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FCC-ee optics tuning WG meeting
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FCC-ee optics tuning WG meeting
Zoom Meeting ID
65518046965
Host
Rogelio Tomas Garcia
Alternative host
Jacqueline Keintzel
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Tuning studyes with Girders, Elaf
V23 is being used however there is a technical problem and studies are still withi V22 lattice.
Attaching BPMs to quads or sextupoles does not show significant differences before using girder simulations (no error between BPM and quad/sext is assumed).
Nominal simulations with girders (150um) are also done. Emittance is satisfactory but DA shows some deterioration, however no seeds fails in the girders case.
DA is computed starting tracking from cavity.
Progress with tuning simulations and with IP tuning knobs, Satya
Tuning steps include optics corrections during the sextupole strenght ramp-up (Elaf only does this when sextupoles are at 100%).
40% seeds failed with 130 um in the arcs and 40um in the IR. DA looks OK for some seeds.
IP tuning knobs are added to regular tuning successfully.
Oide-san comments than in reality we might not know waist or Dx,y at the IP -> Use advanced measurements and or luminosity scans.
DA seems to improve for the seeds with poor DA after the IP tuning. This should be better shown next time as this was one of the pending topics to demonstrate.
Ballistic optics, Cristobal
Switching off quads and sextupoles in about 200m around the IP allows for alignment and BPM calibration. Peak beta functions are reduced in the IR. Vertical chromaticity goes down from -3500 to -500. DA is very much improved however momentum acceptance is deteriorated, which should be improved by optimazing sextupoles. Strategy to optimize MA could be similar to how Oide-san does it: Scanning each family one-by-one, or combine families to cancel up to 4th or 5th order (interesting to try).
Studies with relaxed optics, Kyriacos
Two relaxed optics: one from Oide-san (high beta*, V24) and the previous ballistic optics. These are needed because when reducing sextupoles for tuning DA and MA reduce dramatically in the nominal lattice.
The high beta* lattice and half sextupole has acceptable but short lifetime for trheading however equilibrium emittance are orders of magnitude larger than nominal.
Witih the ballistic optics and half sextupoles the lifetime is double than for the high beta* optics and the emittances are close to nominal. In addition optimazing the sextupoles in the ballistics optics would further improve. All studies are for Z.
In summary there is a sequence of commissioning optics however optics measurements should be targeted when sextupoles are at 100% or close to it.
Beam based alignment test proposal at KARA, Christian
Plan to performed BBA interleaved with optics measurements to monitor possible improvements or magnetic hysteresis. Simulations results show good BBA performance of about 10-20 um for 3 quad families. This deteriorates a lot for families with worse configuration BPM/quad. Adding BPM noise deteriorates performance to 40-50um. With good orbit control this might improve significantly. Parallel BBA should also be explored in KARA.
Sextupole BBA is also studied in KARA. The proposed method in the H plane uses orbit bumps and orbit correctors to cancel the induced orbit perturbation when the beam goes off-center in the sextupole. In the V plane the change of coupling would be used. Simulations show large variations of BBA performance in H in some seeds (rms values betwee several um and few mm.). In V performance is more homogeneus reaching ~60um. The required coupling measurement resolution is challenging. Simulations were done only with single sextupoles.
Sextupoles are powered in families. Jorg asks why the orbit correction is needed, just orbit perturbation could be more accurate. Reason is to separate better offset sextupole by sextupole. Concerning coupling Jorg says that closest tune approach is not accurate. Local coupling RDTs would be the only way to distinguish between sextupoles. Jorg suggests to try tests in the LHC as the KARA configuration is not closer to FCC than LHC , plus LHC tools are very performant.
BPM resolution needs, Jacqueline
Motivation is requirements in phase advance resolution needed as computed by Kyriacos. The phase mesaurement in the IRs seems very challenging, specially in the IR. Maybe 1um resolution and 50000 turns could do reach target in the IRs. In the arcs few um resolution is enough. Resolution can be achieved by kicking many bunches.
Christian proposes to correct for the systematic error in the vertical plane. Rogelio mentioned the studies from Lukas and Andrea Franchi on this. Hopefully 50000 turns could still do it at low amplitude kicks. Oide-san mentioned that the spread also increases with amplitude which is a bit more puzzling.
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