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    Special beam-beam meeting for FCC-ee

    → Europe/Zurich
    6/R-012 - conference room (CERN)

    6/R-012 - conference room

    CERN

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    Minutes from the FCC-ee Accelerator Design meeting of 5 February 2016

    Participants: Sandra Aumon, Anton Bogomyagkov (part time), Andreas Doblhammer, Bernhard Holzer, Salim Ogur, Katsunobu Oide, Dmitry Shatilov, Jorg Wenninger, Frank Zimmermann

    Remote participants: Sergey Glukhov, Kazuhito Ohmi

    1) Beam-beam simulations for FCC-ee

    Demin Zhou presented results obtained by running Kazuhito Ohmi’s beam-beam simulation code BBWS for the newly proposed baseline parameters (Dec. 2015). Beamstrahlung is included. Crab waist for the strong beam is not considered.

    First results for the top quark indicate a good working pint at [0.54, 0.57]/IP. Luminosity is correlated with bunch length and vertical beam sizes. Peak luminosity around 1.3e34/cm^2/s

    Next results are for the ZH production mode. A good working point is [0.53, 0.56]/IP. With crab waist turned on the good luminosity area is increased. Best luminosity around 4.9e34

    Following results were for WW threshold. Crab waist helps a lot. Peak luminosity of 1.9e35/cm^2/s.

    Final result is for ZZ. Crab waist has a big effect. Peak luminosity of 6.8e35/cm^2/s.

    Crab waist is more effective at lower beam energy.

    Now specific luminosity and luminosity were presented as a function of the bunch current product.

     

    Frank Zimmermann asked why the beamstrahlung blows up the horizontal beam size in case of crab waist. Dmitry Shatilov pointed out that with larger vertical beam size (as in the case w/o crab waist) the beamstrahlung increases as is also seen in the bunch length. Demin Zhou will discuss the horizontal phenomenon with Kazuhito Ohmi.

    Frank Zimmermann suggested plotting the luminosity vs bunch current product at constant total beam current (i.e. changing the number of bunches to keep the total current constant).

    The simulations also allow a look at the beam tails. At low energies, especially at the Z pole, the beam tails are dramatically large without crab waist.

    The luminosity is calculated from folding weak beam with the strong beam. Much better luminosity values than in the proposed new baseline table can be obtained by optimizing working and point and using full strength of crab waist. Effectiveness of crab waist strongly depends on lattice nonlinearity. The study of interplay of beam-beam and lattice nonlinearity might be more important in ZZ/WW modes than for tt/HH collider.

    Kazuhito Ohmi commented that only the bunch length is treated self consistently in the simulation.

    Frank Zimmermann proposed updating the transverse beam sizes of the strong beam together with its bunch length.

    Bernhard Holzer asked whether the optimum working point for beam-beam might have adverse effects on dynamic aperture or polarization. Jorg Wenninger remarked that the polarization is good near the half integer. Katsunobu Oide clarified that the chosen working point is near the working points us for the dynamic aperture calculation. Dmitry Shatilov explained that a working point below the main coupling resonance would lead to blow if there is any small skew quadrupole error. In addition, with two IPs, the half integer tune would become an integer, which had to be avoided. For crab waist a good tune could be 0.54, 0.55, further away from the half integer. Katsunobu Oide recommended 3 IPs.

    Katsunobu Oide had commented that “ZZ” should be named “Z”, and “HH” should be called “ZH”.

    Kazuhito Ohmi inquired if the horizontal beam size is bigger with or without crab waist. The answer was with crab waist.

    Bernhard Holzer suggested that a tracking campaign be launched based on the real lattice, including all the nonlinearities. Katsunobu Oide commented that Demin Zhou has such a tracking tool available. It is based on a combination of SAD and BBWS. In the past Demin Zhou had already presented some results on the lattice effect for earlier parameters and optics. These calculations could be redone for the new parameters and latest optics versions. Dmitry Shatilov mentioned that he has the equivalent tracking machinery and can also perform this type of simulation.


     

     

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