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FCC-ee optics tuning WG meeting
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CERN
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FCC-ee optics tuning WG meeting
R. Tomas reports that an IPAC paper on the tuning efforts is being prepared, based on the FSR. For SKEKB, many things point that sextupoles abberations play an important role.
M. Takao present simulation studies on LER beam oscillations, commencing at the injection point, aiming to understand higher order modes. Even with only initial horizontal offset, a vertical osciallation is found. Strong resonances up to 13th order are found for large amplitude. At 1 rms momentum deviation resonances are enhanced, especially vertically. Beam-beam enhances them even further. K. Oide suggests to study the effect of crab-waist. R. Tomas suggest to check which non-linearities are included.
Q. Bruant presents tuning studies of the HEB. He reminds that in previous simulations only 78% of seeds survived. One of the main difference in the improved technique is ramping sextupoles in 4 steps, leading to 100% of seeds converging. Most seeds meet requirements on beta-beating, dispersion and emittance. Following a question by R. Tomas, Q. Bruant answers that tapering is not included, without SR. B. Dalena adds that radiation is only included at the end for emittance calculation. Following a question by C. Carli, Q. Bruant answers that no realsitic optics measurements are assumed.
M. Le Garrec presents first results on ground wave studies at FCC-ee Z-mode. The impact of the wave depends on the starting point of the applied wave. Furthermore, he finds that the vertical amplification factor does not depend on the harmonic number. The beam is displaced by twice the set offset. R. Tomas suggest to check this, also at other locations with larger beta. Another study uses measured vibration PSD at DESY, to reconstruct vertical shifts - news tbc.
P. Hunchak present some news on non-linear aspects for the FCC. He explores correcting amplitude detuning and second-order chroma with octupoles, ideally independent of each other. He finds that correcting first second order chroma and then amplitude detuning yields better results than vice-versa. With only 1 octupole severe DA losses are observed, which could be limited by using multiple octupoles distributed around the lattice. R. Tomas suggest to investigate further correction of RDTs, or more distributed scheme for magnet errors.
C. Goffing presents news from KARA including the I.FAST workshop from the previous week. He finds that different used orbits significantly change the measured offset up to 1 mm , suggesting strong contributions from orbit, angles, possibly BPM signals etc. With decreasing the beam current the rms offsets changes by up to 200 um. Going from 2.5 GeV to 2.3 GeV, or GeV 2.2 GeV shows rms errors up to 0.5 mm, one reason being not optimized models at lower energy. For 3 individual BBAs offsets agree within ~30 um with beam currents from 20 to 60 mA. R. Tomas clarifies that 60 um are already too large for the FCC. M. Boland comments that the gain control for BPMs could be checked. M. Boland asks if non-linearities of the BPMs are included, C. Goffing comments that this cannot explain the differences.
E. Musa presents tuning simulations with measurement noise. Until about 1x10-3 not much difference is seen in the final emittance or optics. However, MA starts dropping already from 10-4. R. Tomas comments that this is larger than in previous studies by K. Skoufaris and J. Keintzel. R. Tomas suggests to use new random noise in each step of the correction technique. Following a question by C. Carli, E. Musa comments that no IR errors are applied. C. Carli suggests to use more seeds.
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