account in any realistic attempt to study the beam stability of colliding beams. Due to the complexity of these physics processes, numerical simulations become an important tool for the analysis of this system. Two approaches are proposed in this paper: a fully self-consistent multiparticle tracking including particle-in-cell Poisson solver for the beambeam interactions and a linearized model taking into account finite bunch length effects. To ensure the validity of the results a detailed benchmarking of these models was performed. It will be shown that under certain conditions coherent beam-beam dipole modes can couple with higher order headtail modes and lead to strong instabilities with characteristics similar to the classical transverse mode coupling instability originating from impedance alone. Possible cures for this instability are explored both for single bunch and multibunch interactions. Simulation results and experimental evidences of the existence of this instability at the LHC will be presented for the specific case of offset collisions.