Informal Meeting on Muon Collider Absorber, Vacuum and Cryogenics Integration held on 18th January 2023

Present: P. Borges de Sousa, L. Bottura, D. Calzolari, C. Carli, S. Fabbri, J. Ferreira Somoza, J. Pavan, A. Lechner, M. Rhandi, D. Schulte, K. Skoufaris, R. van Weelderen

Main points from the presentations and discussions and actions:

• David summarizes conclusions from studies on beam coupling impedances and (in-)stabilities: The minimum inner diameter required to ensure beam stability depends on the transverse damper properties, the materiel (Cu better than W) of the innermost layer and the temperature (80 K better than room temperature).
• Kyriacos shows a plot of the aperture diameter needed for the beam (5 times the rms beam size) plus 20 mm absorber (optimistic) for the arc cells of the latest optics version, for which a complete lattice is available. One concludes that a radius of about 42 mm is needed for the beam. Note that the optics is work in progress and apertures needed in the arc cells might become smaller for the next iteration. Other regions (chromatic compensation, matching to arc ..) to be taken into account. The average betatron functions, which are an important parameter needed to assess beam stability, might possibly have changed as well.
  Action: David and Kyriacos keep contact to ensure that realistic (average) betatron functions are used for studies on beam (in-)stability. 
• Daniele summarizes results from FLUKA studies on heat load and possible radiation damage due to muon decay products. The conclusions is that 30 mm or 40 mm thickness W absorbers will probably be required.
   
• Feasibility of a cold (say 80 K) innermost layer: the motivation is to reduce the beam coupling impedance and, in turn, to alleviate requirements for the transverse damper and/or allow to reduce the inner diameter. This would require a thin (<100 um) Cu layer separated by thermal insulation from the Tungsten absorber kept at about room temperature. An issue might be radiation tolerance of the insulation layer. It is excluded to keep all the  absorber materiel at 80 K.
• Feasibility of Cu for the innermost layer (<100 um thickness): Jose has already started investigations (contacted Mauro) on the best way to fix a thin Cu layer on the inside of the W absorber and will further follow up. In principle, no issues are expected. Even a thin Cu tube inside the W absorber would be suitable.
  Action: Jose will continue to follow up options for a thin Cu layer inside the W absorber
• Usage of the W absorber as vacuum barrier: Jose confirms that Tungsten is a material suitable to serve as vacuum barrier. Thus, no need for an additional chamber for the beam vacuum.
• Thickness of the Tungsten absorber: scenarios for different thickness (e.g. 30 mm vs 40 mm) should be studied to understand feasibility and impact on total cost. A thinner absorber implies more radiation and heat deposition inside magnet and enhancing radiation damage and requirements for the cryogenic system. A thicker absorber leads to larger apertures, heavier magnets, lower maximum gradients and an increased circumference.
• Some discussion on maximum acceptable heat load, superconductor cooling in general, dependance on type of superconductor (HTS vs Nb3Sn).
• Luca suggests to come with different options for the “radial built”, i.e., a list containing the various layers needed together with their thickness and radia. A first version with two options has been sent by him and is available on the INIDCO page of the meeting.
• Rob volunteers to investigate cryogenics solutions together with other members of the team. 
  Action: Rob aims at coming back with first results and proposals before the end of February.