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FCC-ee optics tuning WG meeting
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Europe/Zurich
R. Tomas reminds that we aim to write an IPAC paper on the progress on the FCC-ee tuning studies, presented by L. van Riesen-Haupt. Furthermore, an extended version will be submitted to PRAB.
S. Liuzzo presents a comparison of sensitivity studies for CEPC ttbar and FCC-ee lattices. It is concluded that for most parameters at ttbar, LCCO is more resilent over V22 over CEPC. Final focus quadrupoles refer to all elements in the EIR. Following a question by K. Oide, S. Liuzzo replies that 10 error seeds are used, and K. Oide suggests to also show the error bars. G. Roy notices that LCCO has 152 FF sextupoles, while V22 has only 16. This was identified as a typo after the meeting.
Random errors in the order of up to 1um are applied, which is the maximum without performing beam threading. Sextupoles are on. AT, MAD-X and PTC give slightly different results for the FCC lattice (about 1 um). PTC is closer to AT. The descrepancies stem occur in the EIR, while the arcs agree very well. C. Carli asks why a sudden jump is observed at 0.8 um using AT, which remains to be understood. R. Tomas mentioned that likely this could be mode swapping between X and Y due to coupling. After orbit corrections with SVD (using different steering magnets and SVD values) MAD-X gives larger errors and requires stronger steering magnets. G. Roy comments that the MAD-X output contains numerous information where many used SVD parameters are summarized. Changing in MAD-X the SNGVAL does not seem to change the correction result, where it is suggested to send a request to the MAD-X team.
C. Carli suggests to use commissioning optics for these studies. T. Charles comments that in her studies this did not make a difference. It is noted that in previous simulations by T. Charles has introduced errors with adiabatically increasing strengths and corrections in between. R. Tomas comments that also in other machines errors have been adiabatically introduced, since otherwise for commissioning simulations a more complex beta-squeeze should be introduced.
S. Liuzzo recommends to also name the elements in the FF elements for the FCC IR, as done for the CEPC.
T. Charles points out that an Australian Synchrotron AS2.0 is currently being designed and synergies with the FCC could possibly be found. Furthermore, she updates on tuning studies with MAD-X for ttbar. Correctors, BPMs, trim quadrupoles and skew quadrupoles are introduced. For tune correction she uses the quadrupoles in the RF sections. K. Oide suggests that special quadrupoles in the straight sections (QI, QU, QR) should be used. While linear optics seems good, DA is very low. However, with chromaticity correction a large fraction of seeds start to fail and the vertical chromaticity is drastically increased. K. Oide suggests to also correct chromatic phase advance.
S. Liuzzo comments that one synergy could be commissioning simulations.
A. Hussain presents on tolerances of mulipole errors at Z and ttbar in V22. Following a question by J. Keintzel, it is confirmed that a reduction starting from about 5 sigma reduction is currently considered significant, however further studies with combined errors might change this assumption. K. Oide suggests to not evaluate DA and MA at the IP, which is currently being done. The following tolerances for Z-lattice are TO BE CHECKED AND TABLE ADDED
- b3 random errors in the arc dipoles is 4 units
- b3 systematic error in the arc dipoles is 14 units
- b4 random errors in the arc dipoles is greater than 4 units
- b4 systemtic errors in the arc dipoles is greater than 10 units
- b3 random errors in the IR dipoles is 6
- b3 systematic errors in the IR dipoles is 2
- b5 random errors in the IR dipoles is 2 units
- b5 systematic errors in the IR dipoles is 2 units
- b3 random errors in the arc quads is 1 unit
- b3 systematic errors in the arc quads is 3 unit
- a3 random errors in the arc quads is 1.5 units
- a3 systematic errors in the arc quads is 0.5 units
- b6 random errors in the arc quads is 1 units
- b6 systematic errors in the arc quads is 1 units
- b5, a5, a4 random errors in the arc sext is larger than 6 units
- b5, a5, a4 systematic errors in the arc sext is larger than 6 units
For ttbar the tolerances are more relaxed.
Y. Wu presents updates on orbit corrections for polarization studies in MAD-X and BMAD. SR and sextupoles are on, which are switched on adiabatically. With increasing random errors (30 to 100 um) in non-IR elements the achievable polarization has a wider spread over the seeds, caused by larger vertical orbit or dispersion. With BPM errors (10% missing and 1 um resolution and 1 % scaling error) the final closed orbit is larger. With additional IR misalignments of 10 um about 20% of seeds fail. Increasing to 20 um half of the seeds succeed. With additional long range misalignemnts sextupoles could not be switched on. With long range misalignments the rms vertical error after correction is about a facto 2 to 3 larger. Investigations to switch the sextupoles on are ongoing, including the impact on polarization. S. Liuzzo comments it is shown that with long range misalingments a vertical rms orbit of 40mm is found, which, nevertheless could be corrected which is promising. He suggests to also assume that the BPMs are following the long range misalignments. Yet, as mentioned by R. Tomas, it could be that best target orbit is with in a plane and K. Oide suggests to use survey measurements to identify location of the ideal plane. J. Keintzel suggests to also study the impact on the spin tune.
S. Liuzzo reports that adiabatically increasing sextupoles can also be implemented in his future studies.
B. Dalena comments that for the booster studies segment-by-segment is used, which is similar than beam threading. That technique together with sextupole ramping is found essential to obtain a closed orbit of most of the seeds.
Concerning the long term misalingments, F. Carlier asks if the model is accurate enough, since the IR has tighter tolerances. To be investigated.
The next meeting is aimed to take place in about 2 weeks, where X. Huang could present sextupole BBA studies.
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