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23th FCC-ee Accelerator Meeting
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Europe/Zurich
6/R-012 - conference room (CERN)
Minutes from the FCC-ee Accelerator Design meeting of 8 February 2016
Participants: Sandra Aumon, Nicola Bacchetta, Michael Benedikt, Alain Blondel, Anton Bogomyagkov, Andy Butterworth. Andreas Doblhammer, Bernhard Holzer, Patrick Janot, Mike Koratzinos, Luis Medina, Salim Ogur, Katsunobu Oide, Dmitry Shatilov, Jorg Wenninger, Ted Wilson, Frank Zimmermann
Remote participants: Manuela Boscolo, Eugene Bulyak, Sergey Glukhov, Mauro Migliorati, Suzie Sheehy
1) New design of the Interaction Region for FCC-ee
Anton Bogomyagkov presented a new optics with a full crossing angle of either 26 or 30 mrad, and a slightly asymmetric layout. Maximum transverse deviation of the lepton arm from the hadron arm is less than 7.8 m or 5.3 m, respectively, depending on the crossing angle. Within 130 m from the IP the critical energy is below 100 keV. Further away from the IP it increases in steps to about 200, 300 and 500 keV. Helmut Burkhardt had asked for energies below 100 keV within +/-250 m from the IP. The overall footprints look OK. The energy acceptance is -1.6% to 2.2%.
The combination/crossing in the RF straight still needs to be developed.
A new QD0 quadrupole prototype was constructed at BINP, which meets the target field and has an aperture of 2 cm, slightly smaller than the required minimum of 2.6 cm.
Feedback is required on the critical photon energies.
Patrick Janot announced that a fellow will start in March to work on the SR related background, who could compare the two (or three) optics. An answer might be available in about a year’s time.
Anton Bogomyagkov clarified that the lower crossing angle does not change the critical photon energies, but the separation between the lepton and hadron beam lines.
Mike Koratzinos asked if anybody had simulated the SR flux hitting the final quadrupoles. The answer was that realistic SR simulations with masks have not yet been done.
Mike Koratzinos inquired whether a measured field map of the QD0 could be provided for the BINP quadrupole.
Alain Blondel cautioned that the luminosity monitor should not be too close to the collision point.
Katsunobu Oide commented that he had allowed for an additional 20 cm of space, with the final quadrupole starting at 2.2 m from the IP.
Alain Blondel asked if there is a limit on the minimum crossing angle from parasitic crossing. The answer was no.
Anton Bogomyagkov presented a parameter table with low beta* values. The analytical luminosity values are slightly larger with the smaller crossing angle. Katsunobu Oide commented that the beam-beam parameter should be kept the same in order to make a fair comparison.
Dmitry Shatilov remarked that at the higher energy the smaller crossing angle will be beneficial.
Frank Zimmermann suggested that the dynamic aperture be calculated for this new optics with the small beta* values (0.5 m, 1 mm) as well as with the relaxed beta* (1 m, 2 mm), for the Z pole and for the t-tbar energy.
Jorg Wenninger asked if there is a strong need of minimizing the tunnel size already. Michael Benedikt replied yes, in the standard tunnel only 3 m will be available for the accelerator.
Mike Koratzinos commented that in his helical quadrupole the aperture inner bore is 36 mm diameter, significantly larger than the 20 mm for the BINP QD0 prototype, while at least 24 or 26 mm would be required for the beam.
Frank Zimmermann and Andy Butterworth pointed out that the RF system favours a larger number of bunches at the Z energy, in order to limit the HOM losses.
2) FCC-ee lattice update
Katsunobu Oide presented a fine tuning of the lattice to the hadron ring. The relative distance between one lepton ring and the hadron ring was shown. A tiny adjustment of the electron bending radius results in a perfect match of the beam lines. Near the straight sections there is an offset at the hadron-beam dispersion suppressors. The lepton ring crossover at the RF sections was included in this matching. About 1060 m of wider junction cavern are needed on either side of each of the two IPs. The dynamic aperture at small beta* is OK for the Z pole, but appears slightly marginal (~12 sigma) for 175 GeV.
The emittance at the Z pole can easily be controlled with a wiggler inserted after the RF section, with almost no increase of the total energy loss.
Katsunobu Oide observed that last April Helmut Burkhardt had stipulated critical photon energies less than 100 keV over +/-500 m from the IP. In October he had imposed this same request only over +/-250 m. He wondered which argument had led to the change in requirement.
Alain Blondel remarked that at LEP the distance with low photon energies had been less than 500 m, and that the LEP detectors used gas chambers, which are more sensitive to synchrotron radiation.
3) Beam-beam simulation update
Dmitry Shatilov discussed the mechanism behind the flip-flop effect. An example from the October presentation was shown. There are several ways to model the vertical emittance. If the actual horizontal emittance is considered fir the coupling the flip-flop instability is stronger. Recently a more realistic model of betatron coupling has been implemented, with intermediate matrices obtained from SAD, supplemented by damping and diffusion matrices. The coupling is introduced between RF and crab waist. Several random samples with misalignments were tested for the equilibrium emittance. An additional phase advance can be added. The onset of the flip flop was found, for 45 GeV and beta_x*=50 cm, by scanning with a 10% difference in current, considering four combinations of emittance coupling for the strong and weak beam. The onset of the flip flop appears as a strong transverse blow-up
Bernhard Holzer asked for the size of sextupole misalignments. Katsunobu Oide replied 10 micron.
Mike Koratzinos suggested also studying a 20% asymmetry in bunch current.
Katsunobu Oide commented that according to Demin Zhou there should also be a strong sensitivity to the transverse working point.
Dmitry Shatilov highlighted that for larger beta_x* the flip flop effect occurs much earlier, even after shifting the working point to a better location.
In conclusion, a decrease of beta_x* to 50 cm is useful for two reasons: narrower footprint, and higher flip-flop threshold.
Alain Blondel commented positively on the emittance wigglers. Dmitry Shatilov recommended matching the values of the longitudinal overlap area and the beta_y* by wiggler-based emittance blow up. Katsunobu Oide suggested using the emittance wiggler for feedback against the flip flop effect. Katsunobu Oide proposed a scan of the phase advances between the two IPs.
4) AOB
Katsunobu Oide announced that the optics meetings will continue on Friday morning for the Asian colleagues, but from 9 am instead of 9:30 am.
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