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14th FCC-ee Optics meeting
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Europe/Zurich
6/R-012 - conference room (CERN)
Minutes of the 14th FCC-ee Optics Design meeting
Participants: Dmitry Shatilov, Bastian Haerer, Mike Koratzinos, Katsunobu Oide, Pavel Piminov, Anton Bogomyagkov, Ivan Koop, Eugene Levichev, Sergey Sinyatkin, Andreas Doblehammer, Uli Wienands, Sandra Aumon.
Luminosity optimization at 45GeV, by Dmitry Shatilov
Following the review, Dmitry continued to optimize the luminosity at low energy. He studied the dependence of the hour glass factor (L/beta_y*) on the luminosity in the beam beam simulations: the transverse phase space density plot is less deformed and more stable for large horizontal emittance (larger L/beta_y*). Dmitry recommands to increase the horizontal emittance if the vertical one is 1pm, in order to get a better luminosity at 45GeV. 100cm beta_x* increases the hour glass factor (the factor should be increase as much as possible if it is <1.), but Dmitry does not recommand to do so.
Dmitry showed the lattice parameters from Anton (October 2015 versus February 2014). Mike asked why the momentum compaction increased between the two versions of the lattice. Anton replied that the hor. dispersion function changed. Dmitry presented results with betatron coupling and an asymmetry in intensity within the colliding bunches (10%), - asymmetry due to losses - This effect degradates the luminosity. The vertical emittance is formed by two components: betatron coupling, which is proportional to the actual horizontal emittance, and vertical dispersion. To account in simulations the horizontal emittance growth caused by beam-beam interaction, Dmitry readjusted the vertical noise. The reason is that in the present (simplified) model there is no explicit coupling, but it needs to be taken into account. Katsunobu said that this approach is argumentable, since beam-beam is not a noise. Dmitry replied that he does have a real model for coupling and this will come later. 10% of asymmetry (and depending of the coupling model) is enough to blow up the horizontal emittance and therefore the vertical emittance. To alleviate the flip flop instability, Dmitry proposed to decrease the beta_x* from 1m to 50cm. This cure would suppress the flip-flop instability and should work at every energy. Dmitry proposed to keep 1mm beta_y* at low energy, considering that a good energy acceptance can be achieve. Katsunobu corrected that Masamitsu Aiba needs 0.02 in energy acceptance at injection, instead of 0.01.
Mike asked for the threshold in asymmetry between colliding bunches and if the injectors are able to achieve symmetric bunches . Dmitry replied that keeping asymmetry within 10-15% should be enough to avoid flip-flop.. Katsunobu said that the B-factory is able to monitor the intensity bunch by bunch in routine. However, they do not have so large number of bunches. This has to be checked with Yannis.
IR magnet design: first step, by Mike Koratzinos
Mike started to work on the IR magnet design and is building a very preliminary magnetic model based on the solenoid from Sergey: solenoid, anti-solenoid and final focus quadrupoles. Mike simulated the longitudinal and the horizontal magnetic field along the orbit as well as their projected components. He computed the resulting vertical emittance which blows up around 1.8m (from the fringe field of the solenoid).
Future plans would be to try a design with double helical magnets.
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