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FCC-ee optics tuning WG meeting
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FCC-ee optics tuning WG meeting
J. Keintzel and R. Tomas report the following:
The briefing book for the ESPPU is released: https://cds.cern.ch/record/2944678?ln=de
WG2a bullet point list will be released on 17 October.
It is aimed to work further together with SKEKB colleagues.
A meeting with survey took place to understand alignment tolerances and requirements. Follow-up meetings are foreseen.
C. Goffing presents updates on the pseudo-ballistic optics for GHC and LCC. In GHC, outer IR quadrupole (individual magnets) misalignment needed to be reduced to achieve closed orbit. For LCC nothing between the crab-sextupoles have been misaligned. P. Raimondi comments that in principle 50um to quadrupoles and sextupoles after the vertical chromaticity correction could be applied. Versions 25 are used for LCC and GHC. For LCC more BPMs and correctors needed to be added compared to the official versions distributed for the comparison. For now no BPM to quadrupoles offsets are applied. P. Raimondi suggests looking at offsets relative to the sextupole.
In GHC applying a Gaussian fit, the rms beam offset in quadrupoles relative to the magnetic center is in the order of 10ish µm. In LCC, calculating the rms gives values up to 100 µm horizontal and 44 µm vertical, driven by quadrupoles which do not have a BPM and corrector next to them. P. Raimondi points out that this could also be bad for polarization, due to strong additional kicks of half a mm. R. Tomas comments that Y. Wu had about the same orders-of-magnitude. J. Wenninger asks about optics corrections, where C. Goffing replies only orbit threading and correction is performed before evaluating the relative offset with respect to the magnetic center. J. Wenninger points out that it will probably be more iteratively between threading and BBA, which should be decided in the next years.
Concerning the threading, J. Wenninger asks how orbit threading could be performed without sextupoles. R. Tomas explains that the orbit is stable enough with the ballistic optics without sextupoles, while, indeed nominal optics without sextupoles is not stable. J. Wenninger explains that LEP used MICADO instead of SVD for beam threading. K. Skoufaris explains that for LCC with sextupoles beam threading could be performed. R. Tomas suggests to check again GHC too. P. Raimondi adds that for EBS, in the end sextupoles were switched on for threading, but then for closed orbit, SVD with only half of the eigenvalues were used. J. Wenninger adds that it is similar to LHC, with only few eigenvalues, and once orbit is established, more eigenvalues are used.
V. Gawas presents linear transfer-maps in Xsuite. To generate coupling, skew quadrupoles are inserted before the final doublet or after the crab-sextupoles. It is shown how much the IP parameters (beta, alpha) change with powering this skew quadrupole.Changing the crab-sextupole strength also effects the components of the coupling matrix slightly. Furthermore, the coupling RDT at the IP is also modified with the crab-sextupoles or introducing coupling. R. Tomas asks if this could be from an offset of the sextupoles or in them, which should be checked.
R. Tomas comments that it could be that twiss is not found because of a numerical effect. He adds that these studies is the first step of performing the scans.
A. Hussain presents updates on field error tolerances without beam-beam for V25.3 GHC. He compares a factor with a step approach for chromaticity correction, where no significant difference is found. Systematic b3, b4 and b5 are studied. Comparable results are obtained for this version and the older GHC version. Random errors are bit more relaxed than systematic ones.
Concerning the dispersion error, so the uncertainty of the dispersion measurement is probably larger than 1 µm, but better than 1 mm. It should be reviewed. If beam is displayed by 0.5 %, assuming BPM resolution, the dispersion resolution should be studied. P. Raimondi estimates that 10 µm should be achievable. C. Carli suggests looking at the maximum corrector strength.
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