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FCC-ee tuning WG meeting

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R. Tomas reports that M. Hofer will leave the team and G. Roy will take over the responsibility of the GIT repository. G. Roy reports that questions can be addressed to fcc.optics@cern.ch.

M. Hofer states that the repository to generate a grid with different errors of different length scales. Running the lra_studies notebook up to cell 7 should generate a grid.csv file. R. Tomas already used it and he should be also able to help. Also best to discuss with S. Liuzzo on how he implemented them in AT (see last tuning meeting slides): https://gitlab.cern.ch/mihofer/fccee_xample_longrange_alignment 

I. Agapov thinks that long-range misalingments (above 2-3 times the betatron wavelenghts) should not have a major impact on the optics. R. Tomas comments that recent  simulations by S. Liuzzo have shown unexpectedly strong impacts. To be clarified in future studies.

IPAC papers related to FCC-ee tuning:

  • All: The status of the FCC-ee tuning studies
  • S. Sai: Simulated performance of FCC-ee IP tuning knobs
  • X. Huang: Beam based alignment simulations in FCC-ee

Y. Wang reports that the closed orbit could not be found using AT for the FCC-ee lattice. He reports that misalignments of the girders and the elements are applied on top of each other and it is found that the tune moves to integer or half-integer values, considered being the main problem. LOCO is applied, following by Dy and tune correction. This was not observed for the CEPC which uses SAD. Y. Wang will use the FCC SAD lattice for future studies.

E. Musa presents updates on tuning simulations using AT for V22 and HFD79. No BPM errors are included. Girder misalignments are not considered. The first case studies miaslingments without rotations: In V22 only applying up to 20 µm , all seeds yield acceptable vertical emittance about 1pm. With 50µm or larger errors, seeds start to fail. In HFD79 errors impact the optics less and up to 70 µm yield sufficiently low vertical emittance of about 1pm. IR elements are more sentitive to erors. In V22 without orbit correction only 0.4 µm could be applied to the IR components. With orbit correction up to 8µm could be applied. Beam tests at PETRA could be performed applying LOCO. HFD79 does not include the crab sextupole. Following a question by B. Dalena, E. Musa clarifies that all arc quadrupole and sextupoles are used for tune and linear chromaticity correction, and the same tuning procedure is used. G. Roy suggests generating dedicated knobs for tune and chromaticity corrections to keep e.g. phase advance constraints.

I. Agapov comments that only with 8 µm IR misalignments and orbit corrections a closed orbit could be found seems worrying. R. Tomas comments that CEPC studies have demonstrated 100 µm misalignment, even in the IRs. Dedicated IR tuning knobs are being developed by S. Sai.

Furthermore, these corrected lattices should be used for beam-beam studies. 

Q. Bruant presents updates on orbit correction studies for the HEB using MAD-X and CpyMAD. Girder rotations or long range misalignements are not yet included.  Two optics versions (FODO and HFD) lattice are tested. The HFD lattice is found to be more flexible and less impacted by misalignment errors, and closed orbit is found for all seeds (only 81 for FODO). Similar corrector strengths are required for both lattices. Without optics corrections the HFD featues factor 2 lower vertical beta-beating and vertical dispersion and 33% lower horizontal beta-beating. It is suggested to apply an iterative optics tuning appraoch, similar to other studies. I. Agapov suggests to evaluate which optics corrections and emittance tunings are required.

E. Ahmadi presents on tolerances of systematic multipole errors for DA using X-Suite and considering SR for V22. At Z-mode the multipole errors (especially b4, b6) in the IR quadrupoles (QC*, QY*) have a strong impact. It is shown that only 0.1 units of b4/b6 are acceptable for DA. For the arc quadrupoles, systematic multipole errors have no significant impact on DA. One unit of b3 or b5 in the IR dipoles with SR gives about 20 sigma horizontal and ~50 sigma vertical. R. Tomas comments that 6D DA in X-Suite for the ideal lattice should also be computed, to allow direct comparison. Furthermore, he recommends checking each magnet one-by-one and the impact on the various multipoles. This should also include checking the MA. 

M. Hofer reports on behalf of A. Hussain  concerning studies on tolerances of mulitpole errors in the Z-lattice and ttbar-lattice. Z: Systematic and random errors are considered. No measures to correct these errors are applied. 2 units b4 errors arc dipoles reduce the DA and MA drastically. 2 units b3 errors in arc quadrupoles do not affect the DA and MA very much. About 2 units of a3 in the arc quadrupoles reduces only the MA. 2 units of b6 in arc quadrupoles do not impact DA or MA. 2 units of a4/a5/b5 in arc sextupoles do not impact the DA or MA. ttbar: Thanks to the fast damping time of about 50 turns tested errors of 2 units do not seem to impact the DA or MA significantly. M. Hofer suggests to also check the interplay of various multipole errors. 

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