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FCC-ee optics tuning WG meeting
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FCC-ee optics tuning WG meeting
Satya, Optics tuning with Xsuite
BPMs are placed at quadrupoles and correctors at sextupoles. Arc sextupoles are ramped-up slowly as orbit is being corrected. IR sextupoles are ramped-up slowly with optics corrections interleaved. BPM is solidary with quadrupole with a 10um error from BBA resolution. FF quads have 10um misalignment errors. Rest of IR has 30um and 50um as we approach the arc.
90% of the seeds successfully corrected without radiation. DA is above 17sigma for 4 seeds.
IP tuning knobs (beta*, waist and Dx) have been developed. All knobs worked as expected. IP1 and IP2 have a max 10% beating after correction. DA was not affected by applying the knobs.
With mean radiation only 80% of the seeds were successful. DA is above 15sigma in H for 4 seeds.
Next steps: move to RDT correction for coupling.
Vertical emittance not shown, to be added.
Barbara asked about the realism of the sextupole ramping. Rogelio commented that indeed the sextupole ramping has to be done with the ballistic optics as DA drops to zero for low sextupoles with the nominal optics.
Jack comments that there are now better ways to include SR in Xsuite. Quantum fluctuations to be looked into too.
Anton, DA with Quantum radiation
Studies are done with FCCee lattice from 2019, which had 2 IPs. A clear shrink of DA is observed when adding SR. Close to DA boundary particles spiral as damping but eventually particle amplitude bursts and get lost.
Within the DA some particles damp following the exponential law but at larger amplitudes the particles do not follow the exponential, being clearly slower in the start. At the DA the is no damping for a long time until the burst. Similar phenomena was observed in LEP but the damping-antidamping was not seen.
The power radiated by a 50sigmay particle in quads is dominated by the FF quads.
Analytical equations are derived included SR in quads and simulated motion is reproduced by the eqs.
The quantity B2 appears to determine the vertical damping or anti-damping, defining the DA. The phenomena relates to a parametric resonance. Phase advance between quads of pi/2 should help.
In horizontal plane the phenomena is different and relates to tune shift towards resonances. This is minimized by reducing the chromatic amplitude detuning.
Pantaleo mentions that a check for LCC could be useful. Kevin actually did calculations showing good agreement between prediction and simulation. Exact cancellation between FF quads and chromatic sextupoles does not appear for B2 as there is a phase term going with s/R. Rogelio mentioned that a compromise could be found for this phase or for the phases between IPs as the preferred phase between IPs should be close to pi/2 with the extra contribution from the s/R term.
Kevin, LCC tuning simulations
Similar errors as Satya, excluding IRs and BPMs. 3 seeds have poor vertical emittance. DA is good for all cases. Very good beta-beating and dispersion beating. Strange beta-beating centered at -0.2%. IP parameters are well under control without IP knobs.
Considering long-range errors that zero at the IRs will be considered at a later stage.
Correction strength will be added to the slides as this input is needed for the magnet experts.
Other correction configuration should be explored with H corrector at dipole edge, quad corrector at quad, etc. (Pantaleo suggested that it is easy to include V corrector also at the dipole edge).
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