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FCC-ee optics tuning WG meeting
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FCC-ee optics tuning WG meeting
R. Tomas opens the meeting and shows schematically how the arc alignment could look like, including girders. The vertical dipole and skew quadrupole could be combined with the sextupoles. The horizontal orbit correctors could be attached to the dipoles. No movers are foreseen in the MTR, and therefore the orbit between sextupole and quadrupole would be offset by a few µm. This results in a tradeoff between quadrupole and dipole corrector strength. S. Liuzzo comments that the reference orbit or golden orbit could refer to the actual orbit after BBA, while survey orbit could refer to the one of the ideal machine. Following a question by B. Dalena, R. Tomas clarifies that in the MTR dipoles over 4 arc cells are powered in series.
H. Sugimoto presents BBA experience in SuperKEKB. For quadrupole BBA dedicated power supplies are used. BPMs are attached to the quadrupoles. For quadrupoles the orbit change in all BPMs is measured if one quadrupole is modulated and the resulting offset is determined as the average of estimated offsets. Magnetic interference between HER to LER is observed close to the IP, and thus, LER and HER BBA are performed seperately. The measurement reproducibility is in the order of 50 to 100 µm. One possible limitation are orbit fluctuation during measurements. C. Carli suggests to analyse the difference between 2 measurements to confirm if it stems from orbit drifts. R. Tomas comments that this means that the machine could move by a few µm over a few minutes. X. Huang comments that if one modulates one quadrupole, the orbit changes and thus he suggests to use multipole correlated BPMs. Horizontal offset is larger (50-60 µm) than vertical (25-40 µm), which could be caused by the mounting structure between BPMs and quadrupoles. The shift of the quadrupole center during modulation is not considered.
For sextupole BBA skew and/or normal quadrupole coils could be used. BBA for crab sextupoles has been performed in 2018, yielding horizontal and vertical offsets up to ~ 400 µm. Using skew quadrupoles seems to be preferable. Following a question by R. Tomas, for sextupole BBA the orbit is corrected towards the center of the skew quadrupolar coil. In the future one could explore using the sextupole to perform BBA.
X. Huang presented BBA for the FCC-ee. Initially the induced orbit shift (IOS) by sextupole modulation has been corrected using orbit bumps at sextupoles, which does not scale with many sextupoles. For the new method sextupoles are modulated and the center of the sextupoles is obtained using a response matrix method. The resulting precision is in the order of a 20 to 50 µm using 6 sextupoles. It is reminded that for quadrupoles it is in the order of 10 µm. It is suggested to evaluate the difference between sextupole and skew quadrupole in the FCC magnet design.
S. Liuzzo presents updates on the code comparison between MAD-X and AT for tuning studies in 4D simulations. Both codes agree well with increasing sextupole and quadrupole errors. It is noted that the emittance computation starts to fail with larger errors. The optics including RDTs are sucessfully corrected. Sextupoles are ramped in 5% steps. Applied corrections seem to be sufficient to improve DA until about 15 µm (with IP errors). Without IP errors large errors (80 µm) could be applied and resulting optics errors and DA are improved. This suggests that dedicated IP corrections must be derived.
S. Sai shows progress on the IP tuning knobs. He generates knobs based on T. Charles studies and a lattice with 95 km circumference. After orbit and optics corrections using knobs at 1 IP is shown successful. However, they seem to break once they are at multipole IPs at the same time, which is caused by knobs that are not fully closed in an lattice with errors. Orbits around 1 µm are present after optics tuning studies. Improving that value with additional orbit corrections in the IR could help improving the IP tuning knobs. With improved knobs all IPs could be corrected successfully at the same time. R. Tomas comments that the studied lattice is about a factor 10 better than in her latest studies. S. Liuzzo comments that in his studies with half of the error strengths, the rms orbit is a factor 10 larger. Following a question by S. Liuzzo, R. Tomas replies that e.g. the beta-waist could be measured based on luminosity. More detailed considerations how to measure IP parameters should follow in the future.
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