29th meeting on HEMAC discussions

Tuesday 11 Mar 2025, 16:30 → 17:30 Europe/Zurich

17/1-007 (CERN)

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

1 Updated transverse stability criteria for cavity HOMs in RCS¶

Speaker: David Amorim (EPFL)

2025-02-25_updated_HOM_criteria_v2.odp

2025-02-25_updated_HOM_criteria_v2.pdf

2 Transverse impedance of the RF system¶

Speaker: Leonard Thiele (Rostock University (DE))

Cavity_Impedance.pdf

Cavity_Impedance.pptx

3 Discussion and AOB¶

Speakers: Alexej Grudiev (CERN), Antoine Chance (CEA Irfu), David Amorim (EPFL), Elias Metral (CERN), Elleanor Lamb, Fabian Batsch (CERN), Heiko Damerau (CERN), Leonard Thiele (Rostock University (DE)), Lisa Soubirou, Simon Albright (CERN)

AOB_RF_cryo_plant.pdf

AOB_RF_cryo_plant.pptx

4 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)