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FCC-ee optics tuning WG
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Europe/Zurich
354/1-016 (CERN)
FCC-ee optics tuning WG meeting
J. Keintzel opens the meeting. She reminds that a tuning paper and an optics paper will be prepared for IPAC. The list of FCC week participants will be announced soon.
F. Zimmermann states that the FCC CAS will take place in November in Prague.
L. Todorovic presents on FCC main dipoles. Most challenging would be b2. Prototypes for 2 magnets are studied with different material properties, which give different b2 errors. a2 in the order of 5 units from errors. P. Raimondi suggests to shim geometry to reduce the errors, M. Liebsch agrees but comments that this comes with a cost. Tentative sensitivity study went up to 1 unit b2 systematic only which included rematching, no random errors were studies so far. Current magnet design proposed 10 units systematic and 5 units random b2 to be studied by the tuning team. Measurements of every magnet and then sorting is excluded. Following a question by F. Zimmermann, M. Liebsch states they will not be laminated. SImply geometry could be done rather easily, but magnetic measurements seem difficult. a2 during measurements could stem from not controlled bus bars.
Other multipoles seem not as challenging as quadrupole errors. Higher than b4/a4 should be 0.1 systematic and random.
P. Raimondi suggests producing several prototypes and then measure random values.
C. Carli questions the precision of the measurement. M. Liebsch states very high precision.
Following a question by R. Tomas, b2, b4 (all even harmonics) alternate between apertures. All skew swap sign between apertures.
For higher energies, errors are tentatively better. For now it is decided to keep one table at all energies. 1mm vertical tolerance for bus bars are assumed.
R. Kieffer presents thoughts on luminosity monitoring and IP feedback.
No one from BI has studied the BPM next to the IP. IR and arc BPMs will have a different design, possibly also in the LSSs. R. Tomas stresses to also look at the BPMs at the final doublet. Input from the optics tuning team regarding tolerances and specifications on BPMs (arc, IP, LSSs, especially between sextupoles) and feedback are required (location, resolution, etc).
F. Zimmermann comments it might be better to use the bunch charge instead of BhaBha to define the top-up charge. The rate will be sub MHz, so 1% error within 1s.
Work on the beamstrahlung monitor has started too. Following a question by M. Koratzinos, 1mm is the accuracy on the position measurement. Aim is bunch-by-bunch, but will depend on method. M. Koratzinos suggests using diamond detectors. R. Kieffer agrees that such detectors will be studied. C. Carli raises the question how a vertical offset will change the beamstrahlung photon position and angle - to be discussed with beam-beam experts.
P. Raimondi suggests using ionization of beamstrahlung on the extraction channel, similar to what is done in LHC. R. Kieffer suggests, could potentially be studied, but more expensive.
F. Zimmermann suggests to also use the LumiCal. R. Kieffer says the current design is slow. M. Koratzinos states that at DELPHI it provided real-time information.
E. Musa presents news on W tuning for GHC. SR is included, QF is not. R. Tomas suggests to also show coupling RDTs. DA is very poor.
Following a question of P. Raimondi, E. Musa comments that in the past she used V22 for Z, which yielded better results than now for W-lattice.
Regarding the sequence, P. Raimondi suggests using the corrections for Z, and then load the W lattice. R. Tomas adds that the change between energies will be investigated and reported to magnet experts. Corrected Z lattices should be used and then the energy increased until 80 GeV are reached.
K. Oide points out that right and left of IP have different strengths but should be more similar.
It is suggested that E. Musa will look at Z with QF and S. Jagabathuni W with QF in xsuite.
Due to the very low MA it is suggested one could try injecting on momentum.
Commissioning simulations should be performed for relaxed and/or ballistic.
S. Liuzzo presents on behalf of K. Andre.
At Z, with new correctors settings more seeds failed. 20mT/m (scaled to 182.5 GeV) is the maximum corrector strength (3 sigma value).
For ttbar the same locations as in the comparison report are used. DA is very poor, also design does not match between pyAT and xsuite design. 21mT/m is the maximum corrector strength. A possible reason could be not optimized RF settings - tbd.
K. Oide suggets to improve MA rather on the negative momentum offset than positive. M. Jebramcik comments that he always gets a better acceptance to positive values. P. Raimondi suggests to increase negative acceptance.
Regarding the conversion between pyAT and xsuite great progress is done and conversion could now be tested by other colleagues. This is also done for Z-lattice. At the moment xsuite is slower than pyAT.
R. Tomas suggests to clarify importance of corrector location choice. P. Raimondi suggests to add errors introduced by correctors - tbd. Ideally by mid March.
P. Raimondi suggests to not use trims, but rather use clever orbit steering.
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