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FCC-ee optics tuning WG
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6/R-012 - conference room (CERN)
FCC-ee optics tuning WG meeting
News, Jacqueline
Very interesting SuperKEKB presentations yesterday in the design meeting, connected to today's presentation from Mael.
SuperKEKB non-linear measurements, Mael
On the 9th of December there were studies at squeezed optics on non-linar topics including cancel coil error. However action is low and not good RDT/Amp det measurements are seen (linear optics measurements OK). From the November studies RDTs can be observed, which is with relaxed optics beta*y=8mm.
OMC (LHC) GUI can be used now for SuperKEKB data. Resynchronization of BPM turn done in the past as provided by Jacqueline.
Good beta-beating measured below ~10%. Small changes in beta-beating from cancel coil change in the vertical plane. It should be possible to compare to model predictions with misaligned cancel coil. Jack mentioned that this shift might not be easy as the model is complicated. Oide san clarified that a thin magnet is inserted to fake the cancel coil, hence this could be shifted for the study.
For Oide-san dispersion beating seems OK, but units should be checked. Cross-check with other methods possible.
For November measurements with beta*y=8 mm RDTs are measured. Discrepancy of factor 2.4 is observed in f1200. Tracking is with Xsuite single particle. Comparisons to SAD tracking have to be done. Jack remarked that this model is not fully rematched so some small discrepancies should be expected. The phase of f1200 agrees much better than amplitude. Similar conclusions for f3000, but closer amplitude. For f1300 the amplitude difference is very large (factor 3), but phase again has good agreement. Could BPM noise do this? Is there global problem with action or other amplitude issue? For f4000 the mesaurement disagrees more heavily. Being close to the resonance might enhance amplitude discrepancies and less phase discrepancies.
Similar conclusions go for LER, but not comparison to model available.
The kick amplitudes in 2025 were about factor two lower than in 2024. This is not understood as kicker and optics did not change.
Careful model checks should be done.
a3 in FCC-ee at Z, Wietse,
Wietse presents current sensitivities of DA to errors. Pantaleo highlighted that 'tolerances' should be replaced by 'sensitivities' and tolerances will come once correction techniques are studied, which is the matter of today.
Resonance analysis points to f2001 being relevant for DA (but not so limiting), and 2nd order dispersion together with chromatic coupling might relevant for MA.
First correction was attemped to correct a3 effects adding many skew sextupoles (~20 per octant) looks very successful in restoring MA. Correction is computed via response matrix correction. The correction needs to correct RDTs and chromatic terms at the same time as partial corrections deteriorate either DA or MA.
First correction schemes were tried with few number of correctors. 8 skew sextupoles separated by 90 degrees showed promise. Challenge to make this scheme at all energies as arc phases differ.
Pantaleo proposes to correct chromatic aberrations only at the IR. Studies with relaxed beta* could also help understanding.
Beam-beam optics corrections, Wietse
Optics is studied by introducing a thin less beam-beam element at the IPs. An optics rematching is performed to compensate the optics distortions from beam-beam. This avoids distorting optics at e.g. wiglers. This raises questions on the applicability of this approach as beam-beam is amplitude dependent and therefore particles at large amplitudes would see the optics deviations.
In tracking simulations correcting the beam-beam linear optics improves the vertical emittance partially, improving also luminosity by ~10%. Pantaleo clarified that full recovery is not expected.
Rogelio suggested to use seeds from Kevin to have a realistic source of coupling.
Pantaleo clarifies that this approach corrects beam-beam beta-beating but not tune shift. Changes on injection efficiency and interplay with solenoid could be expected. Frank asks about beta-function at collimators as this introduces a large beta-beating.
Simulations of AC dipole in FCC-ee, Dot
Realistic tracking studies are performed with AC dipole. Effects of amplitude detuning and SR are clearly visible in the resulting forced amplitude. Suitable tune separation settings are identified for different energies. Strong damping of the forced oscillations are observed when turning on SR. Similar effects are observed both for GHC and LCC.
Beta-functions measurements using beta-from-phase have a ~2% error only from SR at tt. This needs to be understood.
Exciting in H induces an oscillation in V. This is not understood.
Studies for actual beta-beating errors should be revisited controling forced amplitude.
Wietse asked about improving adiabaticity, to be studied.
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