Present:  Christian Carli, Maria Carmen Giordano, Jose Ferreira Somoza, Alexey Vorozhtsov, Madeleine Watson

Main points from the discussion:

• NEG coated vacuum chambers considered baseline. This avoids electron cloud build-up in case many bunches have to circulate simultaneously (requirement depending on specification from collider operation and energy calibration).
  • Conservative assumption on apertures based on reserving 5 times the rms beam size for the beam plus 5 mm for orbit perturbations and misalignments: Circular vacuum chamber with 1.5 mm thickness and 25 mm outer radius  plus 10 mm for bake-out equipment and tolerances leads to magnet apertures of 35 mm.
  • Options to reduce apertures if needed:
    • Reducing the space reserved for the beam and vacuum chambers adapted to the beam size ratios (e.g. elliptic chambers in bending magnets)
    • Reduced thickness of bake-out equipment using developments made for the FCC-ee collider (in case this comes in time)
    • Bake-out of the chambers before installation into precisely (on girder) aligned opened magnets.
• Strength requirements for orbit correctors will be determined and provided.
• Beam sizes (5 times the rms size) will be added to the sketches with machine elements.
• Space for vacuum equipment and connections within arc cells could be provided by combining horizontal and vertical correctors
• Bent vacuum chambers inside bending magnets: should be feasible without particular difficulty. Nevertheless, the sagitta will be computed and provided (small with many short bendings).
• Impedance and instabilities mitigation requirements (e.g. coatings, shielding of bellows with RF fingers) have to be clarified.
• At the present stage, both permanent magnets and electro-magnets should be considered as options.
  • In case of permanent magnets, tuning requirements have to be addressed. 
• Longer polarizer ring possibly installed in the same tunnel than the damping ring: indeed an option considered (in such a ring, the beam would be accelerated ruling out permanent magnets and massive yokes or require asymmetric wigglers reducing the equilibrium polarization) which should be discussed separately once first designs are available.

Next steps:

• Hardware experts will have a look and come back with their conclusion and proposals
• Maria Carmen will look into synchrotron radiation photon generation and consequences (using the latest lattice version, which will be provided my Maddie).
• Alexey will look into the feasibility of the required magnets. The minimum distance between elements (can distances be reduced) is important into for further design optimizations.