Erk Jensen Jacek Sekutowicz Ulla van Rienen Hot-Topic Discussion: Which directions should future HOM R&D focus on, i.e. where is rapid progress needed? Erk Jensen Jacek Sekutowicz Ulla van Rienen 04/05/2019

Can we still use “classical” HOM couplers? Trends: Shorter and shorter bunches CW High currents Can we still use “classical” HOM couplers? How far do we get with beam pipe absorbers? Handling of high power losses and HOM beam loading Achromatic telescopic squeeze (ATS)  kilometre-range beta functions amplify effect of transverse impedance (see next slide) Measurement techniques – Where are the limits of HOM- based diagnostics? 04/05/2019

𝑍 ⊥ effect amplified with large 𝛽-functions – e. g 𝑍 ⊥ effect amplified with large 𝛽-functions – e.g. with Achromatic Telescopic Squeeze ATS is used in LHC (HL-LHC, FCC, …) to reach very high luminosity. ATS Reference: https://journals.aps.org/prab/pdf/10.1103/PhysRevSTAB.16.111002 To get very small beams in collision ( 𝛽 ∗ in the order of 10 cm), very large 𝛽-functions are required elsewhere in the machine. Effect of 𝑍 ⊥ amplified by 𝛽-function at impedance location. 𝑥′ 𝑥′ 𝜀 𝛽 ∗ kick 𝜀 𝛽 trajectory 𝑥 𝑥 𝛽 ∗ 𝜀 𝛽𝜀 04/05/2019

Discussion: Beam Measurements ▪ Learn about HOMs with beam - only find dangerous modes for a certain facility? (keep in mind other possible op. modes) - how big effort is reasonable? - relevance to other facilities? - characterize HOMs with (and without) beam - what about cavities w/o HOM couplers ▪ Measure effect on beam

Discussion: Beam Measurements (2) ▪ Benchmark simulations with measurements ▪ Measurements w/ beam combined with simulations, lab tests, cold measurements w/o beam ▪ Common efforts / Learn from each other ▪ Other aims: Cavity alignment, straightness, beam diagnostics etc.

Outcome of the discussion How about SRF structures at frequencies ≫1.3 GHz – possibility of multi-cell structures with large number of cells? On-cell coupling? Superstructure? For many of absorber materials, there are issues with thermal conductivity/power density. How about completely new materials? HOM coupling techniques: on-cell coupling, hook/antenna coupling, absorbing tiles in beam tube, waveguide coupling. HOM-free cavities – need large beam pipes Need funding for “true R&D”, high-potential/high risk studies. This should be easier if we work together between labs/Universities  set up an interest group? Power levels at HOM couplers (≈1 kW) reach similar levels as at FPC’s; similar technologies should be used/applied. Issue: ageing/cleanliness/particulates. HOM-based diagnostics – use the wealth of available information – also here: more money/manpower needed to deepen R&D 04/05/2019