Beam-Beam head-on Limit J. Barranco, X. Buffat, T. Pieloni, C. Tambasco, J. Qiang, K. Ohmi, M. Crouch for the Beam-Beam team BI BSRT Team George and Enrico

Head-on beam-beam limit Pushing Luminosity  Large Beam-Beam parameter  bb Are there show stoppers? Beam-Beam Limit MD 2011  No fast blow-up  Saw problems of fast blow-up not fully understood for Qy=0.32 (moved it to 0.315) 10 th order resonances?  Achieved  bb = 0.01 per IP (2 IPs)  This could be a source of emittance blow-up during collisions needs to be explored in controlled way  HL-LHC relies on  bb = 0.01 and several Ips

Head-on beam-beam limit Pushing Luminosity  Large Beam-Beam parameter  bb  Characterize losses versus  bb and working point  Characterize emittance slow growth versus parameters and working points to define limits  Explore tune diagram to find limits Simulations: Slow emittance growth (4-10%/h) due to resonances (COMBI simulations) Losses versus working point (DA tune scans) Tune scan down the diagonal to see the limit Coherent beam-beam modes and noise- how do they contribute?

HL-LHC transverse offsets and emittance growth Leveling with transverse offset still option for HL-LHC Long range interactions will give orbit effects at the IPs. How large the effect? Emittance growth is induced by small offsets at collision and has intensity threshold! Is HL-LHC safe? 2012 LHC test not conclusive LHC nominal Slow Emittance growth

1st part : injection energy tune scan 8h Incoherent HO limit (resonances): Simulations: Emittance effects from HO beam-beam is independent on energy and  *  Injection (coll tunes) can help exploring the limits  Beams: single bunches as bright as possible (3-2-1 e11 ppb smallest emittances)  Growth and losses versus BB parameter and working point BSRTs & Wire scanners (monitor emittances in time at different working points) ADT Feedback on: ensure no instabilities Collide Head-on and Observe all bunches Changing Tune scan along diagonal:  Monitor emittances and Losses  Luminosity measurements from Experiments Possibly test separation at 0.5 sigma both IPs and 0.8 sigma

2 nd step: need to ramp, squeeze and collide high brightness beams at top energy  Top Energy  Beams: single bunches as bright as possible (3-2-1 e11 ppb smallest emittances)  Need to guarantee single bunch stability (not obvious)  Growth and losses versus BB parameter and working point BSRTs & Wire scanners (monitor emittances in time at different working points) ADT Feedback on: ensure no instabilities Collide Head-on and Observe all bunches Possibly test separation at 0.5 sigma both IPs and 0.8 sigma Changing Tune scan along diagonal:  Monitor emittances and Losses  Luminosity measurements from Experiments