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FCC-ee optics tuning WG meeting
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FCC-ee optics tuning WG meeting
News and introduction, Jacqueline
The alignment group has approached us with some questions trying to approach the alignment approaches and the assumptions in the tuning simulations. First longitudinal misalignments from CERN experience could aim at 0.5 mm (1 sigma).
SuperKEKB plan is shown: 3 days for MD is shown from Dec. 8th. Proposals being prepared by Vahibhavi, Mael, Wietse, Patrick, and Christian.
Ilya: Elaf has been lately focusing on PETRA and could have some time now with more advanced tools. One option could be to study long-range misalignments and another is study the W for both GHC and LCC. Limitation from pyAT could be beam-beam. New files should be posted soon in the repository. Thanks to Ilya and Elaf!
Tuning the GHC lattice with Xsuite, Satya,
GHC v22 Z-mode. BPMs and correctors next to each quad. Normal and skew quad at all sextupoles. Arc quads and sexts are used for chromaticity and tunes. Total: 1856: BPMs and orbit corrs. 632 normal and skew quads.
Errors are introduced sequentially (first arcs, then IR, etc., at full value), followed by threading, orbit and tune corrections, and then sextupole ramp-up.
Relative strength error for dipoles is 0.001, and 0.0002 for all other elements.
Relative measurement errors for phase and Dispersion have been used, but these should be added in an absolute way, not relative (until calibration errors are introduced).
Usual error table is assumed with 0.1 mm of longitudinal error.
20% of the seeds fail. Linear optics and emittances are well preserved after corrections. Some visible reduction of DA is observed of about 3-4 sigmas in H.
Maximum orbit corrector strength of about 30-40 urad is used in the IR, while a factor 10 lower strength is required in the arc.
IP optics parameters are ensured with IP knobs.
By reducing the IR dipole misalignments to 0.1 mm 8 seeds were recovered. Another combination could be tried to probe the dipole tolerance in the IRs.
A more realistic error sequence could be to apply 1st full set or arc errors and then full set of IR errors.
Javier asks about how robust is the IP tuning versus BPM resolution, however this is not yet probed and simulations are still ideal at this phase.
The 0.1mm longitudinal alignment error was a first attempt. It can be increased.
Tuning studies with pyAT for FCC at Z, Kevin
Similar errors as Satya in the arcs.
Total: 2696 BPMs. All correctors first in the sextupoles (1864).
Emittance after correction is well recovered (only arc errors no BBA uncertainty): 30-40 urad corrector strength needed. Beta-beating well below 1%.
Adding BBA uncertainty of 10um does not affect DA and emittance (only 1 seed). H DA is reduced max by 1mm over 6mm.
Putting orbit correctors in quads maximum value goes down to 10urad max.
Adding (misalignment) errors in the IRs but not FD similarly as Satya (no strength errors): H DA is reduced by max 2mm from 6mm, but still in 20sigma acceptance). More seeds exceed the design value. Chromaticity is to be added.
Adding FD errors (complete IR errors): Number of singular values is critical for emittance performance and not so many successful seeds. Work is in progress. Jacqueline comments that the new upcoming LCC lattice should have a weaker FD which will probably help.
Satya asks if the Crab Sextupoles are on or off? They are ON (as Satya). They could be off as a first part of tuning.
Nobody is using IR strength errors. Work in progress.
Xiaobiao comments about the very large vertical emittance before correction, which cannot come from coupling. Kevin comments that emittance is probably dominated dominated orbit.
Orbit correction is extremely good for Kevin when using all quad orbit correctors (2600). Orbit
Kevin has octupoles and decapoles powered in the IRs.
BBA simulations for FCC-ee, Christian
Simulation procedure is performed up to orbit correction for high beta* optics. Errors are close to the usual ones but with 100um BPM-Quad error. After BBA many BPMs are aligned within 20um but many seem also to reach up to 250um.
Quadrupole and BPM centers are ~7um off due to the beam angle (BPM is at the edge of the quad). This value is similar between GHC and LCC.
Applying BBA 2 iterations reaching BBA of ~10 um or below, compatible with the beam angle systematic error.
In GHC 44/51 seeds were successful, while in LCC 33/51 but with 10-50um BPM-Quad error.
LCC IR is very sensitive to misalignments. Maybe diagnosing with errors only in the arc could help, or moving to new LCC optics.
BBA measurements in SPEAR3, Xiaobiao
Difference of BBA results for different Kmodulation depth is measured to be up to 10um. The results appear not to be systematic but this is not fully confirmed, more measurements are needed even at same Kmod levels. Systematic might come quad center shifts with K-mod.
Multipole tolerances, Wietse
After 5 units of systematic b3 in the dipoles clear signs of degradation in DA and other observables are seen. Maybe other effects should be included as a misaligment of 1mm in the dipoles.
Non-linear optics for the GHC lattice, Patrick
Simulations of the RDT measurement are presented. Sextupolar f1200 RDT OMC3 results align well with MAD-NG both for GHC and LCC lattices (factor 10 larger in GHC). f1020 RDT was not measurable as vertical excitation is very low.
Dipole b3 errors at 10 units have a moderate impact on the RDTs (mostly raising the floor values). Adding 1um BPM error compromises the results implying that amplitude and number of turns should be scanned.
Studying random errors in dipole b3 had significant impact at few units. Effect was stronger for the systematic b3 in GHC, however this should be better from other studies (to close check with Kyriacos).
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