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v3.3.6-pre
Participants: Sandra Aumon, Anton Bogomyagkov, Andreas Doblhammer, Bastian Haerer, Ivan Koop, Roman Martin, Luis Eduardo Medina, Katsunobu Oide, Pavel Piminov, Dmitry Shatilov, Serguey Sinyatkin.
Follow-up of DA estimation, by Luis Eduardo Medina
Luis presented the Dynamical Aperture (DA) studies done on the FCC-ee lattice from Katsunobu Oide with crab-waist (CW) with non-interleaved sextupoles - FCCee_t_26_cw_nosol. He compared his results computed with MADX-PTC with SAD. He performed the study for two different emittance ratios (0.1 and 20%), including fringe fields and RF cavities.
For a emittance ratio of 20% at injection, the momentum acceptance of +/-2% is not reached. Fringe effects do not have a significant influence on any of the two different emittance ratios cases. DA loss due to RF cavities is larger for the case of emittance ratio of 20% at injection than for the nominal values. However, no large DA is required at injection. For a emittance ratio of 0.1%, the momentum acceptance of +/-1.3% is reached , the RF reduces the DA. Katsunobu Oide said that it could be due to the high synchrotron tune.
Concerning the lattice FCC_ee_t_35_11_cw, the beam is unstable even for on momentum particles, within 30 turns. Katsunobu suggested to check the optics (ie beta function etc..). Anton asked whether pairs of sextupole are used. Luis replied positively.
Crab waist interaction region for FCC-ee (one quarter of the ring IR: v. 7, arc: v16) from Anton Bogomyagkov
Anton presented the last version of the lattice with 4 IPs with crab-waist, the arcs remains the lattice of Bastian. The horizontal beta function at the IP was increased from 0.5 to 1m. Anton added that one of his step would be to increase the length of the quadrupoles in the Final Focus. The quadrupoles are splitted with a sextupoles at the middle.
Anton presented the lattice functions where the maximum vertical beta-function goes from 7 to 4 km in the vertical chromaticity correction section. Compared to his last version, the horizontal chromaticity correction section was removed.
For a working point of 107.54,87.57, one family of sextupole was used.The energy acceptance is 10 times smaller than required, here +/-0.2% instead of 2%, the limitation comes from fourth order chromaticity in both planes. For a working point of 107.74,87.57, The situation is a bit better, but the energy acceptance remains very low +/-0.7%.
Another working point was tested (121.54,121.57) with two families of horizontal sextupoles per arc, and the energy acceptance is about +/-0.3%. Bastian asked why does the W-functions are not matched further down. Anton answered that he does match the Montague functions, but the chromaticity. As a next step, Anton will reinstall the horizontal chromaticity correction sections.
FCC-ee parameters optimization at high and low energies, different schemes of crossing, by Dmitry Shatilov
Dmitry reminds that the luminosity at high energies (120 and 175 GeV) is strongly limited by the beamstrahlung. The beamstrahlung life time has been reduced by making the bending radius larger but also by increasing slightly the vertical beta* up to 2mm.
Dmitry performed optimization and comparison for 4 collision schemes (head-on , 11 and 30 mrad crossing angle, w and w/o crab-waist) at several energies. The code Lifetrac and the model quasi strong-strong for beam-beam simulations were used. The goal is to maximize luminosity and life time. Beta functions at the IP of 0.5m and 1mm were first used, with the 4 IPs case.
Head-on beam-beam simulations at 175GeV for different RF voltage, which modifies the bunch length. 10.5GV looks to be optimal in terms of luminosity and life time. 9.5GV is not optimal for the hour-glass and tails are creating in the vertical plane. The number of particle per bunch is as well adjusted. As already discussed in previous meeting (beam-beam simulations by K. Ohmi) the beta stars 0.5m/1mm are not suitable for the desired bunch length (loss of luminosity), except if the RF frequency is increased to 800MHz.
Dmitry introduced an asymmetry in the bunch current (weak-strong beam beam) and in phase advance: 0.002 in phase advance and 10% in bunch current are acceptable.
With a crossing angle of 11mrad, at 175GeV, alternate the sign of the crossing angles is good for the stability of the beam due to bunch crabbing. At 175 GeV everything is limited by beamstrahlung lifetime. There is no need in crab waist, and crossing angle reduces the length of interaction area. In this case crab-crossing is helpful. At 45.5 GeV bunch lengthening due to beamstrahlung is larger, damping is much weaker, so we need large Piwinski angle to make in order to avoid hour-glass and to implement crab waist.
Dmitry studies then the scheme of crossings for the crab-waist, needed at 45GeV (crossing angle 30mrad). He showed that the use of CW increases significantly the luminosity. The horizontal emittance and the beta function are increased from 0.5m to 1m in order to get the length of the L* of the order of the vertical beta function. The best scheme with 4 IPs would be no additional crossings (+ - + -) and the working point at low energies of about (0.78, 0.55). The luminosity at 175 GeV without additional crossings is higher by 10,20 % due to "beam-beam crabbing" for a working point (0.55, 0.62).