MEIC New Baseline: Performance and Accelerator R&D Yaroslav Derbenev, Fanglei Lin, Vasiliy Morozov, Yuhong Zhang

New MEIC Baseline New major components Energy range Design points Collider ring circumference: 2200 m Electron collider ring: PEP-II magnets and vacuum chambers Electron ring RF: PEP-II, (476 MHz base frequency) Ion collider ring: super-ferric magnets Warm ion linac Single booster ring: 285 MeV to 8 GeV Energy range Electron: 3 to 12 GeV Proton: (8) 20 to 100 GeV Lead ions: up to 40 GeV Design points Low energy: 30 GeV x 4 GeV (space charge dominated) Medium energy: 100 GeV x 5 GeV (beam-beam dominated) High energy: 100 GeV x 10 GeV (SR dominated) Reference: 60 GeV x 5 GeV

Collider Performance: The Big Picture Luminosity Polarization Mitigation Circumference (1.4 km  2.2 km) Space charge Cooling efficiency down up to 33% improving e-pol lifetime Hour-glass Traveling focusing PEP-II magnets High current Large emttiance No 12 GeV for quad Yes Very bad! Different lattice PEP-II RF stations 750 MHz  496 MHz Large bunch charge Down up to 33% Super-ferric magnets Still slow ramping Single booster May crossing γt High energy injection to full size ring More options for cooling crossing γt should have no or small impact imaginary γt Warm ion linac Below are TBD Stack/cooler ring Improve duty factor

Low Energy (Ecm~21.9 GeV) Parameters Detector type Full acceptance Large Acceptance Proton Electron Beam energy GeV 30 4 Collision frequency MHz 476 Particles per bunch 1010 0.41 3 Beam Current A 0.32 2.3 Polarization % > 70 ~ 80 RMS bunch length cm 1.2 emittance, normalized µm rad 0.35 / 0.14 74 / 29.6 0.35 / 0.12 Emittance aspect 2.5 Horizontal and vertical β* 5 / 2 5.8 / 2.3 2 / 0.8 2.8 / 0.93 Spot size at IP, horiz. and vert. µm 23.4 / 9.36 14.8 / 5.9 Vertical beam-beam tune shift 0.015 0.018 Laslett tune shift 0.06 Very small Distance from IP to 1st FF quad m 7 3.5 4.5 Hour-glass effect 0.76 0.49 Luminosity/IP, HG corrected, 1033 cm-2s-1 1.6 2.6 Slide 4 4

Design Considerations: Low Energy Low proton energy, space charge dominated, proton bunch charge limited Optimization: longer bunch  more charge but there is a hour-glass effect double proton bunch length, 2 cm  4 cm, which can double the proton bunch charge, hour glass effect is 0.76 net luminosity boosting: 2 * 0.76 = 1.52

Medium Energy (Ecm~44.7 GeV) Parameters Detector type Full acceptance Large Acceptance Proton Electron Beam energy GeV 100 5 Collision frequency MHz 476 Particles per bunch 1010 0.66 3.9 Beam Current A 0.5 3 Polarization % > 70 ~ 80 RMS bunch length cm 2 1.2 emittance, normalized µm rad 0.35 / 0.07 144 / 28,8 144 / 28.8 Emittance aspect Horizontal and vertical β* 17.5 / 3.5 7.5 / 1.5 9 / 1.8 4 / 0.8 Spot size at IP, horiz. and vert. µm 24 / 4.8 33.2 / 6.6 17.2 / 3.4 24.3 / 4.9 Vertical beam-beam tune shift 0.012 0.033 0.011 0.034 Laslett tune shift 0.027 Very small Distance from IP to 1st FF quad m 7 3.5 4.5 Hour-glass effect 0.87 0.72 Luminosity/IP, HG corrected, 1033 cm-2s-1 5.0 8.0 Slide 6 6

