Summary Beam-beam Session

Only two talks in the BB session. But BB is an issue that permeates in several other sessions. Some material are extracted also from other sessions.

Flip-Flop Instability in FCC-ee at Low Energies -- Dmitry Shatilov

Old flip-flops are 1D effects. A new intriguing 3D flip-flop is discovered for strong-weak cases (asymmetric

beams). Ingredients of the new instability:

1. asymmetry in beam currents2. beamstrahlung3. crossing beams4. x-y coupling

Instability has a current threshold Threshold increases by lowering beta_x* and raising emitt_x

FCC-ee beam-beam strong-strong simulations for all working and mitigation -- Kazuhito Ohmi

For FCCee at Higgs:Strong-weak BB limit xi = 0.6

sharp contrast with prediction by 3D flip-flop sensitive to working point proximity to ½ tunes

Strong-strong BB limit xi = 0.2 insensitive to working point observed strong high-mode coherent oscillations in x-z oscillations

with crossing beams x-z oscillations removed by substantially lowering beta_x* and

raising emitt_x.

Beam-beam limit, which formula to use?The beam-beam limit formulas are different in FCCee and CEPC! (And you think this has long been settled?)FCCee = 0.16

Based on a model treating beam-beam effect as nonlinear resonancesCEPC = 0.11

Based on a model treating beam-beam kicks as random noisePast experience seems to be closer to the CEPC value. Latest simulations seem to predict FCCee. Which is correct?

Long range beambeamNo longer a critical issue with partial double-ring design at CEPC.Beamstrahlung

Beamstrahlung is a new issue, but is now well accepted. Flat beam + a very large energy aperture are rightly taken into design considerations, affecting the design very seriously.Frank Zimmermann slide

Frank Zimmermann slide

On the other hand, beamstrahlung induced background is not considered too serious for FCCee and CEPC.Qinglei Xiu slide

Round beams Becomes 1D, although still nonlinear Higher BB limit for low energy colliders – Dmitry Shwartz Ruled out for FCCee and CEPC by beamstrahlung

Interplay of BB and lattice nonlinearitiesThis issue was mentioned a few times at the workshop. One example is CW sextupoles nonliearities mess up the CW function. Cancellation technique is needed to further improve the CW scheme.

Yukiyoshi Ohnishi slide

Qing Qin slide

Summary

Important progress is being made. New BB effects are discovered and studied.

With performance being pushed so hard, more subtleties that were unimportant in the past now arise. New effects keep being discovered.

(a) the requirement of crab waist (b) residual nonlinearities after the crab waist cancellation(c) beamstrahlung (d) 3D flip-flop instability(e) interplay with lattice nonlinearities (f) interplay with collective effects(g) etc.

Beam-beam issue is more critical than ever.

But not all pieces have been thought through. Expect more serious learning ahead. Should have >10 talks in the beam-beam session 2018.

SUPERKEKB and HL-LHC should play important roles in the learning process.