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FCC-ee optics tuning WG
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Europe/Zurich
354/1-016 (CERN)
FCC-ee optics tuning WG meeting
LCC tuning and corrector update, Kevin
Failing seeds recovered by introducing the random quadrupole change used by kostas in Petra IV simulations.
Different corrector configurations are presented, which clearly have an impact on correction performance. The newly proposed baseline (v orbit correctors only at sextupoles) does not work well. Adding extra v correctors in the cells without sextupoles (QD1AM) fixes the problem (still poor emittance at tt after correction). Changing BPM locations does not have an impact on performance (baseline is BPMs at quads).
To fix the emittance problem at tt kevin proposes moving tunes away from Qx-2Qy (upwards).
Alexey comments that the requested vertical corrector strength is too strong for the sextupole length. Pantaleo proposes to reduce the number of eigenvalues to ~1200 to reduce the used strength before concluding. Other possibilities are independent correctors or vertical correctors in the bends. The skew quadrupole seems also too strong, to be reviewed.
Another solution proposed by Rogelio is to move all vertical orbit correctors to the quadrupoles. This will also liberate space for skew quadrupole coils and make feasible the skew at sextupoles.
Summary: Vertical corrector in quad 40mT, skew quad to the sextupole 150 mT , normal quad in quad , Horizontal corr in main bend.
Sofia proposed to look at a stand-alone HV orbit corrector that is needed in the straight sections.
Jorg explained that he used the Alternative corrected lattices and he sees negative polarization. Earlier studies in GHC had polarization between 20 and 80%. This needs further investigations.
Optics tuning with Xsuite of LCC, Georgios,
Georgios presents that a full framework in Xsuite is up to speed that shows very similar performance than Kevin's simulations. The framework is going to be published very soon. Georgios needs to finish thesis but maybe Satya could produce comparisons in close collaboration with Kevin to show that both codes agree on the same plots.
Optics tuning of GHC in Xsuite, Satya,
Dy measurement error is increased to 50um. DA and MA after correction look reasonable with some degradation, specially in MA. Including IP tuning knobs has no impact on MA.
A detailed description of the python codes used is given.
The impact on DA/MA from tune correction was negligible.
It is recommended to have a single code for tuning in Xsuite.
Impact of systematic b2 and tapering, Wietse
b2 errors introduce 20% beta-beating. Correcting using simply kf and kd reduces beating to 1% and uses 1% strength of quads. Using cell symetry provides a better correction down to 0.3% and 0.6mm in Dx with 0.1% strength. Dispersion suppressor needs to be explored. DA and partition numbers need to be checked. Errors and corrections should be part of the design using trim variables for quads.
Partial tapering of quads does not deteriorate DA and MA, although baseline is to have individual powering of quads.
FCC-ee non-linear studies, Patrick
Systematic b3=0.5 of dipoles reduces DA but it is recovered with Q' correction. Random b3 has no impact.
Similarly systematic b4 affects DA and RDTs. However measuring RDTs represent an important challenge. Pantaleo proposes to correct b4 by unbalancing strength sextupolar families. Patrick explored dedicated octupolar knobs. In particular correcting amplitude detuning and Q'' shows very efficient.
Probably octupoles (and skew sextupoles, from Wietse's presentation in last meeting) will be needed anyway at entrance and exit of FFS.
Optics aberrations tolerances from Beam-beam, Vaibhavi.
Tolerances are given for all optics aberrations at the IP. Tight waist shift, Dy and coupling tolerances are found (slide 20). These tolerances could be optimistic as lattice transport is not tracked. Future tuning simulations should use this reference table to compare performance. The simulation showing that the flip-flop instability could be recovered by correcting coupling every 200 turns might not be representative of reality (real lattice tracking with aperture constraints, 4 IPs, etc.). The goal would be to devise a tuning procedure to guarantee all the IP parameters to be within tolerances (this tuning could include luminosity scans at a single IP).
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