Design Considerations: Medium Energy Optimization: not match the spot size of two colliding beams strong-weak beam-beam regime electron beam-beam tune-shift is an order of magnitude smaller than the design parameter limit (0.018 vs. 0.15) Electron emittance is very large, so the beam spot size is very large Will loss luminosity (about 15% to 26%) if matching the large electron beam spot size We don’t match the spot size of two colliding beams (ratio is 0.72) Electron β*y is reduced to 1.5 cm

High Energy (Ecm~63.3 GeV) Parameters Detector type Full acceptance Large Acceptance Proton Electron Beam energy GeV 100 10 Collision frequency MHz 159 Particles per bunch 1010 2.0 2.8 Beam Current A 0.5 0.72 Polarization % > 70 ~ 80 RMS bunch length cm 3.5 1.2 emittance, normalized µm rad 0.5 / 0.1 1152 / 230 Aspect ratio 5 Horizontal and vertical β* 22.5 / 4.5 7.5 / 1.5 12 / 2.4 4 / 0.8 Spot size at IP, horiz. and vert. µm 32.6 / 6.5 66.4 / 13.3 15.9 / 3.2 27.2 / 5.4 Vertical beam-beam tune shift 0.003 0.027 Laslett tune shift 0.033 Very small Distance from IP to 1st FF quad m 7 4.5 Hour-glass effect 0.75 0.58 Luminosity/IP, HG corrected, 1033 cm-2s-1 0.97 1.4 Slide 8 8

Design Considerations: High Energy Optimization: reducing bunch repetition rate (number of bunches in ring) electron synchrotron radiation dominated regime electron current is limited to 0.72 A at 10 GeV L ~ f N1 N2 ~ I1 I2 / f Proton bunch length needs to be adjusted to satisfy the boundaries of beam effects (space charge, etc.) Optimization: not matching beam spot size at IP weak beam-beam regime electron beam-beam tune-shift is an order of magnitude smaller than the design parameter limit (0.006 vs. 0.15) Proton beam-beam tune-shift is close to the limit We don’t match the spot size of two colliding beams (ratio is 0.5) Electron β*y is reduced to 1.5 cm

Reference Point Parameters Detector type Full acceptance Large Acceptance Proton Electron Beam energy GeV 60 5 Collision frequency MHz 476 Particles per bunch 1010 0.52 3.9 Beam Current A 0.4 3 Polarization % > 70 ~ 80 RMS bunch length cm 2 1.2 emittance, normalized µm rad 0.35 / 0.07 144 / 28,8 144 / 28.8 Emittance aspect Horizontal and vertical β* 15/ 5 7.5 / 1.5 6 / 1.2 4 / 0.8 Spot size at IP, horiz. and vert. µm 28.7 / 5.7 38.4 / 7.6 18.1 / 3.6 24.3 / 4.9 Vertical beam-beam tune shift 0.013 0.024 Laslett tune shift 0.06 Very small Distance from IP to 1st FF quad m 7 3.5 4.5 Hour-glass effect 0.89 0.71 Luminosity/IP, HG corrected, 1033 cm-2s-1 3.1 (5.5) 6.1 (12.2) Slide 10 Old design 10

Limiting Factors & Design Optimization SR power of the high energy electron beam  limiting average current, but permitting higher bunch charge  Reducing bunch repetition rate can increase luminosity as long as ion space charge is OK Space charge effect for low energy proton beam  limiting bunch charge, but permitting higher average current (higher bunch frequency)  Increase bunch length for storing more charge in a bunch (hour glass effect)  Increase bunch repetition rate Beam-beam effect  limiting bunch charge, but permitting higher average current (bunch frequency)

Limiting Factors & Design Optimization Unusually higher electron emittance  Due to the large bending angle of PEP-II dipoles and also its lattice  Squeezing through the IR magnets (final focusing) is a challenge Ion beam formation/electron cooling Space charge limit at low energy Cooling time (increasing by 33%) Electron beam instabilities Too long damping time at low energy (3, 4 GeV) Interaction region

