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    Beam parameter meeting

    → 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 28 September 2015

    Participants: Sandra Aumon, Andreas Doblhammer, Bastian Haerer, Mike Koratzinos, Katsunobu Oide, Frank Zimmermann

    Remote participants: Anton Bogomyagkov, Dmitry Shatilov,

    Excused: Michael Benedikt

    1) Arc parameters

    Katsunobu Oide suggested choosing target values for the emittance and for the bending radius. Then the cell length is (pretty much) determined. He also made a lattice with 55m cell length.

    The discussion focused on the questions: Should 1 nm be the target emittance at 175 GeV? How much margin is needed for errors?

    Mike Koratzinos advocated a target value of 1 nm at 175 GeV.

    Katsunobu Oide underlined that intrabeam scattering (IBS) is already important at 45.5 GeV, with an emittance scaled down from 1.8 nm at 175 GeV, and IBS will get even more important for a lower emittance optics.

    Bastian Haerer commented that there are some differences in his arc optics from the one used by Anton Bogomyagkov, e.g. the phase advance per cell. Anton Bogomyagkov stressed that the phase advance has a great effect on the emittance.

    Sandra Aumon reported that alignment errors increase the simulated horizontal emittance by a few per cent.

    Dmitry said at 45 GeV with Crab waist, the horizontal emittance can be increased by a factor 4 or larger, if necessary.

    2) Beam-beam parameters

    Dmitry Shatilov explained that the optimum betay* depends on the horizontal emittance. A smaller betay* is advantageous for small emittance.

    Frank Zimmermann pointed out the large range of synchrotron tunes considered by the three simulators (Dmitry Shatilov, Kazuhito Ohmi, and Demin Zhou).

    There was almost a consensus reached that betay* = 2 mm could be defined as the baseline. However, Mike Koratzinos was dissenting, and supported the choice betay* =1 mm.

    It was pointed out that 1mm might be better for performances but the beam is killed by beam-beam effects, i.e.  beam-beam tune shift, whereas 2mm reduces this beam-beam tune shift.

    At least the crossing angle of 30 mrad has been agreed upon.

    Dmitry Shatilov highlighted that the optimum working point depends on the crossing scheme (equal crossing, or alternating crossing).

    Frank Zimmermann pointed out that a smaller emittance at 45.5 GeV not only faces challenges from IBS, but also requires a larger number of bunches, which could easily exceed the harmonic number.

    In addition, in the beam-beam simulations of Demin Zhou, an emittance of 2 nm, together with a larger betay* of 2 mm, and a larger bunch population of 2.6e11 resulted in the highest luminosity. Therefore, there was no obvious merit in aiming at smaller values.

    Frank Zimmermann added that the larger horizontal emittance also helps reducing the effect of beamstrahlung, while it does not preclude the achievement of a small vertical emittance.


     


     

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