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FCC-ee optics tuning WG meeting
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FCC-ee optics tuning WG meeting
Goal: rediscuss with magnet experts on the status of magnetic tolerances and review progress for the comparison report.
Jeremie comments that the knowing the orbit corrector strength is very important to completely dimension the sextupoles. The possibility of having the horizontal orbit corrector in the dipole is still an option (in GHC corrector strength assumed was 20mTm). Impact of orbit correctors in sextupoles was given in Rome.
GHC tuning, Satya
The addition of random a2=0.1e-4 errors have been considered in the arc dipoles. Oide-san comments that BPM-to-Quad roll of 10urad might not be realistic. Simulations neglect orbit and coupling measurement errors. 20 seeds were tuned successfully. The a2 errors increase significantly the Dy after correction. Vertical emittance is OK. Strong degration in both DA an MA. IP knobs do not degrade or improve DA/MA.
For orbit correction 10mrad are used in the arcs.
It seems not strong degradation is seen from adding a2=0.1 random in dipoles, but a closer look is needed (starting point for tracking has been changed since last presentation).
Field quality tolerances, Kyriacos, Abid
LCC has many more sextupoles than GHC.
Tolerances are computed for 5% and 10% drop of MA area for both GHC and LCC applying one error at a time. Clearly, LCC has tighter tolerances in the IR with more of them being below 1 unit.
First allowed harmonic in sextupoles, b9, would be needed.
Oide-san comments that 45 degrees in x/y is a bit heavy for vertical direction, he assumed design emittance ratio.
b1 and a1 quad errors to be discussed in next presentation as it is known to be important in the arc quads.
Magnet point of view, Jeremie
Starting from Kevin's LCC proposal in June (which was similar to GHC).
For b1, a1 quad errors the orbit correction should be dimensioned to control them. Dipole a1 should probably be above 1 unit.
b1 in dipoles could be up to 100 units from magnetic point of view.
Magnet experts would hope to relax a2 from the 1 unit presented by Kevin, however Satya saw a clear impact by adding a 0.1 unit. Guidelines will be provided later on.
b3 of 0.2 seems small, however larger values would require longer sextupoles.
For Abid's a3 in dipoles of 0.1 magnet experts need to do the study to provide a feedback (news in 2026).
Pantaleo comments that distribution along the dipole would be needed to perform more accurate studies.
Daniel proposes for magnet experts to propose what is achievable in terms of field quality (Melvin would be in charge of this with help from Alexey). This will take some time, maybe mid 2026. In parallel Wietse will study possible correction circuits.
LCC tuning, Kevin
Previously it was observed that envelope emittance calculation was giving much larger than tracking. Emittance is now computed from tracking after 6000 turns and 400 macroparticles.
a2=0.1e-4 added in simulations. Version is 106. Sextupoles are ramped from 0 to 100%. Arcs include dispersion suppressor. No chromaticity correction used. Longitudinal errors are included.
2 seeds failed out of 20. Large variation in momentum acceptance. Extremelly small vertical emittance.
Quantum reduces DAs to 15 and 60 sigma x ,y.
The problem with the BPM misalignment seen in last presentation dissappeared when not pushing the orbit correction too much.
Convertor between pyAT and Xsuite is in progress, for next year.
BI assumes one BPM per quad in the arcs. The BPMs in the FD are not yet decided.
BBA, Christian
Simulations for both LCC and GHC. Few more seeds failed for LCC. BBA error ends up being 10 um or below for LCC.
Some issue is seen in GHC as there are more outlayers affecting the rms while fit gives similar 10um resolution.
To be checked if tunes hit a resonance for stronger modulations.
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