Short Term R&D: Reduction of Electron Emittance Problem A factor of 2.5 larger than the previous baseline, requiring large aperture Affecting luminosity (matching beam spot size at IP) It does not meet the requirement of the present IR design Possible mitigation approaches Hardware approach New short dipoles Lattice approach TME lattice Mixed FODO-TME lattice Insertion approach Damping wiggler Related problem: improving damping time for low energy

Parameters with Small Emittance Detector type Full acceptance Large Acceptance Proton Electron Beam energy GeV 100 5 Collision frequency MHz 476 Particles per bunch 1010 0.66 3.9 Beam Current A 0.5 3 Polarization % > 70 ~ 80 RMS bunch length cm 2 1.2 emittance, normalized µm rad 0.45 / 0.09 55 / 11 Emittance aspect Horizontal and vertical β* 10 / 2 7.5 / 1.5 5.5 / 1.1 4 / 0.8 Spot size at IP, horiz. and vert. µm 20.6 / 4.1 15.2 / 3.1 20.5 / 4.1 Vertical beam-beam tune shift 0.015 0.044 0.043 Laslett tune shift Very small 0.027 Distance from IP to 1st FF quad m 7 3.5 4.5 Hour-glass effect 0.89 0.72 Luminosity/IP, HG corrected, 1033 cm-2s-1 8.6 12.8 Slide 14 14

Luminosity with Smaller Emittance Energy Electron emttiance Luminosit at full acceptance detector Luminosit at full at large acceptance detector mm mrad 1033 cm-2s-1 30x4 74 / 28 1.6 2.4 100x5 144 / 55 5 / 8.6 8 / 12.8 100x10 1152 / 440 0.97 / 2.2 1.4 / 3.1 60x5 3.1 / 4.2 6.1 / 8.0

MEIC SC Ion Linac

Up to 200 MeV/u (proton to uranium) CW operation Up to 200 MeV/u (proton to uranium)

CERN Heavy Ion Accelerating Facility MEIC warm ion linac Could we develop a similar ion injector like this? (Optional) an extra compact booster for heavy ions Particularly, having a cost effective warm ion linac MEIC booster ring MEIC collider ring

IR Design for Achieving Higher Luminosity Optimization of electron IR for accommodation of large emittance Different permanent quad covering the energy range Reducing electron β*y to 1.2 cm or even smaller Taking advantage of smaller detector space Traveling focusing (TF) scheme Presently, 10% to 50% loss of luminosity due to hour glass effect Proton bunch length is longer than beta-star Electron beam must be TF scheme friendly (short bunch length and large beta-star) Large Piwinski angle/crab waist

Beam-Beam New design elements in IR Unmatched beam spot sizes at IP Long bunch length and hour-glass effect Traveling focusing Coherent instabilities due to change of harmonic numbers

Consideration of MEIC Upgrade Path Present approach First stage baseline (MEIC) up to 100 GeV x 12 GeV (10 years) Energy upgrade (EIC) up to 250 GeV x 20 GeV Alternative approach First stage baseline (MEIC) up to 100 GeV x 10 GeV (5 years) Luminosity upgrade (HL-MEIC) same, luminosity x 3~4 (5 years) Energy upgrade (HE-MEIC=EIC) up to 250 GeV x 14(?) GeV What is the first stage baseline? Very conservative, all based on “ready-to-build” technologies Acceptable starting performance Significant relaxation of the machine design Minimum technical uncertainty and R&D requirement (weak cooling) Minimum cost (PEP-II RF station (476 MHz)

Consideration of MEIC Upgrade Path What constitutes a luminosity upgrade? (A factor of 4 to 5) Doubled the bunch repetition rate (476 MHz  952 MHz) Increase of the beam current Reduction of beam emittance (better cooling? Better electron lattice?) Redesign of IR (beta-star reduction by a factor 2?), including DA Unmatched beam size? More aggressive hour-glass effect? A little large beam-beam? Space charge compensation? Electron lens? Small cost (a fraction of energy upgrade cost)