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Casual FCC optics review
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Europe/Zurich
6/2-024 - BE Auditorium Meyrin (CERN)
R. Martin - beta* range and IR integration
Presented IR optics based on LATTICE_V4, with the beta-stars: 0.2m, 0.3m and 1,1m
Rogelio commented that a decision is needed on the phase advances, which should be constant between injection and collision.
Q6 goes down by factor 50 from injection to beta*=0.2m
Rogelio, commented this might still be ok, at the LHC the limit is 2% of nominal setting.
Solution for beta*=0.2m seems very unstable
Rogelio asks if for 0.2m it helps if Q4 is moved towards the arcs.
Roman has tried this but it didn't help, but more quads could give more flexibility.
Rogelio suggests that one should try to additionally decrease the triplet strengths in combination with increased drifts from Q4-Q7. Adding quads should be done after one tested
Barbara asks why not to add more quads.
Rogelio replies that this has many implications, e.g. more expensive installation as well as operation.
Horizontal and vertical crossing have different radiation doses in triplets -> imbalance of radiation deposition in IPs.
Different strategies to mitigate this effect were shown.
B. Dalena - ATS compatibility
Chromaticity is corrected with sextupoles in weak sections,
and Montague is corrected with sextupoles in strong sections.
Is ATS squeeze part needed for HL-LHC? -> Rogelio added ATS might not be needed in the squeeze, but we should nevertheless be compatible with it. Barbara clarifies this compatibility concerns mainly the first step of ATS
How big can be the beta-wave due to impedence budget in arcs? -> Rogelio adds it shouldnt be a problem at flattop
Rogelio comments that FCC energy spread should be better than for LHC, to be confirmed.
Results from J.Payet were shown for sextupole, phase advances and chromaticity correction (base layout/ ATS like/ Minimized)
-> tune variation with momentum <0.002 possible.
A. Chance – Dispersion Matching and spurious dispersion correction
Insertion are matched via dispersion suppressor and 2 additional quadrupoles per side if needed.
Tune is matched via the FODO cells in the long arcs only, short arcs stay at 90 degree phase advance.
No specific phase advance between IP and first sextupole yet.
Spurious dispersion peak of up to 5m in the triplet without correction. Non-negligible dispersion beating in the arcs.
Correction scheme: orbit bump in short arcs.
Rogelio agrees to only use short arc for orbit bump
Problems: orbit offset of ca. 9 mm, x/y beta beating of 5/2 %
Rogelio comments that fixed phase advance can reduce orbit and beta beating by ~50%.
A. Chance, B. Dalena and J. Payet should make proposal for IR phase advance and first strong sextupole.
A. Chance - Symmetric vs asymmetric IRs/arcs
Current design: anti-symmetric insertions, symmetric arcs. Number of FODO cells to be discussed. Large number of possibilities. What is the best?
Options:
1. All arcs and insertions anti-symmetric
2. short arcs and insertions anti-symmetric, long arcs symmetric
Tracking studies and higher order optics will help decide on symmetry vs. anti-symmetry.
Implementation of alternatives is not much work.
Antoine ask whether there are preferences for symmetry or anti-symmetry.
Rogelio replies that there are no preferences yet, to be determined as shown in R. Martin's presentation.
Barbara asks if symmetric IRs will be studied. Rogelio replies that IRs will stay anti-symmetric, only (anti)-symmetry of arcs should be studied.
Rogelio comments that 2. of shown options would be best.
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