FCC-ee optics tuning WG meeting
P. Raimondi presents on the updates for the HFD arc, which is now closer to a FODO cell lattice. J. Bauche stresses that having a regular cell with high periodicity is important. He will evaluate the different proposed options and come back to P. Raimondi et al. J. Bauche reminds they are sill looking in detail at the same polarity twin quadrupoles moving from 2D to 3D models. P. Raimondi estimates a new version will be released earliest in 2 weeks. Power consumption will be optimized globally (length of quadrupoles vs RF power, etc.).
S. Liuzzo presents on the multipole tolerances for the LCC. It is noted that the 'larger than' sign is correct implying a target but could be increased taking budget from corrector strengths. M. Liebsch suggests to discuss with magnetic measurement experts. R. Tomas explains that studies are performed multipole by multipole. In general, magnet experts comment that values below 1 unit are small. However, if needed, one can think of mitigation measures.
b2 in quadrupoels: J. Bauche comments that systematic b2 in quadrupoles is the actual quadrupole field. R. Tomas explains that the systematic b2 error could be from e.g. measurements from the probe when determining the quadrupole field. M. Liebisch comments that 10 units systematic b2 seems reasonable yet challenging, and agrees that quadrupules could have a systematic error from the measurement. P. Raimondi comments on the random b2 on quadrupoles: the random b2 corresponds to the measurement error of the quadrupole, since other b2 could be corrected with the trim coils in the quadrupoles. J. Bauche comments that although trims are at every quadrupole, the main question is how should they be connected (e.g. tapering over 4 cells or individual tuning significatnly impacts the costs). P. Raimondi adds that for the LCC he is thinking of having correction trims only at the sextpoles - to be studied. Magnet experts comment that having quadrupole trim coils into sextupoles is more challenging and they recommend having them at quadrupoles. P. Raimondi adds that for GHC trims are certainly required at all sextupoles. R. Tomas suggests to answer the granularity for optics corrections it could be studied using n quads in series. P. Raimondi proposes to explore also instead of using quad correctors, to displace the beam in the sextupoles.
b1 in quadrupoles: For a quadrupole measuring a dipole component b1 is difficult to achieve and is therefore 1 unit is bit too optimistic and 10 units would be more realistic. Orders higher than the main component are typically easier to evaluate than modes lower. P. Raimondi suggets translating b1 error in quadrupole in error of magnetic center measurements. J. Bauche explains that these magnet have horizontal axis shifts due to aperture coupling that changes for different energies. He adds that this orbit shift could possibly be corrected with the orbit correctors. A. Voroszhtsov clarifies that 1 unit is about 1 um and that about 50um shifts are seen in simulations between energies. R. Tomas comments that this contribution then goes into the budget of the orbit corrector strength and reminds that this number could certainly be larger.
b1 in dipoles: J. Bauche comments that random b1 in dipole could go up to 10 units typically. P. Raimondi clarifies that this means that the orbit corrector should be 10 times stronger (for this particular budget item).
b3 in dipoles: Concerning the b3, 0.2 random in dipoles already require 1% sextupole modulation. a2 in arc dipoles should be about 1 unit, where the introduced emittance ratio translates to the minimum coupling which could be achieved after corrections.
b4: Concerning the 0.2 unit tolerances, J. Bauche finds that very challenging. A. Vorozhtsov adds that everything below 1 unit is small. Further reflections upon this will follow in the future.
Correctors in sextupoles: J. Bauche the skew quadrupole correction in the sextupole leads to a 70 units of a4 (relative to the unit of the sextupole), assuming 600 mT integrated strength. J. Bauche reminds that 20mTm was the specificaton for the horizontal and vertical orbit corrector trim circuits and asks if this could be evaluated also for the LCC. P. Raimondi guesses that numbers could be about 20% weaker for LCC. This estimated will be checked in upcoming simulations.
A. Vorozhtsov asks about the good-field region, which clearly needs to be studied too. At the moment studies are per multipole to identify appropriate correction techniques.
C. Goffing presents BBA simulations at KARA. He shows that beam angles at the BPMs determine the achievable BBA accuracy. Simulations in KARA assuming 300 um misalignments give BBA accuaracy of the same order of magnitude.