Preliminary considerations for the final-stage RCS in the LHC tunnel

• David Amorim (CERN)
• Fabian Batsch (CERN)

Discussion Few open points for follow-up: **Priority 1** - *Optimize losses in RCS chain for best overall performance, allowing different transmission in the two or three machines* - Difficult with lots of degrees of freedom: straight-forward only for constant acceleration - Difficulty of different magnets (ramp rate) - How to distribute total RF length over different machines? Minimization of overall total voltage? - *Acceptable longitudinal emittance growth in the RCS chain and how to quantify it exactly?* - Needs figure from low-energy acceleration (low energy), longitudinal emittance for colliders as a starting point. Criteria also on how much bunch is distorted. What is acceptable bunch shape deviation in the collider? - *What are the consequences if we want to use the LHC as the last accelerator ring?* - Check consequences, check Shiltsev proposal: https://iopscience.iop.org/article/10.1088/1748-0221/13/10/T10003 - *Average gradient of RF sections* - Agreed during collaboration workshops to keep two values: 30 MV/m (conservative) and 45 MV/m (aggressive) - *How do define a reasonable synchronous phase?* - First start for constant acceleration, then real ramps - *Power in higher-order modes (HOMs)?* - *Can the ILC cavity (possibly in a modified form) be used for the muon RCS?* - Mauro Migliorati? Presently no code can do it. - David has impedance model for ILC cavities - *Resistive wall impedance?* - David, contact with JPARC, collecting information - Luca will organize a dedicated meeting on 24th November: muon collider magnet working group - *Introduce chromaticity in transverse studies, do we need sextupoles?* **Priority 2** - *Transient beam loading and counter-rotating bunches and multi-turn effects?* - *Cost functions per machine part: could one make an assumption for a cost per unit length of RF (plus some infrastructure cost offset per RF section), idem for unit length of magnets?* - *Would two RCS be sufficient to accelerate from 60 GeV to 1.5 TeV?* - Does not look very attractive - *Is transition gamma of ~20 a good number for the first RCS stage?* - Check various transition gammas and their impact -> David to present - *More detailed lattice model, beyond smooth approximation.* **Further options** - *FFA alternative for RCS?*

Round table

• Fabian Batsch (CERN)
• Heiko Damerau (CERN)
• Ivan Karpov (CERN)
• Alexej Grudiev (CERN)
• Antoine Chance (CEA Irfu)
• David Amorim (CERN)
• Elias Metral (CERN)