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FCC-ee optics tuning WG meeting
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Europe/Zurich
6/2-004 (CERN)
FCC-ee optics tuning WG meeting
R. Tomas presents the sections linked to the tuning team in the FSR:
https://www.overleaf.com/1878582227hgjndpdtjnpn#6df203
X. Huang presents updates on BBA for the ballistic optics using PQMS (Parallel Quadrupole Modulation System). Girder errors of 150 µm are applied and magnet errors of 50µm. R. Tomas comments that the girders should only be ~6m. X.Huang answers that he assumes this to avoid numerical errors in his simulations. Additional rms orbits are 8.3 mm in X plane and 15.7 mm in Y plane, which is corrected by response matrix approach, followed by optics corrections based on LOCO. 10 QF2 are modulated in parallel and 1 µm BPM noise is assumed. Using the response matrix of the lattice with errors yields drastically better results. Also, using BPMs only next to the modulated quadrupole group improves the results. With this, an accuracy between 20 µm and 50 µm is achieved. R. Tomas estimates the rms value to ~15-20 µm. His technique does not require correctors at each quadrupole.
C. Garcia presents the new ballistic optics. In the region of 200 m until the first quadrupole after the first dipole all quadrupoles are switched off. Compared to the squeezed optics DA is increased, but MA decreased. Also all sextupoles in the IRs are switched off. Sextupoles in the arcs are optimized to increase DA and MA. The optimization is perfomed in XSUITE, similar to the technique of K. Oide. K. Oide comments he starts with chromatic correction (tune, and beta-function in the range of +/- 1% in 24 steps; tolerance until ~.01) before sextupole optimization. R. Tomas suggests to add this optimization to the tool. In contrary to K. Oide's optimization, the polarity of the sextuoples is contraint for C. Garcia.
A. Hussain presents updates on field tolerances, including beam-beam. Compared to previous studies tune and chroma is corrected. Tracking is in 6D. R. Tomas comments that 0.2 units systematic b3 error in arc dipoles seems a bit pessimistic and 0.3 seems more realistic. C. Carli comments that the systematic b3 error can also be corrected with adjusted sextupole settings. Maybe also similar for higher-order multipoles. Following a question by T. Pieloni, A. Hussain answers that 2.3e11 particles are used. T. Pieloni suggests to also look at lifetime, since simulations by L. van Riesen-Haupt have shown low dependence of lifetime on MA and DA in beam-beam simulations with errors. R. Tomas suggests to re-run simulations defining tolerances on dispersion errors. Following a quesiton by C. Carli, R. Tomas answers that magnet people are confident to achieve 1 unit.
F. Zimmermann and C. Carli suggest to show absolute, integrated tolerances.
It is suggested to try to define some merit functions based on e.g. area, reduction of tails, largest reduction step, etc.
M. Bai suggests to also calculate some parameters for this seeds, like amplitude detuning, (non-linear) chromaticity, RDTs, etc.
J. Keintzel presents updates on phase advance measurements between crab-sextupoles. 1µm BPM resolution seems ok. It is suggested to try testing these measurements at SuperKEKB. F. Zimmermann asks if this could maybe also be achieved at DAFNE.
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