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    FCC-ee optics tuning WG

    → Europe/Zurich
    354/1-019 (CERN)

    354/1-019

    CERN

    30
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    FCC-ee optics tuning WG meeting
    Zoom Meeting ID
    65516078996
    Host
    Rogelio Tomas Garcia
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    Actions:

    • Simulate failure of correctors and BPMs 
    • Repository for tuning result seeds --> To be followed up to have one repository for tuning and tuned lattices (S. Jagabathuni, J. Salvesen, J. Keintzel, K. Skoufaris, etc)
    • Max BPM to quadrupole alignment --> more studies to be done
    • Including long-range misalignments: --> Values provided by Jürgen and Vincent in ~mid September
    • Tuning simulations with larger H errors after BBA and split errors between arc and IR:
    • a4 correction to be studied:
    • Split arc and IR BPM resolution
    • BPM resolution for RDTs measurements
    • IP dispersion impact

     

    SuperKEKB News, Onishi

    This year was focused on luminosity with little time devoted to studies. Next studies will be in next spring. Currently shutdown to fix cavities. Total integrated luminosity so far reached about 900 fb-1. Number of bunches has almost reached design. Peak luminosity reached another record 5.32e34, still far from design. One limitation is heating from SR, which deforms beam screen and pushes sextupole magnets.

    There is some trouble to control both IP Dy and coupling and global vertical emittance. Vertical emittance is typically 10 - 20 pm in LER, without collisions. However, over time, emittance versus tune changes significantly. 

    In HER emittance was at 60pm until 2026, that reached 40pm for unknown reasons. A fifth order resonance is observed in the emittance versus tune, but this is not understood.

    The luminosity optimization knobs, Dy and coupling, increase emittance in LER. In HER vertical emittance grows versus intensity, due to a poor setting of the feedback.

    Ohmi-san simulations including impedance and full lattice and beam-beam, match measurements at low intensities. At higher intensities vertical LER emittance increase in the machine. Underlying mechanism is not known. This stops. The model has no feedback. SR model should be element-by-element.

    Bunch length is measured by scanning RF phase, agreeing with model.

    Vertical angle at the IP is scanned, reducing overlap of beams, and optimum is close to zero.

    Simulations of DA versus tune with and without a H dispersion error shows impact of synchrobetatron resonances (which reduce MA). DA versus vertical tune shows a continuous degradation. 

    Jack and Giulia plan to visit SuperKEK to study the linac+BT. Jack is involved in the Belle II group, which supports students on machine side. EPFL is interested in participating to experiments in SuperKEKB.

    SuperKEKB model in Xsuite, Jack

    Very fresh release of a Xsuite SuperKEKB lattice with much better accuracy than before.A large range of tools for beamline manipulation is available. Current beta-beating between SAD and Xsuite is below 0.5%. To get it running in 6D a patch with path delay is needed. The improvement was mostly fringe fields, also in multipoles as they contain K0, K1, etc. Identical DAs are reproduced in both codes.

     

    SuperKEKB RDTs analysis with Xsuite, Mael,

    Five horizontal kicks, 2048 turns, analysed with omc3. There is less decoherence or damping for the larger measured kicks, which is not known why. One can see sextupolar RDTs and maybe octupolar.

    Xsuite and SAD tracking give same results and would show much more damping and decoherence. Maybe impedance affects damping time in the measured data. If data is provided this could be added to simulations.

    There is beta-beating in the order of 5-10%. Phase advance differences reach 0.01. A difference between tracking and twiss could come from 6D tracking versus 4D twiss.

    Xsuite and SAD give identical sextupolar RDTs.

    Comparison to measurements now is quite poor (although in the past phase had better agreement with poorer model, !?). Onishi comments that action of simulated particle should be better matched to measurement. Some phases agree locally between measurement and  tracking.

    Ideally field quality errors would be introduced in simulations. The baseline is without the cancel coil error, but this is not the best fidelity.

    -> Check KEK TDR. For new magnets they have tables. We should try it. BPM artifacts might not be excluded either.

    SAD and Xsuite give slightly different amplitude detuning, but very different, including sign, compared to data. Clearly different sextupolar and octupolar sources.

     

    Tuning FCC-ee and dispersion measurement studies, Tirsi

    EPFL is developing a model with corrections, that includes many aberrations and misses high order and systematic errors. These models have been used in different beam-beam, polarization studies, etc. (+ LEP3).

    Third order x dispersion at IP is 20m from the ideal model (!).

    After correction, e.g. Dy at IPs has a spread of 21 um.

    In the measurement of the x dispersion there is a clear effect from the third order dispersion. 

    Without BPM errors, Dy measurement uncertainty is 0.3um (for Dx is 1.7um). For second order Dy it is 100 um.

    A BPM error of 0.1um is needed to keep resolution of 1um of Dy.

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

    Next meetings:

    • 17th September
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