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Beam parameter meeting no 2
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Europe/Zurich
6/R-012 - conference room (CERN)
Minutes from the FCC-ee Accelerator Design meeting of 5 October 2015
Participants: Sandra Aumon, Michael Benedikt, Anton Bogomyagkov, Helmut Burkhardt, Yunhai Cai, Andreas Doblhammer, Bastian Haerer, Bernhard Holzer, Ivan Koop, Mike Koratzinos, Katsunobu Oide, Pavel Piminov, Sergey Sinyatkin, Frank Zimmermann
Remote participants: Manuela Boscolo, Mauro Migliorati, Patrick Janot
0) Comments on the minutes of the last meeting
Mike Koratzinos announced he has requested guidance on physics priorities (Higgs, top and Z).
1) Luminosity calculation
Katsunobu Oide presented a luminosity calculation tool. Increasing beta beta_y* from 1 to 2 mm, the geometric luminosity drops by 30%. A 2% acceptance is assumed. The maximum tune shift is taken to be 0.14. The dependence of the luminosity on horizontal emittance and on beta_x* is weaker. In conclusion the parameters can be relaxed for a luminosity goal of 2e34/cm^2/s at the top threshold, e.g. emittance be increased to 1.67 nm., or beta_y* increased to 1.3 mm.
· Bernhard Holzer remarked that the assumption of 2% constant acceptance is optimistic.
· Dmitry Shatilov commented that in his calculations the beamstrahlung lifetime is the hard limit. He suggested repeating the calculation for fixed beamstrahlung lifetime.
2) FCC-ee parameter optimization
Dmitry Shatilov discussed the luminosity optimization. The vertical emittance needs to be minimized, and interaction region should be maximized. Dynamic emittance and dynamic beta are included. The dynamic emittance also appears in the vertical plane due to coupling. The bunch length is about 2.2 mm without beamstrahlung, and 2.9 mm with beamstrahlung. RF voltage is chosen as 10.5 GV, which was earlier shown to be optimum. A figure of merit is introduced, which indicates the combined effect of luminosity and beam tails. Resonances are revealed by a tune scan in weak-strong simulations. Same-sign crossing and alternate-sign crossing conditions give rise to different resonance behaviour. The plots do not include crab waist.
From the calculation the following recommendations emerge: the horizontal beta_x* should be increased. When the interaction length decrases the optimum beta_y* becomes smaller too.
· Ivan Koop commented that the total tune is important for stability of the orbit.
Automatic crabbing is 8 mrad for the alternate configuration. Despite effective crossing angle decreases crab sextupoles help, due to traveling waist.
Extensive parameter scans were performed. The beamstrahlung lifetime is held at 20 minutes.
· Mike Koratzinos commented that the situation at 120 GeV would be quite different, since the limit then was due to the beam-beam tune shift rather than the beamstrahlung lifetime.
One of the conclusions is that the smaller the emittances the better the luminosity. Alternating crossing is preferable. Crab waist is always better than no crab waist.
· Bernhard Holzer commented that in view of the ongoing tolerance studies it may be impossible to go below 1 nm emittances. 1.3 nm is already a challenge.
· Frank Zimmermann remarked that the acceptance will depend on the beta_y*, which may alter the conclusions.
Bunch current was adjusted to have acceptable lifetime, in most cases limited by beamstrahlung. The main contribution of lattice is the dynamic aperture. Large tune shift may lead to frequent flip-flop situations.
· Mike Koratzinos suggested repeating these simulations for 120 GeV.
· Yunhai Cai commented it would be nice to see the beam-beam parameters.
· Katsunobu Oide remarked that the hourglass factor is included in the beam-beam parameter.
· Dmitry Shatilov commented that the extent of the tune footprint is known.
· Dmitry Shatilov confirmed that flip-flop states can be observed in quasi-strong-strong simulations.
At 45.5 GeV the vertical emittance is determined by synchrotron radiation, and almost not affected by the horizontal emittance. At 45.5 GeV a bunch population of 1e11 yields 26000 bunches and a luminosity close to 2e36/cm^2/s. Crab waist is essential.
· Mike Koratzinos inquired whether the bunch population can be increased further. Dmitry Shatilov replied that the likelihood of flip-flop increases.
Different crossing schemes are compared. The alternating scheme again yields slightly better performance.
· Bernhard Holzer announced that emittances below 1 nm are not feasible as has been demonstrated.
· Anton Bogomyagkov commented on the vertical emittance contribution from the solenoid.
· Anton Bogomyagkov suggested producing a new baseline parameter table and in addition a table with ultimate parameters.
· Bernhard Holzer recalled that the Australian Light Source has reached the quantum limit.
· Mike Koratzinos asked for the change expected when going from 4 to 2 IPs. Dmitry Shatilov responded that the effect was at the level of 5%.
3) Which minimum emittance is achievable?
Sergey Sinyatkin compared several arc cell optics. Emittance is lowered by almost an order of magnitude by shifting quadrupoles horizontally by about 1 cm. Alternatively the energy loss can be reduced further. The horizontal partition number is approximately two for the latter two schemes.
The effect of the solenoid was computed for different configurations.
IBS increases the transverse emittances by about 20% at 45.5 GeV. For other energies the effect of IBS is small.
· Anton Bogomyagkov asked for the experiment’s preferences regarding the solenoid configurations.
· Mike Koratzinos commented that as long as the “100 mrad cone” was not breached the experiments had nothing to say.
· Katsunobu Oide used an L* of 2.2 m, others consider 2 m.
· Frank Zimmermann suggested reducing the solenoid effect by increasing the length of its fringe-field region.
· Anton Bogomyagkov commented that the detectors aims at a luminometer at 1.5 m from the IP.
· Katsunobu Oide commented that beta_y*=1 mm is required for the Zrunning anyhow, and acceptance is not critical.
Conclusions: tentative baseline emitX=1.3 nm (at 175 GeV, scaling with energy squared), beta_y*=2 mm, beta_x*=1 m, equal-sign crossing
· Katsunobu Oide cautioned that a larger beta_x*aso increases the horizontal tune shift.
· Dmitry Shatilov remarked that at low energies a larger beta_x* is preferred to increase the interaction region.
· Anton Bogomyagkov highlighted that neutron production from synchrotron radiation could be a problem in the arcs at 175 GeV.
4) IR layout with solenoids
Mike Koratzinos presented various sketched of the IR layouts with solenoids and antisolenoids.
· Katsunobu Oide pointed out that the alternate-sign crossing is not compatible with the weak bending of the incoming beam.
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