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FCC-ee optics tuning WG
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Europe/Zurich
6/R-012 - conference room (CERN)
FCC-ee optics tuning WG meeting
Quadrupoles and orbit correctors, Nacho Garcia
At tt nominal settings QD has large b1 and some b3. Cut-out design helps.
QFF has larger b1 and b3. Cut-out design helps. No skew components
Trim of 2.5% for tapering and tuning gives a change in b1 and almost no change in b3.
Proposal to cancel b1 using a new coil in the center of te quad assembly. This does not cancel the b1 of 6 units induced by trim.
However, the tapering part is larger than needed, to be reviewed.
The b1 value correspond to 90-150um offsets.
b3 value was 8 or 5 at nominal (systematic) and changes by 0.5 units with trim.
QDD: The V orbit corrector gives a large a3 of 91 units at tt.
QFF at Z gives 138 units.
Bore radius is 37mm while beam screen inner aperture is 30mm, Oide-san wonders if this margin is too large. This could be optimized as affects costs.
Question: Is the systematic quad center shift an issue? Max shift of quad should be 30um
Another question: What is the shift of the quad center during K-modulation of 1%? (send question via email)
Last question: Impact of b3.
Separate HV orbit correctors:
2 motivations: Arcs and straight sections. For the latter the specs could be much more relaxed (larger separation, no winglets in vaccum pipe, etc.).
First design, V4. 300mm length. Large b3, a2 and a3 multipoles.
This is a first design, it can be optimized and it could be H corrector is not needed. To be followed up.
Coupling tolerances for beam-beam, Vaihabi
Clear impact of coupling phase on luminosity in presence of the beam-beam interaction (4 orders of magnitude lower beam-beam removes impact of phase).
Sigma_y dependence with coupling phase is reproduced analytically (large discrepancy on amplitude of f1010 to be understood, probably a square factor, to be checked).
Pantaleo proposes to use skew quads at the crab sextupoles and at the CCX to control coupling a the IP. This was tried in the past.
Full realistic IP coupling parameters have not yet been demonstrated.
Tuning LCC updates with pyAT, Kevin
Pantaleo asks about the optimization of the number of SVD values. Currently for the normal quadrupole #~1500.
Difference in coupling correction between Z and W is seen, while arc is the same. Strange, to be understood. Weights between arcs, IRs, Dy and f1001 to explored.
b1 and a4 from the skew corrector in the sextupoles are added in a random way (not considering proportionality to SQ strength). New dipole multipoles are also included.
The BPM tilt is also increased to more realistic 100 urad.
IP optics parameters are not meeting targets. Need IP tuning knobs.
With new error tables DA and MA are clearly deteriorated, V emittance and IP parameters keep poor.
Clear impact of larger a2 in dipoles (rand) on vertical emittance. The dipoles should be split in 3 to gain sqrt(3) in coupling. Another working point could work.
DA/MA was deteriorated with larger errors, although it is hard to quantify.
a3 in quads and dipoles, Wietse
Adding 16 a3 correctors in the arcs cancels most of aberration but not fully. for a3 in dipoles MA and DA without beam-beam are ok.
Actual performance with beam-beam needs to be included.
Random a3 in quads dominates skew sextupole aberrations compared to dipoles.
First indications that IP aberration is more relevant than arc.
The new lattice with lower phase advance is much more advantageious! To be studied
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