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Beamstrahlung at circular e + e - higgs factories. Marco Zanetti (MIT). Outline. Introduction Beamstrahlung effects, comparision of (T)LEP(3) and ILC Photon spectrum and pairs background Luminosity profile Power BS lifetime How to cope with BS in a e+e- machine

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## Marco Zanetti (MIT)

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**Beamstrahlung at circular e+e- higgs factories**Marco Zanetti (MIT)**Outline**• Introduction • Beamstrahlung effects, comparision of (T)LEP(3) and ILC • Photon spectrum and pairs background • Luminosity profile • Power • BS lifetime • How to cope with BS in a e+e- machine • First attempt of multiturn simulation • Summary**Introduction**• Affordable RF power (O(100) MW per beam) limits the total beam current => high lumi achieved by small beam sizes at IP • Beamstrahlung effects become the limiting factor • Differently than the ILC, beams cannot be disrupted after collision. • BS lifetime needs to be at least larger than burn-out lifetime • Can the parameters be adjusted such that BS doesn’t turn into a showstopper? • Mild BS implies other general good features • In the following results from guinea-pig simulation • TLEP-H and LEP3 considered as examples**Beamstrahlung**• Beamstrahlung dependencies: • Flat beams, vertical size affects only luminosity • For a given bunch length, horizontal size and particles per bunch drive the BS effects • Same dependencies for the BS photon energy • Circular collider parameters designed to lead to smaller BS**BS Photon**• LEP3 BS photon spectrum is much softer than ILC • Tails up to only a few GeV, compared to tens of GeV for ILC • As a consequence much reduced pairs background BS g spectrum pairs spectrum**Luminosity profile**• Softer BS photon spectrum implies much better luminosity profile (L0.01: how much luminosity is delivered within 1% of nominal √s ) • Important for Physics • ISR not included, single crossing simulation • Very similar performances for all circular collider options • To be confirmed by multi-turn simulation**Power**• The spectrum is softer and ng is smaller than ILC, but (T)LEP(3) have up to ~x100 more particles per bunch. • Comparable power dissipation for ILC and circular colliders, O(10) kW • Most of the power dissipated at very small angle LEP3 Power (W/0.2 mrad)**BS lifetime**• Simulate and track O(108) macroparticles and check the energy spread spectrum • Lifetime computed from the fraction of particles beyond a given momentum acceptance (h) • Exponential dependence on h Lifetime>4h h=3% TLEP-H**Dealing with BS**• Scan relevant BS parameters: • B*x to scale horizontal beam dimension • Number of particle per bunch • BS lifetime for nominal parameters (assuming h=0.04): • LEP3: >~ 30 min • TLEP-H: ~day • >4h for h=0.03, ~4 min for h=0.02 LEP3, h=0.02 LEP3, h=0.04**Dealing with BS**• Baseline approach relies on enhanced momentum acceptance • Getting h=4% not straightforward and needs to be worked out: • Possible IR optics, R. Tomas at 2nd LEP3 workshop • Relaxing the requirement on h requires either increasing sx or diminishing N • Same luminosity can be recovered by enhancing the aspect ratio currently very conservative (same as LEP2) • Keep same bx, redistribute particles over more bunches, compensate by reducing ey • Successfully implemented by Uli Weinand et al. (2nd LEP3 workshop) • Keep same bunch pattern and total current, increase bx and compensate with ey • Bottomline: margin is there to cope with BS • TLEP-H is already almost ok!**Multi-turn simulation**• Split the RF system 4 sections (as at LEP2) • Integrate beam-beam simulation (single IP) with simple longitudinal and transverse dynamics • Very preliminary (and not debugged) results: • Very stable longitudinal motion • Transverse motion not yet under control • Vertical coordinate OK • Horizontal coordinate diverging when beams are colliding**Summary**• Beamstrahlung can be a severe issue for circular e+e- machines • Beam parameters (N and sx) need to be such to mild BS effects • Several positive consequences: • Smaller pairs background • Monochromatic luminosity profile • Limited power dissipation • To achieve reasonable BS lifetimes currently a high momentum acceptance (4%) is required • Request can be relaxed by improving aspect ratio (very conservative at the moment) • TLEP-H already almost OK for standard h • To be studied: • Multiturn simulation • Effect of vertical offset (enhancing BS effects)**Multi-turn simulation**• Goal is to check the beam parameters at equilibrium • Integrate beam-beam simulation with simple longitudinal and transverse dynamics • First attempt with only one collision point on the RF system located in a single point • Transverse motion: • Synchrotron motion: • Radiation damping & quantum excitation:**SAPPHIRE**• Simulate laser electron beam compton scattering, then use guinea-pig to collide the backscattering photons • Look at the luminosity profile versus √s • For 3 different effective distances • For 3 different electron polarization • Profile and overall behavior as expected • For Egg>0.6ECM, L=3.6e33 cm-2 s-1

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