BeamBeam and Luminosity studies meeting Monday 7 March

Beam-Beam and Luminosity studies meeting Monday, 7 March The indico page link with the draft agenda can be found at: https: //indico. cern. ch/event/489794/ “Evolution of non-colliding bunches at the LHC flat top” , Fanouria Antoniou “Single beam stability at LHC flat top ”, Elias Metral “Potential impact of Non-colliding bunches in LHC performance at collision” , Tatiana Pieloni

General discussion, Yannis • Lots of debate about: stability and not stability, limit of DA, non-colliding bunches, etc. the diffused information makes people from the background working group asking for the present knowledge about 6 poles and 8 poles limits and BB LR effects with strong 6 poles+8 poles on the DA. • What should be answered by these people is what they prefer in terms of beam parameters (intensity, emit. )

“Evolution of non-colliding bunches at the LHC flat top” , Fanouria Antoniou Comparison with the luminosity model Analysis description: bunch by bunch, compare with the luminosity model (that is iterated bunch by bunch in time-steps of 20 min) prediction, the comparison was done after 3 h at Stable Beams, model includes SR+IBS+lumin. burn off and emittance blow up is the emittance measured over the emittance predicted by the luminosity model. • Non Colliding bunches following the model very well -plots of (e. H* with bright. and lifetime), (e. H* with #LR and bright. ) * bbb after 3 h at SB No clear correlation for #LR, but we see that the emit. blow up corresponds to high brilliance • It was clarified that all the fills shown are after 10/2015 (so no issues with the damper) • Non-Colliding and colliding bunches indistinguishable • Non Colliding bunches getting unstable -At some fills the behaviour of colliding and non-colliding bunches is distinguishable and the blow up can be seen. Conclusions -For most of the Fills analyzed, the non-Colliding bunches follow the model pretty well and have very long lifetimes. However, there are Fills that all bunches (colliding and non-colliding) blow up the same (with respect to the luminosity model). -In two (or at least two) Fills the non-Colliding bunches blow up much more than the Colliding ones in one beam only and seem to get unstable. -In many Fills a (slight) brightness depended correlation of the emittance blow up in the vertical plane is observed. Discussion (questions, comments, etc) Yannis said that macroscopically most fills follow the model, but for colliding bunches there is something extra.

“Single beam stability at LHC flat top ”, Elias Metral SINGLE BUNCH / BEAM STABILITY AT LHC FLAT TOP Plots in slide 2 (see backup slide A and B) using the impedance model in DELPHI (perfect damper and non linear coupling) A • Being down the 150 A->stability (Actually he said that 100 A is enough) • The emit. used is 2 um, for the half of 2 x 150 A=300 A is needed Ilias made a comment/question about the points with small error bars that are not close to the plotted lines. Everyone agreed that this is something to be checked and improved, it happens probably due to noise at these regions. B • Red points (with error bars) due to e- cloud blue points (with error bars) after scrubbing (FT for hor. and vert. plane): see Nicolo. B. talk for LBOC later in March Looking at the effect of coupling for a specific Nb so that it is fast (the one used here is 3*10^11 and it takes about 1 week). Discussion (questions, comments, etc. ) -Stefan comment on the sign of the 8 poles (to be seen). -Yannis asked about the analysis with new eigenmodes that is more complicated (the answer was no, but it should be done), maybe in this way you can see that single bunch/beam gets deformed. -The analysis should be done without damper. Next steps -check amplitude depended coupling -MD for multibunch

“Potential impact of Non-colliding bunches in LHC performance at collision” , Tatiana Pieloni LHC Operational Workshop, EVIAN 2015 • In 2012 it was understood that 4σ (the beam’s one, not the collim. ) is bad because of losses due to BB interactions. • 2015 strategy was the setup at 6σ. 11σ separation proposed to allow maximum 8 pole and high chroma operation! • 2016 Xing proposed 10 sigma BB separation, can also go to 8 sigma. • Tatiana said that when BB dominates the sensitivity in chroma is lowered. She also stated that DA is proportional to lifetime. • Long Range beam-beam effects 2015 -No clear evidence of beam-beam long range and Head-on signature in physics fills - Beautiful Beams and Lumi lifetimes • Plot showing the Beam 1 Intensity lifetimes versus Q’ and 8 poles -Reducing Chroma and 8 poles is equivalent to 2 σ bb separa 3 on! As simula 3 ons predict! Discussion (questions, comments, etc) • Stefan and Yannis asked why don’t we go above 8. 5 sigma? Discussion about the uncertainty from 8. 5 to 11 sigma->answer was 10%. • In the plot mentioned above, why do we see the asymmetries concerning the trapezoid? (to be seen) • Stability (or not) of non-colliding and colliding bunches Conclusions -In 2016 we start with 10 σ separation, assuming high 8 poles and high chromaticity -We might want to push luminosity reducing angle but only and only if: repeat long range crossing angle scan, repeat 8 pole and chromaticity reduction, repeat stability threshold measurements.

Backup slides slide 2 A B