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FCC-ee optics tuning WG
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6/R-012 - conference room (CERN)
FCC-ee optics tuning WG meeting
News, Jacqueline
Timeline is presented. The pre-TDR phase will end towards the end of 2027. Most of the technical aspects of the design, tolerances, optics, procedures, etc. should be well defined by then.
Coupling scans, Vaihabi
To efficiently study coupling and beam-beam interplay it is introduced via ideal matrix transformations to generate it only at the IP. First validations of the model are successfully performed. f1010, sum resonance, has larger impact on luminosity at nominal charge (at lower charge impact of 1010 and 1001 are similar). Bunch length is also observed to grow with coupling. In general tolerance of few 1e-3 is observed at ZH energy, however it should be said that this is an optimistic study. Other energies will be studied soon. This is for single IP. These first tolerance values are more demanding than e.g. for LHC, and in agreement with previous estimates from Dimitry. It should be clarified why Tirsi's simulations feature some coupling.
Optics for simplified IR / Ideas for ttbar optics: more FODO cells or lower lumi?, Pantaleo
Too general title. Studies limit to the control of emittance in tt and possible FF optics improvements.
Excluding FFS: the larger contributors to emittance are DS of RF section, injection dispersion bump dipoles, and coll/diag/Inj DSs. Improving all these section H emittance will be reduced.
An assessment of the total RF voltage requirements is needed. It is possible that larger voltage than presented in the review will be needed.
Including FFS emittance increases by 0.25nm. Increasing arc phase advance could also help (QF stronger by 6%). Improving this and the above will make horizontal emittance comparable to GHC.
Another option to control emittance is to change partition number, e.g. Jx=1.5, as at tt there are no constraints on polarization. This could be tested first independently of technique to increase Jx. Technical solutions at zero cost will be studied later.
The LEP3 final focus is presented as this is a parallel on-going development that features different geographical constraints. DA is actually better than for LCC. Extrapolating this new LEP3 FFS to LCC makes a better FFS, shorter by 500m, and with weaker Curl-H, improving emittance and more symmetric (which could help aberration cancellation). Shorter FFS allows for longer arcs, maybe one cell, that improves various performance aspects.
Jacqueline asks about optics versions, but this is still under development, and about the polarimeter, that needs to be inseted. Rogelio highlights that these are very good improvements, and reducing the length of the FFS is a huge simplification by itself.
LCC tuning updates, tolerances and conversion to Xsuite, Kevin
New locations of some IR BPMs at sextupoles. New sextupole ramping is introduced. BPM orbit errors and coupling resolution error are added for first time. DA and MA have improved.
Pantaleo proposes to use the alignment monument on top of the quadrupole instead of the girder monument. This has to be proposed to the alignment colleagues.
Alexey comments that strengths are about 20 times lower than Kevin. Rogelio notes that ttbar energy requires about factor 4 and Pantaleo suggests using 3 or 4 sigma. Table will be recomputed. Alexey also notes that a normal quadrupole is used in the sextupole which was not in the previous baseline. For us the correction coil could be in the quadrupole. Rogelio asks Kevin if quadrupole correctors could be moved to the quadrupole in simulations, which will be possible. Jorg et al. comment that various resolutions are too optimistic. Scans would be welcome. Dedicated studies are elsewhere ongoing for actual resolution of coupling and dispersion.
Conversion tool from pyAT to Xsuite is developped. First comparison of tuned lattice very promising. Small discrepancy in Dx and DA. Probably one rotation is wrong. Jorg asks for lattices in Xsuite to simulate polarizaiton. Kevin will make this available.
Pantaleo wonders why Xsuite takes a factor 33 longer to track. This is to be checked, could be settings.
Outstanding errors to be included: BPM roll, errors on octupoles and decapoles, FD stength being 1e-4 is very optimistic, measurement errors are all optimistic, RF voltage is 150 MV still (should be 90MV but there was a problem with emittance).
GHC tuning, Satya
Main update is the proper correction of coupling, which indeed now it is well corrected. Emittances are very good but need to be checked with tracking. Both DA and MA results after tuning have improved thanks to the coupling correction. Rogelio will ask Frank to inform reviewers about the new results.
Pantaleo asks Satya to tune the LCC lattice, if possible this is ideal. The problem is finishing the PhD.
Ewen asks about the source of improvement. Satya would need to check. Pantaleo adds that there are extra knobs to tune emittance.
Jorg asks for Satya's tuned files, that he will make available.
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