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FCC-ee optics tuning WG meeting
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FCC-ee optics tuning WG meeting
G. Mitsuka suggests that all beam time requests will be sent to himself and Y. Ohnishi (and possibly others).
J. Salvesen presents the status on the SKEKB lattice. It is available at gitlab:
https://gitlab.cern.ch/jsalvese/superkekb-public/
He asks all collaborators to use if for studies and report bugs, and inconsistencies between SAD and Xsuite models. He warns that e.g. the current lattice is not suitable for spin studies. In the future the goal is to put it in acc-models. An update of the SAD2XS is expected soon. Currently the solenoids cannot be converted. Following a question by R. Tomas, J. Salvesen reports that after importing, re-matching must be performed. The difference stems from the fringes implementation, where JPARC is also working on it. Also, the available scripts only work for lattices that are very similar. The goal is that these scripts are available, ideally, later on this year.
J. Salvesen presents on the luminosity feedback. The SKEKB iBump feedback system has been fully implemented and simulated for SKEKB (with strong-strong, SR, QF, multiparticle, ...). Large residual orbit is possibly only from a mis-match and not from the FB itself.
G. Mitsuka presents status and plans for SuperKEKB in 2025. The 2025c to 2026b run is from 5 November 2025 to 1 June 2026, with the highest priority being integrated luminosity. In total about 150 days are required to achieve the physics goal. 4 days per 3-week cycle are allocated to machine tuning, study, maintenance, etc. For plan A 2-bunch injection for both beams, 0.9mm betay* and mitigation of beam-beam effects are required. Plan B is less aggressive, targeting only 6e34 cm-2s-1 peak luminosity, yet with higher efficiency. The key difference is that lower bunch currents are assumed. J. Salvesen asks about MDs related to the requirements to achieve the luminosity target. G. Mitsuka comments that beam-beam studies are not mentioned, and the first 1 to 2 months of operation are dedicated to single beam studies, beam-beam studies will most likely take place afterwards, early 2026.
First TbT measurements in multi-bunch operation are foreseen from 5 to 17 November 2025 with a different hard-ware system. He explains that this is done with by FEM detuning changing the kick frequency sequentially. Following a question by J. Keintzel, G. Mitsuka explains that although multi-bunches will be present, only 1 bunch will be kicked.
From 19 November to 24 December e.g. correction or cancel coil errors with QC1RE skew sextupoles are foreseen. Following a question by R. Tomas about the skew octupole component from the cancel coil. G. Mitsuka answers it is not yet clear if this will be corrected in addition. R. Tomas comments that it would be good to take TbT measurements with and without correction of the cancel coil.
MD time is from 8 to 11 December.
In December B. Dalena et al, and M. Le Garrec plan to go there in December. Y. Ohnishi is the contact person and beam time requests should be organized with him.
M. Le Garrec presents on vibration studies for the GHC. Inputs has been given by A. Piccini. Transfer fucntions are measured in 2008 in P4 for the LHC, which are used to generate a time series, which are then used for tracking. Frequencies below 1 Hz are assumed to be treated by the feedback system. Each quadrupole (also the FF), are assumed to be on a girder and has its own vibration and thus independent motion in x and y. The beam separation in IP1 is in the order of a few (9 for x 1 for y) nm due to vibrations. Considering the most offset found (18 and 5 nm), in x this corresponds to 0.2% of sigmax, and 16% of sigmay, which, in the worst case, accounts for a 5 % loss, however, 90% of the seeds the luminosity loss is below ~ 1.6%. Angular displacements are not included. J. Salvesen comments on xsuite for the offsets, this is work in progress. Concerning the modelling, the FF quadrupoles dominate. Because of the different arrival time, the effect of the vibrations presented here are possibly a bit underestimated.
J. Salvesen highlights a paper: https://www.slac.stanford.edu/econf/C1009137/PDF/masuzawavibsubmit.pdf The motion of the FF is slightly different stemming from the cryo.
B. Dalena asks about the PSD, if only 1 dimension and no correlation is considered. M. Le Garrec confirms that coupling between the planes is not considered, neither correlation between various magnets, neither coupling in the transfer function. B. Dalena comments that also technical noise (pumps, power converters, etc.) could be studied.
A. Piccini asks if the specifications could be relaxed. R. Tomas comments that they currently could indeed be a bit conservative, but to conclude further studies are needed.
Following a question by P. Lersnimitthum, M. Le Garrec explains that the PSD was measured directly in the LHC tunnel. In these simulations, quiet data, i.e. without noise is used.
V. Gawas presents IP knobs for luminosity tuning, where one input is also beamstrahlung from collisions. She also applies Bayesian optimization. Similar techniques are foreseen to be applied at SuperKEKB. G. Mitsuka adds that a student from Tokyo University is also currently working on it. There is a problem to change the coupling knob in the real machine. He suggests to discuss with MDI teams from SuperKEKB to see the recent status. R. Tomas comments that e.g. the skew quadrupoles in the FD are not really used and can be explored as a coupling knob, also for ML techniques. F. Zimmermann comments that these coils could be maxed out.
C. Goffing presents a measurement proposal for BBA at SuperKEKB. Since BBA at SuperKEKB is typically applied to known bad BPMs, he suggests performing it at a random selection of BPMs. G. Mitsuka comments that some bad BPMs have been reconnected. Possible such BBA measurements can be performed during vacuum scrabbing run.
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