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Beam-beam beta-beating
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6/R-018 (CERN)
Patrick, beta-beating from beam-beam interaction
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MD979 took place in October 2016 aiming at measuring beta-beating from beam-beam in weak-strong regime.
There was an uncertainty in the emittance measurements in the 20-30% level.
In the spectrum we always see the AC dipole excitation. The natural tune was seen only for some large kicks ~2sigma. At smaller amplitudes the situation is not clear. A first beta-beating measurement is accomplished at 2sigma meeting model expectations. Small unexpected discrepancy in IP5 as there is jump in beta-beating but beams were not colliding in principle.
Simulations reproduce tuneshift versus amplitude as expected by formula. Combi simulations show that the damper turns the strong-strong regime into weak-strong for what matters for the spectrum. When intensity ratios are about ~2 the system is closer to strong-strong. The driven oscillations are only in the horizontal plane. Vertical spectrograms are significantly clearer.
Strong-strong cases have a much richer physics.
The MD case is also simulated. An octupolar line is observed, see next talk.
A direct FFT comparison between measurement and Combi at similar conditions would be nice to see.
Exploring the long-range regime might easier in MD. Another possible MD continuation is larger emittance for the strong beam so larger kicks can be used to improve BPM resolution.
A vertical line was observed also in Combi, to be looked at.
Felix, beam-beam RDTs
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-Qx+2Qy octupolar line is directly observed in measurements decreasing versus excitation amplitude. The reduction of the octupolar spectral line is due to the contribution from higher orders. This is reproduced in simulations.
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