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FCC-ee optics tuning WG meeting
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FCC-ee optics tuning WG meeting
J. Keintzel presents general news:
- Alignment working group: Kick-off meeting 06/11/2025: https://indico.cern.ch/event/1606243/
- Comparison report due by 19 December: FCC-ee tuning will contribute. G. Roy suggests including a paragraph on tuning implications due to different design paradigms of GHC and LCC. F. Zimmermann suggests compiling what we have so far. G. Roy adds that it should be presented what we have, also explaining what has not been done, together with open points. Updated results obtained later, should be reported in January.
- A general FCC-ee tuning IPAC paper will be submitted
- WG2a report is online: https://cds.cern.ch/record/2947728
J. Salvesen presents news on the IP feedback studies. IP FB considers only the difference between both beams. Due to the large BB tune shift vertically, with 4 IPs the PI-mode vertical tune is almost at the half integer. With targeting 1nm offset at the IP (driven by EPOL, assuming 1 um Dy at the IP), this poses huge constraints on the IP FB. For noise, LHC tunnel floor and a 20Hz resonance peak (for the FF) is assumed. With the current tune, high frequencies lead to instabilities. With 0.1 lower Qy, these instabilities would disappear since the pi-mode resonance is no longer close to the half integer. Which frequency cut-off line the general orbit feedback will correct remains to be understood. In the assumed noise spectrum, the lowest frequencies, below 1Hz, have the most impact. Reducing the pi-mode tune is essential (either lattice tune or BB tune shift or number of IPs). X. Buffet adds that at 0.1 the lifetime is 0, so better filters would be mandatory. A good trade-off between BB and FB must be found. For FCC-ee TbT FB is required.
J. Salvesen presents on SuperKEKB in Xsuite. First iBump feedback studies have stared in xsuite. A strong-strong simulation approach must be applied. In SKEKB iBump system has limitations because the vertical emittance is not tracked accurately.
K. Andre presents tuning studies for LCC. About 1 damping time (1000-2000 turns) should be tracked for DA. P. Raimondi adds that tracking more turns give the quantum lifetime, whereas when tracking only 256 turns, gives the DA. R. Tomas agrees, but including quantum is essential. pyAT adds 1e-6 to all coordinates in presented MA studies, shown for V105, leading to strong reduction of MA for diagonal kicks. P. Raimondi adds that in V92, this effect is a factor 2 smaller and even smaller in V89; most likely due to the vertical chroma correction section - tbc. These are only at large momenta offsets above 1%. M. Jebramik points out this does not happen when freezing longitudinal coordinates. K. Andre presents when removing this offset MA is restored. S. Liuzzo explains that this was done for EBS to avoid spikes. R. Tomas suggests that if that offset is only 1e-10 does not impact MA too much. M. Jebramcik explains he adds half a sigmay to get rid of the tails. K. Andre adds that BPMs are now moved with the quadrupoles and have 10 um errors. In the last simulations the rms corrector strength is 1e-13um. For some seeds turning on and off SR in the IR doublet the vertical emittance reduces by several folds. In the latest simulations with V92 and BPM errors (still with the 1e-6 offset) in addition many more seeds show low DA and MA (with SR, no quantum). Attaching BPMs to the quadrupoles yield show worse results - tbc. Also vertical emittance is low. When turning off the FF radiation emittance is better. P. Raimondi adds that because of the seen effect, always 20% less are seen. R. Tomas brings up the question which lattice should be used for comparison - it is decided to use the latest version V105. R. Tomas suggests that in about a week or two more tuning news for GHC and LCC will be shown.
K. Skoufaris presents dynamical studies with BB. In Xutil for the LCC the BB tune shift is currenlty being reduced, studies here are shown for more pushed parameters (20% more), at the limit of the BB tune shift.
In GHC (with errors and corrections by Satya), DA shrinks significantly when BB is included. In a good seed with BB, lifetime drops by 10%, whereas in a bad one up to 50%. In FMA with 4D a good seed is even more affected by BB than a bad one. R. Tomas points out that even the x,y=0 particle has a significantly different tune - tbc. S. Jagabathuni comments that no IP tuning is included so far, but 10 um dispersion error and field errors in the arc sextupoles - to be updated.
For tup-up injection, with BB up to 10% of injection parameters could be changed. With injection bump injection efficiency drops and BB reduces it further. The injection bump increases the dispersion in the arcs by a few cm in the arcs.
At Z, LCC has larger DA and MA even with pushed BB. At ttbar reduction is big. M. Jebramcik points out that the injection point in GHC is outside the MA, K. Skoufaris explains it is still marginally in. R. Tomas adds that there is also the hybrid scheme. K. Skoufaris suggests one could also think of exploiting the non-linear dispersion for injection. P. Raimondi adds that with collimators closed, more losses are expected. He adds that because of the large beam size at the FF, transparent injection will not be possible. P. Raimondi will share the lattice with injection kickers.
A. Hussain presents on progress with field errors. Only random values are considered. The cut-off is defined as 95% of the area of DA/MA without errors. Simulations are performed for GHC, with SR without BB. IR without the FF. J. Keintzel points out that arc sextupoles with b5 are always better. In old studies the observable was mainly lifetime. J. Keintzel and K. Skoufaris suggest exploring different observables.
B. Dalena presents on dipole field quality for the HEB, up to 1 unit of b2 to b6 systematic errors are considered. Tune and chroma is corrected. Second order dispersion and montague functions corrections help only marginally. Dipole field errors up to a6/b6 of up to 1 unit seem tolerable. Tuning studies are successful for +1 unit b2a2, but not for -1 (half of the seeds fail) - tbc.
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