26th meeting on HEMAC discussions

Tuesday 16 Jul 2024, 16:30 → 17:30 Europe/Zurich

6/R-012 - conference room (CERN)

https://cern.zoom.us/j/66586421590?pwd=TVZ4WEdtaDRQNFBkNitUTm4rTG83Zz09

16:30 → 17:00 Choice of baseline scenario for RCS chain parameters in existing CERN tunnels

To discuss:
- 3 RCS with: NC RCS in SPS tunnel, NC and hybrid RCS in LHC tunnel
- 3 RCS with: NC and hybrid RCS in SPS tunnel, hybrid RCS in LHC tunnel
- 2 RCS with: NC RCS in SPS tunnel and hybrid RCS in LHC tunnel

Speakers: David Amorim (CERN), Fabian Batsch (CERN)

RCS chain parameters

17:00 → 17:15 Status of RF parameters in RLA2

• Input to parameter table update

Speakers: Leonard Thiele (Rostock University (DE)), Rebecca Taylor (CERN)

RLA_parameters.pdf

RLA_parameters.pptx

Updated Parameters Spreadsheet

17:15 → 17:29 Discussion

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

17:29 → 17:30 Follow-up points

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?

• How do define a reasonable synchronous phase?

  • First start for constant acceleration, then real ramps

• Average gradient of RF sections

  • 1.3 GHz: agreed 30 MV/m (conservative) and 45 MV/m (aggressive)
  • 800 MHz?

• Transient beam loading and counter-rotating bunches and multi-turn effects?

  • Avoid that counter-rotating bunches meet in RF section
  • Impact on placement of RF around the circumference?

• Longitudinal matching

  • How to generate the short bunch length required for the transfer into the collider ring?
  • Longitudinal matching of RCS with pre-acceleration stages (injection) and collider (ejection)

• Can the ILC cavity (possibly in a modified form) be used for the muon RCS?

  • Power in higher-order modes (HOMs) is potential show-stopper
  • Need refined paek and average power calculations for HOM couplers

• Resistive wall impedance?

  • David following chamber materials

• More detailed lattice model, beyond smooth approximation.

  • David developed a MAD-X lattice generator (from high-level machine parameters for muon RCS)

• Introduce chromaticity in transverse studies, do we need sextupoles?

Priority 2

• 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

Further options

• FFA alternative for RCS?

Treated, at least in some detail

• 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
  • 5 TeV cannot reached in the LHC tunnel

• Average gradient of RF sections

  • Agreed during collaboration workshops to keep two values: 30 MV/m (conservative) and 45 MV/m (aggressive)