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FCC-ee optics tuning WG
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6/2-004 (CERN)
FCC-ee optics tuning WG meeting
Jacqueline, News: BPM location options being investigated. To be discussed in a meeting after IPAC
Luka Todorovic, Melvin Liebsh, FCC-ee 2D tolerance analysis
Field strength (gradient):
• DD: 2.1 – 13.5 [T/m]
• FF: 1.4 – 10.2 [T/m]
Magnetic length:
• DD: 1.86 [m]
• FF: 2.89 [m]
Aperture: Ø74 [mm]
Good field region: Ø20 [mm]
Following montecarlo gradient error magnet-by-magnet is 0.01/13 (about 0.1%), but this will be measured. Unclear if this info will be tracked. Multipolar errors syst is given in the bars and uncertainty is in the error bars. Large a2 of 11 units, etc. Summarized in a table on slide 14 (only a2 and b1 change with energy, J1 is tt and J4 is Z, The a1 of 4um should be disregarded).
Quadrupole center shift will have an uncertainty of 60um in X and 20um in Y.
These are proposed values and there could be iterations.
Optimization of geometry could be a done a paticular energy.
In a recent email from Alexey the option of the vertical corrector in the sextupole would need a too long sextupole (>2m), so it is not considered.
Christian Goffing, Sextupole BBA
Challenge is the weak kick applied to the beam by the sextupoles. Instead use tune and RDT measurements.
Single sextupole modulation is not available as they are in series.
Option to use the skew quadrupole modulation to find the center. This gives accuracy of about 10 um.
From Alexey estimates it might be that maximum shift of 0.1mm between the skew quad and the sextupolar centers at Z sext powering but tt corrector powering. Both at Z powering this could be lower, ~10um, but comparable. Hence 20um could be a reasonable target for skewquad/sextupole BBA.
There is a similar issue in MAX4, https://doi.org/10.1107/S1600577518008111 , and attched presentation 'Review of beam based calibration of BPM offsets', M. Sjöström,
where a scan of the aperture (normal quad + sext) is done with beam, where 1mm BBA accuracy was reached in normal operation (as expected from simulations). LINK to come.
For the skew quad BBA change in k is +-1e-4 m^-2.
At ESRF there is an issue with hysteresis, that after BBA shifts in centers of up to 0.5mm were observed (ESRF has sext-skewquad-orbitcorr, maybe more complex). For hysteresis in FCCee the best is to measure the prototypes. This would come on top of the expected 20um.
Giulia Nigrelli, Injection studies
Collimation is neglected here but the effect is expected to be low 1-2%.
Ideal performance with beam-beam is 93% injection efficiency (this is actually a bit hybrid with a fraction of a sigma transversely).
A corrected lattice from Kevin is used. Including aligned beam-beam collisions with BS, efficiency goes down to 71%. Note that IP tuning betas/waists/Dy was not included in the simulations (Tirsi and Kevin are working on this, news on the 27th).
Transient emittance growth is like 30sigma_y, so still OK for the vertical sigma cut of 50sigmay.
IP feedback will not work so well so some offset will deteriorate efficiency of injection.
Concerning SR in the detector even with the large transient in the injected beam emittance the radiation is dominated by the circulating beam.
Patrick Hunchack, Non-linear studies
Studying Z lattice, multipolar errors on dipoles.
b3 error is easily corrected with sextupoles with about 7% strength change (to be studied at tt).
b4 errors the 0.5 units has not large impacts and they can be corrected with sextupole strength changes by 1-2%.
Including misalignments of the dipoles gives feeddown. This gives random 0.15 extra b3/a3 units, 0.1 units extra of a2/b2 (not too serious).
Some systematic effects could come from the sagita, which would require a bit stronger correction of the sextupoles.
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